Practicum Abstracts

Strongly interacting fermionic systems remain a major challenge to many-body theory. Of particular interest are the dynamics of the unitary Fermi gas (UFG), a system characterized by a strong coupling in the form of an s-wave interaction with zero effective range and infinite scattering length. The high-energy (and thus short-range) physics of the UFG is universal and independent of the two-body interaction under study. The UFG is not merely an idealization, but can be also be realized in systems of cold atoms by tuning the scattering length to near infinite values with magnetic Feshbach resonances and controlling the interparticle spacing via cooling. Quantum Monte Carlo (QMC) methods can be used to study the density and spin responses of cold atomic gases. The standard QMC approach is to evaluate the Laplace transform of the response via imaginary time propagation and then use maximum entropy techniques to extract the response; however, the propagation can be costly and can suffer from the fermion sign problem. To circumvent these issues, one can employ the short-time approximation (STA) in which one evaluates real-time matrix elements at short times. This approach is suited to high-energy responses and incorporates the full ground state correlations as well as the final state interactions at the two-particle level. The STA is computationally more efficient than a standard response calculation and it does not suffer as greatly from the sign problem. In this practicum, I began the development of the STA for studying the density response of the unitary Fermi gas. In addition to preliminary computations of STA response densities, I also computed other related physical quantities, such as two-body momentum distributions. Additionally, the plane-wave impulse approximation of the density response, which arises from factorizing the response into one struck particle and a residual spectator system, was evaluated to observe the impact of neglecting two-body physics when approximating the response.

For this project, we used the Los Alamos developed kinetic plasma physics code iFP to explore recently observed anomalous artefacts in the neutron spectra of recent high yield experiments at the NIF. To do so, we utilized the base code along with a fully kinetic neutron spectrum postprocessing package and a recently developed, continuum alpha heating package.

The goal of this practicum was to learn the fundamentals of discrete dislocation dynamics, a technique in which the hosting group and PI are world-class experts. The fundamental knowledge of dislocation physics embedded in the simulations allow for the investigation of plasticity within statistically representative volumes of material in which the evolution of complex dislocation networks control the overall material behavior. Results at the scale of dislocation networks are particularly useful for bridging the gap between predictive atomistic-scale simulations and the continuum grain-level calculations that can be directly compared to macroscale experimental results.

Phase-field dislocation dynamics (PFDD) is an energy-based dislocation simulation tool. The goal was to extend PFDD to include interstitials. Interstitial atoms such as carbon or oxygen can interact with dislocations in a material, drastically affecting the material response. By incorporating PFDD to include interstitial atoms, we can better understand how oxidation affects material response and the underlying mechanisms responsible.

The Detector for Advanced Neutron Capture Experiments (DANCE) is a 4-pi array of BaF2 gamma-ray detectors at the Los Alamos Neutron Science Center (LANSCE). A new spallation target-moderator-reflector-shield (TMRS) assembly has been designed and is expected to be installed at LANSCE in 2021. This new TMRS will offer significantly enhanced neutron flux in the keV-MeV range of interest to the nuclear physics experiments at DANCE and improved neutron energy resolution. However, the new TMRS will also produce a significant gamma flash background. The impact of this increased background upon DANCE has not been studied.

The main goals of my project were (i) to write a code that calculated cross sections with resolved resonances for neutron-induced reactions based on the statistical model of compound nucleus reactions, and (ii) to use this simulated data to verify whether the statistical model fluctuations were reproduced by an R-matrix fit typically used in experimental analysis. The motivation for this project was a recent experimental observation that disagreed with statistical model predictions. I achieved both of these goals. Regarding (i), I wrote a code that calculated cross sections in the resolved resonance region for s- and p-wave incident neutrons based on the Gaussian orthogonal ensemble (GOE) of random-matrix theory. Regarding (ii), I found that an R-matrix fit to a test case calculated with the GOE reproduced GOE fluctuations.

