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 EB: MiniSymposium on Nuclear Structure of Neutronrich Exotic Nuclei II 
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Chair: Toshiyuki Kubo, RIKEN Room: Kohala 2 
Thursday, October 9, 2014 7:00PM  7:30PM 
EB.00001: Shells and shapes in exotic nuclei Invited Speaker: Yutaka Utsuno The shell structure is one of the most fundamental properties in nuclei. It is important not only in the lowlying energy levels but also in nuclear collectivity. This constitutes part of the reason why the evolution of shell structure in exotic nuclei, often called shell evolution, is being extensively studied in exotic nuclei. In this talk, I give an overview of recent progresses in the understanding of shell evolution and its impacts on nuclear collectivity including shapes. First, some examples of shellevolution phenomena are presented, where the importance of the tensor force is stressed. Next, the effect of shell evolution on collectivity is discussed. In the $N\sim 28$ region, the deformations of $^{42}$Si and $^{44}$S are taken. Those nuclei have unusual collective properties due to the quenching of proton and neutron intrashell gaps caused by the tensor force. In the neutronrich Ni region, it is shown that shape coexistence is caused by the shell evolution. Contrary to the usual potential picture, the spherical shell structure strongly depends on the configuration of nuclear states, which is proposed to be called Type II shell evolution. The shapes of strongly correlated nuclei can be examined by using the Monte Carlo shell model. [Preview Abstract] 
Thursday, October 9, 2014 7:30PM  7:45PM 
EB.00002: Results from the LISA Commissioning Experiment on the decay of $^{24}$O$^*$ $\rightarrow$ $^{23}$O + n Warren F. Rogers The Large multiInstitutional Scintillator Array (LISA) at NSCL, Michigan State University was constructed and tested by undergraduate students from several institutions in the MoNA (Modular Neutron Array) collaboration. LISA is used in conjunction with MoNA for detection of neutrons at large angles to the beam axis, corresponding to high energy decays from exotic nuclei at or beyond the neutron dripline. The LISA commissioning experiment was designed to measure and resolve neutron decays from the first two excited states (2$^+$ and 1$^+$) of $^{24}$O to the $^{23}$O ground state, using protonknockout of $^{26}$F on a thin Be target. The trajectories of charged fragments at the site of the decay were identified using the Sweeper Magnet chamber detectors and inversetracking through the magnet. The two scintillator arrays measured the time of flight path of neutrons. Decay energies were determined using these decay kinematics and invariant mass spectroscopy. The two $^{24}$O states were previously observed, but with insufficient resolution to separate the two cleanly. This experiment provided sufficiently resolution to separate the two states cleanly. Results for the decay energies and comparison with Monte Carlo simulations will be presented. [Preview Abstract] 
Thursday, October 9, 2014 7:45PM  8:00PM 
EB.00003: Groundstate energies and charge radii of mediummass nuclei in the unitarymodeloperator approach Takayuki Miyagi, Takashi Abe, Ryoji Okamoto, Takaharu Otsuka In nuclear structure theory, one of the most fundamental problems is to understand the nuclear structure based on nuclear forces. This attempt has been enabled due to the progress of the computational power and nuclear manybody approaches. However, it is difficult to apply the firstprinciple methods to mediummass region, because calculations demand the huge model space as increasing the number of nucleons. The unitarymodeloperator approach (UMOA) is one of the methods which can be applied to mediummass nuclei. The essential point of the UMOA is to construct the effective Hamiltonian which does not induce the twoparticletwohole excitations. A manybody problem is reduced to the twobody subsystem problem in an entire manybody system with the twobody effective interaction and onebody potential determined selfconsistently. In this presentation, we will report the numerical results of groundstate energies and charge radii of $^{16}$O, $^{40}$Ca, and $^{56}$Ni in the UMOA, and discuss the saturation property by comparing our results with those in the other manybody methods and also experimental data. [Preview Abstract] 

EB.00004: ABSTRACT WITHDRAWN 

EB.00005: ABSTRACT WITHDRAWN 
