Bulletin of the American Physical Society
2011 Fall Meeting of the APS Division of Nuclear Physics
Volume 56, Number 12
Wednesday–Saturday, October 26–29, 2011; East Lansing, Michigan
Session GG: Astrophysics III: r-process, s-process |
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Chair: John Ullmann, Los Alamos National Laboratory Room: 105AB |
Friday, October 28, 2011 8:30AM - 8:42AM |
GG.00001: First experimental determination of the $^{59}$Fe($n,\gamma_0$)$^{60}$Fe reaction via Coulomb dissociation E. Uberseder, T. Heftrich, M. Heil, J. Marganiec, R. Reifarth, M. Wiescher The nucleosynthesis of $^{60}$Fe is one of the current outstanding problems in nuclear astrophysics. Observations of galactic radioactivity by $\gamma$-ray telescopes have provided a direct measurement of the $^{60}$Fe/$^{26}$Al$^{g}$ ratio dispersed across the galactic plane. As the two isotopes are produced in similar stellar environments, the ratio provides a unique constraint on current stellar models. Specifically, $^{60}$Fe is created and destroyed by neutron capture on stable iron isotopes. A recent measurement of the $^{60}$Fe(n,$\gamma$)$^{61}$Fe reaction has provided a first experimental quantification of the destruction rate. Currently, no experimental data exist for the $^{59}$Fe(n,$\gamma$)$^{60}$Fe production rate. To address this void, a Coulomb dissociation experiment has been performed at GSI to indirectly measure the ground state neutron capture cross section of $^{59}$Fe. The $^{60}$Fe beam was produced by fragmentation of a 660 AMeV primary $^{64}$Ni beam by a Be target. The $^{60}$Fe fragments were separated using the FRS and impinged on a lead target. The experimental setup provides for an event-by-event reconstruction of the four-momenta of all incoming particles and reaction products. The analysis is currently ongoing, and preliminary results will be discussed. [Preview Abstract] |
Friday, October 28, 2011 8:42AM - 8:54AM |
GG.00002: Neutron Capture from $^{87}$Sr G. Rusev, R. Raut, A.P. Tonchev, W. Tornow, B. Baramsai, J.H. Kelley, G. Mitchell, T. Bredeweg, A. Couture, M. Jandel, J. O'Donnell, R. Rundberg, J.L. Ullmann, A. Chyzh, E. Kwan The neutron-capture resonances of the reaction $^{87}$Sr$(n,\gamma)^{88}$Sr are significant to nuclear astrophysics to estimate the neutron density during the $s$ process, whose path is split by the branching nucleus $^{85}$Kr, and for a possible use of the $^{87}$Rb-$^{87}$Sr chronometric pair to measure the age of our Galaxy. In addition, the $\gamma$ rays of the product nucleus $^{88}$Sr are of importance to nuclear structure and the study of the pygmy resonance observed earlier in $(\gamma,\gamma')$ measurements. We report results from a neutron-capture experiment on $^{87}$Sr carried out with the $4\pi$ BaF$_2$ array, DANCE, at LANL. Spin values of neutron resonances have been deduced using the multiplicity and angular distributions of the cascade $\gamma$ rays following the neutron capture. [Preview Abstract] |
Friday, October 28, 2011 8:54AM - 9:06AM |
GG.00003: Stellar Neutron Sources and s-Process in Massive Stars R. Talwar, G.P.A. Berg, T. Adachi, M. Couder, H. Fujita, J. Gorres, M.N. Harakeh, K. Hatanaka, T. Ito, A. Long, A. Matic, H. Matsubara, J. Matta, M. Nagashima, T. Ogura, D. Patel, Y. Sakemi, H. Schatz, Y. Shimbara, Y. Shimizu, T. Suzuki, A. Tamii, T. Wakasa, M. Wiescher, M. Yosoi Potential stellar neutron sources for the s-process in massive stars are associated with $\alpha $-capture reactions on light nuclei. The capture-reaction rates provide the reaction flow for the buildup of the neutron sources $^{22}$Ne, and $^{26}$Mg during the helium-burning phase in stars. A critical influence on these reactions is expected to come from low-energy resonances at stellar energies between 300 keV and 1500 keV. These resonances are characterized by a pronounced cluster structure near the $\alpha $-threshold. Direct measurements of capture reactions to study the cluster structure are handicapped by the Coulomb barrier and limited detector resolutions. Hence, inelastic $\alpha $-scattering on these nuclei has been used as an alternative tool to probe into the level structure. In reference to this, the experiment performed using the Grand Raiden Spectrometer at RCNP, Osaka will be discussed and preliminary results will be presented. [Preview Abstract] |
Friday, October 28, 2011 9:06AM - 9:18AM |
GG.00004: Impact of beta-decay rate uncertainties on the slow neutron capture process in massive stars Marco Pignatari, Mary Beard, Michael Wiescher, Raphael Hirschi The slow neutron capture process in massive stars is mostly activated during He-burning and C-burning phases. Most of the abundances of elements such as copper or germanium, which have been observed in both the Solar System and Galaxy, are produced in these conditions, together with most of the s-process isotopes between iron and strontium (60 $<$ A $<$ 90). Nucleosynthesis predictions from stellar models depend on the nuclear physics networks used in simulations. A key ingredient in these simulations are the beta-decay and electron capture rates. In particular, the s-process yields beyond iron are affected by the present uncertainties associated with these rates. We aim to present the impact of these rate uncertainties on s-process calculations, focusing the discussion on few examples relevant for their astrophysical impact. [Preview Abstract] |
Friday, October 28, 2011 9:18AM - 9:30AM |
