DREB 2012 - Direct Reactions with Exotic Beams

26 Mar 2012, 08:00 → 29 Mar 2012, 18:00 Europe/Rome

Aula Magna, Faculty of SMFN

Angela Bonaccorso (PI), Meeting Secretariat, Nigel Orr (LPC - Caen)

We are happy to announce that the next Direct Reactions with Exotic Beams Workshop will be held in Pisa, Italy in early Spring 2012.
This meeting will be the 7^th in the series which began in 1999 and focuses on the experimental, theoretical and instrumental aspects of direct reactions with radioactive beams.
The meeting will run over 4 days, from the morning of Monday, March^th through to the evening of Thursday, March 29^th.

The programme will include a free afternoon on the Tuesday and the workshop dinner on the Wednesday.
On Monday afternoon, after the working sessions, a visit to the European Gravitational Observatory (EGO) is organized.

In keeping with the workshop’s tradition, the meeting will be of an informal character: no proceedings will be published and talks will be selected on the basis of contributed abstracts with an emphasis on those presenting new results.

The registration fee is 100,00 € for senior attendees and 50,00 € for students.

The meeting is sponsored by:
Istituto Nazionale di Fisica Nucleare (INFN)
Faculty of Mathematics, Physics and Natural Sciences of the University of Pisa
Enrico Fermi Department of Physics of the University of Pisa
LPC - Caen (France)
CAEN (Italy)

Monday, 26 March

08:30 → 09:20 Registration of Participants

09:20 → 09:25 Welcome + General Information

09:25 → 10:40 Session 1 - Keynote Opening Talks - “Open Questions and Perspectives in DREB Physics”

09:25 Theory

Speaker: Andrea Vitturi (INFN - Padova)

Slides vitturi_dreb2012.pdf

10:00 Experiment

Speaker: Wilton Catford (University of Surrey)

Slides catford-introduction_[modalità_compatibilità].pdf

10:40 → 11:00 Coffee Break

11:00 → 12:40 Session 2

11:00 Ab initio calculations of light-ion reactions

We build a new ab initio many-body approach [1] capable of describing simultaneously both bound and scattering states in light nuclei, by combining the resonating-group method [2] with the ab initio no-core shell model [3]. In this way, we complement a microscopic-cluster technique with the use of realistic interactions, and a microscopic and consistent description of the nucleon clusters. We will present results for proton scattering on 7Be and for the S-factor of the 7Be(p,γ)8B capture reaction important for astrophysics, and for neutron scattering on 8He and 8Li. We will also highlight the first results of the d-3H and d-3He fusion calculations, of the 3He-4He scattering and the developments to include three-body cluster states within our ab initio approach. *Prepared in part by LLNL under Contract DE-AC52-07NA27344. 1. S. Quaglioni and P. Navratil, Phys. Rev. Lett. 101, 092501 (2008); Phys. Rev. C 79, 044606 (2009). 2. K. Wildermuth and Y. C. Tang, A unified theory of the nucleus, (Vieweg, Braunschweig, 1977). 3. P. Navratil, J. P. Vary, and B. R. Barrett, Phys. Rev. Lett. 84, 5728 (2000).

Speaker: Petr Navratil (TRIUMF)

Slides Navratil_DREB12.pdf

11:25 Interior and Exterior Contributions to Transfer Cross Sections

There has been much recent debate concerning what precisely is measured by (d,p) transfer reactions on nuclei. Do experiments measure spectroscopic factors or asymptotic normalization coefficients? This question is related to the relative importance for the transfer amplitudes of the interior and exterior contributions of the target bound-state wave functions. In order to probe this in more detail, for a wide range of transfer reactions we examine the effects of cutting off the lower or upper radial regions of those bound states. We find that the exterior contributions to prior amplitudes are usually very small and hardly affect the cross sections. We find that the the interior contributions to the post amplitudes are small, though give rise to some interference effects in the cross sections. The scheme of [Phys. Rev C 84, 044616 (2011)] decomposes the total amplitude into a sum of the interior post part, the exterior prior part, and a surface integral between. Our results indicate the surface integral will be the dominant contribution. Since that integral depends on the exterior R-matrix parameters of the bound state, the total cross section can be largely written in terms of the exterior properties of the bound-state wave function. This work was performed within the TORUS collaboration under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Speaker: Ian Thompson (LLNL)

Slides Thompson-dr-transfers2d.pdf

11:50 Microscopic calculation of the 3He(alpha,gamma)7Be reaction rate

We calculate the radiative capture cross sections for the 3He(a,g)7Be and the 3H(a,g)7Li reactions [1] in the fully microscopic Fermionic Molecular Dynamics approach [2] using a realistic effective interaction obtained in the Unitary Correlation Operator Method [3]. At large distances bound and scattering states are described by antisymmetrized products of 4He and 3He/3H ground states. At short distances the many-body Hilbert space is extended with additional many-body wave functions obtained by variation after parity and angular momentum projection. These additional configurations are needed to represent polarized clusters and shell-model like configurations. Properties of the bound states like binding energies, charge radii and quadrupole moments are described well, as are the scattering phase shifts. The calculated S-factor for the 3He(a,g)7Be reaction agrees very well with recent experimental data both in absolute normalization and energy dependence. In case of the 3H(a,g)7Li reaction the calculated S-factor is larger than the experimental data by about 15%. We analyze the bound and scattering wave functions using overlap functions and find that a significant part of the cross section is important to treat the wave functions properly at short distances. [1] T. Neff, Phys. Rev. Lett. 106, 042502 (2011) [2] T. Neff, H. Feldmeier, Eur. Phys. J Special Topics 156, 69 (2008) [3] R. Roth, T. Neff, H. Feldmeier, Prog. Part. Nucl. Phys. 65, 50 (2010)

Speaker: Thomas Neff (GSI Darmstadt)

Slides dreb-neff.pdf

12:15 Overlap integrals, spectroscopic factors and asymptotic normalization coefficients for one-nucleon transfer reactions.

Important quantities that input to one-nucleon transfer reactions are one-nucleon overlap functions. Usually, they are modelled by single-particle wave functions that are solutions of one homogeneous Schrodinger equation with some "standard" Wood-Saxon potential. However, they must come from solution of a Schrodinger-like inhomogeneous equation with the source term that contains information about interaction of the removed nucleon with the rest. In the source term approach, the spectroscopic factors are not the same as those given by widely used shell model codes. They follow the highly-discussed trend observed in one-nucleon removal reaction that reveals reduction of spectroscopic strengths in nuclei. Another advantage of the source term approach is that it automatically guarantees the correct asymptotic behaviour of the overlap functions, which is very important for applications in nuclear astrophysics. Examples of application of the method will be given and discussed.

Speaker: Natalia Timofeyuk (University of Surrey)

Slides Natasha_DREB-2012.pdf

12:40 → 14:10 Lunch

14:10 → 15:50 Session 3

14:10 Elastic and inelastic proton scattering of 21Na beam in inverse kinematics

The 18Ne(a,p) reaction is seen to be one of the key reactions determining the break-out from the hot-CNO cycle. There are wide discrepancies between measured cross-sections for this reaction, which may reflect the difficulty in such direct radioactive beam measurements. Moreover, the cross-sections in the literature are inconsistent with measurements of the time-reversed 21Na(p,a) reaction. Elastic and inelastic proton scattering of 21Na has been studied in inverse kinematics using a 21Na beam from the ISAC-2 facility at TRIUMF. Scattered protons were detected in annular silicon detectors and the TIGRESS gamma-ray array was operated in coincidence which allowed the inelastic channel to be cleanly discriminated. Several near-threshold resonances were observed and their properties deduced. These will be compared with previous measurements of excited states in 22Mg in the relevant energy region as well as the results of sd shell model calculations. The perspectives for the 18Ne(a,p) reaction will be given.

Speaker: David Jenkins Jenkins (University of York)

Slides DREB_2012_Jenkins.pdf

14:35 Indirect studies of astrophysical reaction rates - a study of the 18Ne(alpha,p)21Na reaction using the TIGRESS and SHARC detector systems

We here present some of the first direct-reaction data with radioactive ion beams utilising the TIGRESS and SHARC particle-gamma detector systems. In the experiment, transfer reactions in inverse kinematics were used as indirect probes of the 18Ne(alpha,p)21Na and 23Mg(p,gamma)24Al reactions. The SHARC array (Silicon Highly-segmented Array for Reactions and Coulex) is a new multi-purpose device for charged-particle detection. The array is utilised particularly for direct-reaction studies with radioactive-ion beams at the ISAC-II accelerator facility at TRIUMF, Canada [1] in conjunction with the TIGRESS gamma-ray spectrometer [2]. In the present experiment, the properties of states in 22Mg that mediate the 18Ne(alpha,p)21Na Hot-CNO-breakout reaction are studied, with particular emphasis on proton decay to the excited states of 21Na. The states are probed through deuteron transfer from 6Li onto 20Na, identified from the residual alpha particle in the transfer. The 21Na+p decay channel is identified from the emitted proton, for 21Na excited states measured in coincidence with the emitted de-excitation gamma rays. The presentation will describe first results from the experiment, with an outlook towards the wider applicability of direct reactions as indirect probes of astrophysical reaction rates, measured through combined charged-particle and gamma-ray spectroscopy. [1] R. E. Laxdal, Nucl. Inst. Meth. B 204, 400 (2003). [2] C. E. Svensson, et al., J. Phys. G 31, S1663 (2005).

