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- Indico Weeks View
We have carried out measurements, using Miniball, of the $\gamma$-ray de-excitation of $^{222,228}$Ra and $^{222,224,226}$Rn nuclei Coulomb-excited by bombarding $^{60}$Ni and $^{120}$Sn targets. The beams of radioactive ions, having energies of between 4.25 and 5.08 MeV.A, were provided by HIE-ISOLDE at CERN. The purpose of these measurements is to determine the intrinsic quadrupole and octupole moments in these nuclei and look for other cases of permanent octupole deformation to those of $^{224,226}$Ra already reported$^{1,2}$. Another aim of this experiment is to determine the level schemes of $^{224,226}$Rn in order to characterise these isotopes as octupole vibrational or octupole deformed. We present here the preliminary results from these measurements, including the implications for EDM searches.
$^1$ Gaffney L P et al. 2013 Nature $\bf 497$ 199
$^2$ Wollersheim H J et al. 1993 Nuclear Physics A $\bf 556$ 261
Transfer reactions produce a wealth of nuclei in a wide energy and angular range and with cross sections spanning several orders of magnitude. Total angle and energy integrated cross sections for transfer channels have been investigated with spectrometers in various systems close to the Coulomb barrier. Such ingredients allow one to understand how nucleons are exchanged between projectile and target and how energy and angular momentum are transferred from the relative motion to the intrinsic excitation. The recent results of the multinucleon transfer reaction studies with neutron-rich projectile emphasized that these reactions provide a suitable mechanism to populate neutron-rich heavy nuclei [1,2].
The transfer reactions are among the most important tools to probe nucleon-nucleon correlations in nuclear systems. The pairing interaction induces correlations that are essential in defining the properties of finite quantum many body systems in their ground and neighboring states. These structure properties may influence in a significant way the evolution of the collision. Recently, pair correlations were probed in heavy ion collisions by performing studies far below the Coulomb barrier with the PRISMA spectrometer for several systems. The microscopic calculations that incorporate nucleon-nucleon correlations well reproduce the experimental data in the whole energy range, in particular, the transfer probability for two neutrons is very well reproduced, in magnitude and slope [3].
The talk will focus on the main outcome of these recent studies, critically addressing the new achievements, the present problems and new challenges, especially in view of forthcoming experiments to be performed with exotic beams at the radioactive beam facilities.
[1] T. Mijatovic et al., Phys. Rev. C 94 (2016) 064616.
[2] F. Galtarossa et al., Phys. Rev. C 97 (2018) 054606.
[3] D. Montanari et al., Phys. Rev. Lett. 113 (2014) 052601.
The lifetimes of the waiting point nuclei at N = 126 of the rapid neutron capture process (r-process) are important parameters to investigate the astrophysical environment of the r-process. However, the difficulty in the production of those extremely neutron-rich nuclei makes their experimental studies unfeasible. Therefore, the theoretical nuclear models play crucial roles in the simulation of the r-process nucleosynthesis. The experimental studies of lifetimes, masses and nuclear structures of the neutron-rich nuclei around N = 126 provide significant inputs to those theoretical models to improve their predictability for the waiting point nuclei.
We are developing KEK Isotope Separation System (KISS) at RIKEN RIBF facility to produce and separate those neutron-rich nuclei for the measurements of the beta-gamma spectroscopy, the lifetime and the mass [1-2]. The multi-nucleon transfer (MNT) reactions between the Xe-136 beam and the Pt-198 target are employed to produce those nuclei. The MNT reactions were studied at GANIL to investigate their feasibility to produce the neutron-rich nuclei around N = 126, demonstrating its promising potential [3-4]. The KISS consists of an argon-gas-cell-based laser ion source and an isotope separation on-line system to extract a single species of the reaction products. The detector system composed from a multi-segmented gas counter [5] and high-purity germanium detectors makes it possible to perform their beta-gamma spectroscopy and laser ionization spectroscopy.
In this presentation, we will report the present status of the KISS including the recent experimental results of nuclear spectroscopy and the future plan.
[1] Y. Hirayama et al., Nucl. Instrum. and Methods B 353 (2015) 4.
[2] Y. Hirayama et al., Nucl. Instrum. and Methods B 376 (2016) 52.
[3] Y.H. Kim et al., EPJ Web of conferences 66 (2014) 03044.
[4] Y.X. Watanabe et al., Phys. Rev. Lett. 115 (2015) 172503.
[5] M. Mukai et al., Nulc. Instrum. and Methods A 884 (2018) 1.
The physics of nuclear reactions is crucial for understanding element creation in the Universe, and is therefore at the core of science programmes in new generation facilities. I will report on novel theoretical developments in describing low-energy fusion dynamics of heavy ions and weakly bound nuclei using the time-dependent wave-packet method. Topical applications of the method include the incomplete fusion of weakly bound nuclei at Coulomb energies [1] and resonances in stellar carbon fusion [2]. Perspectives of the method for identifying resonant behaviour in nuclear collisions will be discussed [3].
[1] M. Boselli and A. Diaz-Torres, Physical Review C 92 (2015) 044610.
[2] A. Diaz-Torres and M. Wiescher, Physical Review C 97 (2018) 055802.
[3] A. Diaz-Torres and J.A. Tostevin, arXiv: 1809.10517.
The often surprising properties of neutron-rich nuclei have prompted extensive experimental and theoretical studies aimed at identifying the driving forces behind the dramatic changes encountered in the exotic regime. In-beam nuclear spectroscopy with fast beams and thick reaction targets where $\gamma$-ray spectroscopy is used to tag the final state provides information on the single-particle structure as well as on collective degrees of freedom in nuclei that are available for experiments at beam rates of only a few ions/s. This presentation will show how in-beam experiments measure complementary observables that advance our understanding. The interplay of experimental results and theory will be emphasized at the intersection of nuclear structure and reactions in the joined
quest of unraveling the driving forces of shell evolution.
The AGATA array [1], is the European forefront instrument based on semiconductor Germanium detectors, for high-resolution position sensitive gamma-ray spectroscopy.
AGATA is being built in a collaborative effort of more than 40 institutes in 11 countries. The conceptual design of AGATA foresees a 4π array with 60 triple clusters containing 180 Ge encapsulated detectors [2]. Nevertheless, smaller sub- arrays of AGATA have been implemented, first as a prove of concept for a tracking array at INFN-LNL [3] and later to prove the potential of AGATA in different experimental conditions as well as to profit from the scientific possibilities offered by European large scale facilities. Since 2012 AGATA sub-arrays have been installed at the FAIR/NUSTAR-precursor PRESPEC set-up [4], placed at the focal plane of the FRS Fragment Separator in GSI, where experiments with in-flight highly relativistic exotic beams were performed, and in 2014 at GANIL and SPIRAL where experiments with high-intensity stable beams and reaccelerated ISOL radioactive beams are expected to be performed till 2020 [5].In this contribution the AGATA project will be presented, emphasising the capabilities and performance figures, relevant for the present and future European facilities. Finally the recent results of the AGATA experimental activity, coupled with different complementary instruments in the mentioned host laboratories, will be reported.
[1] The AGATA Collaboration, Nucl. Instrum. Methods Phys. Res., Sect. A 668, 26 (2012).
[2] E. Farnea et al., Nucl. Instrum. Methods Phys. Res., Sect. A 621 (2010) 331.
[3] A.Gadea et al., Nucl. Instrum. Methods Phys. Res., Sect. A 654, 88 (2011).
[4] N. Pietralla et al., EJP Web of Conferences 66, 02083 (2014) and http://web-docs.gsi.de/~wolle/PreSPEC/
[5] E. Clément et al., Nucl. Instrum. Methods Phys. Res., Sect. A 855 (2017) 1
In this work we discuss the results of a recent HELIOS [1] measurement of the (d,p) reaction on 18F, from both the ground (1+ ) and isomeric (5+) states, to the members of the 19F ground-state band [2] in the rotational model
We consider the structure of 18,19F in terms of Nilsson single-particle orbits originating from the sd spherical levels coupled to a deformed core, and calculate the (d,p) spectroscopic strengths to 19F from both the ground and isomeric states following the framework reviewed in [3]. Our results show good agreement with the experiment and the shell model.
[1] A. Wuosmaa, et al. Nucl. Instrum. Methods, A580, 1290 (2007).
[2] D. Santiago Gonzalez, et al. Phys. Rev. Lett. 120, 122503 (2018).
[3] B. Elbek and P. O. Tjøm, in Advances in Nuclear Physics,
M. Baranger and E. Vogt eds. (Springer, Boston, MA, 1969).
*This material is based upon work supported by the U.S. DOE, Office of Science,
Office of Nuclear Physics, under Contract No. DEAC0205CH11231.
In recent years, significant progress has been made in ab initio nuclear structure and reaction calculations based on input from QCD employing Hamiltonians constructed within chiral effective field theory. One of the modern approaches is the No-Core Shell Model with Continuum (NCSMC) [1,2], capable of describing both bound and scattering states in light nuclei simultaneously. We will present latest NCSMC calculations of weakly bound states and resonances of exotic halo nuclei 11Be and 15C and discuss the photo-dissociation of 11Be and 14C(n,γ)15C capture. We will also present our results for their unbound mirror nuclei 11N and 15F, respectively. We will point out the effects of continuum on the structure of mirror resonances and highlight the role of chiral NN and 3N interactions. Finally, we will discuss polarization effects in the 3H(d,n)4He fusion [3]. This transfer reaction is relevant for primordial nucleosynthesis and is being explored in large-scale experiments such as NIF and ITER as a possible future energy source.
[1] S. Baroni, P. Navratil, and S. Quaglioni, Phys. Rev. Lett. 110, 022505 (2013); Phys. Rev. C 87, 034326 (2013).
[2] P. Navratil, S. Quaglioni, G. Hupin, C. Romero-Redondo, A. Calci, Physica Scripta 91, 053002 (2016).
[3] G. Hupin, S. Quaglioni, and P. Navratil, Nature Communications (2019) 10:351; https://doi.org/10.1038/s41467-018-08052-6
*Supported by the NSERC Grant No. SAPIN-2016-00033. TRIUMF receives federal funding via a contribution agreement with the National Research Council of Canada.
Accurate studies on 13C spectroscopy have great impact in the present understanding of the role played by extra-neutrons in stabilizing alpha-cluster structures formed in light nuclei. 13C excited states are in fact the simplest systems that can be formed by adding a neutron to a triple-alpha molecular-like structure. Their spectroscopic properties are therefore a fundamental benchmark for theoretical models aiming at describing clustering in light nuclei. To improve our knowledge of 13C structure, we performed a comprehensive R-matrix fit of $\alpha$+9Be elastic and inelastic scattering data in the energy range Ex≈3.5 – 10 MeV at several angles. To carefully determine the partial decay widths of states above the $\alpha$-decay threshold we included in the fit procedure also 9Be($\alpha$,n0)12C and 9Be($\alpha$,n1)12C cross section data taken from the literature. This analysis allows to improve the (poorly known) spectroscopy of excited states in 13C in the Ex≈12-17 MeV region, and tentatively suggests the presence of a large-deformation negative-parity molecular band.
The interaction of neutrons with 7Be that was measured at the SARAF in Israel with a quasi-Maxwellian neutron beam at 49.5 keV reveals a strong B(E1: 2- ---> 2+) ~ 0.04 W.u., decay of the 2- state at 18.91 MeV in 8Be to the alpha-cluster 2+ state at 3.03 MeV [1]. This strong E1 decay leads to large cross section of the 7Be(n,g_1)*8Be(3.03) reaction at the “BBN window". It implies s-waves dominance of the cross section at the “BBN window”, in contrast to previous extrapolations into the “BBN window” from lower energies (the n_TOF measurement [2]) and extrapolation from higher energies (the Kyoto measurement [3]). In addition, the phenomenological structure of all states below 19.5 MeV in 8Be (including the 2- state at 18.91 MeV) provides good evidence for particle-hole (p-h) states in the newly proposed Cluster Shell Model (CSM) of Della Roca and Iachello [4]. The states near the neutron and proton thresholds in 8Be show the characteristic of the p-h states predicted by the CSM. The measured B(E1) of the 2- state at 18.91 MeV is in accordance with other measured decays of the p-h CSM states to the well-known cluster ground-states and 2+ state at 3.03 MeV in 8Be. The new CSM model of Della Roca and Iachello [4] will be introduced with emphasize on the similarity between p-h states in 8Be and single particle states in 9Be.
The material presented in this paper is based upon work supported by the U.S.-Israel Bi National Science Foundation, Award No. 2012098, and the U.S. Department of Energy, Office of
Science, Nuclear Physics, Award No. DE-FG02-94ER40870.
[1] M. Gai, arXiv:1812.09914v1, (2018).
[2] M. Barbagallo et al., Phys. Rev. Lett. 117, 152701 (2016).
[3] T. Kawabata et al., Phys. Rev. Lett., 118, 052701 (2017).
[4] V.DellaRocca and F.Iachello, Nucl. Phys. A 973, 1 (2018).
Low-energy heavy-ion reactions provide us a rich laboratory to study the equilibration dynamics of strongly interacting many-body systems. In particular, these reactions probe an intriguing interplay between the microscopic single-particle dynamics and collective motion at time scales too short for complete equilibration. In this presentation, we discuss recent microscopic studies of equilibration dynamics in deep-inelastic, quasifission, and fusion reactions. In this context we will discuss the equilibration dynamics and time-scales for various quantities that are connected to the experimentally observable entities. These include the study of mass, isospin, and total kinetic energy (TKE) equilibration time-scales. In most of these studies one is essentially dealing with the transport phenomena of isospin asymmetric systems [1,2]. These investigations provide us the ingredients to model such phenomena and help answer important questions about the nuclear Equation of State (EOS) and its evolution as a function of neutron-to-proton $N/Z$ ratio [3].
*This work has been supported by the U.S. DOE under Grant No. DE SC0013847 with Vanderbilt University and by the Australian Research Council Grant No. DP160101254.
[1] C. Simenel and A. S. Umar, Prog. Part. Nucl. Phys. 103, 19 (2018).
[2] K. Godbey, A.S. Umar, and C. Simenel, Phys. Rev. C 95, 011601(R) (2017).
[3] A.S. Umar, C. Simenel, and W. Ye, Phys. Rev. C 96, 024625 (2017).
