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We are pleased to announce the fourth Pre-PAC Workshop for the AGATA physics campaign at LNL. The workshop will take place from the 2nd to the 4th October 2023 at the Legnaro National Laboratories and is planned to be held in-person.
The next LNL PAC meeting is planned for December 2023 and will evaluate experiments to be performed in the first half of 2024. During this period, beams from the entire TANDEM-ALPI-PIAVE accelerator complex will be available at LNL, and AGATA is expected to consist of a minimum of 15 triple clusters (1pi solid angle coverage).
The aim of the Pre-PAC Workshop is to assist the spokespersons in putting the strongest cases for their proposals forward through a discussion of the physics to be investigated, and to assess the feasibility of the proposed experiments. This includes all experiments planning to use stable beams from the TANDEM-ALPI-PIAVE accelerator complex for studies involving AGATA in a possible combination with PRISMA and/or ancillary detectors that are compatible with PRISMA. Any new projects will have to be presented at this workshop before being submitted to the LNL PAC. In most cases, we will not expect presentations of the projects that have already been discussed at the previous Pre-PAC Workshops. If, however, a new project shows a considerable overlap with a previously discussed project of a different group, both parties will be informed and encouraged to participate in the upcoming Workshop.
You are welcome to submit Letters of Intent (LoIs) for the above mentioned AGATA setups with a deadline of September 11th, 2023. They are expected to include, in a pdf file, title, short abstract with a description of the physics case, experimental setup, beam (energy, intensity), target(s), and the justified request of beam time. For the list of available beams from the Tandem- ALPI-PIAVE complex we invite you to consult the LNL website: https://agenda.infn.it/event/36457/attachments/107698/15223/tandem_piave_alpi_beams_agata.pdf
We warmly recommend the LoI spokespersons to contact as soon as possible the experts of the complementary equipment they intend to use. These contact persons are:
PRISMA : L. Corradi, F. Galtarossa
GAL-TRACE : S. Capra
EUCLIDES: J. Pellumaj, D. Brugnara
SPIDER: N. Marchini, M. Balogh
DANTE: K. Rezynkina, J. Benito
Gamma-ray scintillators: S. Pigliapoco, A. Giaz
Plunger: M. Polettini, F. Angelini
Abstract
The 12C + 16O reaction plays a particularly important role in both the carbon and oxygenburning
phases of stars. Fang et al. measured this reaction in a thick-target experiment a few years
ago, with both singles and particle-γ coincidence techniques down to a few nanobarns. However,
the lowest energy points suffer from large experimental uncertainties which prevent discriminating
between the Fowler model and the hindrance approach. Further measurements at low energies
with smaller errors are therefore needed to clarify the underlying physics and allow a reliable
extrapolation to the region of the Gamow peak.
This LoI is the proposal of an experiment on 12C + 16O aiming at the measurement of fusion
cross sections below the μb range with the combined set-up of AGATA and silicon detectors. The
fusion events will be identified by coincidences between the prompt γ-rays and the light charged
particles (p, α) evaporated from the compound nucleus.
The required beam time is 12 days.
The second 0+ state of $^{12}$C at 7.654 Mev (the so-called Hoyle state) is one of the most studied states among all nuclei because of its well defined $\alpha$-cluster structure and for its strong astrophysical implications. Nevertheless, its structure still presents aspects to be
clarified. In particular, the hypotheses of the presence of excitations of the Hoyle state date back to 1956 when Morinaga first suggested it. In recent years, a 2+ [1-5] and a 4+ [6] excited state were found and it was claimed they belong to a rotational band built on the Hoyle state. Such assignment was based on the spin and parity of the states and on their energy.
A stronger proof that these levels are excitations of the Hoyle state may come from the study of their electromagnetic properties, in particular by measuring the B(E2). These are strongly enhanced in transition among states that belong to the same rotational band.
In particular the B(E2,$2^+_2\rightarrow0^+_2$) would provide extremely important information on the nature of the Hoyle state. And even an upper limit on such B(E2) would be enough to put significant experimental constraints to the models that try to describe $^{12}$C structure and in particular the Hoyle state.