Speaker: Christian Diget (University of York)

Slides DREB-2012-Diget.pdf

15:00 New Results with TECSA – the d(26Alm,p)27Al experiment

The detection of gamma rays from the decay of the 26Al ground state in the galaxy gives evidence that nucleosynthesis is occurring in present-day stars, but its origin is not yet clear. This implies that reactions involving 26Al are important for astrophysical processes. In a recent experiment at Texas A&M University, reactions with the ground state and isomeric state of 26Al were investigated with the Texas A&M-Edinburgh-Catania Silicon Array (TECSA). We measured d(26Alg,p)27Al and d(26Alm,p)27Al with an 26Al secondary beam prepared in-flight with the MARS spectrometer. First, the composition of the 26Al beam was determined by measuring the ratio of beta-decays to 26Al ions produced. It was found that at different spectrometer rigidities, beams of 2/3 isomer to ground state ratio or vice-versa could be obtained. Then, in the second part of the experiment, angular distributions were measured for both reactions. The protons were measured in coincidence with timing signals from the beam beam detected by a scintillator and with the cyclotron radio-frequency. Details of the experiment and results from the analysis of the d(26Alm,p)27Al and d(26Alg,p)27Al data will be presented. They will give information about the proton capture reactions 26Alm(p,γ)27Si and 26Alg(p,γ)27Si taking place in stars.

Speaker: Brian Roeder (Texas A&M University)

Slides Roeder_TECSA_DREB2012.pdf

15:25 Measurement of Gamow-Teller transitions from 56Ni

Electron-capture (EC) and beta-decay play important roles in type-II and typeIa supernovae. They occur through the Gamow-Teller (GT) and Fermi transitions in nuclei, which are extensively studied to reliably estimate the weak-interactions rates. Experimentally, a powerful probe to study GT transitions has been provided by the charge-exchange reactions at intermediate energies such as the (p,n), (3He,t) rections. They can selectively excite the GT transitions in a wide excitation energy region. Until recently, such studies have been restricted to stable nuclei because of difficulties in inverse-kinematics measurements with rare isotope beams. In this talk, we present the first study with a rare isotope using the 56Ni(p,n)56Cu reaction at 110 MeV/u in inverse kinematics with a newly developed Low-Energy Neutron Detector Array (LENDA) in combination with the S800 spectrometer. 56Ni is produced in large abundances during the pre-explosion phase of core-collapse supernovae and considered to be as one of the most important contributors to the change in the electron-to-baryon ratio in core-collapse supernovae. In addition, to study the GT transition in 56Ni serves as a stringent test of the effects of the N=Z=28 core not being inert on GT transitions for a large number of nearby nuclei in the Fe region.

Speaker: Masaki Sasano (RIKEN Nishina Center for Accelerator-Based Science)

Slides sasano.pdf

15:50 → 16:10 Coffee Break

16:10 → 17:25 Session 4

16:10 First Measurement with Trojan Horse Method Using Radioactive Ion Beam

For the first time, the Trojan Horse Method (THM) was applied to study a reaction induced by a radioactive beam at low energies relevant for astrophysics. In particular, the 18F(p,α)15O process was investigated via the 18F(d,α15O)n quasi-free reaction. This reaction is interesting for astrophysics, as the γ-ray emission from classical novae is dominated by positron annihilation resulting from the beta decay of radioactive nuclei, and especially of 18F (τ=110 min). A good knowledge of the nuclear reaction rate of production and destruction of 18F is mandatory to calculate the amount of 18F synthesized in novae and the resulting γ-ray radiation intensity. The 18F(p,α)15O reaction, a 18F destruction channel, was studied by many recent experiments. Despite significant progress, the cross section in the energy range covering novae temperatures is still unknown. Moreover, the extrapolation of the reaction rate down to low energies is complicated by the presence of several resonances of the 19Ne nucleus. In this context, the THM was applied to the 18F(d,α15O)n quasi-free reaction. The 18F beam produced at the CRIB facility at RIKEN, Japan, was focused onto a CD2 target and the ejected particles were detected using a new deigned experimental set-up covering a large solid angle. The reaction cross section has been for the first time investigated in the energy range from 0 up to 700 keV, and the predicted 30 keV resonance has been studied. The obtained results will be discussed.

Speaker: Marisa Gulino (LNS)

Slides DREB2012-gulino-1.pdf

16:35 LUNA: Laboratory for Underground Nuclear Astrophysics

LUNA (Laboratory for Underground Nuclear Astrophysics) is a nuclear astrophysics experiment running at the INFN Laboratori Nazionali del Gran Sasso (LNGS) [1]. Aim of the experiment is to measure the cross section of fusion reactions that take place inside the stars and that, in the past, dominated the Big Bang nucleosynthesis (BBN). The low background of the LNGS underground laboratory allows LUNA to investigate these reactions in the solar and Big Bang energy windows, where the cross sections drop down to values from pb to fb and even smaller, without the necessity of an extrapolation from the highest energies as usually done in laboratories at the Earth's surface. Recently, the LUNA collaboration has been engaged in the study of the 2H(α, γ)6Li reaction. This reaction is fundamental in the BBN and determines the amount of primordial 6Li in the Universe. The measured quantity of 6Li has been found to be higher (two-three orders of magnitude) than expected from the present cross section estimate [2]. In order to clarify this discrepancy, the 2H(α, γ)6Li reaction has been directly investigated at LUNA down to 130 keV. The experimental set-up will be described and the preliminary data will be discussed. Other recent measurements at LUNA and future possibilities will be also outlined. [1] C. Broggini et al., Ann. Rev. Nucl. Part. Sci., 60 (2010) 53 [2] C. Angulo et al., Nucl. Phys. A 656 (1999) 3

Speaker: Davide Trezzi (INFN - Milano)

Slides DREB2012-trezzi.pdf

17:00 Low-energy d+d fusion reactions via the Trojan Horse Method

The knowledge of 2H(d,p)3H and 2H(d,n)3He fusion cross section at low energies is of interest for pure and applied physics. Both reactions belong to the network of processes to fuel the first magnetic- and inertial-confinement fusion reactors in the range of kT= 1 to 30 keV. As for the Standard Big Bang Nucleosynthesis (SBBN), the region of interest ranges from 50 to 300 keV and experimental data at least up to 1 MeV are required for an accurate calculation of the reaction rate. Direct data are available below 200 keV, but not always in agreement within each other indicating large systematic errors in some of these data sets. The reaction rate has been calculated using theoretical curves fitting available low-energy data. Recently, we have carried out a new investigation of both 2H(d,p)3H and 2H(d,n)3He reactions throughout a d-d relative energy range from 1.5 MeV down to 2 keV, by means of the Trojan Horse Method (THM) applied to the quasi free 3He+d interaction at 18 MeV. As known, the THM [8-10] brings directly to the extraction of the low-energy bare nucleus cross section, free of Coulomb suppression and electron screening effects. Note that for a plasma plasma the value of the bare nucleus cross section must be known because screening in plasma is different from that in the laboratory. The measured S(E) factors, show deviations by more than 15% from previous estimates. The d+d results will be presented and discussed together with the basic features of the THM.

Speaker: Aurora Tumino (INFN - LNS)

Slides DREB2012-Tumino.pdf

17:25 → 20:25 Visit to Virgo

17:30 - Buses to Virgo
Tour of Virgo and Reception
20:30 Return to Pisa

Tuesday, 27 March

09:00 → 10:40 Session 5

09:00 Fingerprints of core polarization in two-nucleon transfer reactions of halo nuclei

We discuss recent exclusive transfer experiments performed with low-energy beams of two-neutron halo nuclei (11Li, 12Be). We compare the measured absolute differential cross sections with theoretical results that take into account all the processes up to second order within the distorted wave Born approximation. These calculations are based on a structure model of halo nuclei, which takes into account correlations beyond those included in three-body models. We find that effects associated with core ground state fluctuations and the coupling of valence nucleons with the excited states of the core, namely neutron self-energy and the interaction induced by phonon exchange, are crucial to reproduce both structure and reaction data.

Speaker: Ricardo Broglia (INFN - Milano)

Slides Pisa2_Broglia.pdf

09:25 Nuclear response to two-neutron transfer via the (18O,16O) reaction

A study of the structure of different nuclei was pursued at the Catania INFN-LNS laboratory by the (18O,16O) two-neutron transfer reaction at 84 MeV incident energy. The experiments were performed using several solid targets from light (9Be, 11B, 12,13C, 16O, 28Si) to heavier ones (58,64Ni, 120Sn, 208Pb). The 16O ejectiles were detected at forward angles by the MAGNEX magnetic spectrometer. Exploiting the large momentum acceptance (20%) and solid angle (50 msr) of the spectrometer, energy spectra were obtained with a relevant yield up to about 20 MeV excitation energy, with an energy resolution of about 100 keV. Several known low lying and resonant states of the product nuclei have been observed in the energy spectra . A common feature observed with light nuclei is the appearance of unknown resonant structures at for example 10.5 and 13.6 MeV in 15C and 16 MeV in 14C. The strong population of these latter together with the measured width can reveal the excitation of a collective mode connected with the transfer of a pair. Considerations based on kinematical matching conditions and on the shell configuration of the explored nuclei explain why such a mode is so excited in such reactions. In addition the measured angular distributions seems to indicate a transfer of a correlated neutron pair in L = 0 configuration, compatible with the Giant Pairing Vibration mode. Theoretical calculations have been performed in order to estimate the contribution of the two neutrons break-up.

