Showing 1–50 of 60 results for author: Mumpower, M R

ENDF/B-VIII.1: Updated Nuclear Reaction Data Library for Science and Applications

Authors: G. P. A. Nobre, R. Capote, M. T. Pigni, A. Trkov, C. M. Mattoon, D. Neudecker, D. A. Brown, M. B. Chadwick, A. C. Kahler, N. A. Kleedtke, M. Zerkle, A. I. Hawari, C. W. Chapman, N. C. Fleming, J. L. Wormald, K. Ramić, Y. Danon, N. A. Gibson, P. Brain, M. W. Paris, G. M. Hale, I. J. Thompson, D. P. Barry, I. Stetcu, W. Haeck , et al. (84 additional authors not shown)

Abstract: The ENDF/B-VIII.1 library is the newest recommended evaluated nuclear data file by the Cross Section Evaluation Working Group (CSEWG) for use in nuclear science and technology applications, and incorporates advances made in the six years since the release of ENDF/B-VIII.0. Among key advances made are that the $^{239}$Pu file was reevaluated by a joint international effort and that updated… ▽ More The ENDF/B-VIII.1 library is the newest recommended evaluated nuclear data file by the Cross Section Evaluation Working Group (CSEWG) for use in nuclear science and technology applications, and incorporates advances made in the six years since the release of ENDF/B-VIII.0. Among key advances made are that the $^{239}$Pu file was reevaluated by a joint international effort and that updated $^{16,18}$O, $^{19}$F, $^{28-30}$Si, $^{50-54}$Cr, $^{55}$Mn, $^{54,56,57}$Fe, $^{63,65}$Cu, $^{139}$La, $^{233,235,238}$U, and $^{240,241}$Pu neutron nuclear data from the IAEA coordinated INDEN collaboration were adopted. Over 60 neutron dosimetry cross sections were adopted from the IAEA's IRDFF-II library. In addition, the new library includes significant changes for $^3$He, $^6$Li,$^9$Be, $^{51}$V, $^{88}$Sr, $^{103}$Rh, $^{140,142}$Ce, Dy, $^{181}$Ta, Pt, $^{206-208}$Pb, and $^{234,236}$U neutron data, and new nuclear data for the photonuclear, charged-particle and atomic sublibraries. Numerous thermal neutron scattering kernels were reevaluated or provided for the very first time. On the covariance side, work was undertaken to introduce better uncertainty quantification standards and testing for nuclear data covariances. The significant effort to reevaluate important nuclides has reduced bias in the simulations of many integral experiments with particular progress noted for fluorine, copper, and stainless steel containing benchmarks. Data issues hindered the successful deployment of the previous ENDF/B-VIII.0 for commercial nuclear power applications in high burnup situations. These issues were addressed by improving the $^{238}$U and $^{239,240,241}$Pu evaluated data in the resonance region. The new library performance as a function of burnup is similar to the reference ENDF/B-VII.1 library. The ENDF/B-VIII.1 data are available in ENDF-6 and GNDS format at https://doi.org/10.11578/endf/2571019. △ Less

Submitted 5 November, 2025; originally announced November 2025.

Comments: Article associated with the ENDF/B-VIII.1 release, submitted to Nuclear Data Sheets and currently under second round of referee review. 222 pages, 61 tables, 227 figures

Paths to Superheavy Nuclei

Authors: K. Godbey, F. M. Nunes, M. Albertsson, K. J. Cook, J. M. Gates, K. Hagel, K. Hagino, M. Kowal, Jin Lei, J. Lubian, A. Makowski, P. McGlynn, M. R. Mumpower, W. Nazarewicz, G. Potel, J. L. Pore, J. Rangel, K. Sekizawa, A. S. Umar

Abstract: This document summarizes the discussions and outcomes of the Facility for Rare Isotope Beams Theory Alliance (FRIB-TA) topical program "The path to Superheavy Isotopes" held in June 2024 at FRIB. Its content is non-exhaustive, reflecting topics chosen and discussed by the participants. The program aimed to assess the current status of theory in superheavy nuclei (SHN) research and identify necessa… ▽ More This document summarizes the discussions and outcomes of the Facility for Rare Isotope Beams Theory Alliance (FRIB-TA) topical program "The path to Superheavy Isotopes" held in June 2024 at FRIB. Its content is non-exhaustive, reflecting topics chosen and discussed by the participants. The program aimed to assess the current status of theory in superheavy nuclei (SHN) research and identify necessary theoretical developments to guide experimental programs and determine fruitful production mechanisms. This report details the intersection of SHN research with other fields, provides an overview of production mechanisms and theoretical models, discusses future needs in theory and experiment, explores other potential avenues for SHN synthesis, and highlights the importance of building a strong theory community in this area. △ Less

Submitted 23 October, 2025; originally announced October 2025.

Report number: KUNS-3075

Gamma-Ray Spectra of $R$-Process Nuclei

Authors: Axel Gross, Samuel Cupp, Matthew R Mumpower

Abstract: The radioactive decay of unstable nuclei created in the rapid neutron capture process release a large amount of $γ$-rays. When the ejecta is optically thick, these $γ$-rays may contribute to an associated kilonova. Once transparent, prominent spectral features will be directly observable in current and future $γ$-ray detectors. In this work, we study and compare the $γ$-ray spectra of a limited, w… ▽ More The radioactive decay of unstable nuclei created in the rapid neutron capture process release a large amount of $γ$-rays. When the ejecta is optically thick, these $γ$-rays may contribute to an associated kilonova. Once transparent, prominent spectral features will be directly observable in current and future $γ$-ray detectors. In this work, we study and compare the $γ$-ray spectra of a limited, weak, strong, and extended $r$-process across a broad timescale, identifying the nuclei which significantly contribute. We discuss these findings in the context of observability, noting that there are several practical challenges to connecting observed signatures to specific nuclei. However, if these challenges can be overcome, direct observation of $γ$-rays from $r$-process sites can provide insight into the fundamental physics underpinning the $r$-process. △ Less

Submitted 9 October, 2025; originally announced October 2025.

Comments: 40 pages, 7 figures, 1 table, submitted to APJS

Report number: LA-UR 25-29994

Kilonovae and Long-duration Gamma-ray Bursts

Authors: Marko Ristić, Brandon L. Barker, Samuel Cupp, Axel Gross, Nicole Lloyd-Ronning, Oleg Korobkin, Jonah M. Miller, Matthew R. Mumpower

Abstract: Recent detections of kilonova-like emission following long-duration gamma-ray bursts GRB211211A and GRB230307A have been interpreted as originating from the merger of two neutron stars. In this work, we demonstrate that these observations are also consistent with nucleosynthesis originating from a collapsar scenario. Our model accurately predicts the observed optical and infrared light curves usin… ▽ More Recent detections of kilonova-like emission following long-duration gamma-ray bursts GRB211211A and GRB230307A have been interpreted as originating from the merger of two neutron stars. In this work, we demonstrate that these observations are also consistent with nucleosynthesis originating from a collapsar scenario. Our model accurately predicts the observed optical and infrared light curves using a single weak $r$-process component. The absence of lanthanide-rich material in our model, consistent with the data, challenges the prevailing interpretation that a red evolution in such transients necessarily indicates the presence of heavy $r$-process elements. △ Less

Submitted 3 September, 2025; originally announced September 2025.

Comments: Submitted to ApJL; comments welcome!

Report number: LA-UR-25-28887

Multi-messengers from the radioactive decay of $r$-process nuclei

Authors: Axel Gross, Samuel Cupp, Matthew R. Mumpower

Abstract: The radioactive $β$-decay of nuclei synthesized in the rapid neutron capture process ($r$-process) releases a variety of particles, including electrons, $γ$-rays, neutrinos, and neutrons. These particles provide a rich set of multi-messenger signals that carry information about the astrophysical environments where neutron-rich nucleosynthesis occurs. In this work, we calculate from first principle… ▽ More The radioactive $β$-decay of nuclei synthesized in the rapid neutron capture process ($r$-process) releases a variety of particles, including electrons, $γ$-rays, neutrinos, and neutrons. These particles provide a rich set of multi-messenger signals that carry information about the astrophysical environments where neutron-rich nucleosynthesis occurs. In this work, we calculate from first principles the emission spectra resulting from the $β$-decay of $r$-process nuclei. Our approach incorporates detailed nuclear structure and decay data to model the energy distributions of each particle species. We couple the spectra with a nuclear reaction network simulation to obtain the temporal evolution of these distributions. We find that the emission distributions vary significantly in time and are non-thermal, with substantial average energies. We investigate these nuclear signals as a direct probe of heavy element formation and show that they are complementary observables to kilonova. △ Less

Submitted 29 August, 2025; originally announced September 2025.

Comments: 10 pages, 5 figures, submitted to ApJL

Report number: LA-UR-25-28481

Reducing parametric uncertainties through information geometry methods

Authors: M. Imbrišak, A. E. Lovell, M. R. Mumpower

Abstract: Information geometry is a study of applying differential geometry methods to challenging statistical problems, such as uncertainty quantification. In this work, we use information geometry to study how measurement uncertainties in pre-neutron emission mass distributions affect the parameter estimation in the Hauser-Feshbach fission fragment decay code, CGMF. We quantify the impact of reduced uncer… ▽ More Information geometry is a study of applying differential geometry methods to challenging statistical problems, such as uncertainty quantification. In this work, we use information geometry to study how measurement uncertainties in pre-neutron emission mass distributions affect the parameter estimation in the Hauser-Feshbach fission fragment decay code, CGMF. We quantify the impact of reduced uncertainties on the pre-neutron mass yield of specific masses to these parameters, for spontaneous fission of ${}^{252}$Cf, first using a toy model assuming Poissonian uncertainties, then an experimental measurement taken from Göök et al., 2014 in EXFOR. We achieved a reduction of up to $\sim15\%$ in CGMF parameter errors, predominantly in $w_0^{(1)}$ and $w_1^{(0)}$. △ Less

Submitted 12 September, 2025; v1 submitted 26 August, 2025; originally announced August 2025.

