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 HG: Astrophysics IV: alpha-Reactions, Capture Rates, etc. |
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Chair: Joe Natowitz, Texas A&M University Room: 105AB |
Friday, October 28, 2011 10:30AM - 10:42AM |
HG.00001: First experimental results of the $^{33}$S($\alpha$,p)$^{36}$Cl cross section for production in the early Solar System Matthew Bowers, Philippe Collon, Yoav Kashiv, William Bauder, Karen Chamberlin, Wenting Lu, Daniel Robertson, Christopher Schmitt The existence in the early Soalr System (ESS) of the now extinct $^{36}$Cl (t$_{1/2}$ = 3.01 $\times$ 10$^5$ yr) has been determined from correlation between isotopic enrichment of its daughter isotope, $^{36}$S, and Cl abundance in meteorites. The relatively high inferred initial Solar System $^{36}$Cl/Cl ratio strongly suggests that $^{36}$Cl was produced in the ESS by bombardment of solar energetic particles on gas and dust in the protoplanetary disc. However, no experimental data are currently available for the relevant production reactions cross sections. Instead, models of ESS production use Hauser-Feshbach approximations. The $^{33}$S($\alpha$,p)$^{36}$Cl reaction is calculated to have the largest cross section at bombardment energies $<$ 5 MeV/A. Here we report first results of a measurement of the averaged reaction cross section in the energy range 1.93- 1.95 MeV/A. Our result, 191 $\pm$ 33 mb (1$\sigma$), is significantly higher than results of previous calculations, 102 and 34 mb. [Preview Abstract] |
Friday, October 28, 2011 10:42AM - 10:54AM |
HG.00002: Experimental results for studies of the $^{40}$Ca($\alpha $,$\gamma )^{44}$Ti reaction rates Daniel Robertson, Hans-Werner Becker, Matt Bowers, Philippe Collon, Joachim Goerres, Wenting Lu, Chris Schmitt, Michael Wiescher Observational studies of galactic $\gamma $ emitters such as $^{44}$Ti have highlighted their use in nucleosynthesis studies of massive stars in their late stage stellar evolution and final explosive demise in core collapse supernova events. Models used in the simulation of such $\gamma $ emitters rely heavily upon reliable reaction rates for both the creation and annihilation of these isotopes over large temperature ranges. The production of $^{44}$Ti mainly through the $^{40}$Ca($\alpha $,$\gamma )^{44}$Ti reaction is thought to take place primarily in the $\alpha $-rich freeze out phase of a core collapse supernova. However, current supernova models predict lower $^{44}$Ti to $^{56}$Ni ratios than observed, creating a need for more information about its production mechanism. A number of previous studies include prompt $\gamma $-ray measurements, recoil mass separator experiments and the use of AMS, all giving greatly different reaction rates. Aiding in the refinement of these needed rates, the results of experiments at the DTL, Bochum and NSL, Notre Dame will be presented against the backdrop of these previous measurements. [Preview Abstract] |
Friday, October 28, 2011 10:54AM - 11:06AM |
HG.00003: Effects of Initial Composition on A=28-48 Reaction Flows in Thermonuclear Supernovae David A. Chamulak, Edward F. Brown, Alan C. Calder, Aaron P. Jackson, Brendan K. Krueger, F.X. Timmes, Dean M. Townsley Type Ia supernovae (SNe Ia) are the main distance indicator for cosmological studies and a primary source of the iron peak elements in the solar system. However the progenitor systems for this type of supernovae remain loosely understood. Numerical modeling can now probe the connection between the properties of the progenitor and the outcome of the explosion. We have performed numerical calculations to examine the nucleosynthesis in SNe Ia. Detailed yields resulting from explosive burning of the carbon/oxygen plasma in our models are examined using post-processing through a 532-nuclide reaction network. We explore how the production of elements from silicon to titanium varies with the composition of the progenitor star. Our calculations identify the reactions that most effect the final yields. These yields may be observable, allowing nuclear physics to constrain the astrophysical scenario. [Preview Abstract] |
Friday, October 28, 2011 11:06AM - 11:18AM |