Speaker: Diana Carbone (Università di Catania / LNS-INFN)

Slides DREB_2012_Carbone.pdf

09:50 Multi-nucleon transfers using two-neutron halo 6He on 12C at 30 MeV using the SHARC and TIGRESS arrays at TRIUMF ISAC-II

The 12C(6He,4He)14C and 12C(6He,8Be)10Be multi-nucleon transfer reactions are studied at the TRIUMF ISAC-II facility using SHARC (Silicon Highly-segmented Array for Reactions and Coulex), a compact charged particle silicon detector array, together with TIGRESS (TRIUMF-ISAC Gamma-Ray Escape Suppressed Spectrometer), a high-efficiency germanium γ-ray detector array. The (6He,4He) transfer reaction is studied to investigate its possible advantages over (t,p) as a surrogate reaction for two-neutron transfer in a well-known (t,p) case [1]. Due to the close proximity of the 14C excited states, charged particle – gamma coincidences are necessary and obtained through the powerful combination of SHARC and TIGRESS. The (6He,8Be) transfer reaction on 12C is suggested to be a two-proton transfer [2], yet another possibility is that it actually is an alpha transfer from 12C on 6He. Such a mechanism could provide additional insights on the 3α structure of 12C with possible nuclear astrophysics implications. This presentation will discuss the preliminary results obtained in both cases, as well as elastic and inelastic scattering data. [1] F.Ajzenberg-Selove et al., Phys.Rev.C17, 1283 (1978); [2] M.Milin et al., Phys.Rev.C70, 044603 (2004)

Speaker: F. Sarazin (Colorado School of Mines)

Slides Sarazin_DREB2012.pdf

10:15 Study of the nuetron rich nuclei via heavy-ion double charge exchange reaction

We are proposing a new powerful probe of heavy-ion double charge exchange (HIDCX) reaction, the (18O,18Ne) reaction, for the study of neutron rich nuclei. Ground states of 18O and 18Ne are among the same super-multiplet and the transition between them is just double spin-isospin flips keeping the spatial wavefunction unchanged. One can expect the transition is simple and its transition amplitude is large. As a first step to establish the HIDCX reaction, 12Be and 9He nuclei were studied vai the 12C(18O,18Ne)12Be and 9Be(18O,18Ne)9He reactions at 80 MeV/nucleon. The nucleus 12Be is one of the symbolic nucleus evidencing disappearance of N=8 magicity. Spin-parities for low-lying states in 12Be are well understood. The nucleus 9He is, however, unbound and its spin-parities have not certainly been determined. Thus, by making use of shape of angular distribution of the differential cross section in 12Be, we may make a new assignment of spin-parities in 9He. We performed the HIDCX experiment at the RCNP, Osaka University. The high resolution spectrometer Grand Raiden provide us to detect 18Ne particles which were well isolated from other isotopes. This is due to its unique A/Q value of 9/5. In the 12Be spectrum, three peaks were observed and show different angular distribution shapes, indicating different spin-parities. In the 9He spectrum, however, we did not observe any peaks. In the presentation, I will report the detail of the experiment and the result.

Speaker: Hiroaki Matsubara (CNS - University of Tokyo)

Slides Hiroaki_Matsubara.pdf

10:40 → 11:00 Coffee Break

11:00 → 13:00 Session 6

11:00 Electron Scattering - Hofstadter's experiment for short-lived nuclei -

Electron scattering is known to be the best probe for structure studies of atomic nuclei. It has been consistently playing a key role for our understanding of the internal structure of stable nuclei. We have proposed a novel internal-target scheme, SCRIT (Self-Confining RI Target) [1,2], which enables us to realize never-yet-performed electron scattering for short-lived nuclei. Feasibility studies using stable Cs ions demonstrated that this scheme works [3], and a luminosity of higher than 10^26 /cm^2/s was achievable with only 10^6 ions [4] with the trapping time of 50 ms. Note that the luminosity of an order of 10^26-27 /cm^2/s is required to perform the Hofstadter’s experiments, elastic electron scattering, for exotic nuclei, where their charge form factor are determined. We are now constructing an world’s-first electron-scattering facility for short-lived nuclei in RIKEN RI Beam Factory. The facility consists of an electron accelerator, and ISOL system and an electron detection system. The accelerator has been already installed and commissioned. The first collision of electrons and short-lived nuclei will take place in 2014. In my talk, I will discuss on physics motivation of electron scattering for short-lived nuclei, the brand-new technique and the new electron scattering facility under construction. In addition, I will briefly touch upon future perspectives of structure studies of exotic nuclei by electron scattering with higher luminosities.

Speaker: Toshimi Suda (Tohoku University)

Slides DREB_suda_sub.pdf

11:25 Direct Reactions and Decay Spectroscopy using the MSU High Resolution Array

The High Resolution Array (HiRA) constructed as a collaborative effort between Washington University, St. Louis, Indiana University, Bloomington and INFN, Milano, Italy has been used successfully in several direct reaction decay spectroscopy experiments with exotic beams. This talk will review and discuss the physics of some of measurements performed with this array. These include programs in transfer reactions with inverse kinematics using Ar and Ni isotopes, single and multi-particle-decay spectroscopy, knockout reactions, and astro-physically important mass measurements. Special emphasis will be placed on new results. This work is supported by the National Science Foundation under Grant No. PHY-1102511.

Speaker: Jenny Lee (RIKEN)

Slides DREB12-HiRA-final.pdf

11:50 Reactions on light neutron rich nuclei with CHIMERA detector at LNS

Relatively large yields of various neutron rich exotic beams, produced through the in flight fragmentation method are available at LNS especially after the successful upgrading of the fragmentation beam line [1]. Using the CHIMERA detector [2] implemented by a recent upgrade of Pulse Shape method in silicon detector [3], we started a campaign to study transfer, pick-up, knock-out reactions, with proton and deuteron enriched targets. Preliminary measurements on elastic scattering with nuclei as 10,11Be, 13B, 16,17C demonstrated the suitable performances of the detection system [4]. New data on neutron transfer/pick-up reactions will be presented with a preliminary analysis of their cross sections. The complete analysis of these data will be part of a systematic research for a dependence of such cross sections by the observed or claimed halo structures of light neutron rich nuclei. References: [1] see http://fribs.lns.infn.it/upgrade-results.html [2] A.Pagano et al, Nucl. Phys. A 734 (2004) 504 [3] A.Pagano ''Studies of nuclear reactions and time scale with the 4π detector CHIMERA'', Nuclear physics News International, in press. [4]L.Grassi phd thesis http://www.lns.infn.it/index.php?option=com_jombib&Itemid=91

Speaker: Giuseppe Cardella (INFN - Sezione di Catania)

Slides cardella-dreb-2012-web.pdf

12:15 Experiments with a Double Solenoid System

RIBRAS [1] is presently the only experimental equipment in South America capable of producing secondary beams of rare isotopes. It consists of two superconducting solenoids, presently installed in one of the beam lines of the 8 MV Pelletron Tandem accelerator of the University of São Paulo. The exotic nuclei are produced by the collision between the primary beam of the Pelletron Accelerator and the primary target, placed just before the first solenoid. The secondary beam is produced by the in-flight technique and is usually contaminated by particles coming from scattering and reactions in the primary target such as 7Li, alpha and other light particles such as protons, deuterons and tritons. The main contamination of the comes from 7Li2+ particles coming from the primary beam. A degrador placed between the two solenoids is able to separate those particles from the 6He beam providing an additional charge exchange 7Li2+-->3+. In addition, the differential energy loss in the degrador provides further selection of the light particles as protons, deuterons, tritons and and alpha particles by the second solenoid. Here we present the results of the first experiment performed a t RIBRAS using both solenoids. A pure 6He beam was produced and the reaction 6He+p was measured using a thick CH2 target. 1. R. Lichtenthäler et al., Eur. Phys. J. A 25,s01,733 (2005)

Speaker: Rubens Lichtenthäler (University of São Paulo)

Slides dreb-2012-lichten_[modalità_compatibilità].pdf

12:40 Digital Signal Processing for Physics Applications

Speaker: Matteo M. Angarano (CAEN S.p.A.)

Slides CAEN_Angarano_DREB2012_120327.pdf

13:00 → 14:00 Poster Session and Sandwich Lunch

13:00 Probing the 17F+p optical potential at near barrier energies in a microscopic approach

Data of a recently measured proton elastic scattering angular distribution of 17F+p, at 4.3 MeV/u, were analyzed in a double folding microscopic approach and the optical potential as well as the structure of this proton rich nucleus was probed. The data were collected over a wide angular range by means of the DINEX Si-detector array with an overall solid angle of ~0.8 sr. The 17F radioactive beam was produced at the EXOTIC facility of LNL Italy by means of the in-flight technique and the reaction 1H(17O,17F)n. The solid angle and angular calibration of the DINEX-array was obtained via an additional run of 17F+198Au at a sub-coulomb energy. For our theoretical analysis we adopted the JLM model, successful in describing elastic proton and neutron scattering from medium and heavy mass stable nuclei. This work will provide the plain ground to test the applicability of the model for drip line nuclei and energies well below E=10 A MeV while it will put in severe test the local density approximation, complementing existing work on stable projectiles. It should be noted that the microscopic approach involves folding procedures, where density distributions are considered explicitly. In this respect, we have been using densities of three different models to be confronted with our present data. Densities of 17O are also used by interchanging proton and neutron distributions for probing mirror symmetry. Total reaction cross sections are also deduced. The results will be discussed.