Comments: 10 pages, 11 figures, updated email address

An optical-lensing inspired data thinning method for nuclear cross section data

Authors: M. Imbrišak, A. E. Lovell, M. R. Mumpower

Abstract: In the study of nuclear cross sections, the computational demands of data assimilation methods can become prohibitive when dealing with large data sets. We have developed a novel variant of the data thinning algorithm, inspired by the principles of optical lensing, which effectively reduces data volume while preserving critical information. We show how it improves fitting through a toy problem and… ▽ More In the study of nuclear cross sections, the computational demands of data assimilation methods can become prohibitive when dealing with large data sets. We have developed a novel variant of the data thinning algorithm, inspired by the principles of optical lensing, which effectively reduces data volume while preserving critical information. We show how it improves fitting through a toy problem and for several examples of total cross sections for neutron-induced reactions on rare-earth isotopes. We demonstrate how this method can be applied as an efficient pre-processing step prior to smoothing, significantly improving computational efficiency without compromising the quality of uncertainty quantification. △ Less

Submitted 12 September, 2025; v1 submitted 26 August, 2025; originally announced August 2025.

Comments: 10 pages, 6 figures, updated email address

Weighted Levenberg-Marquardt methods for fitting multichannel nuclear cross section data

Authors: M. Imbrišak, A. E. Lovell, M. R. Mumpower

Abstract: We present an extension of the Levenberg-Marquardt algorithm for fitting multichannel nuclear cross section data. Our approach offers a practical and robust alternative to conventional trust-region methods for analyzing experimental data. The CoH$_3$ code, based on the Hauser-Feshbach statistical model, involves a large number of interdependent parameters, making optimization challenging due to th… ▽ More We present an extension of the Levenberg-Marquardt algorithm for fitting multichannel nuclear cross section data. Our approach offers a practical and robust alternative to conventional trust-region methods for analyzing experimental data. The CoH$_3$ code, based on the Hauser-Feshbach statistical model, involves a large number of interdependent parameters, making optimization challenging due to the presence of "sloppy" directions in parameter space. To address the uneven distribution of experimental data across reaction channels, we construct a weighted Fisher Information Metric by integrating prior distributions over dataset weights. This framework enables a more balanced treatment of heterogeneous data, improving both parameter estimation and convergence robustness. We show that the resulting weighted Levenberg-Marquardt method yields more physically consistent fits for both raw and smoothed datasets, using experimental data for ${}^{148}$Sm as a representative example. Additionally, we introduce a geometric scaling strategy to accelerate convergence -- a method based on the local geometry of the manifold. △ Less

Submitted 12 September, 2025; v1 submitted 26 August, 2025; originally announced August 2025.

Comments: 14 pages, 10 figures, changed email address

Nuclear $β^{-}$-decay with statistical de-excitation

Authors: M. R. Mumpower, T. Kawano, O. Korobkin, G. W. Misch, T. M. Sprouse

Abstract: The accurate description of nuclear $β^{-}$-decay has far-reaching consequences for applications spanning nuclear reactors to the creation of heavy elements in astrophysical environments. We present the nuclear particle spectra associated with the $β$-decay of neutron-rich nuclei calculated with the well benchmarked coupled Quasi-particle Random Phase Approximation and Hauser-Feshbach (QRPA+HF) mo… ▽ More The accurate description of nuclear $β^{-}$-decay has far-reaching consequences for applications spanning nuclear reactors to the creation of heavy elements in astrophysical environments. We present the nuclear particle spectra associated with the $β$-decay of neutron-rich nuclei calculated with the well benchmarked coupled Quasi-particle Random Phase Approximation and Hauser-Feshbach (QRPA+HF) model. This approach begins with the population of the daughter nucleus via semi-microscopic Gamow-Teller or First-Forbidden strength distributions (QRPA) and follows the statistical de-excitation (HF) until the initial available excitation energy is exhausted. At each stage of de-excitation the emission by neutrons and $γ$-rays is considered obeying quantum mechanical selection rules. For completeness we also provide parsed Auger and Internal Conversion (IC) electron spectra from Evaluated Nuclear Data Files (ENDF). Our results are tabulated and provided in parsable ASCII formatted tables that are suitable for inclusion in various applications. △ Less

Submitted 4 July, 2025; originally announced July 2025.

Comments: 18 pages of text; 150 page table; 7 figures; published in ADNDT

Multidisciplinary Science in the Multimessenger Era

Authors: Eric Burns, Christopher L. Fryer, Ivan Agullo, Jennifer Andrews, Elias Aydi, Matthew G. Baring, Eddie Baron, Peter G. Boorman, Mohammad Ali Boroumand, Eric Borowski, Floor S. Broekgaarden, Poonam Chandra, Emmanouil Chatzopoulos, Hsin-Yu Chen, Kelly A. Chipps, Francesca Civano, Luca Comisso, Alejandro Cárdenas-Avendaño, Phong Dang, Catherine M. Deibel, Tarraneh Eftekhari, Courey Elliott, Ryan J. Foley, Christopher J. Fontes, Amy Gall , et al. (60 additional authors not shown)

Abstract: Astrophysical observations of the cosmos allow us to probe extreme physics and answer foundational questions on our universe. Modern astronomy is increasingly operating under a holistic approach, probing the same question with multiple diagnostics including how sources vary over time, how they appear across the electromagnetic spectrum, and through their other signatures, including gravitational w… ▽ More Astrophysical observations of the cosmos allow us to probe extreme physics and answer foundational questions on our universe. Modern astronomy is increasingly operating under a holistic approach, probing the same question with multiple diagnostics including how sources vary over time, how they appear across the electromagnetic spectrum, and through their other signatures, including gravitational waves, neutrinos, cosmic rays, and dust on Earth. Astrophysical observations are now reaching the point where approximate physics models are insufficient. Key sources of interest are explosive transients, whose understanding requires multidisciplinary studies at the intersection of astrophysics, gravity, nuclear science, plasma physics, fluid dynamics and turbulence, computation, particle physics, atomic, molecular, and optical science, condensed matter and materials science, radiation transport, and high energy density physics. This white paper provides an overview of the major scientific advances that lay at the intersection of physics and astronomy and are best probed through time-domain and multimessenger astrophysics, an exploration of how multidisciplinary science can be fostered, and introductory descriptions of the relevant scientific disciplines and key astrophysical sources of interest. △ Less

Submitted 3 April, 2025; v1 submitted 5 February, 2025; originally announced February 2025.

Comments: This white paper is the product of the Third Time-Domain And Multimessenger Astrophysics workshop: Multidisciplinary Science in the Multimessenger Era, hosted by Louisiana State University with additional support from DOE, NSF, and NASA. Updated arXiv versions fix minor issues (authors, incorrect images)

Emulation of the final r-process abundance pattern with a neural network

Authors: Yukiya Saito, Iris Dillmann, Reiner Krücken, Matthew R. Mumpower, Rebecca Surman

Abstract: This work explores the construction of a fast emulator for the calculation of the final pattern of nucleosynthesis in the rapid neutron capture process (the $r$-process). An emulator is built using a feed-forward artificial neural network (ANN). We train the ANN with nuclear data and relative abundance patterns. We take as input the $β$-decay half-lives and the one-neutron separation energy of the… ▽ More This work explores the construction of a fast emulator for the calculation of the final pattern of nucleosynthesis in the rapid neutron capture process (the $r$-process). An emulator is built using a feed-forward artificial neural network (ANN). We train the ANN with nuclear data and relative abundance patterns. We take as input the $β$-decay half-lives and the one-neutron separation energy of the nuclei in the rare-earth region. The output is the final isotopic abundance pattern. In this work, we focus on the nuclear data and abundance patterns in the rare-earth region to reduce the dimension of the input and output space. We show that the ANN can capture the effect of the changes in the nuclear physics inputs on the final $r$-process abundance pattern in the adopted astrophysical conditions. We employ the deep ensemble method to quantify the prediction uncertainty of the neutal network emulator. The emulator achieves a speed-up by a factor of about 20,000 in obtaining a final abundance pattern in the rare-earth region. The emulator may be utilized in statistical analyses such as uncertainty quantification, inverse problems, and sensitivity analysis. △ Less

Submitted 23 December, 2024; originally announced December 2024.

Comments: 22 pages, 5 figures

Let there be neutrons! Hadronic photoproduction from a large flux of high-energy photons

Authors: Matthew R. Mumpower, Tsung-Shung H. Lee, Nicole Lloyd-Ronning, Brandon L. Barker, Axel Gross, Samuel Cupp, Jonah M. Miller

Abstract: We propose that neutrons may be generated in high-energy, high-flux photon environments via photo-induced reactions on pre-existing baryons. These photohadronic interactions are expected to occur in astrophysical jets and surrounding material. Historically, these reactions have been attributed to the production of high-energy cosmic rays and neutrinos. We estimate the photoproduction off of proton… ▽ More We propose that neutrons may be generated in high-energy, high-flux photon environments via photo-induced reactions on pre-existing baryons. These photohadronic interactions are expected to occur in astrophysical jets and surrounding material. Historically, these reactions have been attributed to the production of high-energy cosmic rays and neutrinos. We estimate the photoproduction off of protons in the context of gamma-ray bursts, where it is expected there will be sufficient baryonic material that may be encompassing or entrained in the jet. We show that typical stellar baryonic material, even material completely devoid of neutrons, can become inundated with neutrons in situ via hadronic photoproduction. Consequently, this mechanism provides a means for collapsars and other astrophysical sites containing substantial flux of high-energy photons to be favorable for neutron-capture nucleosynthesis. △ Less

Submitted 4 July, 2025; v1 submitted 18 November, 2024; originally announced November 2024.