The practicum project focused on studying high energy density physics (HEDP) experiments using a combination of a magnetohydrodynamics (MHD) code called GORGON with a radiation-hydrodynamics (rad-hydro) code called RAGE. While GORGON is useful in studying problems in which magnetic forces and fields contribute dominantly to material dynamics, it is an Eulerian code with fixed cell size which severely limits its ability to simulate centimeter-scale targets whilst simultaneously resolving fine, micron-scale features. Such features include various stages of instability development in the presence of strong shocks, a process which can contribute significantly to the dynamics of HEDP experiments as well as inertial confinement fusion (ICF) experiments. RAGE is an Eulerian code which does not simulate magnetic fields or forces (only radiation transport and hydrodynamics) but utilizes adaptive mesh refinement (AMR) which increases resolution at defined, localized sections in the simulation region and enables study of evolving micron-scale features. To study magnetically-driven HEDP experiments such as the solid metallic liner implosion experiments that are executed on the Z Facility, a combination of the capabilities from these codes is desirable. A simulation can be initialized and driven by the MHD code GORGON and then, at a time in the experiment when the magnetic drive no longer dominates the liner dynamics (determined by the user), the GORGON output can be exported to RAGE, allowing the problem to continue running in RAGE with the added benefit of AMR. A link between GORGON and RAGE was created by the practicum supervisor (F. Doss) and developed via simulation test problems by the fellow (G. Shipley). The link was used to simulate an experiment on Z designed to study growth of Richtmyer-Meshkov and Rayleigh-Taylor instability structures for the first time with micron-scale resolution. In the future, the link can be used to study other Z experiments, particularly experiments that are relevant to the thesis work of the fellow. Though the fellow has served as the principal investigator for experiments on Z prior to this project, the research practicum facilitated the development of simulation and modeling expertise in support of HEDP experiments on the Z Facility.

This project was an introduction to looking at the uncertainties within the models for fracture networks (in terms of hydrocarbon production, i.e. fracking). The goal was to understand the physical processes that contribute to the long-term tail of the production curve, since this region is generally under-estimated by the discrete fracture networks that the group typically uses. We explored diffusion, the height of the damage zone, and the percentage of free gas in the fracture network. At the same time (but along a separate line of reasoning), we explored using a simple diffusion model to describe this tailing behavior, to look at folding in uncertainties in the fracture length distributions.

The 6Li(n,t) cross section is important for weapons physics, nuclear reactors, neutron detection, and astrophysics. The uncertainties in the current ENDF evaluation are as high as 15% above a few MeV due to conflicting results from various experiments. The goal of this project was to provide additional information for the evaluators and minimize the systematic errors in the R-Matrix fit. The experiment had three 6Li detectors one meter away from a 252Cf fission chamber to use the time of flight method to determine the neutron energy. The 6Li in the detectors acted as the target material.

We performed global multidimensional simulations of the nascent disks surrounding remnants of neutron star mergers by utilizing the general relativistic radiation magnetohydrodynamics (GRRMHD) code HARM. Particular interest was given to the morphological and nucleosynthetic evolution of these systems.

I quantified the amount of Cu corrosion that occurred during various high pressure, high temperature runs in hydrous media with clays used in engineered barrier systems that are part of nuclear repositories. I took reflected light images and and used the SEM to image the corrosion and then quantified the amount of corrosion by taking over 800 measurements of corrosion pitting. These measurements help us understand the extent of corrosion in Cu materials that may be used in nuclear repositories.

Data collected from NIF diagnostics contains inherent uncertainties due to random noise introduced by the diagnostic and data acquisition equipment. A monte carlo model of the Gamma Reaction History (GRH) diagnostic was built to evaluate the extent to which instrument noise affects inferred nuclear reaction conditions. A gamma burst from a DT fusion reaction was taken as the model's starting point and used to generate noisy oscilloscope traces. Reaction parameters were then inferred from these scope traces and compared to the true values from the starting point. This provided an estimate of the errors associated with GRH data.

For this project we used the LANSCE facility to irradiate Cd, Zn, and Nb foils with neutrons. We then counted these irradiated foils on a low background Ge detector at WIPP near Carlsbad, NM. From these data we will be able to determine the production cross sections for various radioactive isotopes from neutron activation of Cd, Zn, and Nb.

During the first few minutes of a neutron stars life, it cools from its initial extremely high temperature state by emitting a prodigious number of neutrinos. To study in detail how this cooling occurs, we developed a general relativistic neutrino transport /stellar structure code. To accurately track the evolution of the young neutron star, we employed state of the art microphysics for both the nuclear equation of state and the neutrino opacities. The effect of hydrodynamic instabilities in the interior on the cooling timescale of the young neutron star were then investigated by implementing a one-dimensional prescription for energy transport by convective motions. These results will help us to understand how hydrodynamic instabilities and the nuclear equation of state affect the spectra of the emitted neutrinos. If neutrinos are observed from a nearby supernova, this will give us a direct window into the high density physics that governs the young neutron star, which is in a regime that cannot be accessed in the laboratory.

I performed my practicum under the supervision of Dr. David Vieira of the Nuclear and Radiochemistry Group at Los Alamos National Laboratory. I worked on analyzing data taken on the Detector for Advanced Neutron Capture Experiments (DANCE). The DANCE array is a highly segmented, high efficiency barium fluoride gamma ray detector array designed to measure the gamma cascade following neutron capture on small (mg quantity) isotope-enriched stable and radioactive (T_1/2>100 d, <1 Ci) targets. The data I specifically analyzed was measured to obtain the neutron capture cross section of arsenic-75 as a function of neutron energy. The measurement of this quantity is important for the fields of nuclear astrophysics and stockpile stewardship science.