Bulletin of the American Physical Society
4th Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 59, Number 10
Tuesday–Saturday, October 7–11, 2014; Waikoloa, Hawaii
Session CB: Mini-Symposium on Using Particle-gamma Coincidences to Study Nuclear Reactions and Structure I |
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Chair: Kathrin Wimmer, Central Michigan Univ Room: Kohala 2 |
Wednesday, October 8, 2014 7:00PM - 7:30PM |
CB.00001: Using particle-gamma coincidences to study nuclear reactions and structure with fast radioactive ion beams Invited Speaker: Nobuyuki Kobayashi Recently, spectroscopic studies on neutron rich nuclei have made considerable progress on the change of single particle levels, so-called shell evolution. On the studies, the method of particle-$\gamma$ coincidences with fast radioactive ion beams has been a powerful spectroscopic tool. The nucleon knockout reaction with $\gamma$-coincedence on a light target has been used to determine the spin parities of final states by measuring the momentum distribution of the particles. In addition, combined analysis of nuclear- and Coulomb-dominated 1$n$ removal reactions on light and heavy targets, respectively, can investigate the single particle orbitals of the incoming and outgoing nuclei. In this talk, I will overview the method of particle-$\gamma$ coincidences, and then I will focus on our recent results. Our present work addresses the spectroscopy of neutron-rich nuclei around the island of inversion, specifically, the halo, shell and deformation properties of $^{31}$Ne [1,2] and $^{37}$Mg [3]. For this purpose, the nuclear- and Coulomb-dominated reaction probes are used at energies around 240 MeV/nucleon. The present analysis exploits the different sensitivities of these reaction mechanisms to obtain the ground state separation energy, spin parity and the spectroscopic factors of these projectiles. The observables obtained were the nuclear- and Coulomb-dominated 1$n$-removal cross sections of $^{31}$Ne and $^{37}$Mg on C and Pb targets, and the parallel momentum distributions of the residues from the C target. We report also the recent analysis for the $^{20}$C momentum distribution in two-neutron removal reactions from $^{22}$C [4].\\[4pt] [1] T. Nakmaura, N. Kobayashi {\em et al}., Phys. Rev. Lett. {\bf 103}, 262501 (2009).\\[0pt] [2] T. Nakmaura, N. Kobayashi {\em et al}., Phys. Rev. Lett. {\bf 112}, 142501 (2014).\\[0pt] [3] N. Kobayashi {\em et al}., Phys. Rev. Lett. {\bf 111}, 242501 (2014).\\[0pt] [4] N. Kobayashi {\em et al}., Phys. Rev. C {\bf 86}, 054604 (2012). [Preview Abstract] |
Wednesday, October 8, 2014 7:30PM - 7:45PM |
CB.00002: In-beam gamma-ray spectroscopy of one-neutron knockout channel from $^{36}$Mg S. Momiyama, P. Doornenbal, H. Scheit, S. Takeuchi, N. Aoi, K. Li, M. Matsushita, D. Steppenbeck, H. Wang, H. Baba, E. Ideguchi, N. Kobayashi, Y. Kondo, J. Lee, S. Michimasa, T. Motobayashi, M. Takechi, Y. Togano, K. Yoneda, H. Sakurai An in-beam gamma-ray spectroscopy via one-neutron knockout reaction of $^{36}$Mg was performed at RIBF to clarify the neutron single-particle configuration in $^{36}$Mg. Neutron-rich Mg isotopes around $^{32}$Mg are known to have a collective nature in spite of the shell-model magic number $N = 20$. In the more neutron-rich region than this area so-called ``island of inversion,'' the deformation persists toward the $N = 28$ region. The large collectivity in the island of inversion is well explained with $\nu(sd)^{-2}$-$(fp)^{+2}$ particle-hole intruder configurations across the $N = 20$ shell gap. One-neutron knockout reactions are sensitive to the neutron single-particle configuration, and may therefore reveal the microscopic driving force for the large collectivity in the neutron-rich Mg isotopes towards $N = 28$. The level scheme of $^{35}$Mg deduced experimentally will be discussed. [Preview Abstract] |
Wednesday, October 8, 2014 7:45PM - 8:00PM |
CB.00003: In-beam $\gamma$-ray spectroscopy of exotic $N=32$ and 34 nuclei David Steppenbeck Recent investigations of exotic $N=32$ and 34 nuclei have highlighted the presence of sizable subshell closures that are absent in stable isotones. The presence of a new subshell closure at $N=34$ was recently reported in $^{54}$Ca, while previous studies focused on the development of the $N=32$ gap in Cr, Ti and Ca. On the theoretical side, these subshells were investigated, for example, in the framework of tensor-force-driven shell evolution, and were attributed to a reordering of the $\nu f_{5/2}$ orbital relative to the $\nu p_{3/2}-\nu p_{1/2}$ spin-orbit partners as protons are removed from the $\pi f_{7/2}$ state. It was also reported that no significant $N=34$ gap exists in Ti isotopes despite the fact that an inversion of $\nu f_{5/2}$ and $\nu p_{1/2}$ has been noted. The strength of the $N=34$ closure in Sc isotopes provides further input on the evolution of the $\nu f_{5/2}$ orbital in exotic systems. Moreover, the structures of Ar isotopes, which are presently reported to $^{48}$Ar, provide information on the magnitude of the $N=32$ closure ``below'' Ca isotopes. In the present work, the low-lying structures of $^{50}$Ar and $^{55}$Sc are reported to investigate further the evolution of the respective $N=32$ and 34 subshell closures in nuclei far from stability. [Preview Abstract] |
