3rd SUNFLOWER Workshop

15 Sept 2014, 08:00 → 16 Sept 2014, 16:35 Asia/Tokyo

Koshiba Hall (The University of Tokyo)

Megumi NIIKURA (Department of Physics, University of Tokyo), Pieter Doornenbal (RIKEN)

Following previous workshops at RIKEN and the TU Darmstadt, we would like to announce the 3rd SUNFLOWER workshops to be hosted at the University of Tokyo, Japan on September 15-16, 2014. The meeting's main purpose is to discusss the status and future physics opportunities of in-beam gamma-ray spectroscopy experiments at the Radioactive Isotope Beam Factory (RIBF) of the RIKEN Nishina Center. In-beam gamma-ray spectroscopy experiments are performed on a regular basis at the RIBF since 2010 with campaigns employing 48Ca, 70Zn, 124Xe, and 238U primary beams. This year, the physics program Shell Evolution And Search for Two-plus energies At the RIBF (SEASTAR) was initiated. SEASTAR takes advantage of the liquid hydrogen target MINOS (MagIc Numbers Off Stability) coupled to the gamma-ray spectrometer DALI2 and successfully performed its first campaign this spring. In addition, also physics opportunities with next generation high resolution gamma-ray spectrometers may be discussed. The objectives for the SUNFLOWER workshop thus include: i) Status of the SUNFLOWER collaboration, ii) Status report on individual performed in-beam gamma-ray experiments, iii) Status of the SEASTAR collaboration including aims for the next campaign, iv) Upcoming experiments, v) Physics proposals and ideas for near future experiments, and vi) High resolution in-beam gamma-ray spectroscopy at the RIBF. Proponents intending to submit a SUNFLOWER proposal to the next RIBF Nuclear Physics Program Advisory Committee, held towards the end of this year, are encouraged to attend the workshop and present their proposal.

Monday, 15 September

09:30 → 10:45 Session 1

Convener: Hiroyoshi SAKURAI (RIKEN Nishina Center / University of Tokyo)

09:30 Welcome

Speaker: Hiroyoshi Sakurai (RIKEN Nishina Center for Accelerator-Based Science)

09:35 Introduction and Overview of the SUNFLOWER activities

An overview of the activities of the SUNFLOWER activities will be given.

Speaker: Pieter Doornenbal (RIKEN)

10:05 Status report of experiment NP1306-RIBF31R1

Speaker: He Wang (RIKEN Nishina Center)

10:25 Structure of light Sn isotopes: report on results from RIBF70 and 74

Speaker: Alexandre Obertelli (CEA Saclay)

10:45 → 11:15 Coffee Break

11:15 → 12:35 Session 2

Convener: Ken-ichiro YONEDA (RIKEN Nishina Center)

11:15 Overview of the experiment RIBF30 ; In-beam gamma-ray spectroscopy for vicinity of 78Ni

This experiment was performed at RIBF in 2011. The physical goal of this experiment is understanding nuclear structure and nuclear force in the region of around 78Ni. Excited energy levels of nuclei around 78Ni was studied by knock-out reaction impinging RI-beam on Be target(1889 mg/cm2). The gamma-ray was mesured by DALI2 at F8 detector and residual nuclei was identified by ZeroDegree spectrometer. We obtained a lot of spectra of nuclei of around 78Ni. I will discuss the shell structure evolution of the magic number Z=28 and N=50 based on obtained results.

Speaker: Shiga Yoshiaki (Rikkyo)

11:35 In-beam gamma-ray spectroscopy next to 78Ni: the nuclei 77,79Cu (Status report for data analysis of RIBF30 experiment)

The question of the shell closure stability near the neutron drip-line has been in the forefront of the nuclear structure research for a long time. The next major neutron shell closure occurs at N=50, which is investigated in the vicinity of the possible doubly magic 78Ni. The doubly magic nature of 78Ni depends on the strength of the N=50 and Z=28 shell closures. The shell closures can be described by the shell gaps, determined by the energy differences between proton and neutron single particle states. The Z=28 shell gap can be deduced from single particle energies of heavy Cu isotopes. Copper isotopes next to 78Ni have been studied by use of in-beam γ-ray spectroscopy. The experiment was performed at RIKEN RI Beam Factory where a high-intensity 238U primary beam with energy of 345 MeV/u impinged on a 925 mg/cm 2 primary beryllium target. The produced RI beams were separated and identified using the BigRIPS spectrometer by the ∆E−ToF−Bρ method. As a next step the tagged fragments were collided with a 1900 mg/cm 2 secondary beryllium target for the nucleon-removal reactions. The reaction channel selection after the secondary reaction was carried out by the ZeroDegree spectrometer. The NaI(Tl)-array called DALI2 surrounding the secondary target detected the γ-rays emitted by the secondary fragments. During the data analysis a strict time gated procedure has been applied on the DALI2 part of the events. Using this method several gamma-ray transitions could be assigned to 77,79Cu from single proton removal reaction channels. Level schemes have been built and compared with the shell model calculations. Stability of the Z=28 shell gap and decrease of the f5/2-f7/2 spin orbit splitting with increasing mass number has been observed. In the presentation the details of analysis will be discussed.