Comments: Published version appearing in ApJ; fixed TSHL's acknowledgements

Report number: LA-UR-24-32250

Mass measurements of neutron-rich nuclides using the Canadian Penning Trap to inform predictions in the $r$-process rare-earth peak region

Authors: D. Ray, N. Vassh, B. Liu, A. A. Valverde, M. Brodeur, J. A. Clark, G. C. McLaughlin, M. R. Mumpower, R. Orford, W. S. Porter, G. Savard, K. S. Sharma, R. Surman, F. Buchinger, D. P. Burdette, N. Callahan, A. T. Gallant, D. E. M. Hoff, K. Kolos, F. G. Kondev, G. E. Morgan, F. Rivero, D. Santiago-Gonzalez, N. D. Scielzo, L. Varriano , et al. (3 additional authors not shown)

Abstract: Studies aiming to determine the astrophysical origins of nuclei produced by the rapid neutron capture process ($r$ process) rely on nuclear properties as inputs for simulations. The solar abundances can be used as a benchmark for such calculations, with the $r$-process rare-earth peak (REP) around mass number ($A$) 164 being of special interest due to its presently unknown origin. With the advance… ▽ More Studies aiming to determine the astrophysical origins of nuclei produced by the rapid neutron capture process ($r$ process) rely on nuclear properties as inputs for simulations. The solar abundances can be used as a benchmark for such calculations, with the $r$-process rare-earth peak (REP) around mass number ($A$) 164 being of special interest due to its presently unknown origin. With the advancement of rare isotope beam production over the last decade and improvement in experimental sensitivities, many of these REP nuclides have become accessible for measurement. Masses are one of the most critical inputs as they impact multiple nuclear properties, namely the neutron-separation energies, neutron capture rates, $β$-decay rates, and $β$-delayed neutron emission probabilities. In this work, we report masses of 20 neutron-rich nuclides (along the Ba, La, Ce, Pr, Nd, Pm, Gd, Dy and Ho isotopic chains) produced at the CAlifornium Rare Isotope Breeder Upgrade (CARIBU) facility at Argonne National Laboratory. The masses were measured with the Canadian Penning trap (CPT) mass spectrometer using the Phase-Imaging Ion-Cyclotron-Resonance (PI-ICR) technique. We then use these new masses along with previously published CPT masses to inform predictions for a Markov Chain Monte Carlo (MCMC) procedure aiming to identify the astrophysical conditions consistent with both solar data and mass measurements. We show that the MCMC responds to this updated mass information, producing refined results for both mass predictions and REP abundances. △ Less

Submitted 12 November, 2024; v1 submitted 9 November, 2024; originally announced November 2024.

Comments: 11 pages, 8 figures

Kilonova Emissions from Neutron Star Merger Remnants: Implications for Nuclear Equation of State

Authors: Kelsey A. Lund, Rahul Somasundaram, Gail C. McLaughlin, Jonah M. Miller, Matthew R. Mumpower, Ingo Tews

Abstract: Multi-messenger observation of binary neutron-star mergers can provide valuable information on the nuclear equation of state (EoS). Here, we investigate to which extent electromagnetic observations of the associated kilonovae allow us to place constraints on the EoS. For this, we use state-of-the-art three-dimensional general-relativistic magneto-hydrodynamics simulations and detailed nucleosynthe… ▽ More Multi-messenger observation of binary neutron-star mergers can provide valuable information on the nuclear equation of state (EoS). Here, we investigate to which extent electromagnetic observations of the associated kilonovae allow us to place constraints on the EoS. For this, we use state-of-the-art three-dimensional general-relativistic magneto-hydrodynamics simulations and detailed nucleosynthesis modeling to connect properties of observed light curves to properties of the accretion disk, and hence, the EoS. Using our general approach, we use multi-messenger observations of GW170817/AT2017gfo to study the impact of various sources of uncertainty on inferences of the EoS. We constrain the radius of a $\rm{1.4 M_\odot}$ neutron star to lie within $\rm{10.19\leq R_{1.4}\leq 13.0}$~km and the maximum mass to be $\rm{M_{TOV}\leq 3.06 M_\odot}$. △ Less

Submitted 20 August, 2025; v1 submitted 14 August, 2024; originally announced August 2024.

Comments: 12 pages, 4 figures

Report number: LA-UR-24-24836

Nuclear uncertainties associated with the ejecta of a neutron-star black-hole accretion disk

Authors: M. R. Mumpower, T. M. Sprouse, J. M. Miller, K. A. Lund, J. Cabrera Garcia, N. Vassh, G. C. McLaughlin, R. Surman

Abstract: The simulation of heavy element nucleosynthesis requires input from yet-to-be-measured nuclear properties. The uncertainty in the values of these off-stability nuclear properties propagates to uncertainties in the predictions of elemental and isotopic abundances. However, for any given astrophysical explosion, there are many different trajectories, i.e. temperature and density histories, experienc… ▽ More The simulation of heavy element nucleosynthesis requires input from yet-to-be-measured nuclear properties. The uncertainty in the values of these off-stability nuclear properties propagates to uncertainties in the predictions of elemental and isotopic abundances. However, for any given astrophysical explosion, there are many different trajectories, i.e. temperature and density histories, experienced by outflowing material and thus different nuclear properties can come into play. We consider combined nucleosynthesis results from 460,000 trajectories from a neutron star-black hole accretion disk and the find spread in elemental predictions due solely to unknown nuclear properties to be a factor of a few. We analyze this relative spread in model predictions due to nuclear variations and conclude that the uncertainties can be attributed to a combination of properties in a given region of the abundance pattern. We calculate a cross-correlation between mass changes and abundance changes to show how variations among the properties of participating nuclei may be explored. Our results provide further impetus for measurements of multiple quantities on individual short-lived neutron-rich isotopes at modern experimental facilities. △ Less

Submitted 3 April, 2024; originally announced April 2024.

Comments: 10 pages, 8 figures; comments welcome

Report number: LA-UR-24-22685

Astromers: Status and Prospects

Authors: G. Wendell Misch, Matthew R. Mumpower

Abstract: The extreme temperatures and densities of many astrophysical environments tends to destabilize nuclear isomers by inducing transitions to higher energy states. Those states may then cascade to ground. However, not all environments destabilize all isomers. Nuclear isomers which retain their metastable character in pertinent astrophysical environments are known as astrophysically metastable nuclear… ▽ More The extreme temperatures and densities of many astrophysical environments tends to destabilize nuclear isomers by inducing transitions to higher energy states. Those states may then cascade to ground. However, not all environments destabilize all isomers. Nuclear isomers which retain their metastable character in pertinent astrophysical environments are known as astrophysically metastable nuclear isomers, or "astromers". Astromers can influence nucleosynthesis, altering abundances or even creating new pathways that would otherwise be inaccessible. Astromers may also release energy faster or slower relative to their associated ground state, acting as heating accelerants or batteries, respectively. In stable isotopes, they may even simply remain populated after a cataclysmic event and emit observable x- or $γ$-rays. The variety of behaviors of these nuclear species and the effects they can have merit careful consideration in nearly every possible astrophysical environment. Here we provide a brief overview of astromers past and present, and we outline future work that will help to illuminate their role in the cosmos. △ Less

Submitted 28 March, 2024; v1 submitted 10 January, 2024; originally announced January 2024.

Comments: 39 pages(10 of the 16 of them references), 5 figures

Report number: LA-UR-24-20183

Element abundance patterns in stars indicate fission of nuclei heavier than uranium

Authors: Ian U. Roederer, Nicole Vassh, Erika M. Holmbeck, Matthew R. Mumpower, Rebecca Surman, John J. Cowan, Timothy C. Beers, Rana Ezzeddine, Anna Frebel, Terese T. Hansen, Vinicius M. Placco, Charli M. Sakari

Abstract: The heaviest chemical elements are naturally produced by the rapid neutron-capture process (r-process) during neutron star mergers or supernovae. The r-process production of elements heavier than uranium (transuranic nuclei) is poorly understood and inaccessible to experiments, so must be extrapolated using nucleosynthesis models. We examine element abundances in a sample of stars that are enhance… ▽ More The heaviest chemical elements are naturally produced by the rapid neutron-capture process (r-process) during neutron star mergers or supernovae. The r-process production of elements heavier than uranium (transuranic nuclei) is poorly understood and inaccessible to experiments, so must be extrapolated using nucleosynthesis models. We examine element abundances in a sample of stars that are enhanced in r-process elements. The abundances of elements Ru, Rh, Pd, and Ag (atomic numbers Z = 44 to 47, mass numbers A = 99 to 110) correlate with those of heavier elements (63 <= Z <= 78, A > 150). There is no correlation for neighboring elements (34 <= Z <= 42 and 48 <= Z <= 62). We interpret this as evidence that fission fragments of transuranic nuclei contribute to the abundances. Our results indicate that neutron-rich nuclei with mass numbers >260 are produced in r-process events. △ Less

Submitted 11 December, 2023; originally announced December 2023.

Comments: Authors' version of manuscript published in Science on December 07, 2023

Journal ref: Science, 382, 1177-1180 (2023)

Thallium-208: a beacon of in situ neutron capture nucleosynthesis

Authors: Nicole Vassh, Xilu Wang, Maude Lariviere, Trevor Sprouse, Matthew R. Mumpower, Rebecca Surman, Zhenghai Liu, Gail C. McLaughlin, Pavel Denissenkov, Falk Herwig

Abstract: We demonstrate that the well-known 2.6 MeV gamma-ray emission line from thallium-208 could serve as a real-time indicator of astrophysical heavy element production, with both rapid (r) and intermediate (i) neutron capture processes capable of its synthesis. We consider the r process in a Galactic neutron star merger and show Tl-208 to be detectable from ~12 hours to ~10 days, and again ~1-20 years… ▽ More We demonstrate that the well-known 2.6 MeV gamma-ray emission line from thallium-208 could serve as a real-time indicator of astrophysical heavy element production, with both rapid (r) and intermediate (i) neutron capture processes capable of its synthesis. We consider the r process in a Galactic neutron star merger and show Tl-208 to be detectable from ~12 hours to ~10 days, and again ~1-20 years post-event. Detection of Tl-208 represents the only identified prospect for a direct signal of lead production (implying gold synthesis), arguing for the importance of future MeV telescope missions which aim to detect Galactic events but may also be able to reach some nearby galaxies in the Local Group. △ Less

Submitted 17 November, 2023; originally announced November 2023.