An abnormal production of events with almost equal-sized fragments was theoretically proposed as a signature of spinodal instabilities responsible for nuclear multifragmentation in the Fermi energy domain. On the other hand finite size effects are predicted to strongly reduce this extra production. High statistics quasifusion hot nuclei produced in central collisions between Xe and Sn isotopes at 32 and 45 MeV per nucleon incident energies have been used to definitively establish, through the experimental measurement of charge correlations, the presence of spinodal instabilities. N/Z influence was also studied. The nature of the dynamics of a phase transition i.e. the fragment formation was the last missing piece of the puzzle concerning the liquid-gas transition in nuclei.
Ref. B. Borderie et al., INDRA coll., Phys. Lett. B 782 (2018) 291.
S. Pirrone(1), B. Gnoffo(1),(2), , G. Politi(1),(2) E. De Filippo(1) P. Russotto(3), G. Cardella(1), F. Favela(1), E. Geraci(1),(2) N. S. Martorana(2),(3) A. Pagano(1),(2), E.V. Pagano(3) E. Piasecki(4) L. Quattrocchi(1),(2) F. Rizzo(2),(3) M. Trimarchi(1),(5) and A. Trifirò (1),(5)
(1) INFN, Sezione di Catania -Catania, Italy
(2) Dipartimento di Fisica, Università degli Studi di Catania - Catania, Italy
(3) INFN, Laboratori Nazionali del Sud- Catania, Italy
(4) Heavy Ion Laboratory, University of Warsaw, Warsaw, Poland
(5) Dipartimento di Scienze Matematiche e Informatiche, Scienze Fisiche e Scienze della Terra, Università, Messina - Messinia, Italy
The reactions 78Kr + 40Ca and 86Kr + 48Ca at 10 A MeV, have been studied in Catania at LNS with the 4π multi detector CHIMERA.
For these systems, we have already analyzed the fusion-evaporation and fission-like processes, [1,2,3]. In this work we present a new study concerning the break-up of the Projectile-Like (PLF) into two fragments, following more violent deep-inelastic collisions.
A selection method has been developed, in order to discriminate PLF break up events from those due to other mechanisms which populate the same region of the phase-space.
A preference for PLF aligned break-up, along the direction of the PLF-TLF separation axis with the light fragment emitted in the backward part, has been evidenced, suggesting the presence of some dynamical effects.
As the isospin is expected to play a crucial role in the onset of this process; a comparison between the neutron-rich 86Kr +48 Ca and neutron-poor 78Kr +40 Ca systems will be presented.
[1] Gnoffo B., Il Nuovo Cimento C, 39 (2016) 275.
[2] Pirrone S. et al., Journal of Physic: Conf. Series, 515 (2014) 012018.
[3] Politi G. et al., JPS Conf. Proc., 6 (2015) 030082.
The study of the emitted particles, comparing pre-equilibrium and thermal components, is a useful tool to examine the nuclear structure. Possible clustering effects, which may change the expected decay chain probability, could be highlighted on the competition between different reaction mechanisms. The NUCL-EX collaboration (INFN, Italy) has carried out an extensive research campaign on pre-equilibrium emission of light charged particles from hot nuclei [1]. In this framework, the reactions $^{16}$O+$^{30}$Si, $^{18}$O+$^{28}$Si, $^{19}$F+$^{27}$Al at 7 MeV/u and $^{16}$O+$^{30}$Si at 8 MeV/u have been carried out using the GARFIELD+RCo array [2] at Legnaro National Laboratories, as a first step, where the fast emission mechanisms could be kept under control.
After a general introduction on the experimental campaign performed on different systems, which have evidenced anomalies in the $\alpha$-particle emission channel, this contribution will focus on the analysis results obtained in the measurement reported above, showing in an exclusive way the observed effects related to the entrance channels. The experimental results will be compared to model prediction, for which the same filtering and complete event selection have been applied.
[1] T. Marchi et al., F. Gramegna et al. - Nuclear Particle Correlations And Cluster Physics – Chapter 20 –pag. 507 (2017) –ISBN 978-981-3209-34-3; L. Morelli et al., Journ. of Phys. G 41 (2014) 075107; L. Morelli et al., Journ. of Phys. G 41 (2014) 075108; D. Fabris et al., PoS (X LASNPA), 2013, p. 061.D; V.L. Kravchuk, et al. EPJ WoCs, 2 (2010) 10006; O. V. Fotina et al., Int. Journ. Mod. Phys. E 19 (2010) 1134.
[2] F. Gramegna et al., Proc. of IEEE Nucl. Symposium, 2004, Roma, Italy, 0-7803-8701-5/04/; M. Bruno et al., M. Eur. Phys. Jour. A 49 (2013) 128.
The Radioactive Isotope Beam Factory (RIBF) at RIKEN provides the world’s highest intensity beams for the production of radioactive isotopes by in-flight fragmentation and fission. Stable beams at 345 MeV/u are impinging on primary targets and secondary beams are separated and identified in the BigRIPS fragment separator. In-beam gamma-ray spectroscopy towards the drip-lines utilizes the DALI2 array for maximum efficiency. To overcome the limited resolution, we currently constructing a germanium-based gamma-ray spectrometer composed of the MINIBALL clusters and several Ge tracking detectors from Japan, Europe, and the USA for experimental fast beam campaigns. The status of the project and the physics program will be presented.
Collective states in cold nuclei (yrast region) are represented by a wave function that assigns coherent phases to the participating nucleons. The degree of coherence decreases with excitation energy above the yrast line because of coupling to the increasingly dense background of quasiparticle excitations. The consequences of this damping mechanism will be discussed with a perspective on applications in nuclear astrophysics and technology.
For isoscalar quadruple vibrational multiplets, the rapid decoherence of the low-spin members will be contrasted with the coherent tidal wave motion of the yast members. The rapid decoherence or even absence of the beta vibration will be addressed. The screening of an oblate band in 137Nd from rotational damping by the prolate quasiparticle background will be discussed. The completely incoherent low energy M1 radiation and the scissors mode of warm nuclei will be addressed.
The microscopic self-consistent mean-field (SCMF) framework based on universal energy density functionals provides an accurate global description of nuclear ground states and collective excitations, from relatively light systems to super-heavy nuclei, and from the valley of beta-stability to the particle drip-lines. Based on this framework, structure models have been developed that go beyond mean-field approximation and include collective correlations related to restoration of broken symmetries and fluctuation of collective variables. In particular this includes i) generator-coordinate method with projections on particle number, angular momentum and parity, ii) implementations for the solution of the collective Hamiltonian for quadrupole and octupole vibrational and rotational degrees of freedom, iii) microscopically determined interacting boson model. These models have become standard tools for nuclear structure calculations, able to describe new data from radioactive-beam facilities and provide microscopic predictions for low-energy nuclear phenomena of both fundamental and practical significance.
In this talk some of the recent applications of the SCMF framework will be highlighted: studies of shape evolution and coexistence, quadrupole and octupole shape phase transitions and SCMF based analysis of the dynamics of spontaneous fission process. Finally, perspectives for future calculations will be discussed.
Nuclear fission, one of the oldest if not the oldest challenge to theoretical many-body physics in literature, is still awaiting a fully quantum microscopic description with a robust predictive power. Since its experimental discovery in 1939 only a few theoretical results have been firmly established in the quantum theory of fission, while many phenomenological and microscopic models, based on untested assumptions have been suggested. The evolution of the compound nucleus from the moment the neutron is absorbed until the saddle is reached was left basically in the dark by theory, and most of the attention was concentrated on the evolution of the nucleus from the saddle-to-scission, where fission fragment properties are defined. The main assumption was that this process is slow and moreover adiabatic, an assumption which allowed the separation of the degrees of freedom into collective and intrinsic. Being slow does not imply adiabaticity however. In a new time-dependent energy density formalism, free of any restrictions and assumptions, we demonstrate that the fission dynamics from saddle-to-scission is slow and even overdamped, but the intrinsic system gains a lot of entropy, and the energy gained from the collective degrees of freedom is never relinquished. The fission dynamics from the saddle-to-scission is much slower than the adiabatic assumption would imply, the collective flow energy never exceeding 1-2 MeV and the rest of the difference between the potential energy at the saddle and at the scission point is almost entirely converted into intrinsic energy or heat. This finding requires a complete retooling of most theoretical and phenomenological approaches, as the introduction of a potential energy surface and inertia tensor is completely illegitimate, the role of collective inertia is negligible. Agreement with experiment is surprisingly good, in spite of the fact that no parameters have been fitted and the results are rather stable with parameter changes.
The phenomenon of hindrance in sub-barrier heavy-ion fusion will be introduced and several experimental evidences show that it is a general phenomenon. It is recognized in many cases by the trend of the logarithmic slope of excitation function and of the S factor at low energies. The comparison with standard Coupled-Channels calculations is a more quantitative evidence for its existence.
Hindrance is observed in light systems, independent of the sign of the fusion Q-value, with different features. In the case of the $^{12}$C+ $^{30}$Si system the hindrance effect is small but it is clearly recognized. Near-by cases show evidence for systematic behaviors. A very recent experiment has concerned the lighter case $^{12}$C+ $^{24}$Mg where hindrance shows up clearly, because a maximum of the S-factor appears already at a relatively high cross section $\sigma$=1.6 mb. The consequences for the dynamics of stellar evolution have to be clarified by further experimental and theoretical work.
Possible interpretations of hindrance will be shortly illustrated, including a recent suggestion on the possible influence of Pauli blocking in the fusion dynamics.
Indeed in many heavier systems the hindrance effect has been recognized with different features depending on the various couplings to the inelastic and transfer channels. When transfer channels with positive Q-value are available, their effect is often important at low energies where it can compete with hindrance.
The 12C+12C fusion reaction is one of the key reactions governing the evolution of massive stars as well as being critical to the physics underpinning various explosive astrophysical scenarios [1]. Our understanding of the 12C+12C reaction rate in the Gamow window – the energy range relevant to the different astrophysical scenarios – is presently confused. This is due to the large number of resonances around the Coulomb barrier and persisting down to the lowest energies measured. In usual circumstances, where the fusion cross-section is smooth it can be readily extrapolated from the energy range measured in the laboratory down to the Gamow window but this is not possible for 12C+12C.
Jiang et al. have developed a new experimental approach to study of the 12C+12C reaction which can circumvent issues related to target contamination [2]. They used the Gammasphere array to detect fusion gamma rays in coincidence with detection of evaporated charged particles using annular silicon strip detectors [2]. This technique has shown considerable promise in essentially removing experimental background from the measurement [2].
The STELLA experiment has been established at IPN Orsay. A intense 12C beam from the Andromede accelerator is incident on thin self-supporting 12C foils. A target rotation system can allow for cooling supporting μA beam currents. Evaporated charged particles are detected with a dedicated silicon array while gamma rays are detected in coincidence with an array of 30 LaBr3 detectors [3]. The design and status of STELLA will be presented along with results on the cross-sections and astrophysical S-factors obtained down into the Gamow window for massive stars.
REFERENCES
[1] A. Chieffi et al., Astrophys. J 502, 7373 (1998).
[2] C.L. Jiang et al., Nucl. Instrum. Meth. A 682, 12 (2012).
[3] M. Heine et al., J. Phys. Conf. Ser. 763, 012005 (2016).
Recent developments of the relativistic nuclear field theory on the finite-temperature formalism will be presented. The general non-perturbative framework, which advances the nuclear response theory beyond the one-loop approximation, is formulated in terms of a closed system of non-linear equations for the two-body Green’s functions. This provides a direct link to ab initio theories and allows for an assessment of accuracy of the approach. This framework has been extended recently to the case of finite temperature, for both neutral and charge-exchange channels [1-3]. For this purpose, the time blocking approximation to the time-dependent part of the in-medium nucleon-nucleon interaction amplitude is adopted for the thermal (imaginary-time) Green’s function formalism. The method is implemented self-consistently on the base of Quantum Hadrodynamics and designed to connect the high-energy scale of heavy mesons and the low-energy domain of nuclear medium polarization effects in a parameter-free way, now also at finite temperature. In this approach we investigate the temperature dependence of nuclear spectra in various channels, such as the monopole, dipole, quadrupole and charge-exchange ones, for even-even medium-heavy nuclei. The special focus is put on the giant dipole resonance’s width problem, the low-energy strength distributions and the influence of temperature on the equation of state. The temperature dependence of the spin-isospin excitations is studied for its potential impact on the astrophysical modeling of supernovae and neutron-star mergers.
References
[1] E. Litvinova and H. Wibowo, Phys. Rev. Lett. 121, 082501 (2018).
[2] H. Wibowo and E. Litvinova, arXiv:1810.01456, submitted to Phys. Rev. C.; E. Litvinova and H. Wibowo, arXiv:1812.11751, submitted to Eur. Phys. J. A.
[3] E. Litvinova, C. Robin and H. Wibowo, arXiv:1808.07223.
$\alpha$ cluster structures are well known to appear in excited states of lighter mass nuclei. According to recent studies, the isoscalar monopole (IS0) and dipole excitations (IS1) are considered to be important probes to identify the alpha cluster struture. We have calculated the continuum IS0 and IS1 transitions in the $^{44}$Ti = $\alpha$ + $^{40}$Ca system. We will demonstrate that the prominent enhancement will occur in the lower excitation energy than the single particle excitation energy due to the development of the alpha cluster structures. We have also extend the similar calculation to the much heavier systems, such as the Te isotopes with the $\alpha$ + Sn structure in the mass range from A=104 to A=110. From a series of our calculations, the systematic enhancement in the IS0 and IS1 strengths has been confirmed in the lower excitation energy of $E_x\leq$ 15 MeV.
Furthermore, the dissociation strength of $^{135}$Cs into $\alpha$ + $^{131}$I, which is induced by the electric dipole (E1) field, will also be discussed. The $^{135}$Cs nucleus is a kind of long lived fission products (LLFPs) in nuclear wastes. From the viewpoint of the alpha cluster structure, there is a possibility that the low-lying E1 transition will be effective for the transmutation of $^{135}$Cs.
Heavy-ion multinucleon transfer reactions at around the Coulomb barrier offer unique opportunity to study a variety of non-equilibrium nuclear dynamics, such as energy dissipation, nucleon transfer, shape evolution, fusion, and so on. Besides the fundamental interest into the underlying reaction mechanism, it possesses substantial importance as a means for producing new, neutron-rich heavy nuclei, whose properties are crucial to figure out the detailed scenario of the r-process nucleosynthesis. Aiming at prediction of optimal reactions for producing yet-unknown neutron-rich unstable nuclei, I have extensively developed and applied methods based on the microscopic framework of the time-dependent Hartree-Fock (TDHF) theory. In this talk, I will review our recent works and progress, showing how the theory works in practice, making possible comparisons with available experimental data.