Speaker: Zsolt Vajta (Atomki)

11:55 Neutron momopole drift towards 78Ni

We would like to present about our current investigation on neutron momopole drift towards 78Ni by performing gamma-ray spectroscopies in the odd-mass N=49 isotone; 79Zn. The study of single-particle levels in 79Zn will provide critical data to understand the neutron monopole evolution in the vicinity of 78Ni. Some of excited levels in 79Zn were identified by a β-γ spectroscopy performed as a part of the EURICA campaign at RIBF in 2012. In addition, we propose to analyze by-products of the SEASTAR experiment to confirm the nature of the observed excited states. In the workshop, preliminary results of the EURICA experiment and purpose of submitted analysis proposal will be discussed.

Speaker: Megumi NIIKURA (Department of Physics, University of Tokyo)

12:15 Status Report of RIBF73

Speaker: Satoshi TAKEUCHI (RIKEN Nishina Center)

12:35 → 14:00 Lunch Break

14:00 → 15:20 Session 3

Convener: Tohru MOTOBAYASHI (RIKEN Nishina Center)

14:00 Quadrupole collectivity in Island-of-Inversion Nuclei

Quadrupole deformation of the Island-of-Inversion has been studied by using in-beam gamma-ray spectroscopy via proton inelastic scattering. This study was performed in RIPS beamline at approximately 50A MeV to attack the neutron-rich side of the deformation region, such as 30Ne and 36Mg. This talk will present experimental deformation lengths of Ne and Mg isotopes and will discuss on their systematics.

Speaker: Shin'ichiro MICHIMASA (CNS, Univ. of Tokyo)

14:20 Overview of NP0702-RIBF32 and NP0906-RIBF02

An overview of experiments NP0702-RIBF32 and NP0906-RIBF02 aiming for new excited states within the Island of inversion will be given. The report will include spectroscopy on 34,36,38Mg, 33,34,35Na, as well as Coulex on 30Ne and 36Mg. Furthermore, results on measured momentum distributions following 1p and 1n-knockout reactions towards 33Mg will be presented.

Speaker: Pieter Doornenbal (RIKEN)

14:40 In-beam gamma-ray spectroscopy of knockout reaction from nuclei around the island of inversion

An in-beam gamma-ray spectroscopy via one-neutron knockout reaction of 36Mg and one- or two-proton knockout reaction of 34Mg was performed. Nuclei around 32Mg, so-called "island of inversion", are known to have a large collectivity. In the more neutron-rich region the deformation persists toward the shell-model magic number N = 28. The large collectivity in the island of inversion is well explained with two-particle-two-hole neutron configuration across the N = 20 shell gap. Knockout reactions are sensitive to the nucleon single-particle configuration, and may therefore reveal the microscopic driving force for the large collectivity in this region. The experiments were performed at RIBF. 36Mg and 34Mg secondary beams were made in BigRIPS with the 48Ca primary beam. Secondary targets were C for 36Mg secondary beam and Be for 34Mg respectively. The NaI(Tl) gamma-ray detector array DALI2 was placed around the secondary target. Knockout residues were separated and identified by the ZeroDegree Spectrometer. The status of data analysis of these experiments will be presented in this workshop.

Speaker: Satoru Momiyama (Department of Physics, University of Tokyo)

15:00 Intruder configurations in the ground state of 30Ne

Speaker: Hongna Liu (Peking University / RIKEN Nishina Center)

15:20 → 15:50 Coffee Break

15:50 → 18:00 Session 4

Convener: Tomohiro UESAKA (RIKEN Nishina Center)

15:50 SEASTAR: overview of the first campaign and outlook

Speaker: Alexandre Obertelli (CEA Saclay)

16:20 SEASTAR Status report on 66Cr and 70,72Fe

During the first SEASTAR campaign last spring, in-beam spectroscopy of very exotic nuclei via proton-knockout reactions were successfully performed with the coupling of DALI2 with the new MINOS device. The use of a thick 100 mm liquid hydrogen target with a Time Projection Chamber allowed an increase of luminosity while maintaining a Doppler correction as good as with a thin target thanks to the reconstruction of the interaction vertex. Among others, the first spectroscopy of 66Cr and 70,72Fe was obtained by (p,2p) and (p,3p) knockout. We will report on the status of the analysis and discuss the collectivity of neutron-rich nuclei beyond N=40.

