Bulletin of the American Physical Society
2020 Fall Meeting of the APS Division of Nuclear Physics
Volume 65, Number 12
Thursday–Sunday, October 29–November 1 2020; Time Zone: Central Time, USA
Session LD: Nuclear Astrophysics III |
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Chair: Stephanie Lyons, PNNL |
Saturday, October 31, 2020 10:30AM - 10:42AM |
LD.00001: Measurement of low-energy resonances in the $^{18}$O($\alpha$,$\gamma$)$^{22}$Ne reaction Alexander Dombos, Daniel Robertson, Thomas Kadlecek, Manoel Couder, Joachim Görres, Mark Hanhardt, Rebeka Kelmar, Orlando Olivas-Gomez, Anna Simon, Ed Stech, Frank Strieder, Michael Wiescher The $^{18}$O($\alpha$,$\gamma$)$^{22}$Ne reaction is part of a reaction chain that produces the $^{22}$Ne($\alpha$,$n$) neutron source for the slow neutron-capture process. However, the astrophysically relevant resonances in the $^{18}$O($\alpha$,$\gamma$)$^{22}$Ne reaction are difficult to measure due to their small resonance strengths. An experiment optimized for background reduction and detection efficiency was recently performed to measure the resonance strengths of these low-energy resonances. The experiment was performed at the Sanford Underground Research Facility (SURF), in the 4850-feet underground cavern dedicated to the Compact Accelerator System for Performing Astrophysical Research (CASPAR). The experimental end station used the $\gamma$-summing High Efficiency Total Absorption Spectrometer (HECTOR). Preliminary results from this experiment will be presented. [Preview Abstract] |
Saturday, October 31, 2020 10:42AM - 10:54AM |
LD.00002: Measurement of low-energy resonances in the $^{22}$Ne($\alpha$,n)$^{25}$Mg Reaction Thomas Kadlecek, Frank Strieder, Daniel Robertson, Mark Hanhardt, Tyler Borgwardt, Manoel Couder, Michael Wiescher The $^{22}$Ne($\alpha$,n)$^{25}$Mg reaction is a key neutron source for the slow neutron-capture process. The reaction rate at stellar energies is most likely dominated by a resonance at 832 keV which experimental strength determination carries a large uncertainty. Due to low resonance strengths at lower energies only upper limits have been previously determined for resonances below 832 keV. Recent measurements were completed for the strengths of those low-energy resonances. These measurements utilized the Compact Accelerator System for Performing Astrophysical Research (CASPAR), located on the 4850-feet level of the Sanford Underground Research Facility (SURF). Preliminary results will be presented. [Preview Abstract] |
Saturday, October 31, 2020 10:54AM - 11:06AM |
LD.00003: Measurement of the $^{25}Mg($\alpha$,n)$^{28}$Si reaction cross-section at low energy Shahina Shahina, A. Boeltzig, R.J. deBoer, K.T. Macon, M. Wiescher, M. Febbraro, R. Toomey There is uncertainty regarding the available neutron flux for the weak s-process in massive stars. In order to correctly model the s-process nucleosynthesis, one key ingredient is the rate of neutron producing reactions. The $^{22}$Ne$(\alpha,n)^{25}$Mg is the main neutron source, but other reactions also contribute. In the present work we study one such reaction, namely $^{25}$Mg$(\alpha,n)^{28}$Si, which acts as a potential neutron source for weak s-process and destroys the strongest neutron poison $^{25}$Mg. Previous measurements for this reaction suffered from two shortcomings: they were not performed at low enough energies relevant for weak s-process in massive stars and measurements using neutron counters were hindered by the contamination of targets with lower Z material. In this work, we used two different setups consisting of deuterated liquid scintillator detectors for neutrons and LaBr$_3$ for $\gamma$-rays in order to measure the $^{25}$Mg$(\alpha,n)^{28}$Si cross-section in the Gamow range 1.4-2.6 MeV. The neutron spectroscopy was performed via neutron spectrum unfolding technique which allows for a clear separation of the signal and the background. Preliminary results, including cross-sections determined from gamma-ray and neutron spectroscopy will be presented. [Preview Abstract] |
