Bulletin of the American Physical Society
APS April Meeting 2014
Volume 59, Number 5
Saturday–Tuesday, April 5–8, 2014; Savannah, Georgia
Session J6: Mini-Symposium on Nuclear Physics: Sensitive Input for Understanding Nucleosynthesis I |
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Sponsoring Units: DNP Chair: Kelly Chipps, University of Tennessee and Oak Ridge National Laboratory Room: 200 |
Sunday, April 6, 2014 10:45AM - 11:21AM |
J6.00001: The sensitivity of $r$-process nucleosynthesis to individual nuclear properties Invited Speaker: Rebecca Surman Calculations of rapid neutron capture, or $r$-process, nucleosynthesis require nuclear data for thousands of nuclei far from stability. We currently have experimental information for only a handful of these nuclei, though many more neutron-rich species are within the reach of current and next generation experimental facilities. Sensitivity studies are one way to get at which of these thousands of nuclear properties are the most crucial to measure for the $r$ process. Our $r$-process sensitivity studies examine the roles of individual nuclear masses, beta decay rates, neutron capture rates, and beta-delayed neutron emission probabilities in $r$-process simulations in a variety of potential astrophysical environments. Here we will point out the pieces of nuclear data with the greatest impact on the final $r$-process abundance pattern and describe the mechanisms by which this influence occurs. [Preview Abstract] |
Sunday, April 6, 2014 11:21AM - 11:33AM |
J6.00002: An astrophysical engine that stores gravitational work as nuclear Coulomb energy Donald Clayton I describe supernovae gravity machines that store large internal nuclear Coulomb energy, 0.80Z$^{\mathrm{2}}$A$^{\mathrm{-1/3}}$MeV per nucleus. Excess of it is returned later by electron capture and positron emission. Decay energy manifests as (1) observable gamma-ray lines (2) light curves of supernovae (3) chemical energy of free carbon dissociated from CO molecules (4) huge abundances of radiogenic daughters. I illustrate by rapid silicon burning, a natural epoch in SN II. Gravitational work produces the high temperatures that photoeject nucleons and alpha particles from heavy nuclei. These are retained by other nuclei to balance photoejection rates (quasiequilibrium). The abundance distribution adjusts slowly as remaining abundance of Z$=$N $^{\mathrm{28}}$Si decomposes, so p, n, $\alpha $ recaptures hug the Z$=$N line. This occurs in milliseconds, too rapidly for weak decay to alter bulk Z/N ratio. The figure displays those quasiequilibrium abundances color-coded to their decays. Z$=$N$=$2k nuclei having k\textless 11 are stable, whereas k\textgreater 10 are radioactive owing to excess Coulomb energy. Weak decays radiate that excess energy weeks later to fuel the four macroscopic energetic phenomena cited. How startling to think of the Coulomb nuclear force as storing cosmic energy and its weak decay releasing macroscopic activation to SNII. [Preview Abstract] |
Sunday, April 6, 2014 11:33AM - 11:45AM |
J6.00003: Benchmarks of results obtained in the new finite-range droplet model Peter Moller The FRDM(1992) mass table has an accuracy of 0.669 MeV respect to a 1989 mass evaluation. The FRDM(2012) has an accuracy of 0.5595 MeV with respect to the AME2003 evaluation. There are several reasons for this improvement. A few are: 1) we calculate the potential energy in a 4D deformation space with densely spaced grid points, 2) we include axial asymmetry, and 3) we have improved the calculation of ground-state zero-point energies, A brief summary is in Phys.\ Rev.\ Lett.\ {\bf 108} (2012) 052501. Locally, substantial improvements are achieved, mainly in regions of shape coexistence. A troublesome staggering in the neutron separation energies in FRDM(1992) has almost disappeared. The $Q_{\alpha}$ values compare very well with experimental data up to $Z=118$, which are very far (about 40 units in $A$) from the data to which the model was adjusted. This may bode well for reliability in the perpendicular direction towards very neutron-rich nuclei. We compare calculated masses, $\beta$-decay half-lives, $\beta$-delayed neutron-emission probabilities, ground-state spins and other quantities to experimental data. Also of interest to nucleosynthesis studies are our calculated fission-barrier heights and fission-fragment mass distributions in the heavy r-process region. [Preview Abstract] |
Sunday, April 6, 2014 11:45AM - 11:57AM |