Speaker: Clementine Santamaria (CEA Saclay, SPhN)

16:40 Status report of the SEASTAR experiment: DALI calibration for 78Ni excited states

Since 78Ni is the most neutron rich doubly-magic-nuclei humankind can reach, the information of the shell structure of 78Ni is an important clue to examine the "shell evolution". The experiment named as "Shell Evolution And Search for Two-plus energies At RIBF" (SEASTAR) was done in May 2014 to obtain the informaition of the excited states of 78Ni. In this experiment, the nuclides of 79Cu were produced by BigRIPS separator and bombarded with a thick liquid hydrogen target named as MINOS. Dexcited gamma-rays were measured by DALI2 detector array, which consist of around 200 NaI(Tl) scintillators surrounding MINOS. After the reaction at MINOS, one-proton knockout reaction was identified by tagging 78Ni with ZeroDegree spectrometer placed at the downstream of the target. Because the measurement run for 78Ni was lasted almost 6days, fluctuations of gains of all detectors should be checked. The gain of DALI2 gamma-ray detector was shifted depending on time, because it was perhaps activated by high-rate beam. In this presentation, as the current status of analysis for the excited state of 78Ni, the gain shift of DALI2 detector will be shown and discussed.

Speaker: Ryo Taniuchi (Department of Physics, Graduate School of Science, The University of Tokyo)

17:00 Discussions and Conclusion

Tuesday, 16 September

09:00 → 10:55 Session 5

Convener: Paul FALLON (Lawrence Berkeley Laboratory)

09:00 Physics Opportunites using Compton Suppressed Ge Clover Array, CAGRA

The construction of a Compton suppressed Germanium clover array (CAGRA) by a U.S.-Japan collaboration is on going. High-precision capabilities of existing devices at RCNP can be combined with ultra-high precision γ-decay measurements to gain access to observables at an unprecedented level of detail. Three experimental sites are foreseen: at the EN beam line, where beams of rare isotopes are available; the Grand Raiden Spectrometer, where high-precision coincidence experiments utilizing light-ion reactions can be performed; and the muon beam facility at RCNP. A wide variety of important scientific questions will be addressed, such as the detailed nature of Pygmy dipole and Gamow-Teller resonances, the shell-evolution across the chart of nuclei, searches for superdeformed states, as well as astrophysical applications. In the presentation, an overview of CAGRA project will be presented and the scientific opportunities using CAGRA will be discussed.

Speaker: Eiji Ideguchi (RCNP, Osaka University)

09:30 New energy-degrading beamline for low-energy reaction measurements of RI beam

The recent developments of technique in providing RI beams have been made many advances in radioactive isotope science. The RI beam facility (RIBF) has expanded the variety of nuclides. However, available beams are restricted to an energy region above 100 AMeV or stopped beams. The variety of reaction has not been necessarily expanded on this point. The deceleration of intense RI beams provided in RIBF enable the further research based on exotic nuclei/exotic states by employing low-energy reactions, such as transfer reaction, fusion reaction and others. CNS has set up OEDO (Optimized Energy Degrading Optics for RI beam) project for production of high-quality low energy RI beams. Regarding OEDO project, the basic idea, performance study by simulation and design is introduced in this presentation.

Speaker: Masafumi Matsushita (CNS, Univ. of Tokyo)

10:00 Gamma ray spectroscopy with AGATA with slow and fast beams

Gamma ray spectroscopy is a very usefull tool for nuclear structure and nuclear physics in general. The direct view inside the nucleus gives important information on the nuclear properties. Progress in detector technology allways resulted in new results in nuclear physics. For this reason the development of next generation HPGe detectors started. The Advanced GAmma Tracking Array (AGATA) [1,2,3] is a European project to develop and operate the next generation gamma-ray spectrometer. AGATA is based on the technique of puls shape analysis, spacial detection of the interaction points of the impinging gammas and gamma-ray energy tracking in electrically segmented high-purity germanium crystals The advanced gamma ray tracking array AGATA will be discussed in his Technical details, use at LNL and GSI, type of experiments, results and future Physics campaign @ GANIL and different facilities. [1] "Conceptual design and infrastructure for the installation of the first AGATA sub-array at LNL"; A. Gadea, et al. NIM A 654 (2011) 88–96 [2] "AGATA – Advanced Gamma Tracking Array"; S. Akkoyuna, et al, Nucl.Instrum.Meth.A668:26-58,2012 [3] "On the Road to FAIR: 1st Operation of AGATA in PreSPEC at GSI"; N. Pietralla, et al, EPJ 66 (2014)02083