Deuteron-induced reactions have a long and fruitful tradition in nuclear physics as an experimental tool for spectroscopy. They have been extensively used to study in detail the single-particle nature of the low-lying spectrum of the nuclear quantum many-body system. Standard reaction theory describing the direct population of sharp bound states have been very successful in extracting detailed structural information from the experimental data, in the form of spin, parities, spectroscopic factors, etc., of the populated bound states. The advent of high intensity exotic beams have granted experimental access to weakly bound systems with a Fermi energy close to the neutron-emission threshold, where the role of the continuum becomes important. Within this context, new theoretical developments are called for, such as a reaction framework able to account for the population of resonant and non-resonant states of the continuum, adapted to the associated structure description of the target-neutron interaction. Aside from paving the way
to the description of (d,p) reactions in exotic loosely bound nuclei in terms of state-of-the-art neutron-target interactions, such a framework can also be used to describe the formation of a compound nucleus in the neutron+target channel. The formalism presented here is thus also an important theoretical ingredient for the use of (d,p) reactions as surrogates for neutron capture processes.
Transfer reactions have always been of great importance for nuclear structure and reaction mechanism studies. With heavy ions it becomes feasible to transfer several nucleons and a considerable amount of energy and angular momenta from the relative motion to the intrinsic degrees of freedom. So far proton pickup channels have been identified in atomic and mass numbers at energies close to the Coulomb barrier only in few studies.
We will show a comprehensive study of the multinucleon transfer reaction $^{40}$Ar+$^{208}$Pb measured near the Coulomb barrier, by employing the PRISMA magnetic spectrometer. By using the most neutron-rich stable $^{40}$Ar isotope we could populate, besides neutron pickup and proton stripping channels, also neutron stripping and proton pickup channels. Comparison of cross sections between different systems with the $^{208}$Pb target and with projectiles going from neutron-poor to neutron-rich, as well as between the data and GRAZING, will be shown. The results are relevant for future investigations with radioactive beams, especially considering the SPES project.
Multinucleon transfer cross sections have been recently measured for the $^{92}$Mo+$^{54}$Fe reaction, where both proton stripping and pickup channels were populated with similar strength. The excitation function was measured from the Coulomb barrier to far below, by making use of inverse kinematics to detect target recoils at forward angles with PRISMA. We will discuss the yield of the proton transfer channels, whose probability turns out to be stronger than predicted by a simple phenomenological analysis. The measurement followed the successful results recently obtained for the closed shell $^{96}$Zr+$^{40}$Ca [2] and super-fluid $^{116}$Sn+$^{60}$Ni [3] systems were focus was on neutron transfer channels.
[1] T. Mijatovic et al., Phys. Rev. C 94, 064616 (2016).
[2] L. Corradi et al., Phys. Rev. C 84, 034603 (2011).
[3] D. Montanari et al., Phys. Rev. Lett. 113, 052601 (2014).
Nowadays a perspective of production of heavy neutron-enriched nuclides encourages the scientists to investigate theoretically as well as experimentally the multinucleon transfer (MNT) reactions with heavy ions [1,2]. This type of reaction is occurred at low energies and leads to a variety of binary fragments formed around the projectile and target with dozens of transferred nucleons between them.
Usually, yields of the MNT products drops exponentially with increasing the number of transferred nucleons between the colliding nuclei, but their values can be rather high for experimental investigation of yet unknown neutron-enriched nuclei in certain cases. Special attention is paid to the theoretical models of MNT processes able to provide a description of the key features of collision dynamics and make reasonable predictions for distributions of reaction fragments. Among such the models the Langevin-type approaches allow one to achieve a good agreement in complex description of energy, angular and mass distributions of the reaction products. Thus, the various reactions with spherical and statically deformed nuclei such as Sm + Sm, Xe + Pb, Gd + W, U + U and U + Cm have been analyzed within a dynamical Langevin-type approach providing a rather well agreement of calculated and experimental data [3,4].
As the next step, we aimed to analyze the MNT processes in pairs of nuclei with different N/Z ratios. In such combinations the early stage of nucleus-nucleus collisions is characterized by fast redistribution of neutrons and protons called N/Z equilibration or isospin-relaxation. This phenomenon significantly influences the collision dynamics and "neutronrichness" of the fragments that can be visible in isotopic yields.
1. L. Corradi et al., Nucl. Instr. Meth. B 317, 743 (2013)
2. V.I. Zagrebaev and W. Greiner, Phys. Rev. C 87, 034608 (2013)
3. A.V. Karpov and V.V. Saiko, Phys. Rev. C 96, 024618 (2017)
4. V.V. Saiko and A.V. Karpov, Phys. Rev. C 99, 014613 (2019)
The N=50 shell closure above $^{78}$Ni has been the subject of intense experimental efforts. While an initial spectroscopy of $^{78}$Ni itself has been achieved, the rich phenomenology around the neutron shell closure still lacks a comprehensive picture. The parabolic behaviour of the N=50 gap, decreasing from Z=40 to Z=32 and the re-increasing towards Z=30 is not well understood, also in terms of its relation with the appearance of low-lying shape-coexisting states in Se, Ge and Zn isotopes. Similarly, the rapid decreasing of the $\nu$s$_{1/2}$ shell, becoming almost degenerate with the $\nu$d$_{5/2}$ orbital, may have a role in the predicted and observed low-lying E1 strength in $^{83}$Ge.
Recent experimental results will be presented, concentrating at first on N=50 core-breaking states and then on evidences of shape coexistence and triaxiality in the region coming both from in-beam and decay spectroscopy. Results will discussed in the framework of shell-model, mean filed, and weak coupling calculations, pointing out the evolution of neutron effective single-particle energies beyond N=50. It will be shown how heavy-meson exchange may provide a common physics picture to these phenomena. The relation to the possible development of a neutron skin beyond N=50, and hence to the appearance of a pygmy dipole resonance, will also be highlighted.
Future perspectives at new generation ISOL facilities will be addressed.
While the $N=50$ shell-gap evolution towards $^{78}$Ni is presently in the focus of nuclear structure research, experimental information on the neutron effective single particle energy (ESPE) sequence above the $^{78}$Ni core remain scarce. Direct nucleon exchange reactions are indeed difficult with presently available post-accelerated radioactive ion beams (especially for high orbital momentum orbitals) in this exotic region. We have studied the evolution of the $\nu g_{7/2}$ ESPE which is the key to understanding the possible evolution of the spin-orbit splitting due to the action of the proton-neutron interaction terms in the $^{78}$Ni region by measuring the lifetime of excited states in order to distinguish between collective and single-particle states. The evolution of the ESPE of this orbital, characterized by a high orbital momentum $\ell=4$, should indeed be particularly sensitive to tensor effects.
In the continuity of an experiment performed in LNL-Legnaro [1], we performed an experiment at GANIL (Caen, France) with AGATA [2], VAMOS [3] and the Orsay plunger OUPS [4] in order to measure lifetime of Yrast excited states (in peculiar $7/2_1^+$ states) in several $N=51$ isotones populated by the reaction $^{238}$U($^9$Be,f). We particularly focused our study on $^{83}$Ge, the closest $N=51$ odd isotones to $^{79}$Ni for which detailed spectroscopy studies are possible within our experimental conditions. We also performed complementary $\beta$-delayed $\gamma$-spectroscopy of $^{83}$Ge with BEDO [5] at the ALTO ISOL photo-fission facility in Orsay to investigate non-Yrast spectroscopy.
Results from both experiments and future plans at IGISOL will be presented and discussed.
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The Second Random Phase Approximation (SRPA) is a natural extension of Random Phase Approximation obtained by introducing more general excitation operators where two particle-two hole configurations, in addition to the one particle-one hole ones, are considered.
Only in the last years,large-scale SRPA calculations, without usually employed approximations have been performed [1,2].
The SRPA model corrected by a subtraction procedure [2] designed to cure double counting issues and the related instabilities has been recently implemented and applied in the study of different physical cases.
In this talk we report on the most recent results obtained by using this model. In particular, results on the dipole strength and polarizability in 48Ca [3], the enhancement of the effective masses induced by the beyond-mean-field correlations [4] and the effect of two particle-two hole configurations on the monopole response [5] will be presented and discussed.
[1] D. Gambacurta, M. Grasso, and F. Catara, Phys. Rev. C 84, 034301 (2011).
[2] D. Gambacurta, M. Grasso and J.Engel,Phys. Rev. C 81, 054312 (2010); Phys. Rev. C 92 , 034303 (2015).
[3] D. Gambacurta , M. Grasso , O. Vasseur, Physics Letters B 777 163–168, (2018).
[4] M. Grasso, D. Gambacurta, and O. Vasseur, Phys. Rev. C 98, 051303(R) (2018)
[5] D. Gambacurta and M. Grasso, in preparation.
Lifetime measurements with the recoil distance Doppler-shift technique
have been performed to determine yrast E2 transition strengths in 178Pt.
The experimental data are related to those on neighboring Pt isotopes,
especially recent data on 180Pt, and compared to calculations within the
interacting boson model and a Hartree-Fock Bogoliubov approach. These
models predict prolate deformed ground states in Pt isotopes close to
neutron midshell consistent with the experimental findings.
Further, evidence was found that the prolate intruder structure observed in
neutron deficient Hg isotopes that is minimum in energy in 182Hg becomes
the ground state configuration in 178Pt and neighboring 180Pt with nearly
identical transition quadrupole moments. The new data on 178Pt are further
discussed in the context of the systematics along the Pt isotopic chain
with respect to an asymmetry of the level schemes relative to the neutron
midshell that is not expected in collective models. In addition, hints for
a sharp shape transition towards a weakly deformed or a quasi-vibrational
structure in 174,176Pt will be discussed based on existing data where
contradicting model approaches exist.
Supported by the Deutsche Forschungsgemeinschaft (DFG) under Contracts
No. FR 3276/1-1 and DE 1516/3-1.
Experimental studies of nuclei far from stability provide guidance for further development of nuclear models. Simple systems in the proximity of the doubly-magic shell closures are the best cases for testing the predictive power of shell-model calculations. In this context, understanding of the nuclear structure in the closest proximity of the doubly-magic $^{132}$Sn is essential before making extrapolations of the nuclear properties towards more neutron-rich tin isotopes. In this work, the $\beta$ decay of $^{135}$In has been studied for the first time.
Excited states in $^{133}$Sn, $^{134}$Sn and $^{135}$Sn were investigated via $\beta$ decay of $^{133}$In, $^{134}$In and $^{135}$In at ISOLDE Decay Station. Isomer-selective ionization using RILIS enabled the $\beta$ decays of $^{133g}$In (I$^{\pi}$=9/2$^+$) and $^{133m}$In (I$^{\pi}$=1/2$^-$) to be studied independently for the first time. Thanks to the large spin difference of those two $\beta$-decaying states, it is possible to investigate separately the lower- and higher-spin states in the daughter $^{133}$Sn and thus to probe single-particle transitions relevant in the neutron-rich $^{132}$Sn region. Single-hole states in $^{133}$Sn were identified at energies exceeding neutron-separation energy up to 3.7 MeV. Due to centrifugal barrier hindering the neutron from leaving the nucleus, the contribution of electromagnetic decay of those unbound states was found to be significant. The same phenomenon was observed for a new neutron-unbound state identified in $^{134}$Sn. Preliminary results of the first $\beta$-decay studies of $^{135}$In were obtained. Comprehensive description of excited states in $^{133}$Sn and $^{134}$Sn was deduced from both $\beta$ and $\beta$n decay branches of indium isotopes.
The single-particle level structure is essential for the stability and decay properties of the heaviest nuclei. However, the prediction of low-lying single-particle states for heaviest elements remains a very challenging task nowadays (see for example [1 - 3]). Experimental data are scarce in this region and any new data serves as an important anchor for theoretical predictions and a possibility to predict the stabilized regions in the region of superheavy elements. The application of sensitive $\alpha$-, $\gamma$- and conversion-electron (CE) spectroscopy methods allowed us to investigate the structure of very heavy nuclei (A>250).
We performed an extensive program aimed at nuclear structure studies of isotopes above fermium (Z=100) using $\alpha$-CE, $\alpha$-$\gamma$ and CE-$\gamma$ spectroscopy at the velocity filter SHIP in GSI Darmstadt. In these measurements, we obtained enhanced data for many isotopes, which helped us to extend and improve the single-particle level systematics for N = 149, 151 and 153 isotones. Besides $\alpha$-decay spectroscopy, we also performed the very first $\beta$-decay studies in this region of nuclide chart.
Our series of measurements at SHIP provided a substantial body of new data. The most recent results for selected isotopes in very heavy element region will be presented and discussed within different theoretical frameworks. In particular, the observation of new single and multi-quasi particle isomers in $^{255}$Rf [4, 5] the very first EC-decay data for $^{258}$Db and $^{254}$Md will be discussed.
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Nuclei with a large N/Z ratio in this region are of great interest to test nuclear models and provide information about single particle states. During the last two decades there has been a substantial effort directed to gathering information about the region around 132Sn[1-3], the most exotic doubly-magic nucleus presently at reach. 132Sn is itself a very interesting case [4]. The simplest excited levels correspond with particle-hole states where a particle is excited across the energy gap of the closed shell. The identification of the p-h multiplets, provides information on the nuclear two-body elements. This isotope has been studied in detail through the β-decay of 132In [5]. Nevertheless, a lot of the expected p-h multiplet states remained unidentified.
We have used fast-timing and γ spectroscopy to investigate 132Sn. The experiment was carried out at ISOLDE, where the excited states of 132Sn were populated in the β-decay of In isomers, produced in a UCx target unit equipped with a neutron converter. The In isomers were ionized using the ISOLDE RILIS, which for the first time allowed isomer-selective ionization of indium. The measurements took place at the new ISOLDE Decay Station, equipped with four clover-type Ge detectors, along with a fast-timing setup consisting of two LaBr3(Ce) detectors and a fast β detector.
In this work we report on the excited structure of 132Sn, populated in the β-decay of 132In, and also, owing to the RILIS isomer selectivity, separately from the β-n decay of the 133In 1/2- isomer and 9/2+ ground state. We present a preliminary new level-scheme, which have been enlarged with 13 new levels and more than 40 new γ-transitions. These results are completed with new lifetimes values of excited states.