Comments: accepted to PRL

Emergent nucleosynthesis from a 1.2 second long simulation of a black-hole accretion disk

Authors: T. M. Sprouse, K. A. Lund, J. M. Miller, G. C. McLaughlin, M. R. Mumpower

Abstract: We simulate a black-hole accretion disk system with full-transport general relativistic neutrino radiation magnetohydrodynamics (GR$ν$RMHD) for 1.2 seconds. This system is likely to form after the merger of two compact objects and is thought to be a robust site of $r$-process nucleosynthesis. We consider the case of a black-hole accretion disk arising from the merger of two neutron stars. Our simu… ▽ More We simulate a black-hole accretion disk system with full-transport general relativistic neutrino radiation magnetohydrodynamics (GR$ν$RMHD) for 1.2 seconds. This system is likely to form after the merger of two compact objects and is thought to be a robust site of $r$-process nucleosynthesis. We consider the case of a black-hole accretion disk arising from the merger of two neutron stars. Our simulation time coincides with the nucleosynthesis timescale of the $r$ process ($\sim$ 1 second). Because these simulations are time consuming, it is common practice to run for `short' duration of approximately 0.1 to 0.3 seconds. We analyze the nucleosynthetic outflow from this system and compare the results between stopping at 0.12 and 1.2 seconds respectively. We find that the addition of mass ejected in the longer simulation as well as more favorable thermodynamic conditions from emergent viscous ejecta greatly impacts the nucleosynthetic outcome. We quantify the error in nucleosynthetic outcomes between short and long cuts. △ Less

Submitted 14 September, 2023; originally announced September 2023.

Comments: 11 pages, 11 figures, comments welcome

Report number: LA-UR-23-28417

Uncertainty Quantification of Mass Models using Ensemble Bayesian Model Averaging

Authors: Yukiya Saito, Iris Dillmann, Reiner Kruecken, Matthew R. Mumpower, Rebecca Surman

Abstract: Developments in the description of the masses of atomic nuclei have led to various nuclear mass models that provide predictions for masses across the whole chart of nuclides. These mass models play an important role in understanding the synthesis of heavy elements in the rapid neutron capture ($r$-) process. However, it is still a challenging task to estimate the size of uncertainty associated wit… ▽ More Developments in the description of the masses of atomic nuclei have led to various nuclear mass models that provide predictions for masses across the whole chart of nuclides. These mass models play an important role in understanding the synthesis of heavy elements in the rapid neutron capture ($r$-) process. However, it is still a challenging task to estimate the size of uncertainty associated with the predictions of each mass model. In this work, a method to quantify the mass uncertainty using \textit{ensemble Bayesian model averaging} (EBMA) is introduced. This Bayesian method provides a natural way to perform model averaging, selection, calibration, and uncertainty quantification, by combining the mass models as a mixture of normal distributions, whose parameters are optimized against the experimental data, employing the Markov chain Monte Carlo (MCMC) method using the No-U-Turn sampler (NUTS). The average size of our best uncertainty estimates of neutron separation energies based on the AME2003 data is 0.48 MeV and covers 95% of new data in the AME2020. The uncertainty estimates can also be used to detect outliers with respect to the trend of experimental data and theoretical predictions. △ Less

Submitted 27 February, 2024; v1 submitted 2 May, 2023; originally announced May 2023.

Comments: 15 pages, 7 figures. Footnote removed from the previous version. Submitted to PRC

Report number: LA-UR-23-24410, P2400029

Executive Summary of the Topical Program: Nuclear Isomers in the Era of FRIB

Authors: G. W. Misch, M. R. Mumpower, F. G. Kondev, S. T. Marley, S. Almaraz-Calderon, M. Brodeur, B. A. Brown, M. P. Carpenter, J. J. Carroll, C. J. Chiara, K. A. Chipps, B. P. Crider, A. Gade, R. Grzywacz, K. L. Jones, B. P. Kay, K. Kolos, Yu. A. Litvinov, S. Lopez-Caceres, B. S. Meyer, K. Minamisono, G. E. Morgan, R. Orford, S. D. Pain, J. Purcell , et al. (7 additional authors not shown)

Abstract: We report on the Facility for Rare Isotope Beams (FRIB) Theory Alliance topical program "Nuclear Isomers in the Era of FRIB". We outline the many ways isomers influence and contribute to nuclear science and technology, especially in the four FRIB pillars: properties of rare isotopes, nuclear astrophysics, fundamental symmetries, and applications for the nation and society. We conclude with a resol… ▽ More We report on the Facility for Rare Isotope Beams (FRIB) Theory Alliance topical program "Nuclear Isomers in the Era of FRIB". We outline the many ways isomers influence and contribute to nuclear science and technology, especially in the four FRIB pillars: properties of rare isotopes, nuclear astrophysics, fundamental symmetries, and applications for the nation and society. We conclude with a resolution stating our recommendation that the nuclear physics community actively pursue isomer research. A white paper is forthcoming. △ Less

Submitted 20 April, 2023; originally announced April 2023.

Comments: 4 pages including references

Report number: LA-UR-23-22994

Bayesian averaging for ground state masses of atomic nuclei in a Machine Learning approach

Authors: M. R. Mumpower, M. Li, T. M. Sprouse, B. S. Meyer, A. E. Lovell, A. T. Mohan

Abstract: We present global predictions of the ground state mass of atomic nuclei based on a novel Machine Learning (ML) algorithm. We combine precision nuclear experimental measurements together with theoretical predictions of unmeasured nuclei. This hybrid data set is used to train a probabilistic neural network. In addition to training on this data, a physics-based loss function is employed to help refin… ▽ More We present global predictions of the ground state mass of atomic nuclei based on a novel Machine Learning (ML) algorithm. We combine precision nuclear experimental measurements together with theoretical predictions of unmeasured nuclei. This hybrid data set is used to train a probabilistic neural network. In addition to training on this data, a physics-based loss function is employed to help refine the solutions. The resultant Bayesian averaged predictions have excellent performance compared to the testing set and come with well-quantified uncertainties which are critical for contemporary scientific applications. We assess extrapolations of the model's predictions and estimate the growth of uncertainties in the region far from measurements. △ Less

Submitted 17 April, 2023; originally announced April 2023.

Comments: 15 pages, 10 figures, comments welcome

Report number: LA-UR-23-23224

Nucleosynthesis and observation of the heaviest elements

Authors: E. M. Holmbeck, T. M. Sprouse, M. R. Mumpower

Abstract: The rapid neutron capture or 'r process' of nucleosynthesis is believed to be responsible for the production of approximately half the natural abundance of heavy elements found on the periodic table above iron (with proton number $Z=26$) and all of the heavy elements above bismuth ($Z=83$). In the course of creating the actinides and potentially superheavies, the r process must necessarily synthes… ▽ More The rapid neutron capture or 'r process' of nucleosynthesis is believed to be responsible for the production of approximately half the natural abundance of heavy elements found on the periodic table above iron (with proton number $Z=26$) and all of the heavy elements above bismuth ($Z=83$). In the course of creating the actinides and potentially superheavies, the r process must necessarily synthesize superheavy nuclei (those with extreme proton numbers, neutron numbers or both) far from isotopes accessible in the laboratory. Many questions about this process remain unanswered, such as 'where in nature may this process occur?' and 'what are the heaviest species created by this process?' In this review, we survey at a high level the nuclear properties relevant for the heaviest elements thought to be created in the r process. We provide a synopsis of the production and destruction mechanisms of these heavy species, in particular the actinides and superheavies, and discuss these heavy elements in relation to the astrophysical r process. We review the observational evidence of actinides found in the Solar system and in metal-poor stars and comment on the prospective of observing heavy-element production in explosive astrophysical events. Finally, we discuss the possibility that future observations and laboratory experiments will provide new information in understanding the production of the heaviest elements. △ Less

Submitted 4 April, 2023; originally announced April 2023.

Comments: 63 pages, 17 figures

Journal ref: European Physical Journal A, 59, 28 (2023)

The Los Alamos evaluation of $^{239}$Pu neutron-induced reactions in the fast energy range

Authors: M. R. Mumpower, D. Neudecker, T. Kawano, M. Herman, N. Kleedtke, A. E. Lovell, I. Stetcu, P. Talou

Abstract: A major revision of the evaluation of $^{239}$Pu neutron-induced reaction cross sections is reported in the fast energy range. The evaluation starts at 2.5 keV incident neutron energy and has been extended up to 30 MeV. Several other notable changes are included in this evaluation since the release of ENDF/B-VIII.0 including the adoption of the Standards fission cross section, inclusion of new rad… ▽ More A major revision of the evaluation of $^{239}$Pu neutron-induced reaction cross sections is reported in the fast energy range. The evaluation starts at 2.5 keV incident neutron energy and has been extended up to 30 MeV. Several other notable changes are included in this evaluation since the release of ENDF/B-VIII.0 including the adoption of the Standards fission cross section, inclusion of new radiative capture data of Mosby et al., inclusion of the (n,2n) data of Meot et al., in addition to advances in the treatment of reaction modeling. In contrast to previous evaluation efforts, this evaluation is reproducible with detailed information stored chronologically utilizing a Git repository. The final evaluation results have been compiled into an ENDF-formatted file, which has been processed successfully through NJOY, checked for internal consistency, benchmarked versus older evaluations and validated against a suite of critical assemblies and pulsed-spheres. △ Less

Submitted 6 February, 2023; originally announced February 2023.

Comments: 31 pages, 31 figures, internal Los Alamos report

Report number: LA-UR-22-24121

Nuclear data activities for medium mass and heavy nuclei at Los Alamos

Authors: M. R. Mumpower, T. M Sprouse, T. Kawano, M. W. Herman, A. E. Lovell, G. W. Misch, D. Neudecker, H. Sasaki, I. Stetcu, P. Talou

Abstract: Nuclear data is critical for many modern applications from stockpile stewardship to cutting edge scientific research. Central to these pursuits is a robust pipeline for nuclear modeling as well as data assimilation and dissemination. We summarize a small portion of the ongoing nuclear data efforts at Los Alamos for medium mass to heavy nuclei. We begin with an overview of the NEXUS framework and s… ▽ More Nuclear data is critical for many modern applications from stockpile stewardship to cutting edge scientific research. Central to these pursuits is a robust pipeline for nuclear modeling as well as data assimilation and dissemination. We summarize a small portion of the ongoing nuclear data efforts at Los Alamos for medium mass to heavy nuclei. We begin with an overview of the NEXUS framework and show how one of its modules can be used for model parameter optimization using Bayesian techniques. The mathematical framework affords the combination of different measured data in determining model parameters and their associated correlations. It also has the advantage of being able to quantify outliers in data. We exemplify the power of this procedure by highlighting the recently evaluated 239-Pu cross section. We further showcase the success of our tools and pipeline by covering the insight gained from incorporating the latest nuclear modeling and data in astrophysical simulations as part of the Fission In R-process Elements (FIRE) collaboration. △ Less

Submitted 21 October, 2022; originally announced October 2022.