We propose to measure the the B(E2) for the transition from the second 2+ (10.03 MeV) to the second 0+ state (7.654 MeV).
The measurement is very difficult to realize and has not been performed up to now because of the very small gamma decay probability of the state (around 10$^{-8}$) due to the fact that the state has a well pronounced $\alpha$-cluster structure and it is well above the $\alpha$-decay threshold. Only using a large solid angle and high efficiency $\gamma$-detector array, like AGATA, it will be feasible to perform such a kind of measurement.\
The initial state can be populated via an inelastic scattering reaction, $^{12}C(p,p')^{12}C^*$, with a proton energy of 28 MeV. The excited state of interest in the $^{12}$C can decay in two ways: either into three $\alpha$-particles
$^{12}C^* \rightarrow^{4}He+^{8}Be$, or gamma-rays $^{12}C^*\rightarrow^{12}C^* + \gamma$.
By measuring the branching ratio $\gamma / \alpha$ and from the total width of the state known from literature it is possible to extract directly the B(E2).
The $\alpha$-decay will be measured using an array of high-granularity silicon detectors (DSSSD) to be placed inside the AGATA scattering chamber. The $\gamma$-decay will be measured using AGATA in coincidence with the proton ejectile and the three alpha-particles from the breakup of the daughter state following gamma decay: the Hoyle state.
The cross section of the $2^+_2$ state populated via ^{12}C(p,p')^{12}C^*$ has been estimated to be 1 mbar from experimental data taken from two experiments [3,7]. Assuming such a cross-section value, from 4 to 10 gamma per day are expected to be detected depending on the value of the B(E2) used for the calculation.
[1] M. Itoh, et al., Nuclear Physics A, vol. 738, pp. 268–272, 2004.
[2] M. Itoh, et al., Phys. Rev. C, vol. 84, p. 054308, Nov 2011.
[3] M. Freer, et al., Phys. Rev. C, vol. 80, p. 041303, Oct 2009.
[4] M. Freer, et al., Phys. Rev. C, vol. 86, p. 034320, Sep 2012.
[5] W. R. Zimmerman, et al., Phys. Rev. Lett., vol. 110, p. 152502, Apr 2013.
[6] M. Freer, et al., Phys. Rev. C, vol. 83, p. 034314, Mar 2011.
[7] M. HARADA, et al., Journal of Nuclear Science and Technology, vol. 36,
no. 4, pp. 313–325, 1999.
The aim of this letter of intent is to study the structure of 46Ca via the lifetime measurements of the low-lying excited states. Particularly, the lifetime of 2_1+, 4_1+ and 0_2+ states will represent a crucial result to investigate the structure of the competing low-lying configurations, shedding light on the phenomenon of shape coexistence, already established in other Ca isotopes. In order to measure the lifetimes of the low-lying states in 46Ca, a multi-nucleon transfer (MNT) reaction between neutron-rich projectile and target will be performed. Such a technique allows us to directly populate the excited states of the channels, overcoming experimental limitations caused by the presence of low-lying isomers.
In the light Sn isotopes, a systematic deviation between the B(E2; $2_1^+ \to 0_{g.s.}^+)$ values and state-of-the-art theoretical calculations has been observed, and such a discrepancy becomes even more significant when comparing predictions and experimental results for the B(E2; $4_1^+ \to 2_1^+)$ strengths.
Recent works have indicated the importance of the $4_1^+ \to 2_1^+$ transitions to clarify the electromagnetic structure of nuclei around $^{100}$Sn.
However, the presence of low-lying isomers prevents the direct measurement of reduced transition probabilities between these states, particularly for the light even-mass Sn isotopes.
In this regard, the neighboring odd-mass Sn isotopes provide an alternative solution to investigate the electromagnetic properties of low-lying excited states in this region.
Similarly, additional information can be obtained from the study of Sb nuclei, which are one proton above the $Z=50$ shell closure.
The systematic study of electromagnetic properties in both Sn and Sb isotopic chains is crucial to explore the robustness of the $Z=50$ shell closure and investigate the nuclear interaction in proximity of the doubly-magic self-conjugate $^{100}$Sn.
