Bulletin of the American Physical Society
3rd Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 54, Number 10
Tuesday–Saturday, October 13–17, 2009; Waikoloa, Hawaii
Session 2WB: Workshop on Physics Opportunities with GRETINA II |
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Chair: Cornelius Beausang, University of Richmond Room: Kona 4 |
Tuesday, October 13, 2009 2:00PM - 2:30PM |
2WB.00001: Lifetime measurements of RI beam and high-spin studies with degraded beams Invited Speaker: The development of RI beams has opened a wide region to study the nuclear structure far from the stability line. During the extensive studies of neutron-rich nuclei in the light mass region, new phenomena such as the disappearance of N=8, 20 magic numbers associated with the deformed ground states were revealed. Gamma-ray spectroscopy was employed for the study of the deformed structure. Based on the relatively low excitation energy of 2$^{+}$ state and the large B(E2) value, large deformation of the ground state was identified. Observation of the excited levels was thus far limited to the low-lying states, but the study of higher-spin states will be useful to understand the collectivity since a presence of a rotational band is one of the clear evidences of the deformed structure. In order to realize a high-resolution gamma-ray spectroscopy of exotic nuclei, we have developed a segmented Ge detector array, CNS GRAPE, and plan to investigate unstable nuclei in the heavier mass region. To study collective structures of unstable nuclei, we plan to perform life-time measurements of 2$^{+}$ and higher excited sates utilizing direct reactions with high-intensity fast RI beams. At present, RI beam factory (RIBF) at RIKEN has a potential to provide world's highest intensity. In addition, experiments using low-energy reactions are planned to study high-spin states. Previously, we have successfully developed an energy-degraded $^{46}$Ar beam produced by the fragmentation of 64AMeV $^{48}$Ca primary beam. It was used for a fusion-evaporation reaction with a $^{9}$Be target. Gamma rays emitted from high-spin states were clearly observed. Same technique to make low-energy RI beam could be applied to heavier RI beams at RIBF and the study of high-spin states will be widely expanded. In the talk, lifetime measurements and studies of high-spin states of unstable nuclei far from the stability using high-efficiency position-sensitive Ge detector array at RIBF will be discussed. [Preview Abstract] |
Tuesday, October 13, 2009 2:30PM - 3:00PM |
2WB.00002: Nuclear transition moment measurements of neutron rich nuclei Invited Speaker: The Recoil Distance Method (RDM) and related Doppler Shift Attenuation Method (DSAM) are well-established tools for lifetime measurements following nuclear reactions near the Coulomb barrier. Recently, the RDM was implemented at National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University using NSCL/K{\"o}ln plunger device and a unique combination of the state-of-the-art instruments available there. Doppler-shift lifetime measurements following Coulomb excitation, knock-out, and fragmentation at intermediate energies of $\sim$100~MeV/u hold the promise of providing lifetime information for excited states in a wide range of unstable nuclei. So far, the method was used to investigate the collectivity of the neutron-rich $^{16,18,20}$C, $^{62,64,66}$Fe, $^{70,72}$Ni, $^{110,114}$Pd isotopes and also of the neutron-deficient N=Z $^{64}$Ge. A significant fraction of these experiments was performed using NSCL's Segmented Germanium Array instrumented with the Digital Data Acquisition System which enables gamma-ray tracking. The impact of GRETINA and gamma-ray tracking on RDM and DSAM studies of neutron-rich nuclei will be discussed. [Preview Abstract] |
Tuesday, October 13, 2009 3:00PM - 3:30PM |
2WB.00003: Nuclear moment measurements on neutron-rich nuclei Invited Speaker: Two techniques have recently been proved workable to measure the magnetic moments (or g factors) of short-lived excited nuclear states in exotic nuclei produced as radioactive beams. These are the high-velocity transient-field technique (HVTF) [1,2], which has been demonstrated to be applicable to relatively low-Z isotopes produced by fast fragmentation, and the recoil-in-vacuum (RIV) technique, which has been applied to heavier nuclei near $^{132}$Sn [3,4]. This talk will report, firstly, on the analysis of recent moment measurements in neutron-rich nuclei, and, secondly, on the further development of the techniques for applications to new regions of the nuclear chart, with an emphasis on the opportunities opened up by the gamma-ray tracking array GRETINA. Progress on recently performed HVTF measurements, including the neutron-rich isotopes $^{42-46}$Ar at NSCL, will be discussed, along with experimental work to extend the technique towards higher Z and the N=40 region. The feasibility of a HVTF g-factor measurement on the first-excited state in $^{32}$Mg, taking advantage of GRETINA, will also be evaluated. For slower beams with 2 -- 5 MeV/nucleon, the RIV technique has advantages: these g-factor experiments are essentially identical to a B(E2) measurement, but with sufficient statistics to measure the particle-gamma angular correlations. Detailed studies of the free ion hyperfine fields, which must be characterized and understood for these moment measurements, have commenced using stable beams at the Australian National University. The results will be described with the view to future applications to moment measurements in the N=34, N=40 and A$\sim $100 regions, using re-accelerated radioactive beams and GRETINA.\\[4pt] [1] A.D. Davies et al. Phys. Rev. Lett. 96, 112503 (2006).\\[0pt] [2] A.E. Stuchbery et al. Phys. Rev. C 74, 054307 (2006).\\[0pt] [3] N.J. Stone et al. Phys. Rev. Lett. 94, 192501 (2005).\\[0pt] [4] A.E. Stuchbery and N.J. Stone, Phys. Rev. C 76, 034307 (2007). [Preview Abstract] |
