Session DK: Nuclear Theory I

Towards a Langevin Model for the Stochastic Dynamics of Nuclear Fission

A robust description of the process of nuclear fission is essential to many research domains ranging from nuclear energy, national security, and nuclear data. However, owing to the nuclear many-body problem, a description of fission based on nucleon-nucleon interactions is unfeasible given current computational limitations, which has led to a number of alternative methods that greatly reduce the overall complexity of this difficult problem. In this work, we present results of recent efforts to model the process of nuclear fission from the perspective of a microscopic-macroscopic model of the atomic nucleus, where fission proceeds from an initially excited state to scission as a stochastic process according to a progression of increasingly sophisticated treatments of the stochastic dynamics. In contrast with our past work, which has treated fission in the strongly-damped limit described by a random-walk process, this approach furnishes the kinetic energies associated with the nascent fragments, which is subsequently used to model the de-excitation properties of fission fragments. LA-UR-21-29350   

Weak processes in light nuclei with quantum Monte Carlo methods

Measurements of beta decay spectra and searches for neutrinoless double beta decay will address important questions within fundamental symmetries. The interpretation of new physics signals requires an accurate understanding of the underlying nuclear dynamics. We aim to achieve this with quantum Monte Carlo (QMC) methods which allow one to solve the many-body Schr\"{o}dinger equation while retaining the full complexity of the nuclear system. In this talk, we discuss an overview of {\it ab initio} calculations of weak processes using QMC with the Norfolk local chiral interaction and its consistent electroweak currents. We present model validation via calculations of Gamow-Teller matrix elements and partial muon capture rates as well as a prediction of the $^6$He beta decay spectrum.   

Ab Initio Optical Potentials for Elastic Scattering at Low Energies Using the Symmetry-Adpated No-Core Shell Model and Green's Functions

Ab initio optical potentials for elastic scattering at low energy is of particular interest for experiments at rare isotope beam facilities. In this work we combine the ab initio Symmetry-Adapted No-Core Shell-Model nuclear structure results [1,2] with the Green's function evaluation of the optical potential through a self-energy calculation [3,4]. Specifically, we show preliminary total cross sections and phase shift results for neutron elastic scattering off Helium-4, Carbon-12, and Oxygen-16 with projectile energies between 0.5 and 10 MeV. We also discuss the role of collectivity and present comparisons to earlier

models

[1] T. Dytrych, K. D. Launey, J. P. Draayer, et al., Phys. Rev. Lett. 124, 042501 (2020)

[2] K. D. Launey, A. Mercenne, and T. Dytrych, Annu. Rev. Nucl. Part. Sci. (2021)

[3] J. Rotureau, P. Danielewicz, G. Hagen, F. M. Nunes, and T. Papenbrock, Phys. Rev. C 95, 024315 (2017)

[4] A. Idini, C. Barbieri, and P. Navrátil, Phys, Rev. Lett. 123, 092501 (2019)   

Fission Fragment Spin Correlations

In nuclear fission, the primary fragments typically carry half a dozen units of angular momentum whose directions and correlations reflect the underlying dynamical mechanisms. Concentrating on the nucleon-exchange mechanism, this presentation describes how the fragment spins are generated, why they should be expected to be oriented preferentially perpendicular to the fission axis, and how they emerge as largely uncorrelated even though they are built up of highly correlated contributions.

    

Emulating Phenomenological R-Matrix Calculations

Accurately quantifying uncertainties for nuclear reactions frequently requires that an expensive computer code be run many times – more times than is reasonably achievable or even possible. For these cases, sophisticated emulation techniques are necessary. I will discuss different approaches for emulating phenomenological R-matrix calculations, how successful they are in different applications, and what limitations are currently being faced.   

Nucleon-nucleus elastic scattering from a symmetry-adapted framework

Effective interactions (or optical potentials) are frequently used to describe a variety of nuclear reactions. When derived from first principles, effective interactions maintain their predictive power and may be able to provide insight into properties of nuclei beyond the reach of current experimental techniques. Within the spectator expansion of multiple scattering theory, a nucleon-nucleus effective interaction can be constructed by using the relevant nonlocal densities from an ab initio nuclear structure model, e.g. the no-core shell model. Here, we determine the applicability of nonlocal densities calculated from the symmetry-adapted no-core shell model to describe proton elastic scattering in light nuclei at medium energies and compare those results to the results from the complete model space. We then extend into nucleon scattering on intermediate mass nuclei and compare to the available experimental data.

    

A modified Brink-Axel hypothesis for astrophysical Gamow-Teller transitions

Weak interaction charged current transition strengths from highly excited nuclear states are fundamental ingredients for accurate modeling of compact object composition and dynamics, but are difficult to obtain either from experiment or theory.  For lack of alternatives, calculations have often fallen back upon a generalized Brink-Axel hypothesis, that is, assuming  the strength function (transition probability) is independent of the initial nuclear state but depends only upon the transition energy and the weak interaction properties of the parent nucleus ground state. Here is presented numerical evidence for a modified `local' Brink-Axel hypothesis for Gamow-Teller transitions for  pf-shell nuclei   relevant to astrophysical applications. Specifically, while the original Brink-Axel hypothesis does not hold globally, strength functions from initial states nearby inenergy are  similar within statistical fluctuations. This agrees with previous work on strength function moments. Using this modified hypothesis, one can tackle strength functions at previously intractable initial energies, using semi-converged initial states  at arbitrary excitation energy. This work provides a well-founded method  for computing  accurate thermal weak transition rates  for medium-mass nuclei  at temperatures occurring in stellar cores near collapse.