Comments: 6 pages, 5 figures, Nuclear Data (2022) conference proceedings. Comments welcome!

Report number: LA-UR-22-30993

Collective enhancement in the exciton model

Authors: M. R. Mumpower, D. Nuedecker, H. Sasaki, T. Kawano, A. E. Lovell, M. W. Herman, I. Stetcu, M. Dupuis

Abstract: The pre-equilibrium reaction mechanism is considered in the context of the exciton model. A modification to the one-particle one-hole state density is studied which can be interpreted as a collective enhancement. The magnitude of the collective enhancement is set by simulating the Lawrence Livermore National Laboratory (LLNL) pulsed-spheres neutron-leakage spectra. The impact of the collective enh… ▽ More The pre-equilibrium reaction mechanism is considered in the context of the exciton model. A modification to the one-particle one-hole state density is studied which can be interpreted as a collective enhancement. The magnitude of the collective enhancement is set by simulating the Lawrence Livermore National Laboratory (LLNL) pulsed-spheres neutron-leakage spectra. The impact of the collective enhancement is explored in the context of the highly deformed actinide, 239-Pu. A consequence of this enhancement is the removal of fictitious levels in the Distorted-Wave Born Approximation often used in modern nuclear reaction codes. △ Less

Submitted 21 October, 2022; originally announced October 2022.

Comments: 7 pages, 7 figures. Comments welcome!

Report number: LA-UR-22-30081

The Influence of Beta Decay Rates on r-Process Observables

Authors: Kelsey A. Lund, J. Engel, G. C. McLaughlin, M. R. Mumpower, E. M. Ney, R. Surman

Abstract: The rapid neutron capture process (r-process) is one of the main mechanisms whereby elements heavier than iron are synthesized, and is entirely responsible for the natural production of the actinides. Kilonova emissions are modeled as being largely powered by the radioactive decay of species synthesized via the r -process. Given that the r -process occurs far from nuclear stability, unmeasured bet… ▽ More The rapid neutron capture process (r-process) is one of the main mechanisms whereby elements heavier than iron are synthesized, and is entirely responsible for the natural production of the actinides. Kilonova emissions are modeled as being largely powered by the radioactive decay of species synthesized via the r -process. Given that the r -process occurs far from nuclear stability, unmeasured beta decay rates play an essential role in setting the time scale for the r -process. In an effort to better understand the sensitivity of kilonova modeling to different theoretical global beta-decay descriptions, we incorporate these into nucleosynthesis calculations. We compare the results of these calculations and highlight differences in kilonova nuclear energy generation and light curve predictions, as well as final abundances and their implications for nuclear cosmochronometry. We investigate scenarios where differences in beta decay rates are responsible for increased nuclear heating on time scales of days that propagates into a significantly increased average bolometric luminosity between 1-10 days post-merger. We identify key nuclei, both measured and unmeasured, whose decay rates are directly impact nuclear heating generation on timescales responsible for light curve evolution. We also find that uncertainties in beta decay rates significantly impact ages estimates from cosmochronometry. △ Less

Submitted 12 August, 2022; originally announced August 2022.

Report number: LA-UR 22-28160

Physically Interpretable Machine Learning for nuclear masses

Authors: M. R. Mumpower, T. M. Sprouse, A. E. Lovell, A. T. Mohan

Abstract: We present a novel approach to modeling the ground state mass of atomic nuclei based directly on a probabilistic neural network constrained by relevant physics. Our Physically Interpretable Machine Learning (PIML) approach incorporates knowledge of physics by using a physically motivated feature space in addition to a soft physics constraint that is implemented as a penalty to the loss function. W… ▽ More We present a novel approach to modeling the ground state mass of atomic nuclei based directly on a probabilistic neural network constrained by relevant physics. Our Physically Interpretable Machine Learning (PIML) approach incorporates knowledge of physics by using a physically motivated feature space in addition to a soft physics constraint that is implemented as a penalty to the loss function. We train our PIML model on a random set of $\sim$20\% of the Atomic Mass Evaluation (AME) and predict the remaining $\sim$80\%. The success of our methodology is exhibited by the unprecedented $σ_\textrm{RMS}\sim186$ keV match to data for the training set and $σ_\textrm{RMS}\sim316$ keV for the entire AME with $Z \geq 20$. We show that our general methodology can be interpreted using feature importance. △ Less

Submitted 20 March, 2022; originally announced March 2022.

Comments: 5 pages, 3 figures, comments welcome

Report number: LA-UR-22-21855

The need for a local nuclear physics feature in the neutron-rich rare-earths to explain solar $r$-process abundances

Authors: Nicole Vassh, Gail C. McLaughlin, Matthew R. Mumpower, Rebecca Surman

Abstract: We apply Markov Chain Monte Carlo to predict the masses required to form the observed solar $r$-process rare-earth abundance peak. Given highly distinct astrophysical outflows and nuclear inputs, we find that results are most sensitive to the $r$-process dynamics (i.e. overall competition between reactions and decays), with similar mass trends predicted given similar dynamics. We show that regardl… ▽ More We apply Markov Chain Monte Carlo to predict the masses required to form the observed solar $r$-process rare-earth abundance peak. Given highly distinct astrophysical outflows and nuclear inputs, we find that results are most sensitive to the $r$-process dynamics (i.e. overall competition between reactions and decays), with similar mass trends predicted given similar dynamics. We show that regardless of whether fission deposits into the rare-earths or not, our algorithm consistently predicts the need for a local nuclear physics feature of enhanced stability in the neutron-rich lanthanides. △ Less

Submitted 18 February, 2022; originally announced February 2022.

Beta-delayed fission in the coupled Quasi-particle Random Phase Approximation plus Hauser-Feshbach approach

Authors: M. R. Mumpower, T. Kawano, T. M. Sprouse

Abstract: Beta-delayed neutron emission and $β$-delayed fission ($β$df) probabilities were calculated for heavy, neutron-rich nuclei using the Los Alamos coupled Quasi-Particle Random Phase Approximation plus Hauser-Feshbach (QRPA+HF) approach. In this model, the compound nucleus is initially populated by $β$-decay and is followed through subsequent statistical decays taking into account competition between… ▽ More Beta-delayed neutron emission and $β$-delayed fission ($β$df) probabilities were calculated for heavy, neutron-rich nuclei using the Los Alamos coupled Quasi-Particle Random Phase Approximation plus Hauser-Feshbach (QRPA+HF) approach. In this model, the compound nucleus is initially populated by $β$-decay and is followed through subsequent statistical decays taking into account competition between neutrons, $γ$-rays and fission. The primary output of these calculations includes branching ratios along with neutron and $γ$-ray spectra. We find a relatively large region of heavy nuclides where the probability of $β$df is near 100%. For a subset of nuclei near the neutron dripline, delayed neutron emission and the probability to fission are both large which leads to the possibility of multi-chance $β$df (mc-$β$df). We comment on prospective neutron-rich nuclei that could be probed by future experimental campaigns and provide a full table of branching ratios in ASCII format in the supplemental material for use in various applications. △ Less

Submitted 8 January, 2022; originally announced January 2022.

Comments: 12 pages, 7 figures

Report number: LA-UR-21-31648

Nuclear masses learned from a probabilistic neural network

Authors: A. E. Lovell, A. T. Mohan, T. M. Sprouse, M. R. Mumpower

Abstract: Machine learning methods and uncertainty quantification have been gaining interest throughout the last several years in low-energy nuclear physics. In particular, Gaussian processes and Bayesian Neural Networks have increasingly been applied to improve mass model predictions while providing well-quantified uncertainties. In this work, we use the probabilistic Mixture Density Network (MDN) to direc… ▽ More Machine learning methods and uncertainty quantification have been gaining interest throughout the last several years in low-energy nuclear physics. In particular, Gaussian processes and Bayesian Neural Networks have increasingly been applied to improve mass model predictions while providing well-quantified uncertainties. In this work, we use the probabilistic Mixture Density Network (MDN) to directly predict the mass excess of the 2016 Atomic Mass Evaluation within the range of measured data, and we extrapolate the inferred models beyond available experimental data. The MDN not only provides mean values but also full posterior distributions both within the training set and extrapolated testing set. We show that the addition of physical information to the feature space increases the accuracy of the match to the training data as well as provides for more physically meaningful extrapolations beyond the the limits of experimental data. △ Less

Submitted 3 January, 2022; originally announced January 2022.

Comments: 10 pages, 3 figures, under review with Phys. Rev. C

Report number: LA-UR-21-27783

Sensitivity of Neutron-Rich Nuclear Isomer Behavior to Uncertainties in Direct Transitions

Authors: G. Wendell Misch, Trevor M. Sprouse, Matthew R. Mumpower, Aaron Couture, Chris L. Fryer, Bradley S. Meyer, Yang Sun

Abstract: Nuclear isomers are populated in the rapid neutron capture process (r process) of nucleosynthesis. The r process may cover a wide range of temperatures, potentially starting from several tens of GK (several MeV) and then cooling as material is ejected from the event. As the r-process environment cools, isomers can freeze out of thermal equilibrium or be directly populated as astrophysical isomers… ▽ More Nuclear isomers are populated in the rapid neutron capture process (r process) of nucleosynthesis. The r process may cover a wide range of temperatures, potentially starting from several tens of GK (several MeV) and then cooling as material is ejected from the event. As the r-process environment cools, isomers can freeze out of thermal equilibrium or be directly populated as astrophysical isomers (astromers). Two key behaviors of astromers -- ground state<->isomer transition rates and thermalization temperatures -- are determined by direct transition rates between pairs of nuclear states. We perform a sensitivity study to constrain the effects of unknown transitions on astromer behavior. We also introduce a categorization of astromers that describes their potential effects in hot environments. We provide a table of neutron-rich isomers that includes the astromer type, thermalization temperature, and key unmeasured transition rates. △ Less

Submitted 16 March, 2021; originally announced March 2021.