Thursday, October 9, 2014 8:30PM  8:45PM 
EB.00006: Small spectroscopic factors of lowlying positive parity states in 31Mg Nobu Imai, Momo Mukai, Joakim Cederkall, Hossein Aghai, Pavel Golubev, Haakan Johansson, Daid Kahl, Jan Kurcewics, Takashi Teranishi, Yutaka Watanabe The single particle structures of evenodd nuclei around the socalled ``island of inversion'' would give us the direct evidence of such a shell evolution in this region. We measured the proton resonance elastic scattering on $^{30}$Mg reaccelerated upto 2.92 MeV/nucleon by REXISOLDE to study the isobarig analog resonances (IARs) of the lowlying bound states in $^{31}$Mg. The proton resonance elastic scattering is a complementary method of (d,p) reaction. We observed three resonances which can be regarded as the IARs of $^{31}$Mg. The proton widths of the first two resonances give a rise to the spectroscopic factors for the two positive parity states in $^{31}$Mg which were found to be strongly quenched compared to those for the $^{35}$S and $^{37}$Ar. Comparison with a modern shell model calculation suggests that the degrees of the $\nu$(2p2h) configuration in $^{30}$Mg would be less than considered [1].\\[4pt] [1] N. Imai, {\it et al.}, Phys. Rev. C, in press. [Preview Abstract] 
Thursday, October 9, 2014 8:45PM  9:00PM 
EB.00007: Erosion of N$=$28 shell gap and triple shape coexistence in the vicinity of $^{44}$S Masaaki Kimura The broken magic number N$=$28 in the vicinity of $^{44}$S has been a topic of interest and importance addicting many experimental and theoretical studies. In particular, in the case of the Ar, S and Si isotopes, the strong quadrupole correlation amongst protons and neutrons is induced by the quenching of the N$=$28 shell gap and results in a variety of deformed states. In this talk, we will report our recent research results of the lowlying spectroscopy in the vicinity of $^{44}$S nuclei based on the theoretical model of the antisymmetrized molecular dynamics. The focuses of the talk will be the following two; (1) Triple Shape Coexistence. The strong quadrupole correlation triggers the novel shape coexistence phenomena. Three different types of quadrupole deformation, i.e. prolate, oblate and triaxial deformed states coexist at very small excitation energy. (2) Shape Transitions of the Ground States. The deformation of the ground state changes between the oblate and prolate deformations depending on the proton numbers. The underlying shell structure and the protonneutron correlation determine the shape of the ground state. [Preview Abstract] 
Thursday, October 9, 2014 9:00PM  9:15PM 
EB.00008: Groundstate nuclearmoment measurement of neutronrich sulfur isotopes Yuichi Ohtomo, Yuichi Ichikawa, Hazuki Shirai, Hideki Ueno, Youko Ishibashi, Takahiro Suzuki, Takeshi Furukawa, Akihiro Yoshimi, Yasushi Abe, Koichiro Asahi, J.M. Daugasu, Tomomi Fujita, Miki Hayasaka, Kei Imamura, Shota Kishi, Shuichiro Kojima, Daisuke Nagae, Aiko Nakao, Tsubasa Sagayama, Yu Sakamoto, Tomoya Sato Recently the erosion of $N=$28 shell gap has been suggested from several spectroscopic experimental data on neutronrich nuclei. In particular, $^{\mathrm{43}}$S isotope is of much interest since shape coexistence is expected to occur which provides key information to understand the evolution of shell gaps far from the stability. The isomeric state of $^{\mathrm{43}}$S at 320 keV is suggested to have a shape close to sphericity with spinparity of 7/2, but both the spinparity and deformed parameter of the groundstate have not been determined directly. In order to investigate mechanisms leading to such an anomalous nuclear structure, we aim at measuring the groundstate nuclearmoment for $^{\mathrm{41,43}}$S. As the first step, the measurement of $\mu $ moment of $^{\mathrm{41}}$S was performed using the technique of $\beta $NMR method at the RIPS facility at RIKEN. In the presentation, the result of this work will be reported. [Preview Abstract] 
Thursday, October 9, 2014 9:15PM  9:30PM 