Speaker: Prof. ATHENA PAKOU (UNIVERSITY OF IOANNINA)

Poster Prascandolo_pakou4_web.pdf

13:05 Halo structure by the ratio method

The study of exotic nuclear structures, like halo nuclei, is usually performed through indirect techniques, such as reactions. Unfortunately, the complexity of the reaction mechanism and the uncertainty in the choice of projectile-target interactions can cause ambiguities in the analysis of measurements. We present here a new way to extract information about the structure of halo nuclei through reactions [1]. The basic idea of this new technique is to study the ratio of angular distributions for breakup and scattering. These two processes exhibit very similar features that depend mostly on the projectile-target interaction [2]. The recoil excitation and breakup model [3] predicts that their ratio gives access to halo-structure information by removing the major dependence on the reaction mechanism and the projectile-target interaction. We check this within the dynamical eikonal approximation [4] and show that the ratio clearly reveals projectile-structure information such as the binding energy and orbital of the halo neutron. Experimentally, the ratio has the advantage not to depend on the absolute normalisation of the cross sections. References [1] P. Capel, R. C. Johnson, and F. M. Nunes, Phys. Lett. B 705, 112 (2011) [2] P. Capel, M. Hussein, and D. Baye, Phys. Lett. B 693, 448 (2010) [3] R. Johnson, J. Al-Khalili, and J. Tostevin, Phys. Rev. Lett. 79, 2771 (1997) [4] D. Baye, P. Capel, and G. Goldstein, Phys. Rev. Lett. 95, 082502 (2005)

Speaker: Dr Pierre Capel (Université Libre de Bruxelles)

Poster capelDREB12Poster.pdf

13:10 Investigating the single-particle structure in 26Na via one neutron transfer in inverse kinematics using the new SHARC array

For neutron rich nuclei far from stability, it is known that the magic number N=20 disappears. Recent experiments, such as 24Ne(d,p) with TIARA [1] showed the breakdown of the N=20 magic number in favour of the new N=16 magic number. Theoretical work by Otsuka, Utsuno et al[2] has highlighted the importance of sodium isotope structure in quantifying the evolution of single particle energies approaching the N=20 island of inversion. SHARC is a multi-purpose silicon array for charged particle detection and features high spatial resolution and a large solid angle coverage. It has been used for the first time, coupled with the TIGRESS segmented germanium array at the ISAC-2 facility at TRIUMF. The aim is to study the disappearance of the N=20 shell gap in very neutron rich sodium isotopes, as evidenced by the rise in energy for the n(d3/2) orbital and the relative lowering of the n(f7/2) and n(p3/2) orbitals. The experiment was conducted with up to 3x10^7 pps of 25Na at 5MeV/u on a CD2 target. An in-beam scintillator detector, the trifoil, mounted behind a stopping foil was included to identify and reject reactions occurring on the carbon in the target. Results will be shown for background rejection achieved using the trifoil and proton angular distributions obtained by gating on gamma ray transitions. The data from the 25Na(d,p)26Na reaction in inverse kinematics will be presented. [1] WN Catford et al, J Phys G 31 (2005) S1655 [2] Utsuno et al, Phys Rev C70 (2004) 044307

Speaker: Ms Gemma Wilson (University of Surrey)

Poster GWilsonDREB.pdf

13:15 Describing core excitation in the scattering of halo nuclei using a Transformed Harmonic Oscillator basis

It is well known that, an accurate description of reactions involving weakly-bound nuclei, such as halo nuclei, requires the inclusion of the coupling to the continuum (i.e., unbound) states. For two-body projectiles, a successful technique to describe these processes is the Continuum-Discretized Coupled-Channels (CDCC) method. Here the continuum is replaced by a discrete set of functions, each one representative of a region of the continuum relevant for the reaction. The standard CDCC method uses an average of the scattering two-body wavefunctions in each energy interval (named "bin"). Alternatively, the continuum spectrum can be described by the eigenstates of the Hamiltonian in a basis of square-integrable functions, or pseudo-states (PS). In this contribution we present a PS basis obtained performing a simple analytic local scale transformation to the harmonic oscillator basis (THO). This THO basis is easy to calculate and reproduces a wide variety of observables, requiring a small number of functions compared to other bases. Moreover, narrow resonances are well characterized by one or two PS. Recently, we have extended this basis to describe systems with a valence+core structure (such as halo nuclei), taking into account the possible excitations of the core. This is the case of 11Be and odd A carbon isotopes. We have applied this model to the 11Be+208Pb reaction at 70MeV/u, where we found an improvement with respect to the single-particle model based on an inert core.

Speaker: Mr José Antonio Lay (Universidad de Sevilla)

Poster lay_DREB.resize.pdf

13:20 Three-body breakup of 11Li with the eikonal method

We use a four-body eikonal model to analyze the experimental breakup cross section of 11Li on 208Pb at 70 MeV/A [1]. It is a current debate if the large concentration of dipole strength at low energies is due to the 11Li low binding energy or if it is due to a resonance effect. First we compute 9Li+n+n three-body phase shifts for J=0+,1- and 2+. A relatively narrow structure is found in the 1- partial wave at low energies. Then we investigate 11Li breakup through a 9Li+n+n three-body model of the projectile. The dipole strength is computed with two descriptions of the three-body continuum: (i) an exact treatment, involving proper asymptotic conditions [2], and (ii) the simpler pseudo-state method, where the continuum is discretized over a square-integrable basis [3]. We show that significant differences may show up. The experimental determination of the dipole strenghts from breakup cross sections is also discussed. [1] T. Nakamura et al., Phys. Rev. Lett. 96 (2006) 252502. [2] P. Descouvemont, E. Tursunov, and D. Baye, Nucl. Phys. A765 (2006) 370. [3] E.C. Pinilla et al., Nucl. Phys. A865 (2011) 43.

Speaker: E. C. Pinilla (Universite Libre de Bruxelles)

Poster pinilla3-DREB.pdf

13:25 Elastic scattering of the halo nucleus 11Li and its core 9Li on 208Pb at energies around the Coulomb barrier

The discovery of the halo nuclei has brought renewed interest in the modeling of nuclear reactions. This structure will affect the reaction properties at near Coulomb barrier energies. Therefore we have studied, for the first time, the dynamics of the halo nucleus 11Li in presence of a strong electric field of 208Pb at energies below, 24.2 MeV, and around, 29.7 MeV, the Coulomb barrier at the ISACII facility at TRIUMF. To disentangle the halo contribution in the scattering we have studied the behavior of the core by measuring the 9Li+208Pb reaction at the same center-of-mass energies of 23.0 and 28.3 MeV. We have compared the elastic differential cross section results of 9Li+208Pb with optical model calculations using the double-folding Sao Paulo Potential (SPP) for the real part and a Woods-Saxon potential for the imaginary part, whose parameters are obtained from the fit of the elastic data. In this contribution the angular distribution of the elastic differential cross section of 11Li+208Pb is presented and compared with Continuum-Discretized Coupled-Channel (CDCC) calculations based on a simple two-body model (2n+9Li) for the 11Li nucleus. The coupling to the breakup channels produces a significant reduction of the elastic cross section below the grazing angle at energies around and below the Coulomb barrier. This effect will be discussed in terms of the strong dipole coupling to the states in the low-lying continuum of 11Li.

Speaker: Mr Mario Cubero (Instituto Estructura de la Materia, CSIC, Spain)

Poster Borge-poster-final.pdf

14:00 → 14:30 Meeting of Advisory Committee and Chairs - discussion of bids and decision on hosting DREB2014

14:30 → 17:00 Informal Instrumentation Discussion Session

14:30 → 17:00 Informal Reaction Theory Discussion Session (Moderator: Michele Viviani - Pisa)

slides amoro.pdf

slides capelDREB12final.pdf

slides crespo_[modalità_compatibilità].pdf

17:00 → 20:00 Free evening

Wednesday, 28 March

09:00 → 10:40 Session 7

09:00 Structure of the unbound systems 10,12Li and 13Be

The unbound systems 10,12 Li and 13Be have been studied via nucleon removal from a 14B beam of 35 MeV/nucleon. The invariant mass spectra were reconstructed from the measured 9,11Li-n and 12Be-n coincidences. Simulations, taking into account the effects of the experimental setup and the reaction, were developed. The reconstructed decay energy spectra and momentum distributions were interpreted using theoretical lineshapes folded with the experimental response, as well as uncorrelated distributions obtained from the data by event mixing. In the case of 10Li, the data were reproduced with an s-wave virtual state of scattering length as = −10 +-3 (stat) +-3 (syst) fm, combined with an l = 1 resonance of energy Er = 0.55 +- 0.02 (stat) +- 3 0.05 (syst) MeV and a width of 0.35 ± 0.05 (stat) ± 0.15 (syst) MeV. This result confirms the 1p1/2 -2s1/2 inversion. For 12Li, evidence was found for excited states with an energy between 0.2 and 0.8 MeV above the threshold. In the case of 13Be, the decay energy, as well as the other observables, were reproduced assuming an l = 0 resonance at 0.7+-0.05 (stat) +- 0.07 (syst) MeV and a width of 1.7 +- 0.1 (stat) +- 0.1 (syst) MeV, combined with an l = 2 resonance with energy Er = 2.4 +- 0.1 (stat) +- 0.1 (syst) MeV and a width of 0.6 +- 0.2 (stat) +- 0.1 (syst) MeV. This hypothesis is supported by the selectivity of proton knockout, whereby the neutron configuration of the states populated in 13Be should directly reflect that of 14B.