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The breaking of symmetries in quantum systems is one of the key issues in nuclear physics. In particular, the spontaneous symmetry breaking in rotating nuclei leads to exotic collective modes, like the chiral motion, which is an unique fingerprint of triaxiality in nuclei and have been intensively studied in recent years. We are currently involved in the study of Lanthanide nuclei. New results have been obtained recently and interpreted as the manifestation of a stable triaxial nuclear shape, presenting various types of collective motion, like tilted axis and principal axis rotation, chiral motion, rotation of nuclei with oblate shape at very high spins. Chiral bands in even-even nuclei, which were taught to be unfavored energetically, unstable against 3D rotation and difficult to observe, have been instead identified very recently in 136Nd. The experimental evidence of such bands will be presented and their theoretical interpretation will be discussed. The experimental evidence of multiple chiral bands in several Lanthanides, as well as the presence of competing collective oblate rotation up to very high spins in Nd nuclei will also be discussed.
It is well known that nucleons are arranged in specific shells resulting in greater stability, analogous to the electron shells in the atom and that this shell structure was expected to be very robust in the whole nuclear chart. However, with advance experimental and theoretical works during the last two decays, we are aware that the shell structure changes when moving far away from stability and it is related to the large neutron excess and nuclear forces. In other words, the Shell Model described in 1949 by Mayer and Jensen is not valid throughout the nuclear chart and nuclear forces have to be reconsidered in the nuclear Hamiltonian which was initially described by harmonic oscillator potential and spin–orbit interaction.
The possible consequences expected for neutron-rich nuclei are shell evolution in which changes in ordering and location of the single particle orbits are significant, and the shape coexistence where particle-hole excitations over a major shell and quadrupole correlations are favored due to inversion of orbitals and reduced shell gaps. In extreme cases proven in the lighter mass regions, new magic numbers appear and some other conventional ones disappear and intruder correlations change the ground state deformation, causing the phenomena called island of inversion. In the present manuscript, these aspects will be discussed in the 78Ni region. Recent experiments performed at RIKEN radioactive beam facility using different methodologies will be presented.
Previous investigations of neutron-rich titanium isotopes indicate the development of a subshell closure at $N=32$. However, shell model calculations could not explain this behaviour so far: the excitation energies of the lowest excited Yrast states in these titanium isotopes are reproduced, but not, for example, the trend of the $B(E2;2_1^+\rightarrow 0_\mathrm{gs}^+)$ values as a function of the neutron number. In addition, only few information about $E2$ transition strenghts between higher Yrast states is known. To measure these, excited states in $^{46-54}$Ti were populated by multinucleon transfer reactions and level lifetimes measured by the Recoil-Distance Doppler-shift method were determined. The experiment was performed at GANIL with the detector system AGATA and the spectrometer VAMOS++ for particle identification as well as the Cologne Compact Plunger for deep inelastic reactions. Lifetimes of the $2_1^+$ and $4_1^+$ state as well as upper and lower limits of the $6_1^+$ and $8_1^+$ state in $^{54}$Ti, respectively, could be determined with the differential decay curve method (DDCM) and corresponding $B(E2)$ values were calculated. In addition preliminary lifetime values of excited states of the neighbor nucleus $^{53}$Ti were determined for the first time and will be presented and discussed in the framework of current shell model calculations.
Nuclei in the vicinity of $^{78}$Ni have recently been in focus of many experimental and theoretical investigations. In particular, the neutron-rich Zn isotopes, only two protons above the Ni isotopic chain, are ideally suited to study the evolution of the Z = 28 proton shell gap, and the stability of the N = 50 neutron shell gap. In the last decade, several experiments were performed to study the collectivity in the even-even Zn isotopes between N = 40 and N = 50 [1-4], but their results are not consistent; consequently, the evolution of nuclear structure in the neutron-rich Zn nuclei is not fully understood.
The ISOLDE facility finished in 2015 the first phase of a major upgrade in terms of the energy of post-accelerated exotic beams bringing it up from 3 MeV/u to 5.5 MeV/u. The increased beam energy strongly enhances the probability of multi-step Coulomb excitation, giving experimental access to new excited states and bringing in-depth information on their structure.
The very first HIE-ISOLDE beam experiment in October 2015 and its continuation in 2016 have been dedicated to the study of the evolution of the nuclear structure along the zinc isotopic chain. The preliminary results discriminate between the two experimental values of B(E2; 4$^{+} \to$ 2$^{+}$) in $^{74}$Zn, and yield for the first time a B(E2; 4$^{+} \to$ 2$^{+}$) value in $^{76,78}$Zn.
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It is shown that the pairing correlation is very important for the two-neutron transfer reactions, for reaction induced by 84 MeV 18O on several targets with low collectivity in its ground state (spherical), proceeding through the one-step process (concerning to transfer process). For the transition to lower excited states, the one-step process also dominated, for the final nuclei having also low collectivity. On the contrary, if the collectivity of these states is considerable, the two-neutron transfer reaction is dominated by a two-step process through an intermediate partition. We present our results for 12,13C(18O,16O) 12,13C[1,2], 16O(18O,16O) 18O[3,4], 64Ni(18O,16O)66Ni[5] and 28Si(18O,16O)30Si[6] by analysing the two-neutron transfer angular distributions. We compare our results with similar results for the 206Pb(18O,16O)208Pb[7] and 7Be(9Be,7Be)9Be[8] reactions, and with the analysis of the quasi-elastic barrier distributions for the 63Cu +18O system [9]. We also show the evidences recently found for the observations of Giant Pairing Vibrations in the 12,13C(18O,16O) 12,13C reactions [10]. Some preliminary results of the effect of pairing correlations in two-protons transfer reactions are also shown. Our ability to describe microscopically multi-nucleon transfer reactions that compete with the double-charge exchange reactions within the NUMEN project [11] will be also discussed.
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11. F. Cappuzzello, et al., Eur. Phys. J. A 54, 72 (2018).
The properties of low-lying states in N=7 isotones have been studied theoretically, going from $^{10}$Li to $^{13}$C.
To reproduce in detail the changes of structure in these nuclei going towards the neutron drip line represents a considerable challenge for many-body theories.
In particular, this concerns the inversion of parity between the ground and first excited state observed going towards the drip line, which is experimentally well established in $^{11}$Be but is under discussion
in the case of the unbound nucleus $^{10}$Li, while the normal sequence is observed in $^{12}$B and $^{13}$C.
The effects of many-body renormalization processes are considered in detail, and transfer reactions are calculated, showing that the cross sections observed in recent $^{9}$Li(d,p)$^{10}$Li one–neutron transfer experiments [1,2] are consistent with, or better, require the presence of a virtual 1/2+ state [3]. Furthermore, theoretical cross sections for reactions leading to low-lying resonant states in $^{11}$Be are successfully compared to data [4].
[1] H.B. Jeppesen et al, Phys. Lett. B, 642(2006)449
[2] M. Cavallaro et al, Phys. Rev. Lett. 118 (2017) 012701
[3] F. Barranco, G. Potel, R. A. Broglia, and E. Vigezzi, Phys. Rev. Lett. 119 (2017) 082501
[4] F. Barranco, G. Potel, R. A. Broglia, and E. Vigezzi, arXiv:1812.01761
In near-barrier fusion reactions with heavy-ions, the coupling effect of the positive Q-value neutron transfers (PQNT) is still a complex and unsolved problem. For studying this effect, the fusion excitation functions of the typical systems, such as 32S+90,94,96Zr, 112,116,120,124Sn, were measured by using an electrostatic deflector setup at CIAE. In this talk, the recent experimental results measured at CIAE will be reviewed, with special emphasis on the effect of the positive Q-value neutron stripping channels of 18O+50Cr,58Ni,74Ge.
Additionally, considering the current inconsistent experimental data and theoretical analysis, the concept of residual enhancement (RE)[1] that mainly aims for reducing the additional uncertainties was proposed to extract a reliable quantitative PQNT effect. More details will be given in this talk.
Reference
[1] H. M. Jia, C. J. Lin, L. Yang et al., Phys. Lett. B 788,43 (2016).
The collinear resonance ionization spectroscopy experiment (CRIS) at ISOLDE-CERN has been developed developed as a sensitive technique to access to electromagnetic properties of exotic nuclei. This technique provides observables that are key for our understanding of the nuclear many-body problem; nuclear spins, electromagnetic moments, and changes in the root-mean-square charge radii. This contribution will present the results from recent experimental campaigns in the vicinity of the so-called doubly magic nuclei: $^{52}$Ca, $^{78}$Ni, $^{100}$Sn and $^{132}$Sn. The relevance of these results, in connection with recent developments in nuclear theory, will be discussed.
Hyperfine structure measurements of the neutron-deficient indium ($Z=49$) isotopes, approaching the heaviest self-conjugate doubly-magic nucleus $^{100}\mathrm{Sn}$, have been performed using collinear resonance ionization spectroscopy [1]. These measurements provide an important benchmark in the development of many-body methods, which are now able to predict properties around the $Z=N=50$ shell-closure [2,3].
States in previously measured odd-even In isotopes have shown a remarkably simple single-particle behaviour, whether this trend in the electromagnetic moments continues will give insight into the strength of the shell closure. Isomeric spin assignments in the odd-odd isotopes also help pin down the ordering of the neutron $d_{5/2}$ and $g_{7/2}$ orbits [4,5]. This first experimental determination of ground-state electromagnetic moments and changes in mean-square charge radii of neutron-deficient $^{101-103}\mathrm{In}$ will shed light on the evolution of nuclear structure around $^{100}\mathrm{Sn}$.
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The structure of deformed, neutron-rich nuclei in the rare-earth region is of significant interest for both the nuclear-structure and astrophysics fields. Although much progress is being made in our understanding of the r-process, a satisfactory explanation for the elemental peak in abundance near A=160 is still elusive. Understanding the origin of this peak may be a key to correctly identifying the astrophysical conditions for the r-process. Theoretical models of element production are dependent on masses and lifetimes of neutron-rich, deformed rare-earth nuclei in this region where little or no information is known. The available nuclear structure information is also scarce, owing to difficulties in the production of these nuclei.
In order to address these issues, an experimental program has been initiated at Argonne National Laboratory using high-purity radioactive beams produced by the CARIBU facility. Mass measurements using the Canadian Penning Trap (CPT) and beta-gamma coincidence studies using the SATURN moving tape system and the X-Array spectrometer, comprising of five Ge clover detectors, were carried out. A number of two-quasiparicle isomers were discovered in odd-odd nuclei using CPT and in several cases their properties were elucidated by complementary beta-decay studies. Evidences were found for changes in the single-particle structure, which in turn resulted in the formation of a sizable sub-shell gap at N=98 and large deformation.
Results from these measurements will be presented, together with predictions based on deformed shell model that includes effects of pairing and spin-depended, nucleon-nucleon interactions. The newly-commissioned beta-decay station at Gammasphere will also be discussed and results from the first experimental campaign will also be presented.
This work is supported by the U.S. Department of Energy, Office of Nuclear Physics, under Contracts No. DE-AC02-06CH11357.
The Ion Guide Isotope Separator On-Line (IGISOL) facility in the JYFL Accelerator Laboratory offers versatile possibilities for nuclear structure studies via high-precision mass measurements as well as via decay and laser spectroscopy. In this presentation, I will focus on mass measurements recently performed with the JYFLTRAP Penning trap mass spectrometer. These include for example measurements on the neutron-rich rare-earth isotopes close to N=100 as well as nuclides close to 78Ni. In addition to the ground states, information on long-living isomeric states has been obtained. Many of the studied nuclides were measured for the first time and therefore provide essential data for nuclear structure far from stability as well as for nuclear astrophysics.
I report about the calculation of the nuclear matrix element involved in
neutrinoless double-β decay within the framework of the realistic shell
model.
Starting from a realistic nucleon-nucleon potential, the effective
shell-model Hamiltonian and 0νββ-decay operator are derived by way of the
many-body perturbation theory.
The contributions to the effective shell-model operator due to short-range
correlations and to the Pauli-principle violations are taken into account.
Attention will be focussed on some 0νββ-decay candidates with mass ranging
from A = 48 up to A = 136.
Borromean nuclei are unique bound quantum systems with unbound sub-systems, that tend to appear in neutron-proton asymmetric isotopes at the edges of the nuclear landscape. Such weakly bound few-body systems can provide sensitive grounds for understanding the nuclear force through their structural properties and interaction. This presentation will describe different techniques of reaction spectroscopy measurements with re-accelerated beams at TRIUMF and in-flight beams at RIBF to explore the ground and excited states of these drip-line nuclei.
At the proton drip-line, spectroscopy of $^{20}$Mg from inelastic scattering with a solid D$_2$ target at the IRIS facility at TRIUMF will be discussed. The observation of new states will be presented and compared to new $\it ab ~initio$ theory predictions. The reaction spectroscopy also offers potential to investigate collectivity that will be discussed to understand shell evolution. The presentation will show how a strong sensitivity to the nuclear force emerges from proton elastic scattering of $^{10}$C.
In the neutron-rich domain, defining the low-Z end of the island of inversion around $N$ = 20 remains as an open problem. The presentation will discuss exploration of the ground state features of the drip-line nucleus $^{29}$F using intermediate energy in-flight beams at RIBF.
The degree to which isospin symmetry is maintained across an isospin multiplet, and hence the extent to which the isospin quantum number can be considered pure, is matter of much contemporary interest. Tests of isospin purity have traditionally been undertaken through examination of the behaviour of the Isobaric Multiplet Mass Equation (IMME), with parabolic behaviour of the IMME of the lowest energy states of a multiplet being considered as a strong evidence for isospin purity. For excited states of multiplets, an alternative approach would be to examine electromagnetic transition matrix elements between analogue states, for which isospin selection rules impose specific behaviour as a function of Tz.. The E2 transition matrix element, in the limit of pure isospin, should be exactly linear with Tz for a T=1 triplet. The measured proton matrix element for the lowest transition, the E2 from the first excited T=1 2+ state to the first T=1 0+ state, can be used as a test of this rule. In this work, we present the results of an experiment to measure this B(E2) strength in the T=1 A=46 triplet
The experiment was performed at GSI, Darmstadt, using the AGATA array in conjunction with the Fragment Separator and the LYCCA array. For two members of the triplet, 46Cr and 46Ti, relativistic Coulomb excitation was used to determine the B(E2), whilst for 46V and 46Ti, lifetimes were measured using a new Doppler-shift technique which we call the stretched-target method.