EB.00009: Behavior of Neutral $s$States in Loosely Bound Nuclei C.R. Hoffman, B.P. Kay, J.P. Schiffer We have shown\footnote{C. R. Hoffman, B. P. Kay, and J. P. Schiffer, Phys. Rev. C {\bf 89}, 061305(R) (2014).} that the variation in binding energy of 1/2$^+$ and 5/2$^+$ states in isotopes of Be to O with $N=5$ to 13 can largely be described in simple geometrical terms. The behavior of neutral $s$states is qualitatively different from other $\ell$ values, showing a tendency to linger below threshold whereas others do not, and nor do proton $s$states. This observation is supported by a wealth of experimental data obtained from radioactive ion beams experiments. While calculations, such as those using the shell model, may reproduce the data, it is likely this simple geometrical effect is subsumed in the effective interactions used. The lingering of neutral $s$states is the same mechanism responsible for neutron halos, and leads one to speculate the existence of heavier halo nuclei close to $^{78}$Ni. This work is supported by the U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DEAC0206CH11357. [Preview Abstract] 
Thursday, October 9, 2014 9:30PM  9:45PM 
EB.00010: Neutronrich nuclei from the nuclear force Naofumi Tsunoda, Takaharu Otsuka, Kazuo Takayanagi, Morten HjorthJensen, Toshio Suzuki, Noritaka Shimizu The properties of neutronrich nuclei starting from the nuclear force with two and threebody force is presented. Recent experimental and computational development makes the neutronrich nuclei a big frontier where the fundamental treatment is important. We aim to study neutronrich nuclei starting from the nuclear force and the theory to derive the effective interaction not empirically but via the microscopic theory. We construct the effective interaction for the shellmodel using newly developed Extended KuoKrenciglowa (EKK) method. The major difference from KuoKrenciglowa (KK) method is that EKK method can be applied to nondegenerate model space where KK method inevitably experience the divergences. In addition, we include the effects of three body force by part via the effective twobody force. With this microscopically derived nuclear force, we discuss the shell structure of neutronrich nuclei and the role of threebody force. [Preview Abstract] 
Thursday, October 9, 2014 9:45PM  10:00PM 
EB.00011: Largescale shellmodel calculation of unnatural parity highspin states in neutronrich Cr and Fe isotopes Tomoaki Togashi, Noritaka Shimizu, Yutaka Utsuno, Takaharu Otsuka, Michio Honma We have investigated unnaturalparity highspin states in neutronrich Cr and Fe isotopes with largescale shellmodel calculation. This shellmodel calculation has been carried out within the model space of the $fp$shell + $0g_{9/2}$ +$1d_{5/2}$ orbits with the truncation of $1\hbar\omega$ excitation of a neutron. The effective Hamiltonian consists of GXPF1Br for the $fp$shell orbits and $V_{\mbox{\scriptsize MU}}$ with the modification for the other parts. This shellmodel calculation has described and predicted the energy levels of both natural and unnatural parity states up to the high spin in Cr and Fe isotopes with $N\le35$. The total energy surfaces have presented the overall prolate deformation and indicated that the excitation into a $\nu0g_{9/2}$ orbit plays the roles of inducing the prolate deformation for the unnatural parity states in these nuclei. It has been found that the excitation energy of $9/2_{1}^{+}$ dropping down with the increase of neutrons in neutronrich oddmass Cr and Fe isotopes is linked to the Fermi surface approaching the neutron shell orbits. [Preview Abstract] 
Thursday, October 9, 2014 10:00PM  10:15PM 
EB.00012: BetaDelayed Neutron Spectroscopy with Trapped Fission Products A. Czeszumska, N.D. Scielzo, E.B. Norman, G. Savard, A. Aprahamian, M. Burkey, S.A. Caldwell, C.J. Chiara, J.A. Clark, J. Harker, S.T. Marley, G. Morgan, R. Orford, S. Padgett, A. Perez Galvan, R.E. Segel, K.S. Sharma, K. Siegl, S. Strauss, R.M. Yee Characterizing $\beta$delayed neutron emission ($\beta n$) is of importance in reactor safety modeling, understanding of rprocess nucleosynthesis, and nuclear structure studies. A newly developed technique enables a reliable measurement of $\beta n$ branching ratios and neutron energy spectra without directly detecting neutrons. Ions of interest are loaded into a Paul trap surrounded by an array of radiation detectors. Upon decay, recoiling daughter nuclei and emitted particles emerge from the center of the trap with minimal scattering. The neutron energy is then determined from the timeofflight, and hence momentum, of the recoiling ions. I will explain the details of the technique, and present the results from the most recent experimental campaign at the CARIBU facility at Argonne National Laboratory. [Preview Abstract] 
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