GG.00005: Development of AMS procedure for measurement of $^{93}$Zr Wenting Lu, Philippe Collon, Yoav Kashiv, Matthew Bowers, Daniel Robertson, Christopher Schmitt The procedure for measuring $^{93}$Zr (t$_{1/2}$ = 1.5 Ma) by AMS is currently being developed at the Nuclear Science Lab at the University of Notre Dame and we report on first experiments performed in this direction. AMS detection of $^{93}$Zr can potentially be applied to address astrophysical and environmental issues: (1) the measurement of the $^{92}$Zr(n,$\gamma )^{93}$Zr reaction cross-section at nucleosynthesis $s$-process relevant temperatures, (2) the search for potential live $^{93}$Zr from a supernova in deep sea sediments, (3) hydrological and radioactive waste tracing. The measurement of $^{93}$Zr requires adequate separation from its stable isobar $^{93}$Nb. We are currently working on optimizing this separation by using the GasFilled Magnet technique with additional multiple dE measurements in a focal plane ionization chamber. [Preview Abstract] |
Friday, October 28, 2011 9:30AM - 9:42AM |
GG.00006: Double magicity of N=Z nuclei near the rp-process path discerned Madan M. Sharma, Jagdish K. Sharma We have investigated the experimental isotope shifts in Kr nuclei near the proton drip-line within the framework of the deformed relativistic Hartree-Bogoliubov theory. In this work, we have attempted to answer the question as to why the charge radius of $^{72}$Kr shrinks significantly as against its expected swelling in approaching the proton drip line. It is shown that it happens due to the shell closure observed at N=Z=36 in deformed space, which compactifies the charge radius of $^{72}$Kr. Consequently, we have discerned that N=Z rp-process nuclei $^{68}$Se, $^{72}$Kr, $^{76}$Sr and$^{ 80}$Zr exhibit shell closures both at the proton and neutron numbers in the deformed space with the consequence that pairing correlations for protons and neutrons vanish. This lends a double magicity to these nuclei. Thus, N=Z rp-process waiting-point nuclei are shown to exhibit a magic character similar to that shown by the r-process waiting-point nuclei in the neutron-rich region. A significant number of nuclei in rp-process region are also shown to exhibit neutron magicity at N= 34, 36, 38, and 40 in the deformed space. [Preview Abstract] |
Friday, October 28, 2011 9:42AM - 9:54AM |
GG.00007: Formation of the Rare Earth Peak: Gaining Insight Into Late-Time r-Process Dynamics Matthew Mumpower, Gail McLaughlin, Rebecca Surman The classical r-process is thought to be responsible for approximately half of the neutron-rich elements above iron. While many studies of r-process environments have focused on early time behavior, eg. conditions for sufficient neutron-to-seed ratio, less effort has been made studying late-time r-process dynamics. We study the formation and evolution of the rare earth peak which occurs as matter decays back to stability. We show that the rare earth peak is sensitive to the interplay between thermodynamic evolution and nuclear physics input. We highlight the late-time dynamical behavior which is critical for peak formation and show that the final structure of the rare earth abundances depends most strongly on the nuclear physics input. We identify neutron capture rates which are critical to rare earth peak formation. These nuclei lie within 10-15 neutrons from stability. [Preview Abstract] |
Friday, October 28, 2011 9:54AM - 10:06AM |
GG.00008: $\beta$-decay and neutron emission studies of r-process nuclei near $^{137}$Sb Karl Smith, F. Attallah, T. Faestermann, U. Giesen, H. Geissel, M. Hannawald, M. Hausmann, M. Hellstroem, R. Kessler, K.-L. Kratz, Y. Litvinov, H. Mahmud, M.N. Mineva, F. Montes, G. Muenzenberg, B. Pfeiffer, J. Pereira Conca, P. Santi, H. Schatz, C. Scheidenberger, K. Schmidt, R. Schneider, A. Stolz, K. Suemmerer, J. Stadlmann The $\beta$-decays of very neutron rich nuclides in the A=130 region, including the astrophysically relevant nuclei $^{137}$Sb, were studied experimentally at the Helmholtzzentrum f\"{u}r Schwerionenforchung (GSI) using a stack of four 500~$\mu$m thick double-sided silicon strip detectors in conjunction with the Mainz $4\pi$ neutron long counter detector. The system allows the time correlation of ion implant and decay events and the detection of neutrons emitted during the decay. We measured half-lives and branchings for $\beta$-delayed neutron emission ($P_n$ values) for a number of nuclei in the region. The impact of our results on various types of models for the astrophysical rapid neutron capture process (r-process) is explored. [Preview Abstract] |
Friday, October 28, 2011 10:06AM - 10:18AM |
GG.00009: Results of precision mass measurements from CARIBU with the CPT J. Van Schelt, D. Lascar, G. Savard, J.A. Clark, J.P. Greene, A.F. Levand, T. Sun, B.J. Zabransky, S. Caldwell, M.G. Sternberg, A. Chaudhuri, K.S. Sharma, G. Li An array of neutron-rich nuclides from the CAlifornium Rare Isotope Breeder Upgrade (CARIBU) at ANL beyond $^{132}$Sn has been subjected to precision mass measurements with the Canadian Penning Trap mass spectrometer, including many never-before-measured nuclides. Neutron-separation energies calculated directly from these results provide essential input to models of the astrophysical $r$-process. Trends in binding energies far from stability provide input to nuclear mass models and identify regions of deformation. Additional nuclear structure information can be extracted from symmetry energy and observations of isomeric states. Implications for all of these topics will be discussed as well as future plans with the more intense CARIBU source. [Preview Abstract] |
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