Speaker: Giacomo Randisi (LPC Caen)

Slides dreb2012_randisi.pdf

09:25 First observation of ground state di-neutron decay: 16Be

A single-proton knockout reaction from a 53 MeV/u 17B beam was used to populate the ground state of 16Be. The decay of 16Be was reconstructed from the four-momentum vectors of the 14Be fragment and the two emitted neutrons using the MoNA/Sweeper setup. 16Be is bound with respect to the emission of one neutron and unbound to two-neutron emission. The dineutron character of the decay was evidenced by the small relative angle between the two neutrons and by the sharp relative 2n-energy. The 2n-analysis of the 16Be decay will be presented. Results on the 2n-separation energy will be compared to shell model calculations.

Speaker: Artemis Spyrou (NSCL/MSU)

Slides Spyrou_DREB2012.pdf

09:50 Evidence for the ground-state resonance of 26O

Evidence for the ground-state resonance of 26O was observed through a three-body coincidence measurement of 24O+n+n using the Modular Neutron Array (MoNA) at the National Superconducting Cyclotron Laboratory. The ground-state of the unbound 26O was populated through a single proton-knockout reaction from a 82 MeV/u 27F beam. A Monte Carlo simulation, which included the population of an excited state in 26O and the ground-state of 25O through a 1p-1n knockout reaction, was used to fit the data. The 26O ground-state was determined to be unbound by less than 200 keV, in agreement with recent shell-model calculations.

Speaker: Zachary Kohley (National Superconducting Cyclotron Laboratory)

Slides Kohley_O26.pdf

10:15 Multi-neutron detection, reaction mechanism and the quest for 7H

The experimental observation of heavy hydrogen resonances allow the study of the most neutron asymmetric matter states in nature. In my talk I'd like to use the findings in two-neutron + core systems by observing energy and angular correlations iin order to show the prospects when extending the applied methods to the triton + four neutron decay channel of 7H. This extension to the 5 body case would allow for a characterization of the involved continuum structure and to go beyond comparing missing energy spectra to phase space, but will require the detection of four neutrons in coincidence. The current status and shortcomings for the reliable measurement of many neutrons will be presented and the expected improvements e.g. when being ready to use the currently designed NeuLAND detector for R³B at GSI and FAIR will be exemplary discussed. The selective population of the continuum states is another important ingredient for a successful determination of their structure. This will be shown at the example of 13Be* excited states using breakup data, and discussed for the heavy hydrogen systems. The use of kinematical correlations in breakup reactions allows for a further refinement of these selections. Of particular interest is here the quasi-free knockout mechanism where selected examples will be presented.

Speaker: Haik SIMON (GSI)

Slides Simon-DREB2012.pdf

10:40 → 11:00 Coffee Break

11:00 → 12:40 Session 8

11:00 Angular-momentum content of momentum profile in a neutron knockout from 14Be.

A novel method for interpretation of experimental data based on a study of the root-mean-square (r.m.s) of momentum distribution of a (A-2)+n system after one-neutron knockout from a Borromean nucleus of mass A as a function of (A-2)-n relative energy is introduced. This is the first time the momentum profile is used for analyzing experimental data. The method is applied to the analysis of experimental data obtained by ALADIN-LAND collaboration at GSI. Beams of 8He, 11Li and 14Be impinging on a liquid hydrogen target at relativistic energies have been used. The analysis starts with an investigation of applicability of the method by testing it on two well-known unbound nuclei, 7He and 10Li. Finally it is applied to the unbound 13Be using the estimated momentum r.m.s. for a neutron knockout from s- and p-shells in the test nuclei. Contradictive interpretations of different experimental data on 13Be will be discussed.

Speaker: Leonid Chulkov (Kurchatov Institute)

Slides Chulkov_DREB2012.pdf

11:25 Nuclear Breakup of 17Ne and its Two-Proton Halo Structure

17Ne is a proton-dripline nucleus that has raised interest in nuclearstructure physics in recent years. As a (15O+2p) Borromean 3-body system, it is often considered to be a 2-proton-halo nucleus, yet lacking concluding experimental quantification of its structure. We have studied breakup reactions of 500 AMeV 17Ne secondary beams in inverse kinematics using the R3B-LAND setup at GSI. The foci were on (p,2p) quasi-free scattering on a CH2 target, and on one-protonknockout reactions on a carbon target. Recoil protons have been detected with Si-Strip detectors and a surrounding 4pi NaI spectrometer. Furthermore, projectile-like forward protons after one-proton knockout from 17Ne have been measured in coincidence with the 15O residual core. The resulting relative-energy spectrum of the unbound 16F, as well as proton-removal cross sections with CH2 and C targets, and the transverse-momentum distributions of the residual fragments will be presented. Conclusions on the ground-state structure of 17Ne will be discussed.

Speaker: Felix Wamers (Technical University Darmstadt)

Slides FWamers_DREB2012_final.pdf

11:50 Shell evolution in neutron-rich Al isotopes around N=20

The structure of nuclei in the region commonly referred as island of inversion has been extensively studied because of the anomalous breakdown of the N = 20 shell closure. Differently than in Ne, Na and Mg isotopes, the experimental two-neutron separation energy of the Al isotopes do not show anomalies and are well reproduced by large scale shell model calculations involving the full sd proton shell and the pf neutron shell as valence space [1]. Recent magnetic moment measurements performed on the 33,34Al isotopes [2] have shown large discrepancies with shell model predictions, in the sd and sdpf model spaces, implying a possible extension of a deformed region beyond Z = 12. In order to study the evolution of the single particle occupancy in the Al isotopes we have measured the longitudinal momentum distributions of the 33,34,35Al from one-neutron removal reactions and the corresponding cross sections at the Fragment Separator at GSI [3]. The beam energy was around 900 MeV/u. The momentum distribution analysis has been performed in the eikonal framework. Comparing our results with shell model predictions, the inferred 2s1/2 neutron occupancy in the 33Al ground state wave function is 20-40% lower than the predicted one. The inclusive data do not exclude the presence of intruder states. Large l=1 occupancy is found in 34Al although it is smaller than in 33Mg [4]. [1] PRC58(1998)2033; [2] PLB643(2006)257and PLB658(2008)203; [3] NIMB70(1992)286; [4] PLB685(2010)253.

Speaker: C. Nociforo (GSI - Darmstadt)

Slides NociforoDREB12_CN_sent.pdf

12:15 Exclusive measurements of (p,pX) neutron and proton knockout reactions on 57Ni

In this presentation, an exclusive experimental approach for the investigation of the shell structure of exotic nuclei using proton-induced knockout reactions is discussed. The experiment performed at the LAND-R3B facility at GSI allowed for an analysis of (p,2p) as well as for the first time (p,pn) reactions in inverse kinematics from kinematically complete measurements of the incoming and outgoing channels. Hereby, results for one-proton and one-neutron knockout reactions on the radioactive isotope 57Ni will be discussed. The quasi-free-knockout character of the reactions was identified event-by-event by the reconstructed angular correlations of the scattered nucleons. Inclusive momentum distributions of the residual 56Co and 56Ni fragments and corresponding (p,2p) and (p,pn) reaction cross sections were deduced. The experimental setup is also capable to distinguish different reaction channels by observing the gamma decay of excited fragments. Particularly, an analysis of excited 56Ni residues was carried out, allowing the separation of different angular momentum states of the knocked-out neutron in the 57Ni ground state. The results will be compared to knockout reactions induced by a Be target and to theoretical estimates. This work is supported by HIC for FAIR and EMMI.

Speaker: Alina Movsesyan (Technical University Darmstadt)

12:40 → 14:10 Lunch

14:10 → 15:50 Session 9

14:10 Investigations on the excitation of the core and the halo of 11Li

The two-neutron halo is a unique three-body system that has pushed the frontier of nuclear structure to discover new phenomena. An interesting question is whether the correlation of the two-neutrons can sustain a coherent oscillation giving rise to a soft dipole resonance. While this has been a subject of investigation since several years, a conclusive answer is not yet reached. The proton inelastic scattering of 11Li was found to show a peak at 1.3±0.1 MeV that was proposed to be the soft dipole resonance [1]. However, the resolution of this experiment does not allow a clear discrimination on whether the peak is a threshold effect or a dipole state. The recent Coulomb dissociation measurement of 11Li [2] on the other hand reports an enhancement around 0.6 MeV and is discussed to reflect the soft E1 mode. A resonance peak at 1.02±0.07 MeV was reported from the pion capture measurement [3]. Recent high precision inelastic scattering experiment at TRIUMF to investigate on resonance in 11Li will be presented. The formation of the neutron halo in 11Li is closely associated with the dynamic role of the core nucleus 9Li in binding the valence neutrons. We will report the first inelastic scattering measurement of 9Li to study the nature of excitation for its first excited state. [1] A.A. Korshenninikov et al., Phys. Rev. Lett. 78(1997) 2317. [2] T. Nakamura et al., Phys. Rev. Lett. 96 (1998) 252502. [3] M.G. Gornov et al., Phys. Rev. Lett. 81 (1998) 4325.