Speakers: Giacomo De Angelis (INFN LNL), Oliver Wieland (INFN sezione di Milano)

10:25 Status of GRETINA

GRETINA is a first implementation of a gamma-ray spectrometer which is capable of tracking gamma-rays through its active detector volume. It consists of seven, four-crystal modules (6x6 segments). Each crystal is individually encapsulated with all four crystals sharing a common cryostat. The irregular, tapered hexagonal crystals pack into a spherical shell with the seven modules spanning 1π solid angle. GRETINA was constructed and commissioned at LBNL. It completed its first physics campaign at NSCL/MSU in July of 2013 and is now running at ATLAS/ANL. We will give a short overview of the project, and discuss some technical aspects and the performance of the array. Highlights from the experimental program carried out at NSCL, and some initial results from the ATLAS campaign will be presented. Future plans for GRETINA will also be discussed.

Speaker: Augusto Macchiavelli (Nuclear Science Division - Lawrence Berkeley National Laboratory)

10:55 → 11:25 Coffee Break

11:25 → 12:45 Session 6

Convener: Jenny LEE (The University of Hong Kong)

11:25 Spectroscopy of Unbound Excited States of 31Ne with Particle-Gamma Coincedence

Speaker: Nobuyuki Kobayashi (The University Of Tokyo)

11:45 Spectroscopic Studies Close to 100Sn Using Neutron Knockout Reactions

Characterizing the nature of single-particle states outside of double shell closures is essential to a fundamental understanding of nuclear structure. This is especially true for those doubly magic nuclei lying far from stability that are much less studied and where the shell closures influence nucleosynthetic pathways. The region around 100Sn is one of the most important due to the proximity of the N=Z=50 magic numbers, the N=Z line, the proton drip-line, and the end of the rp-process. However, owing to low production rates there is a lack of spectroscopic information and until recently, there were no firm Jπ assignments for odd-mass tin isotopes lighter than 109Sn. There is experimental and theoretical evidence for a reversal of the spins of the ground and excited states in 101Sn compared to 103Sn coming from the near degeneracy of the d5/2 and g7/2 orbitals and the unexpectedly strong pairing for the (g7/2)2 configuration. Recent experiments using the S800 at the NSCL have revealed the nature of the ground and first excited states in 105,107Sn via the momentum distribution of residues from a neutron knockout experiment. We propose one-neutron knockout experiments on beams of 104,102Sn to measure the l-values of the ground and excited states in 101,103Sn, identified via γ rays measured in DALI2 in coincidence with residues detected at the end of the Zero Degree Spectrometer (ZDS). The degree of mixing in the ground states of 102,104Sn between the d5/2 and g7/2 single-particle states will be extracted from the population of the final states compared to reaction calculations.

Speaker: Giordano Cerizza (University of Tennessee)

12:05 The missing link in the Sn chain: Coulomb excitation of 102Sn

Abstract: Recent experiments using both low- and intermediate-energy Coulomb excitation of the unstable isotopes 106,108,110Sn [1-4] indicate a larger than expected collectivity of the first 2+ state in these isotopes. A new experiment for stable 114Sn in inverse kinematics has also further corroborated this picture by confirming the result of an earlier measurement but with a five-fold improvement in precision [5]. A series of experiments on 104Sn have shown a decreasing B(E2) trend towards 100Sn [6-8]. In particular the experiment at RIBF was of unprecedented accuracy [7]. These experimental results are currently not well reproduced by state-of-the-art large-scale shell-model (LSSM) calculations. With this proposal we aim to expand the measurements of the reduced transition probabilities for the first 2+ state in the Sn isotopic chain to 102Sn as well as to measure corresponding quantities in neutron deficient Cd isotopes. The purpose is to investigate correlations across the shell gap and thus to test the robustness of the 100Sn shell closure with respect to quadrupole response. The measurement of the B(E2) value in 102Sn which should exhibit a local minimum will finally answer the question of the similarity of the shell closure in 100Sn to the one in 56Ni. Such a minimum is not observed the N=3, fp shell for the Ni isotopes and for the N = 50 isotones above Z = 28 [9]. The combination of high-efficiency of the DALI2 setup, and increased intensity of 124Xe primary beam puts RIBF in a world-unique position as the only facility where a precision measurement of this kind can be carried out in the near future. [1] A. Banu et al., Phys. Rev. C. 72, 061305(R) (2005) [2] J. Cederkall et al., Phys. Rev. Lett. 98, 172501 (2007). [3] C. Vaman, et al., Phys. Rev. Lett. 99, 162501 (2007). [4] A. Ekstrőm et al., Phys. Rev. Lett. 101 012502 (2008). [5] P. Doornenbal et al., Phys. Rev. C 78, 031303(R) (2008). [6] G. Guastalla et al., Phys. Rev. Lett. 110, 172501 (2013). [7] P. Doornenbal et al., http://arxiv.org/pdf/1305.2877v1.pdf., and PRC(R) submitted. [8] V. M. Bader et al., Phys. Rev. C 88, 051301(R) (2013). [9] T. Faestermann, M. Gόrska, and H. Grawe, Progr. Part. Nucl. Phys. 68, 85 (2013) and ENSDF.