Comments: 52 pages, 13 figures, 2 tables

Report number: LA-UR-21-21559

Isochronic evolution and the radioactive decay of r-process nuclei

Authors: T. M. Sprouse, G. Wendell Misch, M. R. Mumpower

Abstract: We report on the creation and application of a novel decay network that uses the latest data from experiment and evaluation. We use the network to simulate the late-time phase of the rapid neutron capture (r) process. In this epoch, the bulk of nuclear reactions, such as radiative capture, have ceased and nuclear decays are the dominant transmutation channels. We find that the decay from short-liv… ▽ More We report on the creation and application of a novel decay network that uses the latest data from experiment and evaluation. We use the network to simulate the late-time phase of the rapid neutron capture (r) process. In this epoch, the bulk of nuclear reactions, such as radiative capture, have ceased and nuclear decays are the dominant transmutation channels. We find that the decay from short-lived to long-lived species naturally leads to an isochronic evolution in which nuclei with similar half-lives are populated at the same time. We consider random perturbations along each isobaric chain to initial solar-like r-process compositions to demonstrate the isochronic nature of the late-time phase of the r-process. Our analysis shows that detailed knowledge of the final isotopic composition allows for the prediction of late-time evolution with a high degree of confidence despite uncertainties that exist in astrophysical conditions and the nuclear physics properties of the most neutron-rich nuclei. We provide the time-dependent nuclear composition in the Appendix as supplemental material. △ Less

Submitted 7 February, 2021; originally announced February 2021.

Comments: 13 pages, 5 figures, 1 appendix. Submitted to The Astrophsyical Journal

Report number: LA-UR-21-20819

Astromers in the radioactive decay of r-process nuclei

Authors: G. Wendell Misch, T. M. Sprouse, M. R. Mumpower

Abstract: We study the impact of astrophysically relevant nuclear isomers (astromers) in the context of the rapid neutron capture process (r-process) nucleosynthesis. We compute thermally mediated transition rates between long-lived isomers and the corresponding ground states in neutron-rich nuclei. We calculate the temperature-dependent beta-decay feeding factors which represent the fraction of material go… ▽ More We study the impact of astrophysically relevant nuclear isomers (astromers) in the context of the rapid neutron capture process (r-process) nucleosynthesis. We compute thermally mediated transition rates between long-lived isomers and the corresponding ground states in neutron-rich nuclei. We calculate the temperature-dependent beta-decay feeding factors which represent the fraction of material going to each of the isomer and ground state daughter species from the beta-decay parent species. We simulate nucleosynthesis by including as separate species nuclear excited states with measured terrestrial half-lives greater than 100 microseconds. We find a variety of isomers throughout the chart of nuclides are populated, and we identify those most likely to be influential. We comment on the capacity of isomer production to alter radioactive heating in an r-process environment. △ Less

Submitted 5 March, 2021; v1 submitted 23 November, 2020; originally announced November 2020.

Comments: 8 pages, 3 figures, 1 table

Report number: LA-UR-20-29246

Astromers: Nuclear Isomers in Astrophysics

Authors: G. Wendell Misch, Surja K. Ghorui, Projjwal Banerjee, Yang Sun, Matthew R. Mumpower

Abstract: We develop a method to compute thermally-mediated transition rates between the ground state and long-lived isomers in nuclei. We also establish criteria delimiting a thermalization temperature above which a nucleus may be considered a single species and below which it must be treated as two separate species: a ground state species, and an astrophysical isomer ("astromer") species. Below the therma… ▽ More We develop a method to compute thermally-mediated transition rates between the ground state and long-lived isomers in nuclei. We also establish criteria delimiting a thermalization temperature above which a nucleus may be considered a single species and below which it must be treated as two separate species: a ground state species, and an astrophysical isomer ("astromer") species. Below the thermalization temperature, the destruction rates dominate the internal transition rates between the ground state and the isomer. If the destruction rates also differ greatly from one another, the nuclear levels fall out of or fail to reach thermal equilibrium. Without thermal equilibrium, there may not be a safe assumption about the distribution of occupation probability among the nuclear levels when computing nuclear reaction rates. In these conditions, the isomer has astrophysical consequences and should be treated a separate astromer species which evolves separately from the ground state in a nucleosynthesis network. We apply our transition rate methods and perform sensitivity studies on a few well-known astromers. We also study transitions in several other isomers of likely astrophysical interest. △ Less

Submitted 1 November, 2020; v1 submitted 28 October, 2020; originally announced October 2020.

Comments: Accepted to ApJS, see eventual published article for data. 23 pages, 21 figures

Report number: LA-UR-20-22828

Extension of the Hauser-Feshbach Fission Fragment Decay Model to Multi-Chance Fission

Authors: A. E. Lovell, T. Kawano, S. Okumura, I. Stetcu, M. R. Mumpower, P. Talou

Abstract: The Hauser-Feshbach fission fragment decay model, $\mathtt{HF^3D}$, which calculates the statistical decay of fission fragments, has been expanded to include multi-chance fission, up to neutron incident energies of 20 MeV. The deterministic decay takes as input pre-scission quantities - fission probabilities and the average energy causing fission - and post-scission quantities - yields in mass, ch… ▽ More The Hauser-Feshbach fission fragment decay model, $\mathtt{HF^3D}$, which calculates the statistical decay of fission fragments, has been expanded to include multi-chance fission, up to neutron incident energies of 20 MeV. The deterministic decay takes as input pre-scission quantities - fission probabilities and the average energy causing fission - and post-scission quantities - yields in mass, charge, total kinetic energy, spin, and parity. From these fission fragment initial conditions, the full decay is followed through both prompt and delayed particle emissions, allowing for the calculation of prompt neutron and $γ$ properties, such as multiplicity and energy distributions, both independent and cumulative fission yields, and delayed neutron observables. In this work, we describe the implementation of multi-chance fission into the $\mathtt{HF^3D}$ model, and show an example of prompt and delayed quantities beyond first-chance fission, using the example of neutron-induced fission on $^{235}$U. This expansion represents significant progress in consistently modeling the emission of prompt and delayed particles from fissile systems. △ Less

Submitted 26 October, 2020; originally announced October 2020.

Comments: 15 pages, 11 figures, submitted to PRC

Report number: LA-UR-20-27423

Modeling Kilonova Light Curves: Dependence on Nuclear Inputs

Authors: Y. L. Zhu, K. Lund, J. Barnes, T. M. Sprouse, N. Vassh, G. C. McLaughlin, M. R. Mumpower, R. Surman

Abstract: The mergers of binary neutron stars, as well as black hole-neutron star systems, are expected to produce an electromagnetic counterpart that can be analyzed to infer the element synthesis that occurred in these events. We investigate one source of uncertainties pertinent to lanthanide-rich outflows: the nuclear inputs to rapid neutron capture nucleosynthesis calculations. We begin by examining thi… ▽ More The mergers of binary neutron stars, as well as black hole-neutron star systems, are expected to produce an electromagnetic counterpart that can be analyzed to infer the element synthesis that occurred in these events. We investigate one source of uncertainties pertinent to lanthanide-rich outflows: the nuclear inputs to rapid neutron capture nucleosynthesis calculations. We begin by examining thirty-two different combinations of nuclear inputs: eight mass models, two types of spontaneous fission rates, and two types of fission daughter product distributions. We find that such nuclear physics uncertainties typically generate at least one order of magnitude uncertainty in key quantities such as the nuclear heating (one and a half orders of magnitude at one day post-merger), the bolometric luminosity (one order of magnitude at five days post-merger), and the inferred mass of material from the bolometric luminosity (factor of eight when considering the eight to ten days region). Since particular nuclear processes are critical for determining the electromagnetic signal, we provide tables of key nuclei undergoing $β$-decay, $α$-decay, and spontaneous fission important for heating at different times, identifying decays that are common among the many nuclear input combinations. △ Less

Submitted 7 October, 2020; originally announced October 2020.

Comments: 29 pages, 17 figures, 4 tables

Report number: LA-UR-20-22630

Propagation of Hauser-Feshbach uncertainty estimates to r-process nucleosynthesis: Benchmark of statistical property models for neutron rich nuclei far from stability

Authors: S. Nikas, G. Perdikakis, M. Beard, R. Surman, M. R. Mumpower, P. Tsintari

Abstract: Multimessenger observations of the neutron star merger event GW170817 have re-energized the debate over the astrophysical origins of the most massive elements via the r-process nucleosynthesis. A key aspect of such studies is comparing astronomical observations to theoretical nucleosynthesis yields in a meaningful way. To perform realistic nucleosynthesis calculations, understanding the uncertaint… ▽ More Multimessenger observations of the neutron star merger event GW170817 have re-energized the debate over the astrophysical origins of the most massive elements via the r-process nucleosynthesis. A key aspect of such studies is comparing astronomical observations to theoretical nucleosynthesis yields in a meaningful way. To perform realistic nucleosynthesis calculations, understanding the uncertainty in microphysics details such as nuclear reaction rates is as essential as understanding uncertainties in modeling the astrophysical environment. We present an investigation of neutron capture rate calculations' uncertainty away from stability using the Hauser-Feshbach model. We provide a quantitative measure of the calculations' dependability when we extrapolate models of statistical properties to nuclei in an r-process network. We select several level density and gamma-ray strength models appropriate for neutron-capture and use them to calculate the reaction rate for each nucleus in the network. We observe how statistical properties affect the theoretical reaction rates. The rates are then sampled with the Monte Carlo technique and used in network calculations to map the range of possible r-process abundances. The results show that neutron capture rates can vary by a couple of orders of magnitude between calculations. Phenomenological models provide smoother results than semi-microscopic. They cannot, however, reproduce nuclear structure changes such as shell closures. While semi-microscopic models predict nuclear structure effects away from stability, it is not clear that these results are quantitatively accurate. The effect of the uncertainty on r-process yields is large enough to impede comparisons between observation and calculations. Progress in developing better microscopic models of gamma strengths and level densities is urgently needed to improve the fidelity of r-process models. △ Less

Submitted 4 October, 2020; originally announced October 2020.