Wednesday, October 8, 2014 8:00PM - 8:15PM |
CB.00004: Differential spectroscopy using knockout reactions: the case of $^{33}$Mg Daniel Bazin The structure of $^{33}$Mg has been investigated using proton and neutron knockout reactions from radioactive beams of $^{34}$Al and $^{34}$Mg, respectively. The experiment was conducted at the Radioactive Isotope Beam Factory, where the 240 MeV/u radioactive beams where produced in the BigRIPS fragment separator\footnote{T. Kubo {\it et al.}, Prog. Theor. Exp. Phys. {\bf 2012}, 03C003} from the fragmentation of a 350 MeV/u $^{48}$Ca primary beam. $\gamma$-rays emitted during the reactions were recorded in the DALI2 array\footnote{S. Takeuchi {\it et al.}, arXiv:1403.5349v2 (2014)}, while knockout residues were collected and analyzed by the Zero Degree Spectrometer operated in dispersive mode. The analysis of the momentum distributions recorded in coincidence with the $\gamma$-rays, as well as the comparison between the two reactions, yield unambiguous result on the parity assignments of the $^{33}$Mg ground and excited states. In particular, the negative parity of the ground state is firmly established from this data. In addition, absolute partial cross sections are deduced and used with an eikonal reaction model to extract spectroscopic factors. [Preview Abstract] |
Wednesday, October 8, 2014 8:15PM - 8:30PM |
CB.00005: Excited state energies of neutron-rich Fe isotopes from knock-out V. Werner, C. Louchart-Henning, M. Lettmann, P. Doornenbal, T. Uesaka, A. Obertelli, C. Santamaria, A. Corsi, R. Taniuchi, H. Sakurai, M. Niikura, S. Franchoo, L. Olivier, Y. Shiga Within the first SEASTAR campaign at RIKEN-RIBF excited-state energies of neutron-rich Fe and Cr isotopes, as well as $^{78}$Ni have been measured. Experiments were performed using the DALI2 scintillator array in conjunction with the MINOS liquid-hydrogen target and TPC system. The RIKEN-RIBF cyclotrons delivered a 345 MeV/u $^{238}$U beam with an intensity of about 13 pnA impinging on a Be target. Fission fragments were separated and identified using the BigRIPS Spectrograph before the secondary target, and reaction products using the ZeroDegree Spectrograph after the secondary target. Progress in the analysis of Fe isotopes will be presented. [Preview Abstract] |
Wednesday, October 8, 2014 8:30PM - 8:45PM |
CB.00006: Lifetime Measurement of Nickel-58 Using RDM with GRETINA Charles Loelius The structure of nuclei near the doubly magic $^{56}\texttt{Ni}$ has provided a sensitive probe of configuration mixing across the N=Z=28 shell gap. The shell model description of nuclei in this region is well established, with the gxpf1 interaction accurately reproducing the energy levels and transition strengths of Nuclei in the vicinity of $^{56}\texttt{Ni}$. However, there remain open questions as to the effects of higher lying orbitals beyond the $\textit{pf}$ shell. These can be addressed by a study of the B(E2)'s of nuclei in near the shell gap, particularly the B(E2;$4^+\rightarrow 2^+$) where effects of high $\textit{l} $ orbitals may be enhanced. $^{58}\texttt{Ni}$ provides a strong candidate for study, as the only previous B(E2;$4^+\rightarrow 2^+$) measurement using the Doppler Shift Attenuation Method resulted in a B(E2) three times larger than that predicted by theory. In order to determine the possible effects of higher lying orbitals, a second measurement of the lifetime of $^{58}\texttt{Ni}$ was undertaken at the National Superconducting Cyclotron Laboratory using the the Gamma-Ray Energy Tracking in Beam Nuclear Array (GRETINA) and the Recoil Distance Method (RDM). Preliminary results of this measurement will be presented. [Preview Abstract] |
Wednesday, October 8, 2014 8:45PM - 9:00PM |