Saturday, October 31, 2020 11:06AM - 11:18AM |
LD.00004: Commissioning of HECTOR at CASPAR: $^{27}$Al$(p,\gamma)^{28}$Si resonance strength measurements 4,850 feet underground Orlando Olivas-Gomez, Dan Robertson, Alex Dombos, Anna Simon, Rebeka Kelmar, Tom Kadlecek, Joachim Goerres, Mark Hanhardt, Edward Stech, Frank Strieder, Manoel Couder, Michael Wiescher The High Efficiency Total Absorption Spectrometer (HECTOR), is a $4\pi$ $\gamma$-summing detector which specializes in measuring radiative-capture cross sections --- e.g. $(p, \gamma)$, $(\alpha, \gamma)$ --- for reactions related to astrophysical processes. Recently, HECTOR was been moved to the Compact Accelerator System for Performing Astrophysical Research (CASPAR) laboratory, which is located at the Sanford Underground Research Facility, 4850-feet underground. The underground environment provides an optimal background shielding needed to study several radiativate-capture processes at low energies related to the s-process. The commissioning of HECTOR at CASPAR, along with several measurements of resonance strengths below 1 MeV for the $^{27}$Al$(p,\gamma)^{28}$Si reaction will be presented. [Preview Abstract] |
Saturday, October 31, 2020 11:18AM - 11:30AM |
LD.00005: Neutron-Capture Cross Section Constraints for i-process Nucleosynthesis Andrea L. Richard, Sean N. Liddick, Artemis Spyrou, Alexander C. Dombos Neutron-capture nucleosynthesis occurs via a variety of processes depending on the astrophysical sites and conditions. Recent observations and stellar evolution models suggest that an intermediate process, known as the i-process, exists between the s- and r-processes, and is necessary to explain abundances in the Ge-Te region. The abundance patterns of i-process nuclei are greatly impacted by neutron-capture rates. Direct neutron-capture measurements are only feasible for long-lived nuclei, while for short-lived nuclei, indirect techniques are required. One such technique is the $\beta$-Oslo method in which the nuclear level density (NLD) and $\gamma$-strength function ($\gamma$SF) are extracted following the $\beta$-decay of a neutron-rich parent and are used in a statistical reaction model to constrain the neutron-capture cross section. In this work, $^{103,104}$Mo were studied at the NSCL via the $\beta$-decay of $^{103,104}$Nb and detected using the Summing NaI (SuN) detector. Results on the NLD, $\gamma$SF, neutron-capture cross sections, and reaction rates of $^{102}$Mo(n,$\gamma$)$^{103}$Mo and $^{103}$Mo(n,$\gamma$)$^{104}$Mo using the $\beta$-Oslo method, and i-process network calculations from the Nucleosynthesis Grid (NuGrid) Collaboration will be presented. [Preview Abstract] |
Saturday, October 31, 2020 11:30AM - 11:42AM |
LD.00006: Pycnonuclear Fusion and the Shallow Heat Source in Accreting Neutron Star Crusts R. Jain, H. Schatz, R. Lau, M. Beard, S. S. Gupta, A. V. Afanesjev, E. F. Brown, A. Deibel, L. R. Gasques, G. W. Hitt, W. R. Hix, L. Keek, P. Moller, P. S. Shternin, A. W. Steiner, M. Wiescher, Y. Xu Observing X-rays during quiescence from transiently accreting neutron stars provide unique clues about the nature of dense matter. Current models of neutron star crust, however, systematically predict lower temperatures than those observed, and an artificial shallow heat source is required to account for the observations. It has been previously proposed that the shallow heat source could be of nuclear origin, particularly the fusion of lighter elements in the crust. The pycnonuclear fusion reaction rates implemented in our models have large uncertainties spanning several orders of magnitude. We present the sensitivity studies of these pycnonuclear fusion reactions in realistic network calculations and also study their impact on the neutron star cooling curves in quiescence. Although we see a shallower deposition of nuclear heat when pycnonuclear fusion reaction rates are enhanced, we eliminate the possibility that the problem of shallow heating could be attributed to the uncertainties in pycnonuclear fusion reaction rates. [Preview Abstract] |