The results are analysed in the context of all available data for B(E2)s for T=1 triplets. The A=46 case we will present represents one of the most precise tests of the linearity rule (matrix element vs. Tz) to date.
Understanding energy production and nucleosynthesis in stars requires a precise knowledge of the nuclear reaction rates at the energies of interest. To overcome the experimental difficulties arising from the small cross sections at those energies and from the presence of the electron screening, the Trojan Horse Method has been introduced. The method represents one of the most powerful tools for experimental nuclear astrophysics because of its advantage to measure unscreened low-energy cross sections of reactions between charged particles, and to retrieve information on the electron screening potential when ultra-low energy direct measurements are available. This is done by selecting the quasi-free (QF) contribution of an appropriate three-body reaction A+a → c+C+s, where a is described in terms of clusters x⊕s. The QF reaction is performed at energies well above the Coulomb barrier, such that cluster x is brought already in the nuclear field of A, leaving s as spectator to the A + x interaction. The THM has been successfully applied to several reactions connected with fundamental astrophysical problems and recently to resonant ones involving medium-heavy nuclei, such as 12C, 16O and 18,19F. I will recall the basic ideas of the THM and show some recent results.
Proton elastic scattering is a very important process to understand nuclear interactions in finite nuclei. Even if this process has been extensively studied in the last years, a consistent microscopic description is still under development.
We want to study the domain of applicability of microscopic two-body chiral potentials in the construction of an optical potential, derived as the first-order term within the spectator expansion of the multiple scattering theory and adopting the impulse approximation and the optimum factorization approximation.
First, we derive a nonrelativistic theoretical optical potential
from nucleon-nucleon chiral potentials at fourth (N3LO) and fifth order (N4LO).
We check convergence patterns and establish theoretical error bands for
pp and np Wolfenstein amplitudes and the cross sections, analyzing powers,
and spin rotations of elastic proton scattering off some light nuclei at an incident proton energy of 200 MeV [1,2].
Second, the cross sections and analyzing powers for elastic
proton scattering off calcium, nickel, tin, and lead isotopes are presented for several incident proton energies, exploring the range 156 ≤ E ≤ 333 MeV, where experimental data are available. In addition, we provide theoretical predictions for Ni56 at 400 MeV, which is of interest for the experiments at EXL [3].
In addition, we present some preliminary results for antiproton elastic scattering off nuclei at energies close to 200 MeV [4]
Our results indicate that microscopic optical
potentials derived from nucleon-nucleon chiral potentials at N4LO can provide reliable predictions for the cross section and the analyzing power both of stable and exotic nuclei.
Bibliography
[1] M. Vorabbi, P. Finelli, C. Giusti, Phys. Rev. C93, 034619 (2016)
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Most of the attention in the study of quantum phase transitions (QPT) in nuclei, has been devoted to shape phase transitions in a single configuration (denoted Type I), described by a single Hamiltonian, $\hat{H}(\xi) \!=\! \left( 1\!-\!\xi \right)\hat{H}_{1} + \xi \hat{H}_{2}$, where $\xi$ is the control parameter. A different type of phase transitions (denoted Type II) occurs when two (or more) configurations coexist. In this case, the quantum Hamiltonian has a matrix form, with entries: $\hat{H}_{A}(\xi^A),\,\hat{H}_{B}(\xi^B),\,\hat{W}(\omega)$, where the index $A$, $B$ denotes the two configurations and $\hat{W}$ denotes their coupling. As the control parameters are varied, the separate Hamiltonians $\hat{H}_A$ and $\hat{H}_B$ can undergo shape-phase transitions of Type I, which in turn can result in a crossing of configurations $A$ and $B$. In the present contribution, we focus on the $_{40}$Zr isotopes and find a variety of multiple intertwined phase transitions both of Type I and Type II [1]. These isotopes have been recently the subject of several experimental investigations [2] and theoretical calculations [3]. By employing the interacting boson model with configuration mixing, we have calculated the spectra and other observables of the entire chain of Zr isotopes, from neutron number 52 to 70. The latter exhibit a complex phase structure with coexisting Type I and Type II QPTs, and ground state shapes changing from spherical ($^{92-98}$Zr), to X(5)-like ($^{100}$Zr), to axially deformed ($^{102-104}$Zr), and finally to $\gamma $-unstable ($^{106-110}$Zr). This interpretation is corroborated by the evolution along the Zr chain of order parameters and key observables, including B(E2) values, isotope shift and two-neutron separation energies.
[1] N. Gavrielov, A. Leviatan and F. Iachello, submitted (2019).
[2] P. Singh et al., Phys. Rev. Lett. 121, 192501 (2018) and references therein.
[3] See e.g., T. Togashi et al., Phys. Rev. Lett. 117, 172502 (2016).
The occurrence of shape coexistence in nuclei with N = 58 and 59, suggests that the evolution of the deformation is a gradual process. Our goal was to study N = 57, 96Y isotope where only a few states were known. Additionally, we decided to investigate whether deformed structures are present in the 94Y nucleus which lies 5 neutrons away from the N = 60 boundary and in the 97Y with 59 neutrons. During the talk also the new result concerning the enhancement of octupole collectivity in the N=60, 96Zr isotope will be mentioned [1].
The yttrium isotopes have been produced in the fission of 235U active target induced by cold neutron from the reactor at ILL. The level scheme has been established based on multi-fold gamma-ray coincidence relationships measured with the new highly efficient HPGe array FIPPS [2]. For completess also recent data from the previous fission experiment with EXILL spectrometer has been added.
During the analysis, over 50 new gamma transitions in 96Y isotope, have been identified [3, 4]. Additionally, the analysis revealed that the long 8+ isomer is located 400 keV higher than it was reported in NNDC base, which has to be taken into account in reactor antineutrino anomaly calculations [5]. By using the delayed-coincidence method it was possible to identify a few weak transitions above the 201-ns isomeric state, which seem to form a rotational band. In the case of 94Y isotope, 11 new gamma transitions have been identified [6] while in the 97Y, 8 new prompt lines can be observed [4]. Angular correlation analysis supported by shell-model consideration allowed to propose spin-parity assignments for most of the new levels.
[1] Ł.W. Iskra et al., Phys. Lett. B 788, 396 (2019)
[2] C. Michelagnoli et al., EPJ 193, 04009 (2018)
[3] Ł.W. Iskra et al., Europhys. Lett. 117, 12001 (2017) and ILL annual report
[4] Ł.W. Iskra et al., (in preparation)
[5] A.A. Sonzogni et al., Phys. Rec. C 91, 011301(R) (2015)
[6] Ł.W. Iskra et al., Phys. Scr. 92, 104001 (2017)
We show an evolution to derive the shell-model effective Hamiltonian employing two- and three-body interactions based on the chiral effective field theory. A new way to calculate three-body matrix elements of the chiral interaction with the nonlocal regulator is given.
We apply our framework to the p-shell nuclei and perform benchmark calculations to compare our results with those by an ab initio no-core shell-model. We report that our results are satisfactory and the contribution of the three-body force is essential to explain experimental low-lying spectra of the p-shell nuclei. We discuss the contribution of the three-body force on the effective single-particle energy extracted from the monopole interaction.
Next, we investigate the shell evolution on the fp-shell nuclei. We show that the monopole component of the shell-model effective Hamiltonian induced by the three-body force plays an essential role to account for the experimental shell evolution.
Researches on neutrinoless double beta decay have crucial implications on particle physics, cosmology and fundamental physics. It is likely the most promising process to access the absolute neutrino mass scale. To determine quantitative information from the possible measurement of the 0νββ decay half-lives, the knowledge of the Nuclear Matrix Elements (NME) involved in such transitions is mandatory. The use heavy-ion induced double charge exchange (DCE) reactions as tools towards the determination of information on the NME is one of the goals of the NUMEN and the NURE projects. The basic point is that there are a number of similarities between the two processes, mainly that the initial and final state wave functions are the same and the transition operators are similar, including in both cases a superposition of Fermi, Gamow-Teller and rank-two tensor components.
The availability of the MAGNEX magnetic spectrometer for high resolution measurements of the very suppressed DCE reaction channels is essential to obtain high resolution energy spectra and accurate cross sections at very forward angles including zero degree. The measurement of the competing multi-nucleon transfer processes allows to study their contribution and constrain the theoretical calculations.
An experimental campaign is ongoing at INFN-Laboratori Nazionali del Sud (Italy) to explore medium-heavy ion induced reactions on target of interest for 0νββ decay.
Recent results obtained by the (20Ne,20O) DCE reaction and competing channels, measured for the first time using a 20Ne10+ cyclotron beam at 15 AMeV will be presented at the conference.
Double charge exchange excitations (DCX) induced by heavy ion beams at intermediate energies [1],[2] attract a lot of interest in relations with new collective excitations such as double isobaric analog states (DIAS) and double Gamow-Teller giant resonance (DGTR) . This reaction is also closely linked with double beta decay matrix elements. In 1980s, the double charge exchange reactions (DCX) were performed by using pion beams, i.e., $(\pi^+, \pi^-)$ and $(\pi^-, \pi^+)$ reactions. Through these experimental studies, the double isobaric analog states (DIAS), and the double dipole resonance states (DGDR) are identified. However, DGTR were not found in the pion double charge exchange spectra.
A new research program based on a new DCX reaction ($^{12}$C, $^{12}$Be(0$^+_2$ )) is planned at RIKEN RIBF facility with high intensity heavy ion beams at the optimal energy of E$_{lab}$ =250MeV/u to excite the spin-isospin response [1].A big advantage of this reaction is based on the fact that it is a $(2p,2n)$ type DCX reaction and one can use neutron-rich target to excite DGT strength.
In this talk, I will present a microscopic study of DGTR within a framework of microscopic Hartree-Fock+BCS (or Bogolyubov) and QRPA. The results of QRPA will be also examined by analytic formulas to calculate the excitation energies of the DIAS and DGT strength using commutator relations for the double isospin $(t_-)^2$ and spin-isospin operator $(\sigma t_-)^2$. I will give formulas to estimate energies of the DIAS state and DGT states with separable interactions [3].
[References]
[1] M. Takaki, T. Uesaka et al., Proposal for experiment at RCNP, "Search for double Gamow Teller giant resonances in $^{48}$Ti via the heavy-ion double charge exchange reaction" (2015).
[2] F. Cappuzzello et al., Journal of Physics: Conference Series 630, 012018 (2015).
[3] H. Sagawa and Uesaka, Phys. Rev. C94, 064325 (2016) and H. Sagawa, to be published.
Heavy ion charge exchange reactions are of manyfold interest for nuclear reaction and structure physics. In a recent paper [1] a fully microscopic theory of heavy ion single charge exchange (SCE) reactions was formulated. Here, a new theoretical approach is presented, emphasizing the role of single and double charge exchange reactions for probing nuclear response functions of the same type as encountered in single and double beta decay [2]. In particular, a special class of nuclear double charge exchange (DCE) reactions proceeding as a one-step reaction through a two-body process are shown to involve nuclear matrix elements of the same diagrammatic structure as in $0\nu 2\beta$ decay. These correlated Majorana-DCE (MDCE) reactions are distinct from second order DCE reactions which are characterized the best as sequential double single charge exchange (DSCE), thus carrying a close resemblance to $2\nu 2\beta$ decay. The results suggest that ion-ion DCE reactions are the ideal testing grounds for investigations of double-beta decay nuclear matrix elements as proposed by the NUMEN project [3]. Nuclear response functions for $\tau_\pm$ excitations and applications to recent single and double charge exchange data measured by the NUMEN collaboration at LNS Catania are discussed.
References:
[1] H.~Lenske, J.~I.~Bellone, M.~Colonna and J.~A.~Lay,
Phys. Rev. C 98 (2018) 044620
[2] H.~Lenske,
J.Phys.Conf.Ser. 1056 (2018) 012030.
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Eur. Phys. J. A 54 (2018) 72
In a recent ref. [1] group representation methods have been applied to the nuclear point-group symmetries and combined with realistic mean-field calculation results together with the new specifically designed methods of experimental analysis. The authors demonstrated that existing in the literature experimental data on 152Sm are fully compatible with the extremely restrictive group-theory criteria of simultaneous presence of tetrahedral and octahedral symmetries.
We discuss the theoretical predictions related to the systematic presence of these symmetries throughout the periodic table. Interestingly enough, in some nuclei the presence of one of the two symmetries are predicted whereas in some others theory predictions are compatible with the interpretation of spontaneous octahedral symmetry breaking by its tetrahedral partner (tetrahedral symmetry group is a sub-group of the octahedral one). The corresponding theory predictions aim at an optimisation of the propositions of new experiments, which would employ the advanced mass-spectrometry methods, ref. [2] – in view of the new experimental search criteria of ref. [1]. Since part of the predictions indicates that several exotic nuclei are concerned, we employ the parameter optimisation methods based on the so-called inverse problem theory, ref. [3].
The addressed field of symmetry-research presents particularly promising potentialities in the domain of exotic nuclei studies. Indeed, as it can be demonstrated, in the exact tetrahedral and/or octahedral symmetry limits the corresponding nuclei emit neither E2 nor E1 radiation generating isomeric states with lifetimes which are much longer than the related ground states.
Bibliography
[1] J. Dudek et al., Phys. Rev. C 97, 021302(R) (2018)
[2] T. Dickel and Ch. Scheidenberger, private communication
[3] I. Dedes, PhD thesis, University of Strasbourg, https://tel.archives-ouvertes.fr/tel-01724641
Exploiting exact and special symmetries to unmask simplicity within complexity, which remains the ‘holy grail’ of nuclear physics, will be considered within its historical context and as evolving through 21st Century ‘ab initio’ methods, including emerging results linked to the internal structure of nucleons.
Some exemplar results for very light to medium mass nuclei will be presented, and what these may portend for heavier systems, including species beyond known lines of stability, will be proffered.
Pairing correlation produces the odd-even staggering of the binding energies.
In addition to that, it also introduces spontaneous breaking of the gauge symmetry.
The pairing rotation is the Nambu-Goldstone mode associated with the gauge symmetry breaking
in superconducting nuclei, and is measurable experimentally as a pairing rotational band.
In our previous work [1], it was shown that the binding-energy differences,
\delta_{2n}, \delta_{2p}, and \delta V_{pn}, are understood in terms of the
moment of inertia of the pairing rotation.
Conventionally a simple form is assumed for the pairing energy density functional
because of the lack of observables to constrain the coupling constants.