Speakers: Magdalena Gorska (GSI), Pieter Doornenbal (RIKEN)

12:25 Feasibility study of 100Sn spectroscopy with MINOS at RIBF

Speaker: Zhengyu Xu (The University of Hong Kong)

12:45 → 14:00 Lunch Break

14:00 → 16:00 Session 7

Convener: Oliver WIELAND (INFN sezione di Milano)

14:00 Experiments of gamma decay from pygmy resonace

Experiments of NP1012-RIBF62 (T.Aumann), NP1306-RIBF51R1 (O.Wieland) and NP1312-RIBF56R1 (H.Baba) will be performed in 2014 fall at RIBF. Inelastic alpha scattering on 124/128/132Sn, 20/22/24O and Coulomb excitation on 70/72Ni, 20/22/24O isotopes are measured to investigate property of pygmy resonance in neutron rich nuclei. Since energy of gamma ray from pygmy region is relatively high, large volume LaBr3 detectors (HECTOR+) are installed together with DALI. In this presentation, our physics motivation and experimental methods will be introduced.

Speaker: Hidetada Baba (RIKEN)

14:20 Pygmy Dipole Resonances in tin isotopes studied by alpha scattering

We will study the isospin nature of the Pygmy Dipole Resonances in tin isotopes. Therefore, we investigate their excitated states with the alpha-particle as an isoscalar probe. In this contribution we will present the experimental setup consisting of the gamma-detection system DALI2 + large-volume LaBr detectors, and the zero-degree spectrometer. The concept to obtain the single-gamma spectrum and the corresponding E1 transition strength in the three tin isotopes 124Sn, 128Sn, and 132Sn will be discussed.

Speaker: Joachim Tscheuschner (Technische Universitaet Darmstadt)

14:40 Study of dineutron correlation in Borromean nuclei via (p,pn) reaction

Speaker: Yuki Kubota (CNS, University of Tokyo)

15:00 Production of 40Mg Following 2p Knockout from 42Si

The experimentally observed collapse of the N=28 shell closure in 42Si has suggested a large oblate deformation at Z=14 and N=28. The isotonic nucleus 40Mg may be expected to have “mid-shell” character, and a similarly large deformation. Combined with the fact that it may lie at the edge of the predicted neutron drip-line for Z=12, 40Mg is a key nucleus both for understanding single-particle and shape evolution in the sd-fp shell, as well as the possible effects of weak binding. The last neutron in 40Mg is expected to occupy a low-l p3/2 state, which could lead to a neutron halo. The structure of 40Mg provides a benchmark for theory to predict the properties of the most exotic nuclei. The inclusive two-proton knockout reaction cross-section for 42Si into 40Mg has been measured in an experiment performed at the RI Beam Factory, at RIKEN Nishina Center. A secondary radioactive ion beam of 42Si was produced following fragmentation of 48Ca on a Be target, and identified through the BigRIPS fragment separator. Following reactions on a thick (~4g/cm2) 12C target, five 40Mg were uniquely identified in the ZeroDegree spectrometer, providing the first measurement of the inclusive two-proton knockout cross-section into 40Mg. Comparison with theoretical shell-model predictions suggests that the observed cross-section is consistent with a drastic change in nuclear shape between the 42Si and the 40Mg ground states. We will describe the results and interpretation of our first measurement in 40Mg, and discuss the planned follow-up experiment, focusing on the measurement of the energy of the first 2+ excited in this most exotic N=28 isotone.

Speaker: Heather Crawford (Ohio University)

15:20 Discussion and Closing