Comments: Submitted to Phys. Rev. C (abstract adapted to arXiV character limit and style)

Improvements to the macroscopic-microscopic approach of nuclear fission

Authors: Marc Verriere, Matthew R. Mumpower

Abstract: The well established macroscopic-microscopic (mac-mic) description of nuclear fission enables the prediction of fission fragment yields for a broad range of fissioning systems. In this work, we present several key enhancements to this approach. We improve upon the microscopic sector of nuclear potential energy surfaces by magnifying the resolution of the Lipkin-Nogami equations and strengthening t… ▽ More The well established macroscopic-microscopic (mac-mic) description of nuclear fission enables the prediction of fission fragment yields for a broad range of fissioning systems. In this work, we present several key enhancements to this approach. We improve upon the microscopic sector of nuclear potential energy surfaces by magnifying the resolution of the Lipkin-Nogami equations and strengthening the Strutinsky procedure, thus reducing spurious effects from the continuum. We further present a novel deterministic method for calculating fission dynamics under the assumption of strongly damped nucleonic motion. Our technique directly determines the evolution of the scissioned shape distribution according to the number of random walk steps rather than the statistical accumulation of fission events. We show that our new technique is equivalent to the Metropolis random walk pioneered over the past decade by Randrup and colleagues. It further improves upon it, as we remove the need for altering the nuclear landscape via a biased potential. With our final improvement, we calculate fission fragments mass and charge distributions using particle number projection, which affords the simultaneous calculation of both mass and charge yield distributions. Fission fragments are thus calculated from the quantum mechanical $A$-body states of the potential energy surface rather than the collective mass asymmetry variable ($α_{\rm g}$) of the Finite-Range Liquid-Drop Model (FRLDM) used in past work. We highlight the success of our enhancements by predicting the odd-even staggering and the charge polarization for the neutron-induced fission of ${}^{233}$U and ${}^{235}$U. △ Less

Submitted 19 January, 2021; v1 submitted 14 August, 2020; originally announced August 2020.

Report number: LLNL-JRNL-813628

Journal ref: Phys. Rev. C 103, 034617 (2021)

Following nuclei through nucleosynthesis: a novel tracing technique

Authors: T. M. Sprouse, M. R. Mumpower, R. Surman

Abstract: Astrophysical nucleosynthesis is a family of diverse processes by which atomic nuclei undergo nuclear reactions and decays to form new nuclei. The complex nature of nucleosynthesis, which can involve as many as tens of thousands of interactions between thousands of nuclei, makes it difficult to study any one of these interactions in isolation using standard approaches. In this work, we present a n… ▽ More Astrophysical nucleosynthesis is a family of diverse processes by which atomic nuclei undergo nuclear reactions and decays to form new nuclei. The complex nature of nucleosynthesis, which can involve as many as tens of thousands of interactions between thousands of nuclei, makes it difficult to study any one of these interactions in isolation using standard approaches. In this work, we present a new technique, nucleosynthesis tracing, that we use to quantify the specific role of individual nuclear reaction, decay, and fission processes in relationship to nucleosynthesis as a whole. We apply this technique to study fission and $β^{-}$-decay as they occur in the rapid neutron capture ($r$) process of nucleosynthesis. △ Less

Submitted 13 August, 2020; originally announced August 2020.

Comments: 18 pages, 10 figures

Report number: LA-UR-20-24018

Journal ref: Phys. Rev. C 104, 015803 (2021)

Probing the fission properties of neutron-rich actinides with the astrophysical $r$ process

Authors: Nicole Vassh, Matthew R. Mumpower, Trevor M. Sprouse, Rebecca Surman, Ramona Vogt

Abstract: We review recent work examining the influence of fission in rapid neutron capture ($r$-process) nucleosynthesis which can take place in astrophysical environments. We briefly discuss the impact of uncertain fission barriers and fission rates on the population of heavy actinide species. We demonstrate the influence of the fission fragment distributions for neutron-rich nuclei and discuss currently… ▽ More We review recent work examining the influence of fission in rapid neutron capture ($r$-process) nucleosynthesis which can take place in astrophysical environments. We briefly discuss the impact of uncertain fission barriers and fission rates on the population of heavy actinide species. We demonstrate the influence of the fission fragment distributions for neutron-rich nuclei and discuss currently available treatments, including recent macroscopic-microscopic calculations. We conclude by comparing our nucleosynthesis results directly with stellar data for metal-poor stars rich in $r$-process elements to consider whether fission plays a role in the so-called `universality' of $r$-process abundances observed from star to star. △ Less

Submitted 18 June, 2020; originally announced June 2020.

Comments: International Workshop on Fission Product Yields (FPY) 2019 proceedings

Report number: LA-UR-20-22730

129I and 247Cm in Meteorites Constrain the Last Astrophysical Source of Solar r-process Elements

Authors: Benoit Côté, Marius Eichler, Andrés Yagüe, Nicole Vassh, Matthew R. Mumpower, Blanka Világos, Benjámin Soós, Almudena Arcones, Trevor M. Sprouse, Rebecca Surman, Marco Pignatari, Maria K. Pető, Benjamin Wehmeyer, Thomas Rauscher, Maria Lugaro

Abstract: The composition of the early Solar System can be inferred from meteorites. Many elements heavier than iron were formed by the rapid neutron-capture process (r process), but the astrophysical sources where this occurred remain poorly understood. We demonstrate that the near-identical half-lives ($\simeq$ 15.6 Myr) of the radioactive r-process nuclei 129I and 247Cm preserve their ratio, irrespective… ▽ More The composition of the early Solar System can be inferred from meteorites. Many elements heavier than iron were formed by the rapid neutron-capture process (r process), but the astrophysical sources where this occurred remain poorly understood. We demonstrate that the near-identical half-lives ($\simeq$ 15.6 Myr) of the radioactive r-process nuclei 129I and 247Cm preserve their ratio, irrespective of the time between production and incorporation into the Solar System. We constrain the last r-process source by comparing the measured meteoritic 129I / 247Cm = 438 $\pm$ 184 to nucleosynthesis calculations based on neutron star merger and magneto-rotational supernova simulations. Moderately neutron-rich conditions, often found in merger disk ejecta simulations, are most consistent with the meteoritic value. Uncertain nuclear physics data limit our confidence in this conclusion. △ Less

Submitted 2 March, 2021; v1 submitted 8 June, 2020; originally announced June 2020.

Comments: 36 pages, 7 figures, 7 tables

Journal ref: Science 26 Feb 2021: Vol. 371, Issue 6532, pp. 945-948

Markov Chain Monte Carlo Predictions of Neutron-rich Lanthanide Properties as a Probe of $r$-process Dynamics

Authors: Nicole Vassh, Gail C. McLaughlin, Matthew R. Mumpower, Rebecca Surman

Abstract: Lanthanide element signatures are key to understanding many astrophysical observables, from merger kilonova light curves to stellar and solar abundances. To learn about the lanthanide element synthesis that enriched our solar system, we apply the statistical method of Markov Chain Monte Carlo to examine the nuclear masses capable of forming the $r$-process rare-earth abundance peak. We describe th… ▽ More Lanthanide element signatures are key to understanding many astrophysical observables, from merger kilonova light curves to stellar and solar abundances. To learn about the lanthanide element synthesis that enriched our solar system, we apply the statistical method of Markov Chain Monte Carlo to examine the nuclear masses capable of forming the $r$-process rare-earth abundance peak. We describe the physical constraints we implement with this statistical approach and demonstrate the use of the parallel chains method to explore the multidimensional parameter space. We apply our procedure to three moderately neutron-rich astrophysical outflows with distinct types of $r$-process dynamics. We show that the mass solutions found are dependent on outflow conditions and are related to the $r$-process path. We describe in detail the mechanism behind peak formation in each case. We then compare our mass predictions for neutron-rich neodymium and samarium isotopes to the latest experimental data from the CPT at CARIBU. We find our mass predictions given outflows that undergo an extended (n,$γ$)$\rightleftarrows$($γ$,n) equilibrium to be those most compatible with both observational solar abundances and neutron-rich mass measurements. △ Less

Submitted 10 February, 2021; v1 submitted 7 June, 2020; originally announced June 2020.

Report number: LA-UR-20-24194

Journal ref: ApJ 907:98 (2021)

Characterizing $r$-Process Sites through Actinide Production

Authors: Erika M. Holmbeck, Rebecca Surman, Anna Frebel, G. C. McLaughlin, Matthew R. Mumpower, Trevor M. Sprouse, Toshihiko Kawano, Nicole Vassh, Timothy C. Beers

Abstract: Of the variations in the elemental abundance patterns of stars enhanced with $r$-process elements, the variation in the relative actinide-to-lanthanide ratio is among the most significant. We investigate the source of these actinide differences in order to determine whether these variations are due to natural differences in astrophysical sites, or due to the uncertain nuclear properties that are a… ▽ More Of the variations in the elemental abundance patterns of stars enhanced with $r$-process elements, the variation in the relative actinide-to-lanthanide ratio is among the most significant. We investigate the source of these actinide differences in order to determine whether these variations are due to natural differences in astrophysical sites, or due to the uncertain nuclear properties that are accessed in $r$-process sites. We find that variations between relative stellar actinide abundances is most likely astrophysical in nature, owing to how neutron-rich the ejecta from an $r$-process event may be. Furthermore, if an $r$-process site is capable of generating variations in the neutron-richness of its ejected material, then only one type of $r$-process site is needed to explain all levels of observed relative actinide enhancements. △ Less

Submitted 23 January, 2020; originally announced January 2020.

Comments: 12 pages, 5 figures, NPA-IX proceedings

Journal ref: Journal of Physics: Conference Series 1668, 012020 (2020)

First Exploration of Neutron Shell Structure Below Lead and Beyond $\boldsymbol{N=126}$

Authors: T. L. Tang, B. P. Kay, C. R. Hoffman, J. P. Schiffer, D. K. Sharp, L. P. Gaffney, S. J. Freeman, M. R. Mumpower, A. Arokiaraj, E. F. Baader, P. A. Butler, W. N. Catford, G. de Angelis, F. Flavigny, M. D. Gott, E. T. Gregor, J. Konki, M. Labiche, I. H. Lazurus, P. T. MacGregor, I. Martel, R. D. Page, Zs. Podolyák, O. Poleshchuk, R. Raabe , et al. (4 additional authors not shown)

Abstract: The nuclei below lead but with more than 126 neutrons are crucial to an understanding of the astrophysical $r$-process in producing nuclei heavier than $A\sim190$. Despite their importance, the structure and properties of these nuclei remain experimentally untested as they are difficult to produce in nuclear reactions with stable beams. In a first exploration of the shell structure of this region,… ▽ More The nuclei below lead but with more than 126 neutrons are crucial to an understanding of the astrophysical $r$-process in producing nuclei heavier than $A\sim190$. Despite their importance, the structure and properties of these nuclei remain experimentally untested as they are difficult to produce in nuclear reactions with stable beams. In a first exploration of the shell structure of this region, neutron excitations in $^{207}$Hg have been probed using the neutron-adding ($d$,$p$) reaction in inverse kinematics. The radioactive beam of $^{206}$Hg was delivered to the new ISOLDE Solenoidal Spectrometer at an energy above the Coulomb barrier. The spectroscopy of $^{207}$Hg marks a first step in improving our understanding of the relevant structural properties of nuclei involved in a key part of the path of the $r$-process. △ Less

Submitted 3 January, 2020; originally announced January 2020.