I will show that the moment of inertia for the pairing rotation can be used to constrain the coupling constants of the pairing energy density functional, and discuss an extended form of the pairing energy density functional by including the terms with the kinetic pair density and the spacial derivative of the pair density [2].
I will also show a systematic calculation of the pairing rotational moments of inertia from stable to unstable nuclei employing various pairing functionals by performing the linear response calculation using the finite-amplitude method.
[1] N. Hinohara and W. Nazarewicz, Phys. Rev. Lett. 116, 152502 (2016).
[2] N. Hinohara, J. Phys. G 45, 024004 (2018).
A new Skyrme functional devised to account well for standard nuclear properties as well as for spin and spin-isospin properties is presented. The main novelty of this work relies on the introduction of tensor terms guided by ab initio relativistic Brueckner-Hartree-Fock calculations of neutron-proton drops. The inclusion of tensor term does not decrease the accuracy in describing bulk properties of nuclei, experimental data of some selected spherical nuclei such as binding energies, charge radii, and spin-orbit splittings can be well fitted. The new functional is applied to the investigation of various collective excitations such as the Giant Monopole Resonance (GMR), the Isovector Giant Dipole Resonance (IVGDR), the Gamow-Teller Resonance (GTR), and the Spin-Dipole Resonance (SDR). The overall description with the new functional is satisfactory and the tensor terms are shown to be important particularly for the improvement of the Spin-Dipole Resonance results.
Heavy ion fusion reactions play important roles in a wide variety of stellar burning scenarios. $^{12}$C+$^{12}$C, $^{12}$C+$^{16}$O and $^{16}$O+$^{16}$O are the principle reactions during the advance burning stages of massive star. $^{12}$C+$^{12}$C also triggers the happening of superburst and Type Ia supernovae. The heavy ion fusion reactions of the neutron-rich isotopes
such as $^{24}$O are the major heating source in the crust of neutron star. In this talk, I will review the challenges and the recent progress in the study of these heavy ion fusion reactions at stellar energies. The outlook for the studies of the astrophysical heavy-ion fusion reactions will also be presented.
Crucial information on nova nucleosynthesis can be potentially inferred from γ-ray signals powered by 18F decay [1]. Therefore, the reaction network producing and destroying this radioactive isotope has been extensively studied in the last years. Among those reactions, the 18F(p,α)15O cross-section has been measured by means of several experiments, using direct and indirect methods. The presence of interfering resonances in the energy region of astrophysical interest has been reported by many authors including the recent applications of the Trojan Horse Method (THM).
The THM is an indirect method using direct reactions to populate 19Ne states of astrophysical importance, with no suppression by the Coulomb and centrifugal barriers. In this work, we evaluate what changes are introduced by the THM data in the 18F(p,α)15O astrophysical factor recommended in a recent R-matrix analysis [2-4], accounting for existing direct and indirect measurements [5]. We will particularly focus on the role of the THM experiment, since it allowed to cover the 0-1 MeV energy range with experimental data, with no need of extrapolation and with unprecedented accuracy (better than 20%).
Then, the updated reaction rate is calculated and implications of the new results on nova nucleosynthesis are discussed. In particular, while no change on the dynamical properties of the explosion is found, important differences in the chemical composition of the ejected matter is observed, with a net reduction in the mean 18F content by a factor of 2 and a corresponding increase in the detectability distance [4].
[1] J. Josè, Stellar Explosions: Hydrodynamics and Nucleosynthesis (London: Taylor and Francis, 2016)
[2] R.G. Pizzone et al., Eur. Phys. J. A 52, 24 (2016)
[3] S. Cherubini et al., Phys. Rev. C 92 015805 (2015)
[4] M.La Cognata et al., Astrophys. J. 846 65 (2017)
[5] D.W. Bardayan et al., Phys. Lett. B 751 311 (2015)
[6] R.H. Cyburt et al., Astrophys. J. Suppl. 189 240 (2010)
Important roles of Gamow-Teller transitions have been studied for electron-capture and $\beta$-decay processes at stellar environments [1, 2] as well as $\nu$-nucleus reactions [3]. Importance of first-forbidden transitions in $\beta$-decay rates of N=126 isotones have been shown, and the short half-lives obtained were used to study r-process nucleosynthesis in core-collapse supernova explosions (SNe) and binary neutron-star mergers [4].
Here, we focus more on the roles of forbidden transitions in nuclear weak processes. $\nu$-induced reactions on $^{16}$O, where spin-dipole transitions are dominant, are studied with new shell-model Hamiltonians [5] and SN$\nu$ detection and $\nu$ mass hierarchy dependence of the cross sections [6] as well as nucleosynthesis of light elements such as $^{11}$B and $^{11}$C in SNe [5] are discussed.
Next, we study e-capture processes on $^{20}$Ne which become important in late stage of the evolution of O-Ne-Mg cores in stars. The transition to the ground state in $^{20}$F (2$^{+}$) is a second-forbidden transition and is important in certain ranges of densities and temperatures [7]. Electron-capture rates for the transition are evaluated with the multipole expansion method, and compared with a simple evaluation using a constant parametrized strength obtained from the beta-decay experiment [8]. Energy dependence of the second-forbidden transition strength is found to lead to a significant difference in the capture rates from the simple parametrized method.
[1] T. Suzuki, H. Toki, and K. Nomoto, ApJ. 817, 163 (2016)
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[3] T. Suzuki et al., Phys. Rev. C 74, 0407 (2006)
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[5] T. Suzuki, S. Chiba, T. Yoshida, K. Takahashi, and H. Umeda, Phys. Rev. C 98, 034613 (2018)
[6] K. Nakazato, T. Suzuki, and M. Sakuda, PTEP 2018, 123E02 82018)
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[8] O. S. Kirseborn et al., arXiv:1805.19149 (2018)
The nuclear shapes and collective excitations have been one of the most prominent and studied themes of nuclear structure physics. Experiments using radioactive-ion beams allow to study thus far unknown nuclei and also necessitate timely systematic, as well as reliable, theoretical analyses. The interacting boson model (IBM) has been remarkably successful in phenomenological description of low-lying states in nuclei. Microscopic foundation of the IBM, i.e., derivation of the bosonic Hamiltonian from nucleonic degrees of freedom, has been extensively studied in terms of the shell model, but it has been somewhat limited to nearly spherical nuclei.
In this presentation I will focus on a comprehensive method of deriving the Hamiltonian of the IBM from the energy density functional theory (DFT). We begin with the DFT self-consistent mean-field calculation of the potential energy surface with the relevant shape degrees of freedom. The DFT energy surface is then mapped onto the expectation value of the IBM Hamiltonian in the boson condensate state. This procedure completely determines the strength parameters of the IBM Hamiltonian, which is used to compute the excitation spectra and electromagnetic transition rates. Since the DFT framework allows for a global mean-field description of many nuclear properties over the entire region of the nuclear chart, it has become possible to derive the IBM Hamiltonian for any arbitrary nuclei. This has paved the way and allowed unprecedented opportunities to study the spectroscopy of heavy exotic nuclei in an accurate, systematic, and computationally feasible way.
Interesting applications of the mean-field-based IBM calculations include the shape phase transitions and coexistence in neutron-rich isotopes in the mass A~100 region, the possible intruder states in even-even Cd isotopes, and the spectroscopy in heavy odd-A and odd-odd nuclei, in particular, the influence of odd particles on the nature of shape phase transitions.
The Zr isotopes (Z=40) belong to a mass region where shape coexistence has been proposed. These isotopes exhibit a variety of shapes, going from deformation near mid-open-shell (80Zr), through sphericity near the closed neutron shell (90Zr) and sub-shell (96Zr), and then to a sudden reappearance of deformation at 100Zr.
Such a variety of behavior is unprecedented anywhere on the nuclide chart. Shape coexistence has been also suggested by several experimental works, however, direct information on the shape of ground and excited states are still lacking for these isotopes, since multi-step Coulomb Excitation measurements have not yet been performed on these isotopes.
94Zr is particularly interesting because it is thought to be a strong candidate for displaying type-II shell evolution, as recently proposed for the Zr isotopes around N = 56, by state-of-the-art Monte Carlo Shell Model calculations.
As such, a dedicated experiment to study collectivity and configuration coexistence in 94Zr by means of low-energy Coulomb excitation was performed at the INFN Legnaro National Laboratory. The GALILEO-SPIDER setup, which in this instance has been further implemented with 6 LaBr3:Ce scintillators, has been used.
In this talk, I will present the results of the experiment, discussing the information on the shape obtained from the analysis with the GOSIA code. A preliminary comparison with Monte Carlo Shell model predictions will be also shown.
The shape of a nucleus is one of its fundamental properties. The nuclei in the neutron-rich region around mass 100 are well known to exhibit rapid shape changes. The simplest estimate of nuclear deformation in even-even nuclei can be obtained from the energy of the 2+1 state. For Sr (Z = 38) and Zr (Z = 40) isotopes this energy is observed to decrease dramatically at N = 60, while its evolution is much more gradual in Mo nuclei (Z = 42) [1]. Precise lifetime measurements provide a key ingredient in the systematic study of the evolution of nuclear deformation and the degree of collectivity in this region.
Neutron-rich nuclei in the mass region of A = 100-120 were populated through the fusion-fission reaction of a 238U beam at 6.2 MeV/u on a 9Be target. The compound nucleus 247Cm was produced at an excitation energy of ~45 MeV before undergoing fission. The setup used for this study comprised the high-resolution mass spectrometer VAMOS [2] in order to identify the nuclei in Z and A, the Advanced γ-ray Tracking Array AGATA [3] of 35 germanium detectors to perform γ-ray spectroscopy, as well as a plunger mechanism to measure lifetimes down to a few ps using the Recoil Distance Doppler Shift method (RDDS) [4]. In addition, the target was surrounded by 24 Lanthanum Bromide (LaBr3) detectors for a fast-timing measurement of lifetimes longer than 100 ps.
In this contribution, we will report on new lifetime results for short-lived states in neutron-rich A~100 nuclei, with an emphasis on the Zr and Mo chains. We will discuss the experimental techniques used to evaluate the lifetimes as well as their interpretation in terms of state-of-the-art nuclear structure models.
[1] S. Ansari et al. Phys. Rev. C 96, 054323
[2] M. Rejmund et al. Nuclear Instruments and Methods in Physics Research A 646 (2011) 184–191
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[2] A. Dewald et al. Progress in Particle and Nuclear Physics 67, 3
The Zirconium isotopes across the N=56,58 neutron sub-shell closures have been of special interest since years, sparked by the near doubly-magic features of $^{96}$Zr and the subsequent rapid onset of collectivity with a deformed ground-state structure already in $^{100}$Zr. Recent state-of-the-art model approaches [1] did not only correctly describe this shape phase transition in the Zr isotopic chain, but also the coexistence of non-collective structures and pronounced collectivity especially in $^{96,98}$Zr. This transition between different structural realizations within an isotope, first established in $^{96}$Zr [2], was attributed to the reordering of the effective valence spaces. The isotope $^{98}$Zr is located on the transition from spherical to deformed ground state structures. However, information on collectivity of this isotope in terms of E2 observables has been notoriously difficult to obtain, since it is unstable, and the lifetime of its first excited 2$^+$ state turned out to be out of range for fast-timing techniques in decay spectroscopy, only giving an upper bound. In this work a new lower bound on this lifetime will be presented, obtained from Coulomb excitation of a radioactive $^{98}$Zr beam [3]. This data has recently been complemented by a recoil-distance lifetime measurement following a two-neutron transfer reaction. The new data will be brought in context with the discussion of the shape-phase transition and the type-II shell evolution in $^{96,98}$Zr.
Supported by the German BMBF under Grand No. 05P15RDFN1 and DFG within SFB 1245.
[1] T. Togashi et al., Phys. Rev. Lett. 117, 172502 (2016).
[2] C. Kremer et al., Phys. Rev. Lett. 117, 172503 (2016).
[3] W. Witt et al., Phys. Rev. C 98, 041302(R) (2018).
We report here on the measurement of deep sub-barrier fusion cross sections for $^{58}$Ni +$^{64}$Ni. In this system the influence of positive Q-value transfer channels on sub-barrier fusion was evidenced in a famous experiment by Beckerman et al. [1]. Subsequent experiments for the two symmetric systems $^{58}$Ni +$^{58}$Ni and $^{64}$Ni +$^{64}$Ni showed that fusion hindrance is clearly present in both cases. The lowest measured cross section for $^{58}$Ni +$^{64}$Ni, however, was relatively large ($\sim$0.1 mb), so that no hindrance was observed. The present measurements have been recently performed at the XTU Tandem accelerator of LNL and the excitation function has been extended by two orders of magnitude downward.
The case of $^{58}$Ni +$^{64}$Ni is very similar to $^{40}$Ca+$^{96}$Zr [2] because of the flat shape of the two sub-barrier fusion excitation functions, originating from the couplings to several Q$>$0 neutron pick-up channels. $^{40}$Ca+$^{96}$Zr was studied to very small cross sections (2$\mu$b) and fusion hindrance does not show up, suggesting [3] that this unusual behavior is due to the Q$>$0 transfer couplings, since the valence nucleons can flow freely from one nucleus to the other without being hindered by Pauli blocking [4].
Our experiment indicates that the flat trend of the sub-barrier cross sections for $^{58}$Ni +$^{64}$Ni continues down to the level of $\sim$1$\mu$b and fusion hindrance is not observed. This trend at far sub-barrier energies reinforces the suggestion that the availability of several states following transfer with Q$>$0, effectively counterbalances the effect of Pauli repulsion that, in general, is predicted to reduce tunneling probability inside the Coulomb barrier.