To accomplish this objective we propose to measure the lifetime of low-lying states in the $^{107,109,111}$Sn and $^{111,113}$Sb, especially the $2_1^+$- and $4_1^+$-core coupled states (i.e. $9/2^+$, $11/2^+$ and $13/2^+$).
The nuclei of interest will be populated via a multi-nucleon transfer reaction by using a 784-MeV $^{112}$Sn beam impinging on a $^{92}$Mo target and the lifetime of the excited states will be measured via the recoil-distance Doppler-shift technique using the dedicated plunger setup.
The emitted $\gamma$ rays will be detected by the AGATA array and the beam-like reaction products will be identified by the PRISMA spectrometer.
Physics aim
A recent measurement at the Spiral1 facility at GANIL has revealed that the $^{46}$Ar nucleus is a bubble nucleus with a filled $\pi d_{3/2}$ shell and an empty $\pi s_{1/2}$ [1]. This finding also explain the small B(E2) value for $^{46}$Ar 2$^+$ state.
This important discovery is in agreement with ab-initio calculations with the NNLOsat parametrization of the chiral effective Hamiltonian, but in disagreement with shell-model calculations with the well-established SDPF-U interaction. This latter Hamiltonian predicts the $^{46}$Ar 0$^+$ ground state to be an even mixture of the $\pi d_{3/2}$ and $\pi s_{1/2}$ shells. This also somehow what would be suggested by an heuristic reasoning, considering that in $^{47}$K the $\pi d_{3/2}$ and $\pi s_{1/2}$ shells appear to be separated by only 360 keV.
A key point then arises: if the $^{46}$Ar 0$^+$ ground state has the $\pi d_{3/2}^{4} s_{1/2}^0$ proton configuration, at which energy is the second $0^+$ with the $\pi d_{3/2}^{2} s_{1/2}^2$ wave function ? And why it does not mix with the ground state ? A measurement of its excitation energy would provide crucial information to understand if the lack of mixing is coming from a large energy difference between the two $0^+$ levels or from the small diagonal pairing between them.
Proposed measurement
We propose to populate the $\pi d_{3/2}^{2} s_{1/2}^2$ excited $0^+$ state $^{46}$Ar using a $^{48}$Ca($^6$Li,$^8$B)$^{46}$Ar reaction at about 10 MeV/u. Gamma rays will be detected by the AGATA array, while the $^8$B ejectile by a DSSD detector like TRACE or OSCAR. The reaction can be performed in direct kinematics, using a $^6$Li 56 MeV beam from the Tandem and a CaF target or in inverse kinematics with a Tandem-ALPI or PIAVE-ALPI $^{48}$Ca beam at around 500 MeV.
Considering that a cross section of about 1 $\mu$b can be intercepted by the DSSD detector, we predict that about seven days of beam time will be sufficient to detect the desired $\pi d_{3/2}^{2} s_{1/2}^2$ excited $0^+$ state. The use of PRISMA can be considered for ancillary detection of heavy recoils, at least in A/Q.
[1] D. Brugnara et al., to be submitted
In the previously accepted proposal we aimed to study the transition into the N = 20 Island of Inversion by multi-nucleon transfer reactions using a 26Mg beam on a 238U target. The proposal was evaluated by the PAC in December 2022. The experiment was scheduled and performed in April 2023.
In our original submission, we proposed to use a 26Mg beam at 7 MeV/A and a 1 mg/cm2 thick 238U target. Multi-nucleon transfer reactions populate states in nuclei with N > 14. The combination of PRISMA for reaction product identification and measurements of ejectile momenta with AGATA for γ-ray spectroscopy would allow us to measure excited state lifetimes on the order of 1 ps. The main goals of the proposed experiment were:
• to establish negative parity states in Mg isotopes arising from the promotion of an odd number of particles across the N = 20 gap to the fp shell and
• to study quadrupole and octupole collectivity in Ne isotopes.
In addition we aim to perform spectroscopic studies of Na and F nuclei. Since this experiment is part of a proposed campaign to use multi-nucleon transfer reactions with light projectiles we will also benchmark GRAZING calculations which are needed to plan future experiments.