Speaker: Rituparna Kanungo (Saint Mary's University)

14:35 Unbound states of the drip-line nucleus 24O from (p,p') scattering

The studies of the structure and the spectroscopy of neutron-rich nuclei are important for our understanding of the evolution of the nuclear structure towards the drip-line. They have shown that the shell model picture established in the valley of stability is modified, and that local magic numbers may appear. Low-lying resonances or new magic shells represent stringent tests for the nuclear theories and their modeling of the nuclear interaction. The exotic nuclei have few or no bound excited states so the influence of continuum states adds a new challenge for theoretical calculations. These effects can be investigated in the case of the last bound O isotope, associated to the new magic shell gap created at N=16 : oxygen-24. To explore its spectroscopy, we have measured at RIKEN the proton (p,p') scattering reactions at 263 MeV/n on the BigRIPS line, using the unique intensity of the RIBF 24O beam, and the state-of-the-art charged particle detector MUST2. Via the missing mass method the excitation energy spectrum of the 24O was deduced. The spectra for the contaminants (25F and 23O) will also be presented. The talk will explain the specific methods for the extraction of the position of the states, in the case of the spectra of low statistics. The experimental results will be compared to the microscopic calculations recently developed to take into account explicitly the treatment of the continuum-coupling and of the 3-body forces.

Speaker: Simon BOISSINOT (CEA-Saclay IRFU/SPhN)

Slides TalkBoissinotDREB28thMarch12.pdf

15:00 Measurement of the Giant Monopole and Quadrupole Resonances in ⁶⁸Ni using the Maya Active Target

The study of the Isoscalar Giant Monopole Resonance (ISGMR) and the Isoscalar Giant Quadrupole Resonance (ISGQR) in stable nuclei have provided relevant information on both nuclear matter and nuclear structure in the past decades. For instance, the ISGMR centroid can be linked to the incompressibility modulus of the infinite nuclear matter. Values for exotic nuclei would help in constraining it. In unstable nuclei, only one measurement has been performed to date (in 56Ni)[1]. In order to study the evolution of the ISGMR and the ISGQR along an isotopic chain, measurements in neutron-rich Ni isotopes are called for. To reach this goal, a dedicated experiment was recently performed at GANIL. The inelastic scattering of deuteron and alpha particles on 68Ni at 50 MeV/A has been studied in inverse kinematics with the Maya active target. Preliminary results concerning the inelastic scattering reactions using deuterium gas as the target will be shown. [1] C. Monrozeau et al., Phys.Rev.Lett. 100 (2008) 042501.

Speaker: Marine Vandebrouck (IPN Orsay, Université Paris-Sud, IN2P3-CNRS)

Slides MarineVandebrouckDREB.pdf

15:25 Experimental results on the Coulomb excitation of exotic nuclei at the R3B-LAND setup

Coulomb excitation is a powerful tool to investigate the collective response of exotic nuclei, providing a unique insight into the dynamical properties of nuclei located far from stability. The experimental low-lying dipole data obtained in such experiments not only provides valuable information for the nuclear equation-of-state through the observation of the Pygmy Dipole Resonance (PDR), but also for nucleosynthesis scenarios, such as the rp-process. Several experiments have been carried out in the past years using the R3B-LAND setup at GSI in Darmstadt, in which the Coulomb excitation of unstable nuclei has been investigated. The systematic measurement of a neutron-rich portion of the Sn isotopic chain has revealed PDR strength above the one-neutron threshold. The measured strength is used to constrain the asymmetry term of the nuclear equation-of-state, which has an impact on the behavior of not only exotic nuclei, but also on objects of astrophysical interest, such as neutron stars. Preliminary data from more recent experiments will be discussed, namely for neutron-rich Ni and neutron-deficient Ar isotopes. In particular, the differential cross sections for the neutron decay channels of 68Ni will be compared to other experimental data, obtained by virtual photon scattering. Data on the proton decay channels of 32Ar and 34Ar will also be presented, for which a proton-related PDR has been predicted by an RPA calculation.

Speaker: Dominic Rossi (GSI)

Slides dreb2012_rossi_v3.pdf

15:50 → 16:10 Coffee Break

16:10 → 17:50 Session 10

16:10 Dynamical limits of nucleon knockout at intermediate energy.

In the past 15 years, many new features of nuclear shell structure have been discovered via knockout reactions from exotic nuclei at intermediate energies. Cross sections of such processes are usually analysed using the eikonal and adiabatic approximations and give access to experimental spectroscopic factors. The parallel momentum distributions of the projectile-like residues contain information on the intrinsic angular momentum of the removed nucleon. Nevertheless, the limits of the reaction mechanism have to be well understood for a safe extraction of these quantities. To further investigate these limits, we performed the one-neutron and one-proton removal from unstable nuclei with large asymmetry DS=Sn-Sp. Inclusive cross sections and parallel-momentum distributions were measured for incident 14O and 16C at 53 MeV/ nucleon and 75 MeV/nucleon respectively. Strong deviations from standard adiabatic predictions are observed in the case of deeply-bound nucleon removal. The corresponding parallel momentum distributions exhibit a cut-off at high energy and a strong low-energy tail. All these results will be presented and discussed in details.

Speaker: Freddy Flavigny (CEA/IRFU/SPhN)

Slides Flavigny_DREB.pdf

16:35 Correlations in direct two-proton knockout and details of the reaction mechanism

In surface-grazing collisions with a light target nucleus the sudden removal of two protons from an intermediate-energy neutron-rich projectile has been shown to proceed as a direct reaction. In addition to giving spectroscopic information, this type of reaction promises a rather unique tool assign spins by measuring the momentum distributions of the heavy reaction residues. In a two-nucleon removal reaction three reaction mechanisms contribute to the cross section: the inelastic removal of both nucleons, the elastic removal of one nucleon and inelastic removal of the second, and the elastic dissociation of both nucleons. The direct two-proton knockout reaction from a 28Mg beam at 93 MeV/u has been studied at NSCL. First coincidence measurements of the heavy 26Ne projectile residues and the removed protons enabled the relative cross sections from each elastic and inelastic nucleon removal mechanism to be determined. These final-state-exclusive measurements are key for further validation of this direct reaction and its use for quantitative spectroscopy of highly neutron-rich nuclei. The deduced yields for the three contributing mechanisms are compared to recent reaction model expectations based on the use of sd-shell model structure amplitudes and eikonal reaction dynamics. The kinematic correlations of the detected removed protons are also analyzed.

Speaker: Kathrin Wimmer (NSCL - MSU)

Slides wimmer_DREB.pdf

17:00 Eikonal reaction theory for one- and two-neutron removal reaction

Neutron removal reaction is an useful experimental tool to investigate weakly-bound nuclei. The removal cross section and momentum distribution of fragments give us the information about single particle state of the last neutron in projectile. Very recently, we introduced an accurate method of treating the neutron removal reaction at intermediate incident energies induced by both nuclear and Coulomb interactions [1]. In the method, the nuclear and Coulomb breakup processes are consistently treated by the method of continuum discretized coupled channels without making the adiabatic approximation to the Coulomb interaction, so that the removal cross section calculated never diverges and is reliable even in the presence of the Coulomb interaction. This method is called eikonal reaction theory (ERT). In this talk, I present the essence of ERT and this method is applied to one-neutron (1n) removal from 31Ne and two-neutron (2n) removal from 6He and 32Ne. For the analysis of 1n removal, the spectroscopic factor and asymptotic normalization coefficient of the last neutron in 31Ne are evaluated. For the analysis of 2n removal, we focus on the reaction mechanism of 2n removal. Especially, the effect of di-neutron correlation on this reaction is discussed. Reference. [1]M. Yahiro, K. Ogata, and K. Minomo, Prog. Theor. Phys. 126, 167 (2011).

Speaker: Kosho Minomo (Kyushu University)

Slides DREB2012-minomo.pdf

17:25 Study of 16C by neutron knockout reaction

The neutron-rich isotope, 16C has been investigated by neutron knockout reaction of 17C on liquid hydrogen target. Applying the invariant mass method in inverse kinematics and gamma-ray spectroscopy, the energy spectrum was reconstructed, in which neutrons, charged fragments, and gamma-rays originated from the decay of the reaction residue (16C*) were detected in coincidence. A peak at about 0.46 MeV was observed in the invariant mass spectrum in coincidence with a peak at 0.74 MeV in the gamma-ray spectrum, which indicates the presence of an unbound state with an excitation energy of 5.45 MeV. A simple shell model calculation has shown a strong evidence that the spin-parity of the state is likely to be 2-. Derivation of the experimental cross-section and comparison with the theoretical cross-section will be presented.

Speaker: Jongwon Hwang (Seoul National University)

Slides DREB2012-Hwang.pdf

20:00 → 23:00 Workshop Dinner at “Santa Caterina”, including the announcement of the hosts of DREB2014

Thursday, 29 March

09:00 → 10:40 Session 11

09:00 Study of 10He by the 11Li(d, 3He) transfer reaction

Resonant states of unbound 10He have been studied by the missing mass method using the 11Li(d,3He) reaction to investigate 10He structure. This nucleus was first studied at RIKEN in a pioneering work [1] using the invariant mass method in 8He+n+n channel. A recent experiment at GSI [2] leads to compatible results for the first resonant state at 1.2 MeV . An experiment using 8He(t,p) reaction [3] shows no evidence of a state below 3 MeV above the two neutrons threshold. Spectroscopy of resonant states of 10He remains ambiguous. The missing mass method allows to reconstruct the excitation energy spectrum independently of the decay channel. The experiment was performed at the RIKEN RIPS facility, using a secondary beam of 11Li at 50 AMeV impinging on a CD2 target. At forward angle, a wall of four MUST2 telescopes [4] was coupled with four 20 μm thick silicon detectors for light particles identification. Heavy beam-like residues were detected around zero degree using a fifth MUST2 telescope. In addition a 9Li beam at 50 AMeV was used to perform a reference experiment. 8He and 10He excitation energy spectra will be discussed and associated differential cross sections will be compared to DWBA calculations using various wave function overlaps. [1] A.A. Korsheninnikov et al, Phys. Lett. B 326, 31 (1994). [2] H.T. Johansson et al, Nucl. Phys. A Volume 842, 15 (2010). [3] M.S. Golovkov et al, Phys. Lett. B Volume 672, 22 (2009). [4] E. Pollacco et al, Eur. Phys. J. A25, 287 (2005).