[1] M. Beckerman et al. Phys. Rev. Lett. 45, 1472 (1980)
[2] A.M. Stefanini et al., Phys. Lett. B728, 639 (2014)
[3] H. Esbensen et al., Phys. Rev. C 89, 044616 (2014)
[4] C. Simenel et al., Phys. Rev. C 95, 031601(R) (2017)
Interaction of massive nuclei shows a considerable reduction in fusion cross sections at the Coulomb barrier according to a comparison of experimental cross sections with the calculated ones obtained using a barrier passing (BP) model. Lowered fusion cross sections are accompanied by a high probability of deep-inelastic and quasi-fission (QF) processes arising on the way to fusion. The detection of evaporation residues (ERs) resulting from the compound nucleus (CN) formation is an unambiguous sign of the complete fusion, whereas fission events do not specify the CN formation since CN-fission strongly interferes with QF events. Theoretical models developed to describe heavy ER cross sections σ_ER treat them as the product of capture cross-section σ_c relating to a composite-nuclear system formation, of CN production probability P_CN, and of survivability against fission when CN decays W_sv. Most of the models reproduce experimental σ_ER quite well, but they give P_CN differed from each other within several orders of the magnitude. Such a difference implies a corresponding distinction in W_sv. Available data on the excitation functions for fission and ERs obtained in projectile-target combinations with very different mass numbers (very asymmetric ones) can be well described in the framework of the BP and statistical model (SM) approximations. These data allow us to choose SM parameters implying that P_CN=1 and σ_c=σ_bp. Thus, fitting the calculated excitation functions to the measured ones with scaling of macroscopic fission barriers one can get W_sv. Fusion suppression corresponding to P_CN<1 appears in less asymmetric combinations and can be derived using W_sv for very asymmetric ones leading to the same or nearby CN and σ_c obtained in experiments or with the BP model calculations. The work attempts to systemize the data on P_CN derived as described above for projectile-target combinations leading to ERs from Pb to heaviest nuclei produced in (HI,xn) reactions.
Reaction with Exotic Nuclei at Near- and Sub-barrier Energies become a hot topic of current interest in nuclear physics. In the talk, I would like to present recent results obtained in the nuclear reaction group of CIAE.
The first topic is on the optical model potentials (OMPs) of exotic nuclear systems. Due to the limitations of intensity and quality of RIBs, it is difficult to extract the OMPs of exotic nuclear systems by the elastic scattering. For this reason, a transfer reaction method was proposed and applied to extract the OMPs of 6He+12C, 64Zn, 209Bi systems via 11B, 63Cu, 208Pb(7Li,6He) reactions [1]. The threshold anomaly behavior has been obtained in the 6He+209Bi system for the first time [2]. Results show that the dispersion relation is not applicable for the exotic nuclear systems. Possible reasons are discussed but further study is strongly required to discover the underlying physics.
The second topic is on the reaction mechanism of exotic nuclear systems. An important task is to understand the breakup effects as well as its mechanism. To this end, a complete-kinematics measurement method was developed and applied in the 17F+58Ni, 89Y [3], 208Pb and 7Be+208Pb experiments. The processes of elastic scattering, breakup/transfer, and fusion evaporation have been identified successfully. Preliminary results of 17F+58Ni show that elastic breakups are dominant, moreover, the fusions are suppressed above the barrier while enhanced below the barrier.
[1] L. Yang, C. J. Lin, H. M. Jia et al., Phys. Rev. C 96, 044615 (2017); Phys. Rev. C 95, 034616 (2017); Phys. Rev. C 89, 044615 (2014); Phys. Rev. C 87, 047601 (2013).
[2] L. Yang, C. J. Lin, H. M. Jia et al, Phys. Rev. Lett. 119, 042503 (2017).
[3] G. L. Zhang, G. X. Zhang, C. J. Lin et al., Phys. Rev. C 97, 044618 (2018).
The study of fusion reaction for weakly bound nuclei at sub-barrier energies is of large interest, especially those studies of the breakup and transfer effects in weakly bound nuclei. Due to the low breakup threshold, the fusion reactions induced by weakly bound nuclei are complicated processes including complete fusion and incomplete fusion. Also, transfer processes, including the one-neutron stripping, followed by the breakup of the projectile can occur. In all the above reaction channels, the same products can be produced by different mechanisms. So it is fundamental to exprimentally discriminate the different reaction channels to explore the various reaction mechanisms.
In this report we will introduce the study of suppression factor of complete fusion and how to use gamma rays in coincidence with the light charged to discriminte the different reaction channels. On basis of GALILEO array which is a high-efficiency gamma-ray spectrometer coupled with the Si-ball EUCLIDE for the detection of charged particles at Legnaro National Laboratory (LNL) in Italy, the experiments of 6Li+89Y and 6Li+209Bi have been performed. It is indicated that the different reaction mechanisms can be clearly studied. This facility can be used well to explore the fusion reacton mechanisms induced by weakly bound nuclei.
Energy differences between analogue states in the T=1/2 $^{23}$Mg-$^{23}$Na mirror nuclei have been measured along the rotational yrast bands with the EXOGAM + Neutron Wall + DIAMANT setup at GANIL. The nuclei of interest have been populated via the $^{12}$C+$^{16}$O fusion evaporation reaction.
This allows us to search for effects arising from isospin-symmetry breaking interactions (ISB) and/or shape changes. Data are interpreted in the shell model framework following the method successfully applied to nuclei in the $f_{7/2}$ shell.
It is shown that the introduction of a schematic ISB interaction of the same type of that used in the $f_{7/2}$ shell is needed to reproduce the data.
An alternative novel description, applied here for the first time, relies on the use of an effective interaction deduced from a realistic charge-dependent chiral nucleon-nucleon potential.
This analysis provides two important results: (i) The mirror energy differences give direct insight into the nuclear skin; (ii) the skin changes along the rotational bands are strongly correlated with the difference between the neutron and proton occupations of the $s_{1/2}$ “halo” orbit.
Complete spectroscopy for a certain nucleus means that up to a given
excitation energy for each state, spin and parity is determined by
experiment and the composition is described by some theoretical model.
Among heavy nuclei the goal to reach complete spectroscopy is
approached only for $^{208}$Pb.
Knowledge of nuclear states in $^{208}$Pb is gained since 1899. Since
the 1990s the sensitivity of the Munich Q3D magnetic spectrograph [1]
improved and several hundred levels in $^{208}$Pb up to 8 MeV were
found. The shell model describes the majority of nuclear states in
$^{208}$Pb with great success [2].
From the very beginning a few low-lying states were recognized to need
other model descriptions. The qualities of the 3- yrast state were
understood to be peculiar already in the 1950s. Its coupling to 1p-1h
configurations revealed a new class of nuclear excitations [3,4]. The
description of collective states as tetrahedral rotations and
vibrations invented 80 years ago was verified by discovering the 2-
member of the predicted 2+- parity doublet in $^{208}$Pb at Ex = 4.1
MeV [5,6].
In 2016 a major step of complete spectroscopy was reached with the
identification of 151 states below 6.2 MeV with spin, parity, and
major composition [3]. Now below 6.2 MeV nearly 160 states are
observed - including 5 states predicted but not yet clearly identified
[3-6]. The shell model predicts, however, only about 125 states.
Sixteen states are described by coupling 1p-1h configurations to the
3- yrast state, four states as pairing vibrations, nine states as
tetrahedral rotations and vibrations, and six states wait for some
model description.
[1] G. Dollinger and T. Faestermann. Nucl. Phys. News 28:5 (2018)
[2] R. Broda et al. PRC 95:064308 (2017)
[3] A. Heusler et al. PRC 93:054321 (2016)
[4] A. Heusler et al. PRC submitted
[5] A. Heusler et al. EPJ A 53:215 (2017)
[6] A. Heusler et al. PRC(R) submitted
The common treatment of proton-neutron (pn) pairing in N = Z nuclei relies on Cooper pairs and HFB-type models. However, in these nuclei the pn interaction generates quartet correlations of alpha type which compete with the Cooper pairs. In fact, for any T=0 and T=1 pairing interactions the ground state of N = Z systems is accurately described not by Cooper pairs but in terms of collective quartets [1-8]. Alpha-like quartets are relevant degrees of freedom for treating also more general two-body interactions than pairing [9-11]. From this perspective, I will discuss how the quartetting is affecting the competition between the T=0 and T=1 pn pairing correlations in nuclei as well as the contribution of pairing to the Wigner energy.
The treatment of proton-neutron pairing in self-conjugate nuclei in terms of conventional BCS-type approaches has revealed to be problematic. We have shown [1-4] that this form of pairing can be very well accounted for in a formalism of $J=0,T=0$ quartets. We have extended the quartet formalism to the treatment of realistic interactions both in the case of even-even [5,6] and odd-odd [7] self-conjugate nuclei. The role of quartets other than $J=0,T=0$ in the description of these systems has been investigated and it will be illustrated.
The difficulties associated with a microscopic treatment of $N=Z$ nuclei in a formalism of quartets rapidly grow with increasing the number of active nucleons. To make this formalism accessible also to large systems, we have recently explored an approach where elementary bosons replace quartets with $J=0,T=0$ and $J=2,T=0$. This boson architecture, which is clearly analogous to that of the Interacting Boson Model in its simplest formulation (IBM-1), has been employed for an analysis of $^{28}$Si [8]. The boson Hamiltonian has been derived with the help of a mapping procedure and the resulting spectrum and $E2$ scheme have been compared with the experimental data. As a peculiarity, the potential energy surface of this nucleus turns out to be that expected at the critical point of the U(5)-$\overline{\rm SU(3)}$ phase transition of the IBM structural diagram.
[1] N. Sandulescu, D. Negrea, J. Dukelsky, C. W. Johnson, Phys. Rev. C 85 (2012) 061303(R).
[2] M. Sambataro and N. Sandulescu, Phys. Rev. C 88 (2013) 061303(R).
[3] M. Sambataro, N. Sandulescu, and C.W. Johnson, Phys. Lett. B 740 (2015) 137.
[4] M. Sambataro and N. Sandulescu, Phys. Rev. C 93 (2016) 054320.
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[8] M. Sambataro and N. Sandulescu, Phys. Lett. B 786 (2018) 11.
In order to investigate 212Po alpha-structure, the inverse kinematic thick target method has been used to study elastic and inelastic scattering of 208Pb on 4He target. A 208Pb beam produced by the Superconducting Cyclotron (CS), INFN-LNS, at the incident energy of 10 MeV/u was sent onto a 4He gas cell. The gas cell was acting as target and as beam degrader, completely stopping the beam before it reaches the detection system placed at 0° with respect to the beam direction.
The recoiling alpha-particles were measured at forward angles in the center-of-mass system. The 208Pb stopping power in 4He was measured to correctly determine the excitation energy Ex from the detected alpha energy.
In this talk, the experimental technique will be described and the preliminary data analysis of the stopping power and the elastic cross section will be shown.
Deviations from a smooth trend in the separation energy extracted from atomic masses are typically associated with a sudden onset of deformation or the rise of a magic number. This information is limited to ground and isomeric states. A new way to investigate shell effects at high excitation energies is presented here and inferred from empirical drops in nuclear polarizabilities. Deviations from the effect of giant dipole resonances reveal the presence of shell effects in semi-magic nuclei with neutron magic numbers N = 50, 82 and 126. Similar drops of polarizability in the quasi-continuum of nuclei with, or close to, magic numbers N = 28, 50 and 82, could reflect the continuing influence of shell closures up to the nucleon separation energy. These findings strongly support recent large-scale shell-model calculations in the quasi-continuum region, which describe the origin of the low-energy enhancement of the photon strength function as induced paramagnetism, and assert the generalized Brink-Axel hypothesis as more universal than originally expected.
Indication of triaxiality in $^{78}$Ge has recently been presented from a low-energy sequence of strictly $\Delta J=1$ transitions [1]. Neutron-rich Ge and Se isotopes were studied using the Gammasphere Ge-detector array at ANL. Beams of $^{76}$Ge and $^{82}$Se were incident upon thick $^{238}$U and $^{208}$Pb targets in deep-inelastic reactions. New data in $^{80,82}$Se will be presented to clarify $\beta$-decay studies [2,3], and angular-correlation measurements are used to strengthen spin and parity assignments in some cases.
These observations can provide insights into the single-particle and collective properties of these neutron-rich nuclei. NuShellX calculations for the N = 46 and N = 48 Ge and Se isotones will be shown to test the $p_{3/2}f_{5/2}p_{1/2}g_{9/2}$ proton and neutron subspace[4]. Additionally, new insight into the structure of isotonic nuclei will be discussed.
This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under contract Nos. DE-AC02-06CH11357 (ANL) and DE-AC02-98CH10886 (BNL), and grants No. DE-FG02-94ER40834 (Maryland). This research used resources of ANL’s ATLAS facility, which is a DOE Office of Science User Facility.
[1] A. M. Forney, W. B. Walters, C. J. Chiara, R. V. F. Janssens, A. D. Ayangeakaa, J. Sethi, J. Harker, M. Alcorta, M. P. Carpenter, G. G\"urdal, C. R. Hoffman, B. P. Kay, F. G. Kondev, T. Lauritsen, C. J. Lister, E. A. McCutchan, A. M. Rogers, D. Seweryniak, I. Stefanescu, and S. Zhu. Submitted (2018).
[2] J.V. Kratz, H. Franz, N. Kaffrell, G. Hermann. Nucl. Phys. A 250 13-37 (1975).
[3] H. Gausemel, K. A. Mezilev, B. Fogelberg, P. Hoff, H. Mach, and E. Ramstr\"om
Phys. Rev. C 70, 037301 (2004).
[4] B.A. Brown and W.D.M. Rae. Nucl. Data Sheets 120 Supplement C, 115-118 (2014).
An interesting aspect of nuclear structure is the shell evolution for isotopes with extreme isospin values. Experimental evidence show the presence of a sub-shell closure at N = 32 for 52Ca, 54Ti and 56Cr. Mass measurements on 52,53K suggest that this sub-shell closure is maintained below Z=20. For the case of the 48Ar, low lying 2+, 4+ and the second 2+ states, as well as the B(E2)↑ value have been accessed using different techniques, and a triaxial character has been suggested. A recent γ-ray spectroscopy measurement of 50Ar, reported an energy of the first 2+ state of 1178(18) keV. The satisfactory reproduction of this experimental results by shell model calculations indicated the conservation of the shell gap at N = 32 for Ar isotopes. In the same measurement, a tentative transition with energy of 1582(38) keV was suggested to correspond to the 4 +→2+ transition. However, the limited statistics did not allow for a coincidence analysis to obtain any definitive conclusion on the existence of the peak, nor on spin and parity assignments. To further investigate the nature of the N=32 shell gap below Ca, the analysis of different reaction channels populating 50Ar is of high importance.
We will report on the preliminary results of the measurements of proton and neutron knockout reactions as well as inelastic scattering populating 50Ar performed at RIKEN within the third SEASTAR campaign. Isotopes of interest were produced after the fragmentation of a 70Zn beam at 345 MeV/u on a Be target and identified with BigRIPS. Selected isotopes were focused onto the liquid-hydrogen target of the MINOS device and gamma rays from the reactions were detected with the DALI2+ array. Outgoing particles were identified using the SAMURAI magnet and related detectors. Preliminary results on the spectroscopy of low-lying levels for 50Ar will be presented and the cross sections to populate the different states from different reaction channels will be discussed.