J6.00004: Effective Reaction Rates (ERR) for the Helium Burning Reactions Sam M. Austin, Christopher West, Alexander Heger Simulations of helium burning in presupernova stars are subject to uncertainties in the rates of both the triple alpha and $^{12}$C($\alpha,\gamma$) reactions and to approximations in the simulation itself, particularly in the treatment of convection. We have attempted to treat this problem in a consistent manner by introducing ''Effective Reaction Rates'' (ERR) for the two reactions, their parameters fixed by requiring that they reproduce the production of the intermediate mass and s-only isotopes. The process is based upon a data base of $2112$ simulations (West et al., ApJ {\bf 769}, 2 (2013)) in which the two rates are varied by $\pm 2\sigma$ for a set of $12$ stars with masses from $12-30 M_{\odot}$. We find that the abundances are well reproduced for ERR lying along a line $r_{\alpha, \gamma} = r_{3\alpha} + 0.35$. It is a test of the ERR procedure that the ERRs reproduce a variety of observables. For points along the ERR line, the central C fraction at the end of helium burning, the remnant mass after the SN explosion, and the yields of the neutrino isotopes have constant values. [Preview Abstract] |
Sunday, April 6, 2014 11:57AM - 12:09PM |
J6.00005: Sensitive r-process nuclei production at Notre Dame Maxime Brodeur Abundance calculations of the astrophysical rapid-neutron capture process, which is responsible for the synthesis of about half of the elements heavier than iron requires precise and accurate knowledge of ground state properties of neutron-rich nuclei. These sensitive quantities are often uncertain or unmeasured and must be calculated using phenomenological nuclear models. This lack of data is due to a combination of the minute production of these exotic nuclei and a lack of available experimental time. Indeed, all the relevant experimental efforts currently take place a reduced number of large user facilities where strong experimental time competition put a constraint on the number of measurements that can be performed yearly. To mitigate the situation, we propose the implementation of a dedicated radioactive ion beam facility at the University of Notre Dame. Neutron-rich nuclei will be produced in an element-independent manner by the proton-induced fission of actinide targets following the IG-ISOL method. This new facility will not only provide needed radioactive ion beams for research, but will also help reinforce the development of the future scientific workforce. [Preview Abstract] |
Sunday, April 6, 2014 12:09PM - 12:21PM |
J6.00006: $\beta$-decay studies of very neutron-rich Pd and Ag isotopes Karl Smith The rapid-neutron capture process (r-process) is attributed as the source of nearly half the elements heavier than iron. To gain insight into the r-process nucleosynthesis, uncertainties such as the nuclear physics involved must be minimized. An experiment was performed to measure properties of neutron-rich nuclei just below the $N=82$ shell closure believed to be responsible for production of the $A=130$ peak in the solar r-process abundance pattern. $\beta$-decay half-lives and neutron branching ratios, $P_n$ values, were measured for Pd and Ag isotopes at the GSI Fragment Separator (FRS). The FRS provided in-flight separation and identification of fission fragments produced by a 900 MeV/u $^{238}$U beam. Ions of interest were implanted in the Silicon Implantation detector and Beta Absorber (SIMBA) array. The large pixelation of the array allowed for position-time correlation between implants and the corresponding $\beta$-decays. The parent nucleus may decay to an excited state in the daughter, above the neutron separation energy emitting a neutron. These neutrons were moderated and detected in Beta deLayEd Neutron (BELEN) detector surrounding SIMBA. Resulting analysis of half-lives and neutron emission branching ratios including a time-dependent background will be presented. [Preview Abstract] |
Sunday, April 6, 2014 12:21PM - 12:33PM |
J6.00007: Validation of (d,p$\gamma$) as a Surrogate for (n,$\gamma$) A. Ratkiewicz, J.A. Cizewski, A. Adekola, S. Burcher, M.E. Howard, B. Manning, S.L. Rice, C. Shand, J.T. Burke, R.J. Casperson, N.D. Scielzo, R.A.E. Austin, N. Fotiades, R.O. Hughes, T.J. Ross, M. McCleskey, S.D. Pain, W.A. Peters The abundance pattern of nuclei produced in the stellar r-process may be impacted by the rates at which participating exotic nuclei capture neutrons at late times in the process. These neutron capture rates are difficult or impossible to measure directly; therefore a surrogate method to constrain them must be identified. The low-energy (d,p) transfer reaction is a promising candidate for a surrogate, as it shares many characteristics (such as low angular momentum transfer) with the neutron capture reaction. We report on a campaign to validate (d,p$\gamma$) as a surrogate for (n,$\gamma$) using $^{95}$Mo as a target and focusing on excitations in $^{96}$Mo near the neutron separation energy. We will present preliminary results from completed measurements and plans to extend the campaign to an inverse kinematics measurement of $^{95}$Mo(d,p$\gamma$) with techniques being developed for radioactive ion beams. This work was supported in part by the U.S. DOE and the NSF. [Preview Abstract] |
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