CB.00007: GRETINA lifetime measurements: magnetic transition rates and halo structure Hiro Iwasaki Halo nuclei have served as benchmark systems for understanding nuclear structure and correlations at the limit of stability. Valence neutrons with spatially extended wave functions are expected to provide a new degree-of-freedom in collective modes, inducing so-called soft collective motions. However, the experimental information on halo excitation modes has been limited so far to the E1 mode, hampering the detailed characterization. In GRETINA campaign at NSCL, we performed excited-state lifetime measurements to investigate magnetic transition rates of halo nuclei. In this talk, after introducing our program, the GRETINA performance in lifetime measurements as well as our results on the neutron-rich 17C isotope will be discussed. [Preview Abstract] |
Wednesday, October 8, 2014 9:00PM - 9:15PM |
CB.00008: Single-particle structure of neutron-rich N=40 isotopes: A new Island of Inversion Kathrin Wimmer The region around neutron-rich N=40 nuclei has recently attracted a lot of interest. The high-lying 2+ state in 68Ni and its small transition probability to the ground state are a result of the N=40 harmonic oscillator shell gap between the fp shell and the 1g9/2 orbital. This shell gap is reduced for the more neutron-rich Fe and Cr isotopes. In the shell model the increase in B(E2) values and the decrease in 2+ excitation energy can only be reproduced if the neutron 1g9/2 and 2d5/2 intruder orbitals are included in the model space. Spectroscopic studies of neutron-rich nuclei around N=40 have been performed at the NSCL utilizing the S800 spectrometer and the GRETINA gamma detector array. The study focused on the one-neutron removal reactions from 64,66Fe. This experiment employed a new technique of combined prompt and delayed gamma-spectroscopy allowing to quantify the occupancy of the intruder neutron 1g9/2 and 2d5/2 orbitals in 64,66Fe. Comparison of the measured spectroscopic factors with large-scale shell model calculations show a significant occupation of the intruder orbitals across the N=40 sub-shell gap. Therefore the existence of a new ``Island of Inversion'' at N=40 as been experimentally verified for the first time. [Preview Abstract] |
Wednesday, October 8, 2014 9:15PM - 9:30PM |
CB.00009: Shell structure studies in the vicinity of the doubly-magic $^{78}$Ni by in-beam $\gamma$-ray spectroscopy Yoshiaki Shiga In-beam $\gamma$-ray spectroscopy in the vicinity of $^{78}$Ni is reported. In order to pin down the shell evolution on and beyond $N=50$ in neutron-rich nucleus, the energies of low-lying excited states in the isotopes around $^{78}$Ni were measured. The experiment was performed at the RIKEN Nishina Center for Accelerator-Based Science. The secondary beams of isotopes around $^{78}$Ni were produced by in-flight fission reactions of $345$ MeV/u ${}^{238}$U primary beam on a $925$ mg/cm${}^2$ beryllium target. The secondary beams then impinged on a $1889$ mg/cm${}^2$ thick beryllium target to produce the isotopes of interest. The de-excitation $\gamma$-rays were measured with the NaI(Tl) detector array DALI2 in coincidence with the reaction products which were analyzed in the ZeroDegree spectrometer. Doppler corrected $\gamma$-ray energy spectra obtained for the reaction residues Zn, Cu and Ni isotopes will be shown. The nuclear structure in this region will be discussed by the level scheme obtained, e.g. the vibrational mode in $^{80}$Zn, as well as the energy trends of the low-lying states in the isotope chains. [Preview Abstract] |
Wednesday, October 8, 2014 9:30PM - 9:45PM |
CB.00010: Shape coexistence in $^{74}$Rb explored via 2$_{1}^{+}$ state lifetime measurement C. Morse, H. Iwasaki, A. Lemasson, A. Dewald, T. Braunroth, D. Bazin, V.M. Bader, T. Baugher, J.S. Berryman, C.M. Campbell, A. Gade, C. Langer, I.Y. Lee, C. Loelius, E. Lunderberg, F. Recchia, D. Smalley, S.R. Stroberg, R. Wadsworth, C. Walz, D. Weisshaar, A. Westerberg, K. Whitmore, K. Wimmer The self-conjugate nucleus $^{74}$Rb lies in a region of the nuclear chart which is well-known to harbor many intriguing nuclear structure phenomena. The nature of the collectivity in this region changes rapidly, with E2 strengths known to show sudden increases with a small increase in mass along the N$=$Z line. Shape coexistence is also a well-known feature of this mass region, especially in the Kr isotopes. Shape coexistence therefore may also persist in $^{74}$Rb, with N $=$ Z $=$ 37, as it does in the isobaric analogue states of $^{74}$Kr. To investigate these phenomena, the lifetime of the first 2$^{+}$ state in $^{74}$Rb has been measured using the NSCL TRIPLEX plunger and GRETINA. Results of the measurement will be presented and the implications for the structure of $^{74}$Rb will be discussed. [Preview Abstract] |
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