The tin nuclei, representing the longest isotopic chain between two experimentally accessible doubly-magic nuclei, provide a unique opportunity for systematic studies of the evolution of basic nuclear properties when going from very neutron-deficient to very neutron-rich species. A little over a decade ago, they were considered a paradigm of pairing dominance: the excitation energies of the first $2^+$ and $4^+$ states are rather constant along the Sn isotopic chain, and the $B(E2;2^+\to0^+)$ values for isotopes with A>116 present a parabolic behavior expected for the seniority scheme. On the other hand, the $B(E2;2^+\to0^+)$ values measured for neutron-deficient Sn isotopes remain constant with N. Unfortunately, the lack of information on $B(E2;4^+\to2^+)$ strengths in light Sn nuclei, combined with large experimental uncertainties on the $B(E2;2^+\to0^+)$ values, prevent firm conclusions on the shell evolution in the vicinity of the heaviest proton-bound N=Z doubly-magic nucleus $^{100}$Sn.
To remedy this, the first lifetime measurement in neutron-deficient tin isotopes was carried out using the Recoil Distance Doppler-Shift method, providing a complementary solution to the previous Coulomb-excitation studies. Thanks to the unusual application of a multi-nucleon transfer reaction, together with unprecedented capabilities of the powerful AGATA and VAMOS++ spectrometers, the lifetimes of the $2^+$ and $4^+$ states in $^{106,108}$Sn have been directly measured for the very first time.
Large-scale shell-model calculations were performed to account for the new experimental results. In particular, the comparison of the $B(E2;4^+\to2^+)$ values with the theoretical predictions shed light on the interplay between quadrupole and pairing forces in the vicinity of $^{100}$Sn. An interpretation has also been proposed for the anomalous $B(E2;4^+\to2^+)/B(E2;2^+\to0^+)$ ratio observed not only for the Sn isotopes, but also in other regions of the nuclear chart.
Since the 80’s, various mean-field theoretical approaches indicated neutron rich Nickel isotopes among the best candidates for the appearance of the shape coexistence phenomenon, including the possibility of finding its most extreme manifestation, i.e. shape isomerism. Shape isomerism arises from the existence of a secondary deformed minimum at large deformation in the nuclear potential energy surface, separated from the primary energy minimum by a high barrier, what results in a significantly hindered gamma transition between the minima. In an experiment performed in Bucharest [1], we have identified a shape-isomer like structure in the 66Ni nucleus. This is the lightest atomic nucleus exhibiting a photon decay hindered - solely - by a nuclear shape change. Such a rare process, at spin zero, was clearly observed only in actinide nuclei in the 1970’s. 66Ni was populated employing a two-neutron transfer reaction induced by an 18O beam on a 64Ni target, at sub-Coulomb barrier energy. The experimental findings have been well reproduced by the Monte Carlo Shell Model Calculations [1].
Encouraged by the results on 66Ni, we have started a comprehensive gamma spectroscopy investigation of 62Ni, 64Ni and 65Ni at IFIN-HH (Bucharest), ILL (Grenoble) and IPN Orsay, using different reaction mechanisms to pin down the wave function composition of selected excited states. We aim at shedding light on the origin of deformation in neutron-rich Ni isotopes, and at possibly locating other examples of shape isomerism in this region. Preliminary results will be presented and compared with Monte Carlo Shell Model predictions.
Perspectives in the search for shape isomerism in other mass regions will be also discussed, following recent calculations pointing to Pt, Hg and Pb nuclei (with N≈110) and Pd, Cd and Sn (with N≈66) as best candidates. Such systems could be investigated with radioactive beams from HIE-ISOLDE and SPES.
[1] S. Leoni et al., Phys. Rev. Lett. 118, 162502 (2017).
HIE-ISOLDE [1] at CERN reached the end of phase 2 in 2018, operating with four cryomodules for the first time and reaching the original design energy of 10 MeV/$u$ for radioactive ion beams.
Experiments have been focused on two experimental setups so far, with the Miniball HPGe array [2] taking most of the beam time and the Scattering Experiments Chamber (SEC) concentrating on reactions with light nuclei.
The ISOLDE Solenoidal Spectrometer (ISS) [3] was newly commissioned in 2018 for few-nucleon transfer reactions in the magnetic field of a former MRI magnet.
In this talk I will present the HIE-ISOLDE project and the show preliminary status of experiments from three years of operation.
Some of the selected physics cases will be, amongst others, Coulomb excitation at both ends of the Sn isotopic chain and studying octupole collectivity in both the lanthanides and the actinides.
Finally, preliminary results from the first two experiments at ISS will also be discussed, along with plans for the future of the device.
References:
[1] M. Lindroos, P. Butler, M. Huyse, and K. Riisager, Nucl. Instrum. Meth. B 266, 4687 (2008).
[2] N. Warr et al., Eur. Phys. J. A 49, 40 (2013).
[3] S. J. Freeman et al., CERN-INTC 031, 099 (2010).
Since neither of the hydrogen nor helium nuclei have a particle-bound excited state, Li-6 is the lightest nuclide in the entire nuclear chart for which an excited state decays predominantly by gamma-ray emission. The particle-decay of its 0+ state with isospin T=1 at 3563 keV excitation energy is parity-forbidden, and it decays exclusively by a strong isovector M1 transition to the 1+ ground state with isospin T=0. This decay transition represents the M1 analogue to the GT decay of the ground state of He-6 which has recently been measured with spectacular precision [1]. Although the lifetime of the 0+ state of Li-6 has been measured many times since the 1950s there is a disturbing 3-sigma deviation between the error-weighted mean value of the world-data and the measurement which claimed the highest precision. Moreover, the latter [2] has not been a measurement at the photon point but it was an electron-scattering experiment constraining the B(M1, 0+_3653 -> 1+ gs) value from an, in principle, model-dependent extrapolation of electron-scattering data at finite momentum transfers to the photon point. We have re-measured [3] the electromagnetic decay width of the 0+ state of Li-6 with a statistical uncertainty of only 1% with the technique of Relative nuclear Self-Absorption. The data and the technique will be presented and discussed.
[1] A. Knecht at el., Phys. Rev. Lett. 108, 122502 (2012).
[2] J. Bergstrom et al., Nucl. Phys. A 251, 401 (1975).
[3] U. Gayer et al., in preparation.
Excited states of $^{31}$S and $^{31}$P mirror nuclei were recently studied using the same fusion evaporation reaction $^{24}$Mg($^{12}$C, 1$\alpha$1p) and $^{24}$Mg($^{12}$C, 1$\alpha$1n). The 45~MeV beam was delivered by the XTU-Tandem accelerator at LNL Legnaro. The detection system was composed of GALILEO $\gamma$-ray spectrometer coupled to 4$\pi$ Si ball Euclides and to Neutron Wall. Previous studies of A=31 mirror nuclei showed the oscillation behaviour of Mirror Energy Difference values (MED) values for the negative-parity sequence as a function of spin. These oscillation may be explained including in the wave function excitations to the fp shell considering thus the electromagnetic spin-orbit effect. Description of the MED in sd shell nuclei for negative parity and high spin states involving the electromagnetic spin orbit term is up to now only qualitative (because it involves interactions in two main shells). Additionally, shell-model calculations performed using the USD residual interaction and the Monte Carlo shell model with the SDPF-M interaction reproduce well the excitation energies and the reduced transition probabilities for positive-parity states up to the spin $\frac{13}{2}^{-}$. An interesting feature revealed by these calculations is that the yrast negative-parity states show an alternating structure: the $\frac{7}{2}^{-}$ , $\frac{11}{2}^{-}$ , and $\frac{15}{2}^{-}$ states are described by almost equal contributions of the proton and neutron excitation to the fp shell, whereas the $\frac{9}{2}^{-}$ and $\frac{13}{2}^{-}$ states have only a neutron excitation to the f$_{7/2}$ shell. Because experimental MED values are available up to spin J=$\frac{13}{2}$ for both negative and positive parity in our experiment we tried to identify high-spin states if $^{31}$S in order to disentangle the theoretical puzzle. The results of our investigations will be presented.
Alpha decay has been a probe of nuclear structure and clustering in nuclei since the dawn of nuclear physics. However, microscopic description of alpha-decay rates remains to be a challenge. During the talk, the recent observation of the superallowed alpha-decay chain $^{108}$Xe-$^{104}$Te to doubly magic $^{100}$Sn [1], using the recoil-decay correlation technique with the Argonne Fragment Mass Analyzer at ATLAS, will be presented. This is an important stepping-stone towards developing a microscopic model of alpha decay since it is only the second case of alpha decay to a doubly magic nucleus, besides the benchmark $^{212}$Po alpha decay to $^{208}$Pb. The decay properties of $^{108}$Xe and $^{104}$Te indicate that in at least in one of them the reduced alpha-decay width is a factor of 5 larger than in $^{212}$Po. The enhanced alpha-particle preformation probability could be the result of stronger interactions between protons and neutrons, which occupy the same orbitals in N=Z nuclei. During the talk, the alpha emitters in the $^{100}$Sn region will be compared with their counterparts in the $^{212}$Po region, and with the existing alpha-decay models. Prospects for alpha-decay studies in the $^{100}$Sn region will be also discussed.
[1] K. Auranen, D. Seweryniak et al., Phys. Rev. Lett. 121, 182501 (2018)
When the liquid drop fission barrier vanishes in the fermium-rutherfordium region only the stabilization by quantum mechanics effects allows the existence of the observed heavier species. Those are in turn providing an ideal laboratory to study the strong nuclear interaction by in-beam methods as well as decay spectroscopy after separation [1].
Here we focus on the achievements of decay spectroscopy after separation (DSAS) for the deformed nuclei in the region Z=100-112 and N=152-162. They have the potential to provide direct links to the next heavier spherical closed shell nuclei via the investigation of single particle levels [2]. Particularly interesting features are meta-stable states due to nuclear deformation, so-called K isomers, which can be used to trace the spherical superheavy nuclei (SHN) and to locate the island of stability [3]. The application of coincidence and correlation methods, employing the detection of $\alpha$s, $\gamma$s, X-rays, conversion electrons and fission fragments, can be used as powerful tools to separate and study specific decay features like e.g. in the investigation of the $^{258}$Db decay performed by Heßberger et al. [4].
High intensity accelerators, efficient in-flight separators and spectrometers, and highly efficient detectors with fast electronics are the essential ingredients for the success of the field. The new SPIRAL2 facility and, in particular, the separator-spectrometer setup S$^3$ [5] presently under construction at the accelerator laboratory GANIL in Caen, France, will offer great perspectives for the field [6].
[1] D. Ackermann and Ch. Theisen, Phys. Scripta 92, 083002 (2017).
[2] M. Asai et al., Nucl. Phys. A 944, 308 (2015).
[3] D. Ackermann, Nucl. Phys. A 944, 376 (2015).
[4] F.P. Heßberger et al. Eur. Phys. J. A 52, 328 (2016).
[5] F. Dechery et al., Eur. Phys. J. A 51, 66 (2015).
[6] D. Ackermann, EPJ Web of Conf. 193, 04013 (2018).
The properties of the mass and energy distributions of fissionlike fragments formed in the reactions 48Ca,58Fe + 208Pb,36S, 48Ca,48Ti,64Ni + 238U,48Ca + 232Th,244Pu,248Cm at energies around the Coulomb barrier have been analyzed to define the systematic trend of compound nucleus fission and quasifission in cold and hot fusion reactions. The measurements have been carried out at the U400 cyclotron of the FLNR, JINR using the double-arm time-of-flight spectrometer CORSET. The fusion probabilities have been deduced from the analysis of mass and energy distributions. It was found that for the studied reactions fusion probability depends exponentially on mean fissility parameter of the system. For the reactions with actinide nuclei leading to the formation of superheavy elements the fusion probabilities are of several orders of magnitude higher than in the case of cold fusion reactions.
The search for new magic numbers beyond 208Pb, understanding the enhanced stability of superheavy nuclei (SHN) and their existence despite the repulsive Coulomb interaction is an active field of research in both theoretical and experimental nuclear physics. Precise structure studies of quasi-particle excitations in deformed actinide and transactinide nuclei are crucial to this understanding. In the last decades exhaustive investigations have been carried-out on the decay of deformed nuclei in the transfermium region around 254No.
In this contribution, I will first report on the recent results of in-beam spectroscopic studies on the 244Cf (Z=98) nucleus performed at the University of Jyvaskyla using the RITU gas-filled separator, the GREAT spectrometer and the Jurogam germanium array. The ground-state rotational band of the neutron-deficient californium isotope 244Cf was identified for the first time indicating that the nucleus is deformed. The kinematic and dynamic moments of inertia were deduced from the measured gamma-ray transition energies and are compared to theoretical calculations.
I will then present the investigation of the 250No isotope performed at the University of Jyvaskyla using the same set-up. Using fully equipped focal plane detector with digital electronics, we were able to give a definitive answer to the puzzling question concerning the decay path of the isomeric state and the ground state of 250No. Those results will be compared to configuration-constrained PES calculations performed for the 250No and other heavy nuclei.
Finally, I will briefly describe the new focal plane detection set-up SIRIUS that have been built in the framework of Spiral2 coupled with S3 spectrometer. The SIRIUS spectrometer, which has been designed for the identification of fusion-evaporation residue through decay tagging, will provide important information on nuclear deformation, single-particle properties.
Recent high-energy (p,pd) reaction study1 has confirmed the existence of high-momentum correlated pair of nucleons with S=1 and T=0 in ground state of 16O nucleus. The effect of such high-momentum correlated pairs affect the structure of ground and low excited nuclei through the tensor blocking.
A new paradigm of the nuclear structure that includes blocking effects of the tensor interactions is proposed. All of the recently discovered magic numbers (N=6, 14, 16, 32, 34) in neutron-rich nuclei are explained by the blocking effects that occur at specific shell configurations. A large amount of binding energy is gained by high-momentum correlated pairs of nucleon produced by the tensor interaction. Such tensor correlations strongly depend on the configuration space available for exciting 2p-2h states. When additional neutron occupy a new orbital, the configuration that was available before may be lost and result in sudden loss of binding energy otherwise gained by the 2p-2h excitation. Such tensor blocking effects enlarge the energy gaps at all observed new magic numbers. The tensor blocking also explains consistently observed peculiar configurations of neutron rich nuclei at the border of shells. The present study will open new horizon in nuclear physics particularly focusing the high momentum properties in excitation spectra.