The beam time in April 2023 was not successful due to problems with the beam deliv- ery. The ion source was unable to provide 26Mg, probably because the source material was prematurely evaporated. In discussion with the local team, the AGATA physics campaign spokesperson, the PAC chair, and the LNL directorate, we decided to switch the beam to 22Ne. This case was similarly to the original plan presented in the context of the LNL mid-term plan and endorsed by the PAC. Unfortunately, the beam current was highly unstable, and frequent re-tuning and adjustments significantly reduced the effective beam intensity on target.
Nevertheless, we were able to collect important data. Population of Ne isotopes up to N = 16 was observed which gives us confidence that in this proposed experiment we can achieve lifetime measurements in the 4n channel 30Mg. During the run, we gained experience to optimize the PRISMA settings to compensate for the low efficiency of the spectrometer for these light ions. In particular, we successfully tested and alternative way to perform particle identification by correlating the time of flight of the ions with their kinetic energy (E-ToF). The Z reconstruction and the mass separation achieved with this method are shown in Fig. ??.
In summary, we believe that our physics case to study nuclei toward the N = 20 island of inversion is still of high interest and therefore request 7 days of beam time. We will also consider proposing the follow-up experiment using the 30Si beam during the AGATA- PRISMA campaign at LNL once both the AGATA scheduling and beam availability are released.
In the landscape of nuclear shapes, dominated by reflection-symmetric forms leading to either spherical or axially-deformed arrangements, the occurrence of asymmetric pear-like nuclei is present only in selected areas of the nuclear chart, named Islands of Octupole Deformation. A measurement of lifetimes of the first excited states of $^{220}$Ra, being at the edge of the island of octupole deformation, would help assessing the pattern of such deformation at the limit of the region and to assess its quadrupole-octupole degree of freedom.
The lifetimes of the first excited states are expected to be of the tens of ps order, based on the systematics in the region. We propose to perform the measurement with an inverse kinematics reaction, using a $^{208}$Pb beam on a $^{18}$O target, using AGATA+PRISMA with a plunger target.
We propose to Coulomb excite $^{96}$Zr on a $^{116}$Sn target 1-mg/cm$^2$ thick, using the maximum energy of $^{96}$Zr beam available from the TANDEM-ALPI facility (5.5 MeV/u). The data resulting from ``safe'' Coulomb excitation (24$^{\circ} \le \theta_{LAB} \le 27^{\circ}$) will be used to extract $B(E2)$ and $B(E3)$ values. Lifetimes of excited states that are within the sensitivity range of the RDDS method will be measured with aid of a plunger device. This includes the $3^-_1$ state, which lifetime is important in the context of the large $B(E3)$ strength discussed above, as well as the 2$^+_2$ state involved in the type-II shell-evolution scenario. The lifetime, or a limit thereof, will be also measured for the 6$^+$ state, shedding light on its possible character as a two-phonon state related to an octupole vibration. Based on our experience with the same setup, five to seven distances for the plunger in five days of measurement will be sufficient to extract these lifetimes.
Multinucleon transfer reactions are known to provide a useful method of populating
neutron-rich nuclei in gamma-ray spectroscopy experiments. Recent theoretical
calculations have suggested that such reactions could also be used to produce neutron-
deficient nuclei, but this has not been properly tested in experiments. The calculations
suggest that it may be possible to produce the neutron-deficient Z=54 xenon nuclei with
reasonable cross sections. These nuclei are of considerable interest due to their
propensity to develop octupole correlations heading towards N=Z. Here, we propose an
experiment to study the level schemes of the neutron-deficient xenon isotopes 110,112Xe
to further study the evolution of octupole correlations. In the experiment we will also
study the production yields of neutron-deficient nuclei above 100Sn with multinucleon 50
transfer reactions. We will use a beam of 58Ni at 316 MeV on a thin 112Sn target. The 28 50
beam-like reaction products will be identified using the PRISMA spectrometer, and the prompt decays of excited states will be measured using AGATA. To perform this measurement, we request 14 days of beam time.
