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 RQ: Mini-Symposium: New Insights into Fission I |
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Chair: Nicolas Schunck, LLNL |
Sunday, November 1, 2020 8:30AM - 9:06AM |
RQ.00001: The role of fission in understanding the astrophysical origin of the heaviest elements Invited Speaker: Nicole Vassh The heaviest elements are understood to be synthesized by the rapid neutron capture process ($r$ process), but the astrophysical site(s) which host the conditions capable of reaching the heaviest species remains an open question. While there is observational evidence for the synthesis of lanthanides in neutron star mergers, whether heavier elements such as gold, platinum, and the actinides are produced in mergers remains uncertain. Signatures of fission in observables such as abundance patterns and kilonova signals can potentially illuminate the ultimate reach of the $r$ process in candidate astrophysical events. We will review recent work which aims to identify such signatures and discuss their dependence on unknown fission properties of exotic neutron-rich nuclei. [Preview Abstract] |
Sunday, November 1, 2020 9:06AM - 9:18AM |
RQ.00002: Effects of angular momentum in fission Ramona Vogt, Jorgen Randrup In certain environments, both in nature and in laboratory-created systems, there is a finite probability for neutron-induced fission of an excited state, particularly for target nuclei with low-lying isomeric states. We study whether fission and the subsequent de-excitation of the fission fragments from such long-lived isomers result in observable consequences. We focus on possible effects of fission from the first excited state of $^{235}$U, with compound nuclear spin of $J = 0,1$, relative to fission from the ground state, with spin $J=3,4$. We employ the complete event fission model FREYA because we can specify the initial angular momentum of the system. We find that the effects due to the spin of the initial state are less important than the angular momentum imparted to the fragments during scission. The effect of most observables on the magnitude of the initial angular momentum imparted to the system is generally on the few percent level. There is a modest anisotropy in neutron emission that is independent of the value of $J$. We also propose a new method of studying neutron-photon correlations that shows some sensitivity to the angular momentum. [Preview Abstract] |
Sunday, November 1, 2020 9:18AM - 9:30AM |
RQ.00003: Identification of Key Isotopes in Kilonova Heating Kelsey Lund, Yonglin Zhu, Jennifer Barnes, Trevor Sprouse, Nicole Vassh, Gail McLaughlin, Matthew Mumpower, Rebecca Surman The rapid neutron capture process (r-process) is one of the main mechanisms whereby elements heavier than iron are synthesized, and is responsible for the creation of the heaviest stable actinides. Observations of the gravitational wave event GW170817, and its optical counterpart, AT2017gfo, support neutron star mergers as an r-process production site. Accurately and reliably modelling yields and observational signatures from these sites requires inputs from nuclear physics, which introduce potentially large uncertainties. We use nucleosynthesis modeling to evaluate the effect of varying these inputs, including different nuclear mass models, fission decay rates, and daughter product distributions in lanthanide and actinide production. I show that applying different nuclear physics inputs generates discrepancies in abundances of key isotopes which contribute significantly to the overall nuclear energy generation in the merger event, which is a necessary component of kilonova light curve modeling. [Preview Abstract] |
Sunday, November 1, 2020 9:30AM - 9:42AM |
RQ.00004: Fission fragment yield trends Matthew Mumpower Nuclear fission, or the splitting of heavy nucleus into lighter fragments, is a crucial process in many modern applications and scientific pursuits. The prediction of trends in the fragment yields that arise across the chart of nuclides are of particular interest for the rapid neutron capture process of nucleosynthesis as this information dictates the placement of recycled material in lower mass regions. I will discuss recent advances of fission modeling within the theoretical macroscopic-microscopic framework and highlight our most recent calculations. [Preview Abstract] |
Sunday, November 1, 2020 9:42AM - 9:54AM |
RQ.00005: A re-evaluation of the energy released in fission that is converted into heat in a nuclear reactor Alejandro Sonzogni, Elizabeth McCutchan, Andrea Mattera A number of authors have studied the problem of calculating the energy released following the neutron-induced fission that can be converted into heat in a nuclear reactor. These energy values are needed to estimate a nuclear reactor fission rate from its thermal power value. In particular, for nuclear reactor antineutrino experiments, these values are of relevance to normalize the measured IBD spectrum and yield. In this work we will focus on a consistent and well-documented method to obtain above mentioned energies for the thermal neutron induce fission of $^{\mathrm{235}}$U, $^{\mathrm{239}}$Pu and $^{\mathrm{241}}$Pu, and for fast neutrons on $^{\mathrm{238}}$U. The latest available fission yield data and models are used, and realistic estimates of their uncertainties are obtained. Results will be compared with those from the 2004 work of Kopeikin \textit{et al}. and the 2013 work of Ma \textit{et al}. [Preview Abstract] |
Sunday, November 1, 2020 9:54AM - 10:06AM |
RQ.00006: Advances in $^{239}$Pu($n$,$f$) Prompt Fission Neutron Spectrum Measurements from the Chi-Nu Experiment Keegan Kelly, Matthew Devlin, John O'Donnell, Denise Neudecker, Robert Haight, Jaime Gomez, Terry Taddeucci, Ching-Yen Wu, Roger Henderson, Jack Henderson, Amy Lovell, Nikolas Fotiades, John Ullmann, Toshihiko Kawano, Shea Mosby, Morgan White, Hye Young Lee, Patrick Talou The prompt fission neutron spectrum (PFNS) is of vital importance for understanding the behavior of systems driven by neutron-induced fission. The $^{239}$Pu($n$,$f$) PFNS was recently measured at LANL with the Chi-Nu experimental approach in which results from a Li-glass and from a liquid scintillator array were combined to form a single measurement from 10 keV to 10 MeV outgoing neutron energy for each of 20 incident neutron energy ranges from 1-20 MeV. Data analysis incorporated a highly-detailed covariance assessment, including correlations between all 1300 reported PFNS data points. These correlations allowed for the determination of the covariance matrix of the mean spectrum energies as a function incident neutron energy, which had never before been reported for an experimental PFNS measurement. Clear observations of second-chance fission and pre-equilibrium neutron emission was observed, though third-chance fission is much less prominent than predicted in ENDF/B-VIII.0 and other evaluations. [Preview Abstract] |
Sunday, November 1, 2020 10:06AM - 10:18AM |
RQ.00007: the sreft (spatially resolving fission tracker) time projection chamber Esther Leal Cidoncha, Christopher Prokop, Kyle Schmitt A new fission tracking detector SREFT is being developed at Los Alamos Neutron Science CEnter (LANSCE). It will offer the possibility to measure neutron beam imaging and flux monitoring, Fission Fragments Total Kinetic Energy (TKE) for hot samples and Fission Product Yields (FPY), Minor actinide fission Cross Section ratios, Fission Fragment Angular Distributions and anisotropies, and (n,$\alpha$) and (n,x$\alpha$) reactions. This design builds from the current FissionTPC used at LANL, with a focus on fission fragments. Without the requirement to detect recoil protons, this device can operate with modest segmentation (187 channels per anode). This leads to several simplifications and improvements for detector operation. The detector can also operate with gas pressures close to atmospheric, making it possible to use a thin-walled pressure vessel. Its small size allows it to be used for supporting measurements inside or in parallel with another detector. Placing it inside the DANCE detector it can be used to measure (n,f) reactions, this is required to study the fission gammas and subtract them from the capture gammas to study (n,$\gamma$) cross sections. It could also provide a measurement of the beam profile and flux in support for LENZ, that is used to measure (n,z) reactions. [Preview Abstract] |
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