Speaker: Adrien MATTA (IPN Orsay - CNRS/ IN2P3 / Universite Paris-Sud XI)

Slides dreb_matta.pdf

09:25 Studies of neutron rich Beryllium isotopes using transfer reactions.

On behalf of the IS430 and the MINIBALL collaboration. The bound states of neutron rich beryllium isotopes have been studied through scattering and transfer reactions at ISOLDE. A low energy 11Be beam incident on both deuteron and proton targets led to several transfer reactions enabling the study of 10,11,12Be. Both particles and gammas were detected, the latter using the MINIBALL cluster. 12Be is known for the inversion of states, leading to the breaking of the N=8 shell. The bound states of 12Be have been studied in several different experiments and so far four bound states have been identified. The small separation in energy between the three excited states makes them hard to distinguish in a transfer reaction. The MINIBALL makes it possible to distinguish the three states through gamma gates. Spectroscopical factors from the experiment will be presented and compared to a previous experiment at TRIUMF, where experiment and theory did not agree. A fifth bound state (0-) has been suggested from three body calculations. The excitation energy of the state should be close to the excitation energy of the 1- state, hence gamma's will be necessary to distinguish the 0- state from the others. We have found no evidence for a fifth bound state in the experiment, but see indications of a low lying resonance with either 0- or 1-. The presentation will focus on the study of 12Be, but results from the study of 11Be and 10Be will also be presented.

Speaker: Jacob Johansen (Aarhus University)

Slides Johansen_DREB.pdf

09:50 Neutron sd-shell excitations for light nuclei with N ≥8

In light nuclei, the separation between the p and sd shells decreases with increasing N/Z near N=8, and the energy of the 1s1/2 orbital drops below that of the 0d5/2 level. This behavior is largely responsible for the halo-like nature of neutron-rich N=9 nuclei, which contain a loosely-bound 1s1/2 neutron. We have used HELIOS to study neutron transfer with the (d,p) reaction using beams of the unstable nuclei 12B, 13B, and 15C, leading to 13B, 14B, and 16C, corresponding to neutron numbers of N=8, 9, and 10. Of particular interest are the properties of states with one or two neutrons in the 1s0d shell. The exotic beams were produced using the in-flight method. Protons were detected at backward laboratory angles using HELIOS, in coincidence with and forward-recoiling beam-like recoils identified in an array of silicon-detector telescopes. HELIOS is designed specifically to study such reactions in inverse kinematics, and to optimize the excitation-energy resolution in the center-of-mass system. Excitation-energy spectra and angular distributions for these reactions will be presented, as well as relative spectroscopic factors. The data will be compared to the predictions of shell-model calculations for this region. This work was supported by the U.S. Department of Energy, Office of Nuclear Physics under contracts DE-FG02-04ER41320 and DE-AC02-06CH11357, and NSF Grant Nos. PHY-02-16783 and PHY-10-68217.

Speaker: Alan Wuosmaa (Western Michigan University)

Slides DREB_2012_Wuosmaa.pdf

10:15 Transfer reactions into the Island of Inversion

The T-REX setup was built to use the post-accelerated radioactive beams from REX-ISOLDE to study isotopes far from stability using transfer reactions in inverse kinematics. The first experiments performed with T-REX aimed at a better understanding of the "Island of Inversion", a region in the nuclear chart near $^{32}$Mg where the narrowing of the $N = 20$ gap and pairing correlations can lead to deformed ground states with $2p-2h$ configurations. We will present results from d($^{30}$Mg,p)$^{31}$Mg, determining for the first time the negative parity of the second excited state at 221 keV. The experiment also showed a cross section for this second excited state that is a factor four lower than the cross sections of the ground state and the first excited state when compared to DWBA calculations. This might be an indication of a possible shape co-existence of an oblate deformed second excited state and prolate deformed ground and first excited state. The second experiment performed with T-REX in the Island of Inversion was the t($^{30}$Mg,p)$^{32}$Mg experiment which identified the proposed shape coexisting excited $0^{+}$ state to be at 1058 keV. This is much lower than any prediction by theoretical models.

Speaker: Vinzenz Bildstein (University of Guelph)

Slides Bildstein2012.pdf

10:40 → 11:00 Coffee Break

11:00 → 12:40 Session 12

11:00 Evolution of the shell structure in medium mass nuclei: search of the neutron 2d5/2 orbital in 69Ni

The Harmonic Oscillator closed shell at N=40 in 68Ni is weak and loses its strength at two proton-holes distance. This was manifested in 64,66Fe and 60,62Cr nuclei by the deformation of the low-lying states. Calculations performed in this mass region predict a new island of inversion at N=40 similar to the one discovered at N=20. Using a large valence neutron space, the neutron 1g9/2-2d5/2 gap at N=40 is shown to be a crucial ingredient for the understanding of the nuclear structure around N=40. Moreover, its measurment would definitely help to draw preliminary conclusions about the magicity of 78Ni. The neutron 1g9/2-2d5/2 energy difference has been determined in 69Ni using the neutron stripping reaction d(68Ni,p). The experiment has been performed at GANIL in inverse kinematics using the MUST2 array. Angular momenta and spectroscopic factors of the populated states (1g9/2, 2d5/2) were obtained from the comparison between the experimental differential transfer cross-sections as a function of the proton detection angle and DWBA calculations using adiabatic potentials.

Speaker: Mohamad MOUKADDAM (Université de Strasbourg)

Slides DREB12_Moukaddam_29_March.pdf

11:25 Proton-Neutron interaction around N=40 studied at ISOLDE

One of the main goals of modern nuclear structure research is to identify changes in the mean field or residual interaction when going towards exotic systems. To track such changes, different observables are available for the experimentalist, like effective single particle energies for odd nuclei or values for the electric quadrupole transition strength in even-even nuclei. In this talk we show that the isospin degree of freedom gives a similar sensitivity to changes in shell structure. In this case the observable is the low-lying magnetic transition strength. A smooth contribution from the symmetry energy from the Bethe-Weiszäcker mass formula is needed to develop a consistent picture. We demonstrate the sensitivity to (sub)shell closures in case of e.g. N=16 (24O) or N=40 (68Ni). We show first results from a recent ISOLDE experiment using MINIBALL and T-REX. A highly intense 72Zn beam was used to populate nuclei beyond N=40. A tritium-titanium target was used to study both Coulomb excitation and the two-neutron transfer reaction simultaneously. Both these reactions are well suited to populate the low-lying off-yrast states of interest, related to these questions.

Speaker: Dr Dennis Muecher (TU Munich)

Slides dreb_2012_muecher.pdf

11:50 Test of the asymmetry of reduction factors with one nucleon transfer reaction on 14O

Nucleon transfer reactions have been for years a powerful tool to investigate the filling of orbitals issued from the nuclear shell model. Even for the most stable nuclei, spectroscopic factors deviate from unity. It is understood as the result of short or long-range correlations. In the case of exotic nuclei, the variation of spectroscopic factors with the difference in separation energy ΔS = |Sn - Sp| is still an open question, but no strong effect was shown from transfer reaction [1]. However available data are restricted to a limited range in ΔS values. A different picture arises from the one nucleon knock-out performed at higher incident energy where a strong asymmetry was recently found [2] for the reduction factor of spectroscopic factors versus ΔS. As a spectroscopic factor is not an observable, it is important to disentangle structure effects from reaction mechanism. To investigate such an effect, we performed one nucleon transfer and knock-out experiments on the same nucleus 14O corresponding to large positive and negative ΔS values, large enough to significantly test the asymmetry. Transfer results will be shown and compared to previous data obtained for (d,p) transfer and Ar isotopes [1]. [1] J. Lee et al., Phys. Rev. C 83 (2011) 014606 [2] A. Gade et al., Phys. Rev. C 77 (2008) 044306

Speaker: Alain Gillibert (CEA/IRFU/SPhN)

12:15 Asymptotic normalization coefficients of mirror states in 21Al

The structure of nuclei at and beyond the proton-drip line is important in the understanding of the rapid proton capture nucleosynthethic process (rp process). Knowledge of the nuclear properties in the light proton drip line region Z~13 are key inputs to network calculations of the rp-path. However, little spectroscopic information is known about the structure of 21Al which is proton unbound. Up to now only an upper limit of the lifetime of the unbound ground state has been measured [1]. Here we present the asymptotic normalization coefficients (ANCs) for mirror states in 21Al obtained from the neutron transfer reaction 20O(d,p)21O at 10.53 AMeV [2] and using proton resonance energies obtained from microscopic calculations. The analysis shows that the neutron stripping to the ground and first excited state of 21O is rather peripheral enabling the prediction of proton widths in 21Al using the mirror symmetry. M. G. Saint-Laurent, et al., Phys. Rev. Lett 59, 33 (1987). B. Fernandez-Dominguez, et al., Phys. Rev. C 84, 011301(R) (2011).