We propose to conduct an experiment aimed at assessing the performance of AGATA at energy levels of up to 4.8 MeV. Investigating AGATA’s response to highly energetic gamma rays, specifi- cally focusing on efficiency, resolution, and the performance of the tracking algorithm, is a matter of considerable interest within the scientific community. However, such an evaluation has not yet been undertaken. The measurements outlined in this Letter of Intent will be executed in collabora- tion with the utilization of standard, long-lived radiation sources readily available at the Laboratori Nazionali di Legnaro (LNL). These sources include 241Am, 133Ba, 152Eu, 60Co, and 226Ra. Addi- tionally, we plan to procure a short-lived source of 56Co specifically for this experiment.
Much attention has been attracted to the "Southwest" of the double-magic $^{132}$Sn in the past 30 years. Even though the $N$ = 82 shell closure is still robust from $^{132}$Sn to $^{128}$Pd, the evolution of proton and neutron orbitals has been discovered in this region. Our knowledge of the single particle orbitals can help to examine the nuclear force and constrain shell model parameters, which is the general way to explore the "terra incognita" region with a larger $N$/$Z$ ratio. In the Indium isotopes around the neutron-mid shell, the $np$-$nh$ excitation across the proton closed shell plays an important role in the low-lying states, resulting in the emergence of shape coexistence.
In this proposal, we aim to study the exotic region(N~74) in the "southwest" of doubly-magic $^{132}$Sn with the most neutron-rich stable $^{124}$Sn isotope via multinucleon transfer(MNT) reaction. To determine the single particle property of the $\pi$$p_{3/2}$ and $\pi$$p_{1/2}$ orbitals, how will the shape coexistence evolve towards $N$ = 82, the lifetime measurement of 3/2$^{-}$, 7/2$^{+}$, 13/2$^{+}$ and 11/2$^{+}$ states will be performed for odd-A Indium isotopes based on the gamma lineshape analysis. Finally, the B(E2), B(M1) and mixing ratio will be deduced with the help of the large covering angle, highly segmented nature and high efficiency of AGATA. Several new levels, such as the yrast structure in odd-odd Indium isotopes around $N$ = 74 and the intermediate states deexciting to the 1/2$^{-}$ and 9/2$^{+}$ states in $^{119,121}$Ag will be established for the first time. In the meantime, as a byproduct, we will check if the MNT could serve as the appropriate reaction mechanism to produce the $M$1 scissors states in the $^{156}$Gd.
In this letter of intent, we set out a case to search for excited states in the isotopes, $^{135}$Eu, $^{136}$Eu, $^{137}$Eu, $^{138}$Eu and $^{139}$Eu. Information on excited states for most of these nuclides is relatively sparse, presenting a fertile ground for discovery in a region exhibiting isomerism, shape coexistence, and superdeformation. This experiment aims to elucidate and characterise excited states built upon low-lying Nilsson bandheads, with a particular focus on the evolution of their relative excitation energy as a function of neutron number.
We request 12 days of beam time plus 1 day of setup and calibration to carry out a search for the excited states of the nuclides $^{135-139}$Eu, using the AGATA array coupled to the $4\pi$ EUCLIDES charged particle spectrometer for efficient veto of non-alpha evaporation channels and the PRISMA magnetic spectrometer for unambiguous identification of the recoils.
The region of the nuclear chart close to the doubly-magic ($Z=82,N=126$) $^{208}$Pb nucleus is at the center of extensive experimental and theoretical investigations, connected both to nuclear structure, astrophysics and reaction mechanism studies. The experimental information concerning low-lying excited states of nuclei in this region is still extremely scarce and in most cases limited to isomeric states identified in fragmentation reactions.
Multi-nucleon transfer (MNT) reactions at energies close to the Coulomb barrier have been indicated as a promising tool to perform detailed spectroscopy of these nuclei but the best experimental conditions to enhance the survival probability of the primary heavy fragments to secondary processes like neutron evaporation and fission is still under debate.