Comments: 6 pages, 5 figures

Report number: LA-UR-19-30930

Co-production of light and heavy $r$-process elements via fission deposition

Authors: Nicole Vassh, Matthew R. Mumpower, Gail C. McLaughlin, Trevor M. Sprouse, Rebecca Surman

Abstract: We apply for the first time fission yields determined across the chart of nuclides from the macroscopic-microscopic theory of the Finite Range Liquid Drop Model to simulations of rapid neutron capture ($r$-process) nucleosynthesis. With the fission rates and yields derived within the same theoretical framework utilized for other relevant nuclear data, our results represent an important step toward… ▽ More We apply for the first time fission yields determined across the chart of nuclides from the macroscopic-microscopic theory of the Finite Range Liquid Drop Model to simulations of rapid neutron capture ($r$-process) nucleosynthesis. With the fission rates and yields derived within the same theoretical framework utilized for other relevant nuclear data, our results represent an important step toward self-consistent applications of macroscopic-microscopic models in $r$-process calculations. The yields from this model are wide for nuclei with extreme neutron excess. We show that these wide distributions of neutron-rich nuclei, and particularly the asymmetric yields for key species that fission at late times in the $r$ process, can contribute significantly to the abundances of the lighter heavy elements, specifically the light precious metals palladium and silver. Since these asymmetric yields correspondingly also deposit into the lanthanide region, we consider the possible evidence for co-production by comparing our nucleosynthesis results directly with the trends in the elemental ratios of metal-poor stars rich in $r$-process material. We show that for $r$-process enhanced stars palladium over europium and silver over europium display mostly flat trends suggestive of co-production and compare to the lanthanum over europium trend which is often used to justify robustness arguments in the lanthanide region. We find that such robustness arguments may be extendable down to palladium and heavier and demonstrate that fission deposition is a mechanism by which such a universality or robustness can be achieved. △ Less

Submitted 18 June, 2020; v1 submitted 18 November, 2019; originally announced November 2019.

Report number: LA-UR-19-30652

Journal ref: ApJ 896:28 (2020)

Primary fission fragment mass yields across the chart of nuclides

Authors: M. R. Mumpower, P. Jaffke, M. Verriere, J. Randrup

Abstract: We have calculated a complete set of primary fission fragment mass yields, $Y(A)$, for heavy nuclei across the chart of nuclides, including those of particular relevance to the rapid neutron capture process ($r$ process) of nucleosynthesis. We assume that the nuclear shape dynamics are strongly damped which allows for a description of the fission process via Brownian shape motion across nuclear po… ▽ More We have calculated a complete set of primary fission fragment mass yields, $Y(A)$, for heavy nuclei across the chart of nuclides, including those of particular relevance to the rapid neutron capture process ($r$ process) of nucleosynthesis. We assume that the nuclear shape dynamics are strongly damped which allows for a description of the fission process via Brownian shape motion across nuclear potential-energy surfaces. The macroscopic energy of the potential was obtained with the Finite-Range Liquid-Drop Model (FRLDM), while the microscopic terms were extracted from the single-particle level spectra in the fissioning system by the Strutinsky procedure for the shell energies and the BCS treatment for the pairing energies. For each nucleus considered, the fission fragment mass yield, $Y(A)$, is obtained from 50,000 -- 500,000 random walks on the appropriate potential-energy surface. The full mass and charge yield, $Y(Z,A)$, is then calculated by invoking the Wahl systematics. With this method, we have calculated a comprehensive set of fission-fragment yields from over 3,800 nuclides bounded by $80\leq Z \leq 130$ and $A\leq330$; these yields are provided as an ASCII formatted database in the supplemental material. We compare our yields to known data and discuss general trends that emerge in low-energy fission yields across the chart of nuclides. △ Less

Submitted 14 November, 2019; originally announced November 2019.

Comments: 22 pages, 15 figures

Report number: LA-UR-19-26981

Journal ref: Phys. Rev. C 101, 054607 (2020)

Propagation of Statistical Uncertainties of Skyrme Mass Models to Simulations of $r$-Process Nucleosynthesis

Authors: T. M. Sprouse, R. Navarro Perez, R. Surman, M. R. Mumpower, G. C. McLaughlin, N. Schunck

Abstract: Uncertainties in nuclear models have a major impact on simulations that aim at understanding the origin of heavy elements in the universe through the rapid neutron capture process ($r$ process) of nucleosynthesis. Within the framework of the nuclear density functional theory, we use results of Bayesian statistical analysis to propagate uncertainties in the parameters of energy density functionals… ▽ More Uncertainties in nuclear models have a major impact on simulations that aim at understanding the origin of heavy elements in the universe through the rapid neutron capture process ($r$ process) of nucleosynthesis. Within the framework of the nuclear density functional theory, we use results of Bayesian statistical analysis to propagate uncertainties in the parameters of energy density functionals to the predicted $r$-process abundance pattern, by way not only of the nuclear masses but also through the influence of the masses on $β$-decay and neutron capture rates. We additionally make the first identifications of specific parameters of Skyrme-like energy density functionals which are correlated with particular aspects of the $r$-process abundance pattern. While previous studies have explored the reduction in the abundance pattern uncertainties due to anticipated new measurements of neutron-rich nuclei, here we point out that an even larger reduction will occur when these new measurements are used to reduce the uncertainty of model predictions of masses, which are then propagated through to the abundance pattern. We make a quantitative prediction for how large this reduction will be. △ Less

Submitted 29 January, 2019; originally announced January 2019.

Comments: 7 pages, 4 figures

Report number: LA-UR-19-20003

Journal ref: Phys. Rev. C 101, 055803 (2020)

FRIB and the GW170817 Kilonova

Authors: A. Aprahamian, R. Surman, A. Frebel, G. C. McLaughlin, A. Arcones, A. B. Balantekin, J. Barnes, Timothy C. Beers, Erika M. Holmbeck, Jinmi Yoon, Maxime Brodeur, T. M. Sprouse, Nicole Vassh, Jolie A. Cizewski, Jason A. Clark, Benoit Cote, Sean M. Couch, M. Eichler, Jonathan Engel, Rana Ezzeddine, George M. Fuller, Samuel A. Giuliani, Robert Grzywacz, Sophia Han, C. J. Horowitz , et al. (23 additional authors not shown)

Abstract: In July 2018 an FRIB Theory Alliance program was held on the implications of GW170817 and its associated kilonova for r-process nucleosynthesis. Topics of discussion included the astrophysical and nuclear physics uncertainties in the interpretation of the GW170817 kilonova, what we can learn about the astrophysical site or sites of the r process from this event, and the advances in nuclear experim… ▽ More In July 2018 an FRIB Theory Alliance program was held on the implications of GW170817 and its associated kilonova for r-process nucleosynthesis. Topics of discussion included the astrophysical and nuclear physics uncertainties in the interpretation of the GW170817 kilonova, what we can learn about the astrophysical site or sites of the r process from this event, and the advances in nuclear experiment and theory most crucial to pursue in light of the new data. Here we compile a selection of scientific contributions to the workshop, broadly representative of progress in r-process studies since the GW170817 event. △ Less

Submitted 3 September, 2018; originally announced September 2018.

Comments: Proceedings for the FRIB Theory Alliance workshop "FRIB and the GW170817 kilonova", held 16-27 July 2018 at the Facility for Rare Isotope Beams, Michigan State University, East Lansing, MI USA; 44 pages, 14 figures

Report number: LA-UR-18-28022

Actinide Production in Neutron-Rich Ejecta of a Neutron Star Merger

Authors: Erika M. Holmbeck, Rebecca Surman, Trevor M. Sprouse, Matthew R. Mumpower, Nicole Vassh, Timothy C. Beers, Toshihiko Kawano

Abstract: The rapid-neutron-capture ("r") process is responsible for synthesizing many of the heavy elements observed in both the solar system and Galactic metal-poor halo stars. Simulations of r-process nucleosynthesis can reproduce abundances derived from observations with varying success, but so far fail to account for the observed over-enhancement of actinides, present in about 30% of r-process-enhanced… ▽ More The rapid-neutron-capture ("r") process is responsible for synthesizing many of the heavy elements observed in both the solar system and Galactic metal-poor halo stars. Simulations of r-process nucleosynthesis can reproduce abundances derived from observations with varying success, but so far fail to account for the observed over-enhancement of actinides, present in about 30% of r-process-enhanced stars. In this work, we investigate actinide production in the dynamical ejecta of a neutron star merger and explore if varying levels of neutron richness can reproduce the actinide boost. We also investigate the sensitivity of actinide production on nuclear physics properties: fission distribution, beta-decay, and mass model. For most cases, the actinides are over-produced in our models if the initial conditions are sufficiently neutron-rich for fission cycling. We find that actinide production can be so robust in the dynamical ejecta that an additional lanthanide-rich, actinide-poor component is necessary in order to match observations of actinide-boost stars. We present a simple actinide-dilution model that folds in estimated contributions from two nucleosynthetic sites within a merger event. Our study suggests that while the dynamical ejecta of a neutron star merger is a likely production site for the formation of actinides, a significant contribution from another site or sites (e.g., the neutron star merger accretion disk wind) is required to explain abundances of r-process-enhanced, metal-poor stars. △ Less

Submitted 18 October, 2018; v1 submitted 17 July, 2018; originally announced July 2018.

Comments: 15 pages, 19 figures