Saturday, October 31, 2020 11:42AM - 11:54AM |
LD.00007: Mass Measurements of Neutron-Rich Indium Isotopes for Enhanced $r$-Process Studies and Developments for the TITAN Penning Trap Chris Izzo, Eleanor Dunling, Gabriella Kripko-Koncz, Marilena Lykiardopoulou, William S. Porter, Timo Dickel, Iris Dillmann, Ania Kwiatkowski TRIUMF's Ion Trap for Atomic and Nuclear science (TITAN) is among the world leaders in achieving precise mass measurements of exotic nuclei. The TITAN Measurement Penning Trap (MPET) has been used for more than a decade, and the recent addition of a Multiple-Reflection Time-of-Flight Mass Spectrometer (MR-TOF-MS) has expanded the capabilities at TITAN. The TITAN MR-TOF-MS was recently used to measure the masses of neutron-rich indium isotopes in the $N=$82, $Z=$50 region, which is crucially sensitive for the astrophysical $r$-process. Indium masses from $A=$125-134 were measured, including the first ever mass measurements of $^{\mathrm{133,134}}$In. Several isomer masses with half-lives as short as 5 ms were resolved from the ground states as well. These results will be presented with a discussion of their impact for the $r$-process. Additionally, MPET has recently undergone developments for improved measurements of highly charged radioactive ions. These developments include the addition of a cryogenic system to improve the vacuum quality and upgrades to allow mass measurements by Phase-Imaging Ion-Cyclotron-Resonance with MPET, which will allow a substantial increase in mass precision and resolution. A brief overview of these recent MPET developments will be presented. [Preview Abstract] |
Saturday, October 31, 2020 11:54AM - 12:06PM |
LD.00008: Gamma rays from neutron star mergers: a unique signature of r process Xilu Wang, Nicole Vassh, Matthew Mumpower, Trevor Sprouse, Rebecca Surman, Ramona Vogt, Jorgen Randrup Neutron star mergers (NSMs) are one site for rapid neutron capture (r process) nucleosynthesis, which are verified by the multi-messenger observations of the event GW170817. The optical and infrared signals from the event indicated lanthanide production from a NSM. NSM could also emit gamma rays from the heavy isotopes synthesized through the r process in the neutron-rich ejecta. The gamma ray signal may provide a unique probe of the NSM environment as well insight into the nature of the r process. We simulate the gamma-ray light curves and spectra from a NSM event with different astrophysical and nuclear conditions and find that the NSM gamma ray signals encode the ejecta properties and r process features like neutron richness and heavy isotope yields. We also test the detectability of nearby NSMs by making comparisons to the sensitivity limit of the next generation gamma ray detector AMEGO. [Preview Abstract] |
Saturday, October 31, 2020 12:06PM - 12:18PM |
LD.00009: Which Neutron Star Mergers Synthesized the r-Process Elements? Erika Holmbeck The astrophysical origin of the heaviest elements made by rapid neutron-capture (the $r$-process) is unknown, though neutron star mergers (NSMs) are strong possible candidates. Metal-poor stars enhanced with these elements provide some basis of comparison with nucleosynthesis model yields, such as that from NSMs. Currently, many theoretical $r$-process studies take this route of comparing individual model results to observed stellar abundances. However, we take the opposite approach in a way that has so far not been accomplished in literature; we use the observed abundances of metal-poor stars themselves to reconstruct properties of the progenitor neutron star binaries that would have merged to produce those elements. We will present the results of this new analysis and comment on whether the predicted binary systems agree with present-day Galactic populations of neutron stars. We also explore the effect that the nuclear Equation of State (EOS) has on our results and if metal-poor stars can provide an additional, indirect EOS constraints. [Preview Abstract] |
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