Tuesday, October 13, 2009 3:30PM - 4:00PM |
2WB.00004: COFFEE BREAK
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Tuesday, October 13, 2009 4:00PM - 4:30PM |
2WB.00005: Search of neutron-rich superdeformer, superheavy K-isomer, superfast rotor, and chiral wobbler with RIBF and GRETINA Invited Speaker: Nuclear physics has entered a new phase of research with radioactive beams in the 21st century. The scope of the study is originally aimed at the dripline regions, but the study of neutron-rich and proton-rich systems, weakly-bound systems such as halo nuclei are currently the primary targets for the investigation. High-spin nuclear structure physics was developed thanks to the heavy ion collision and subsequent neutron-evapolation reactions. In the 1980s and 1990s', many new forms of exotic excited states were discovered such as superdeformation and magnetic rotation. However, it is still far from completion in understanding the structure of these high-spin states. For example, the decaying mechanism from super to normal deformed states is still unclear. Many of bandheads of discovered superdeformed bands are unidentified. With a combination of GRETINA and RIBF, we can enjoy a new opportunity to go for the study of new physics: high-spin states with radioactive beams. In my talk, I would like to discuss what kind of new high-spin physics can be investigated with this new fascilities, for example, neutron-rich superdeformation, superheavy K-isomers, ultra-fast high-spin states over 100$\hbar$, and more exotics 3D rotating states such as wobbling and chiral rotation. [Preview Abstract] |
Tuesday, October 13, 2009 4:30PM - 5:00PM |
2WB.00006: Spectroscopy of the Heaviest Elements Invited Speaker: The specific ``magic'' proton and neutron numbers, representing major spherical shell gaps, beyond 208Pb are a matter of considerable debate. It is well established that nuclei near Z=100, N=152 (252Fm) have well-deformed prolate shapes. By performing prompt and delayed gamma-ray spectroscopy on deformed transfermium nuclei we can learn about the single-particle structure, shell gaps, pairing correlations, and excitation modes in the heaviest nuclei. After a brief overview of state-of-the-art measurements, I will describe recent results from experiments at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory which use the Berkeley Gas-filled Separator (BGS). I will then discuss the prospects of a new generation of spectroscopy measurements on the heaviest elements when the BGS is used in conjunction with the GRETINA gamma--ray tracking array. [Preview Abstract] |
Tuesday, October 13, 2009 5:00PM - 5:30PM |
2WB.00007: Isomer spectroscopy using RI beam Invited Speaker: We have studied systematically high-spin oblate shape isomers in the $N$=83 isotones, which have revealed the characteristics of nuclear structure, such as the preserving pairing interactions at high-spin states, decrease of $Z$=64 proton shell gap energy as the decrease of proton number from 64 to 60 and so on. Recently, it became possible to search for isomers by the secondary fusion reaction at high-spin states in nuclei, which could not be populated by the stable beam and stable target, using RCNP RI beam line at Osaka University. RI beams enable us to study high-spin states in nuclei in wide mass region. By using the RI beams delivered by RIBF and the high-efficiency $\gamma$-ray detection system GRETINA, it will be possible to investigate nuclei far from the stability line. Single-particle energies and nucleon-nucleon interactions of these nuclei close to drip line are expected to be the test ground of nuclear models, such as shell structures. We have a plan to search for isomers with half lives of $\sim$$\mu$sec to $\sim$msec and to explore the decay mechanism of isomers in the proton-rich nuclei along $N$=$Z$ line with 80$< $$A$$<$100. Moreover we try to search for nuclei beyond the proton drip line, which could be defined that isomeric states would be bound by the centrifugal potential although the ground states would be unbound against the proton emission. Isomers are expected to reveal the following characteristics of these nuclei. (1) Existence of isomers could prove the magicity of $N$=$Z$=50 and the large neutron-proton interaction, as one of the candidates of isomers is spin-gap isomer which is caused by the lowering of excitation energies resulting from the stretch coupling of spins of high-$j$ (g$_ {9/2}$) holes of the $^{100}$Sn core. (2) Isomers could prove the nuclear deformation which is caused by the evolution of shell structure. One of spin-gap isomers in $^{94}$Ag was reported to have large prolate deformation. (3) This mass region is on the way of the rapid proton (rp) synthesis pass. Recently, neutrino reactions in the super novae were reported to play a role of the synthesis of the rp-process nuclei. In the case of no path or slow down of rp process, isomers could contribute to synthesis of rp-nuclei with larger $Z$, although the production rates of isomers are small. [Preview Abstract] |
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