We propose to use the $^{208}$Pb+$^{130}$Te reaction in inverse kinematics at $\sim$10 % above the Bass barrier to populate nuclei close to $^{208}$Pb, in particular with $Z<82,N\geq126$. The main focus of the proposed experiment is the detailed spectroscopy of the $N=126,127$ nuclei $^{206,207}$Hg nuclei and $^{208}$Tl and the determination of the cross sections for the population of neutron-rich nuclei in the $N=126$ region with the selected reaction.
The neutron-rich part of the nuclear chart and, in particular, the regions around double-shell closures have been studied in detail revealing intriguing phenomena. However, around the $^{208}$Pb nucleus, there is still a lack of spectroscopic information, especially regarding lifetimes of nuclear excited states. Large shell model calculations performed for Pb isotopes typically reproduce the experimental level scheme energies but fail to reproduce the reduced transition probabilities.
This proposal aims to investigate the shell evolution in the region of the double magic $^{208}$Pb isotope involving the neutron $g_{9/2}$ shell through the lifetime determination of the lowest lying yrast excited states in $^{211}$Pb. In particular, the main goal will be to determine the $B(E2:17/2^+\rightarrow13/2^+)$ and $B(E2:13/2^+\rightarrow9/2^+)$ transition strengths in $^{211}$Pb which would provide the testing ground of recent large-scale shell-model calculations for evaluating the performance of the effective three-body forces in heavy systems.
We propose lifetime measurements in $^{211}$Pb employing AGATA+Prisma setup coupled to a plunger device in the reverse configuration.
Finally, the evaluation of the production of $^{212}$Pb and $^{208}$Hg isotopes will be done by the study of the emission of the $\gamma$-rays from the $4^+\rightarrow2^+$ and $2^+\rightarrow0^+$ transitions.
The experiment "Probing nucleon-nucleon correlations in the $^{48}$Ca+$^{208}$Pb system below the Coulomb barrier" performed in March 2023 is of significant importance for understanding nucleon-nucleon correlations. It provides a unique opportunity to simultaneously investigate these correlations for a complete set of transfer channels, involving both the addition and removal of neutron and proton pairs. Despite encountering challenges such as target issues and ECR source problems, the data collected showed intriguing insights, especially when comparing observations above and below the barrier.
We propose a direct kinematics experiment by using a $^{48}$Ca beam on a $^{208}$Pb target, employing the superconducting
PIAVE-ALPI accelerator complex of LNL. Our goal is to obtain transfer probabilities for proton transfer channels at larger distances of closest approach for different populated states. Furthermore, we will measure angular distributions and cross sections for all open transfer channels. This direct kinematics experiment is essential for achieving our original research aims and will contribute to the knowledge of MNT's efficacy in populating regions of the nuclear chart south of $^{208}$Pb.
We ask for a total of {\bf 5 days} of beam time with {\bf PIAVE+ALPI}.
The E2 transition probability of the 4+1 state in 74Zn remains a puzzle. The safe Coulomb excitation experiment performed at ISOLDE at 3 MeV/u is compatible with a maximum of collectivity as expected for a mid-neutron g9/2 shell nucleus and supported by the shell model calculations. However, the first plunger experiment performed at AGATA+PRISMA using multi-nucleon transfer (MNT) at LNL in 2010 shown on the contrary an unexpected decrease of the collectivity from 72Zn to 74Zn. The long lifetime of the 4+ state was later confirmed using another MNT reaction and a plunger at AGATA + VAMOS in 2016. Recently, the safe Coulomb excitation was done at higher energy at HIE-ISOLDE leading to an intermediate value. The disagreement is very puzzling since the two MNT reactions with plunger lead independently to the same value. It could be speculated that an unknown transition is feeding the 4+ with a lifetime in the range of >100 ps. We propose to revisit the 74Zn case by MNT in a dedicated experiment, without plunger, to clarify the level scheme and search for a long-lived feeder using the geometrical DSAM effect in AGATA thanks to its unprecedented resolving power coupled to PRISMA. If the statistic is sufficient, an angular distribution will be performed to tentatively assign spin/parity to the state feeding the 4+1 state.
Full proposal at https://u.ganil-spiral2.eu/cloud/index.php/s/SRqbi15C327J7PE