HG.00004: Systematic study of alpha-optical potential near the Z=50 region for p-process Wanpeng Tan, A. Palumbo, A. Best, M. Couder, J. deBoer, S. Falahat, J. G{\"o}rres, P. LeBlanc, H. Lee, S. O'Brien, E. Strandberg, M. Wiescher, J. Greene, Zs. F{\"u}l{\"o}p, Gy. Gy{\"u}rky, G. Kiss, E. Somorjai, G. Efe, R. G{\"u}ray, N. {\"O}zkan Production of proton-rich elements beyond iron in stars proceeds via p-process, i.e., a sequence of photo-disintegration reactions, ($\gamma$,n), ($\gamma$,p), and ($\gamma$,$\alpha$) on heavy elements at temperatures of 2-3 x 10$^9$ K. The involved reaction rates are typically calculated with the statistical Hauser-Feshbach (HF) model. However, the HF model performs poorly in calculating the critical ($\gamma$,$\alpha$) rates due to the uncertainty of the alpha optical potentials applied. To test the reliability of the HF calculations and provide a systematic understanding of the alpha optical potential at energies of astrophysical interest, a series of precision alpha scattering measurements were carried out at the Notre Dame FN tandem accelerator. Specifically, $^{106}$Cd, $^{118}$Sn, and $^{120,124,126,128,130}$Te were studied at energies both below and above the Coulomb barrier. The derived potential was applied for calculating the $\alpha$-induced reaction rates on these nuclei using the CIGAR code. The results were compared to the corresponding experimental rates obtained via activation experiments at Notre Dame and other places. [Preview Abstract] |
Friday, October 28, 2011 11:18AM - 11:30AM |
HG.00005: Correcting for $\beta $-Summing in $\beta $-delayed Proton Detection with Double-Sided Silicon Strip Detectors Zachary Meisel, H. Crawford, G. Grinyer, G. Lorusso, P. Mantica, M. del Santo, H. Schatz In studies of $\beta $-delayed proton emission, detection of the proton energy is often of primary interest. However, once deposited in a thick double-sided silicon strip detector, the parent nucleus emits a positron and proton nearly simultaneously. Simulations must be used to determine the amount of energy the positron contributed to the overall energy detected to allow an accurate extraction of the proton energy. We solve this issue, which we call the $\beta $-summing effect using GEANT4 for the parent nuclei Mg-20, Si-23, and Kr-69. We find the positron's contribution can significantly impact peak energy for a decay event, leading to an incorrect determination of proton energy. The $\beta $-summing primarily depends on implantation depth and weakly on the $\beta $-decay's Q-value. We also find the shape of the summing peak can be used to constrain the parent nucleus's implantation depth. [Preview Abstract] |
Friday, October 28, 2011 11:30AM - 11:42AM |
HG.00006: Photonuclear and radiative capture reaction rates for Nuclear Astrophysics Mary Beard, S. Frauendorf, B Kaempfer, R. Scwengner, M. Wiescher The vast majority of nuclei heavier than iron are synthesisized via the capture of neutrons. There are however 35 naturally occurring nuclei, including isotopes of Mo and La, located on the neutron-deficient size of the valley of stability. It has been proposed that these nuclei, referred to as p-nuclei, are produced via sequential photo-dissociation reactions in the oxygen-neon shell burning regions of a pre-supernova star.~ As such, cross sections for p-nuclei production are particularly sensitive to the gamma-ray strength function, which, though dominated by the giant dipole resonance, may contain extra strength contributions near to the neutron threshold. Recently new ($\gamma $, $\gamma $') cross section measurements have been performed at the ELBE facility at Helmholtz-Zentrum Dresden-Rossendorf for the nuclei $^{92-100}$Mo, $^{88}$Sr, $^{90}$Zr and $^{139}$La probing the photo-absorption cross section over an energy range~ 4.5 - 6 MeV, up to the neutron separation threshold. The use of these measurements as a test of existing gamma-ray strength function models, and the consequent impact on p-nuclei production rates, will be discussed. [Preview Abstract] |
Friday, October 28, 2011 11:42AM - 11:54AM |
HG.00007: $\alpha$-induced cross section of $^{120}$Te for the astrophysical p process S.R. Lesher, A. Arend, A. Aprahamian, S. Almaraz-Calderon, A. Kontos, W.P. Tan, R.T. G\"{u}ray, N. \"{O}zkan There are about 35 nuclei found in nature, which are not susceptible to neutron capture and are explained by the p-process. The modeling for this process requires thousands of nuclear reactions involving both stable and unstable nuclei including ($\alpha, \alpha$), ($\alpha, p$) and ($\alpha, \gamma$) reactions. In a recent experiment, the cross section of the reaction $^{120}$Te($\alpha,p$)$^{123}$I was measured in the energy range of astrophysical interest for the p-process. The $\alpha$ beam from the Notre Dame FN Tandem Van de Graaff accelerator bombarded highly enriched self-supporting $^{120}$Te targets and the $\gamma$-rays from the activated $^{123}$I was counted with a pair of Ge clover detectors in close geometry. Preliminary results will be presented. [Preview Abstract] |
Friday, October 28, 2011 11:54AM - 12:06PM |
HG.00008: ABSTRACT WITHDRAWN |
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