Speaker: Beatriz Fernandez-Dominguez (Universidade de Santiago de Compostela)

12:40 → 14:10 Lunch

14:10 → 15:50 Session 13

14:10 Structure and reactions of three-body exotic nuclei using discretization methods

In order to study weakly-bound systems, such as halo nuclei, it is essential to take into account the unbound states of the system. Since these unbound states form a continuum of energies, their inclusion in reaction calculations requires the introduction of a discretization method, i.e., the representation of the continuum by a finite and discrete basis. Among the different discretization methods in the literature, we will discuss about the binning procedure and the Transformed Harmonic Oscillator (THO) method, the last as an example of Pseudo-State (PS) method. The binning procedure is the discretization method that has been traditionally used within the standard Continuum-Discretized Coupled-Channels (CDCC) formalism for 2-body projectiles (3-body CDCC). Recently it has been extended to 3-body projectiles (4-body CDCC), in order to be applied to Borromean nuclei like 6He and 11Li. Alternatively, the spectrum can be represented using a PS basis. These PS are obtained as the eigenstates of the Hamiltonian of the system in a basis of square-integrable functions. One of these PS methods is the THO, whose basis functions are readily obtained by applying a local scaling transformation to the conventional HO functions. The identification of continuum structures in this PS basis, such as resonances, is discussed and applied to several three-body systems. Elastic and breakup of Borromean nuclei (6He, 11Li) are also presented, comparing the results obtained with both methods.

Speaker: Manuela Rodriguez-Gallardo (Universidad de Sevilla)

Slides DREB_GMD.pdf

14:35 Analysis of breakup channel for the 11Li+208Pb reaction at energies around the Coulomb barrier.

We present new data for the reaction 11Li+208Pb, recently measured at the radioactive nuclear beam facility in TRIUMF (Vancouver, Canada) at energies around the Coulomb barrier (Elab = 24.2 and 29.7 MeV). The halo nucleus of 11Li is composed by a core nucleus of 9Li and two loosely bound neutrons. Due to the weakly bound structure, this nucleus is easily polarizable. Thus, in the presence of a strong electric field this halo nucleus will be distorted and, eventually, it can be broken. This phenomenon, known as dipole Coulomb polarizability, produces a strong reduction of the elastic cross section with respect to the Rutherford prediction. In addition, this phenomenon gives rises to a large breakup probability of the weakly bound projectile. In this contribution we present preliminary experimental data on the angular and energy distributions of the 9Li fragments. These distributions are compared with Continuum-Discretized Coupled-Channels calculations, using a simple two-body model (9Li+2n) to describe the bound and continuum states of 11Li. We find that for small scattering angles, the reduction of the elastic cross section with respect to the Rutherford scattering is mostly due to dipole Coulomb couplings to the breakup channels. At these angles, the ratio between the outgoing 9Li and 11Li is also well reproduced by the calculations. At larger angles, the breakup probability is underestimated suggesting the presence of other mechanisms rather than the direct breakup.

Speaker: J. P. Fernández-García (University of Seville, Spain.)

Slides Presentation_dreb2012_JPFernandez.pdf

15:00 Exploring the coupling to nucleon transfer in fusion involving neutron-rich Sn nuclei at energies near the Coulomb barrier

Neutron-rich fission fragments accelerated to energies around the Coulomb barrier are used for studying the reaction mechanisms of fusion at HRIBF. Fusion excitation functions have been measured using neutron-rich radioactive 132Sn beams incident on Ca and Ni targets. Large sub-barrier fusion enhancement has been observed in the reaction with the 40Ca target. A previously measured fusion excitation function for 40Ca+124Sn suggests that the enhancement is due to multineutron transfer. The Q-values for multineutron transfer in the reaction of 132Sn with 58Ni are comparable to those in the reactions with 40Ca, but the sub-barrier fusion enhancement is significantly smaller. Furthermore, it is a surprise to find that the sub-barrier fusion enhancement for 118Sn+64Ni, which has no positive Q-value for neutron transfer, is comparable to that for 132Sn+58Ni. To investigate the differences in the correlations between transfer and sub-barrier fusion enhancement for Sn+Ca and Sn+Ni systems, the fusion excitation functions for 124Sn+46,50Ti have been measured. The neutrons transferred from 124Sn to 46Ti populate similar orbitals as those in 132Sn+40Ca but different from those in 132Sn+58Ni. A comparison of the fusion excitation functions for Sn+Ca, Sn+Ti, and Sn+Ni will be presented. Coupled-channels calculations to analyze the contributions of coupling to transfer will be discussed. *This research was supported by the US Department of Energy Office of Nuclear Physics.

Speaker: Felix Liang (Oak Ridge National Laboratory)

Slides dreb2012-Liang.pdf

15:25 Fusion reactions and neutron transfer in collisions induced by Li isotopes on Sn targets

Recently experimental evidences have been observed concerning the enhancement of the sub-barrier fusion cross section due to neutron transfer, both in reaction with stable nuclei [1,2] and with weakly bound nuclei [3]. The enhancement seems to be related to the sign of the Q-value for neutron transfer. A new mechanism has been proposed [4] for the sub-barrier fusion of weakly bound nuclei, in which an intermediate rearrangement of valence neutrons with positive Q-value may lead to a gain in kinetic energy of the colliding nuclei and, thus, to enhancement of the barrier penetrability and therefore of the fusion cross-section. To investigate the role played by the coupling to transfer channels having positive Q-value, we proposed to study the fusion of lithium isotopes with a combination of different Sn isotopes:6Li+120Sn,7Li+119Sn,8Li+118Sn and 9Li+117Sn. All these reactions lead to the same compound nucleus but are characterized by different Q-value for neutron transfer.The fusion cross section are measured by using an activation technique where the radioactive evaporation residues are identified by the X-ray emission following their electron capture decay.In this contribution, results of the 6Li+120Sn, 7Li+119Sn which have been recently measured at LNS(Catania)will be discussed. [1]Trotta et al,Phys.Rev.C65,011601(2002) [2] Stefanini et al,Phys.Rev.C74,034606 (2006) [3] Penionzhkevich et al,Phys.Rev.Lett.96,162701(2006) [4] Zagrebaev et al., Phys.Rev.C 67 061601(R) (2003)

Speaker: Maria Fisichella (Università di Messina / LNS-INFN)

Slides fisichella_dreb2012.pdf

15:50 → 16:10 Coffee Break

16:10 → 17:00 Session 14

16:10 Coulomb Excitation of 8Li on a 197Au Target

Current experimental values of the B(E2, 2+->1+) 8Li strongly disagree with theoretical estimates [1-3]. If confirmed this discrepancy would have a strong impact on nuclear models. We have measured inelastic scattering of 8Li (T1/2= 838 ms) on a 197Au target at Elab=26.9 MeV. Elastic scattering has been measured in parallel. The 8Li radioactive beam has been produced at the RIBRAS system (Brazilian Radioactive Ion Beam) from the 9Be(7Li,8Li)8Be reaction. The primary beam of 7Li (Elab = 30.7 MeV) has been extracted from the Pelletron/Linac Accelerator of the Institute of Physics of the University of São Paulo. Use has been made also of 197Au with thickness of 5.0mg/cm2. The angular distributions have been measured within a range of 20-75 degrees by making use of a detection system with four telescopes. Each telescope consisted of two silicon detectors with thicknesses of 20 and 1000 microns. Preliminary results will be presented. [1] J. A. Brown et al., Phys. Rev. Letts., 66, 19, 1991. [2] R. J. Smith et al., Phys. Rev. C, 43, 5, 1991. [3] P. Descouvemont and D. Baye, Phys. Letts. B 292, 235-238, 1992.

Speaker: Marlete Assunção (Universidade Federal de São Paulo)

Slides dreb2012_massuncao_[modalità_compatibilità].pdf

16:35 Scattering of 8He on 208Pb at energies around the Coulomb barrier

The scattering of 8He has attracted the interest of the scientific community due to its particular features. Being a skin nucleus with four valence neutrons and similar binding energies for 1n and 2n, it is very different from 2n halo 6He where the breakup into α+2n is favored. The differences of both helium isotopes are expected to be reflected in the elastic and reaction cross sections for collisions with heavy targets at Coulomb barrier energies. In order to investigate the dynamics of 8He we performed an experiment at SPIRAL/GANIL facility (Caen, France), as part of a large experimental project devoted to study radioctive light isotopes by means of nuclear reactions induced at energies around the Coulomb barrier [1-4]. In this contribution elastic 8He and 6,4He fragments angular distributions produced in collisions of 8He with a 208Pb target at 22 MeV are presented and discussed together with calculations carried out for transfer and break-up processes. The analysis of the experimental data performed so far has shown that the angular distribution of the 8He elastic scattering follows the trend of 6He up to the scattering angles around 80º. For larger angles the absorption becomes even greater what could be interpreted as an effect of 1n transfer reaction. [1] A. M. Sánchez-Benítez et al. Nucl. Phys. A, 803:30, 2008. [2] L. Acosta et al. Phys. Rev. C, 84, 044604, 2011. [3] L. Acosta et al. Eur. Phys. J. A 42 , 461, 2009. [4] M. Cubero, J.P. Fernández-García et al. EPJ

Speaker: Gloria Marquínez-Durán (University of Huelva, Spain)

Slides DREB_GMD.pdf

17:00 → 17:35 Keynote Summary Talk: "Perspectives and Reflections on Workshop"

slides DREB2o12_Summary.pdf

17:35 → 17:40 Closing of Workshop