Karl Smith

Dielectric materials are foundational to our modern-day communications, defense, and commerce needs. Although dielectric breakdown is a primary cause of failure of these systems, we do not fully understand this process. We analyzed the dielectric breakdown channel propagation dynamics of two distinct types of electrical trees. One type of these electrical trees has not been formally classified. We observed the propagation speed of this electrical tree type to exceed 10 million meters per second. These results identify substantial gaps in the understanding of dielectric breakdown, and filling these gaps is paramount to the design and engineering of dielectric materials that are less susceptible to electrostatic discharge failure.

Deuterated stilbene is an organic scintillator that is a desirable material for fast neutron spectroscopy using spectrum unfolding techniques without requiring time-of-flight information. Due to the crystal structure of the material, some anisotropy of the light output exists, which is dependent on the direction of heavy charged particle recoil relative to the crystal structure. The anisotropy of trans-stilbene (hereafter referred to as stilbene) has been well characterized in previous work, but for deuterated stilbene, the anisotropy has only been partially characterized along the a and b crystal axes, while the artificial c ' axis, which shows the largest anisotropy in stilbene, has not been characterized until this publication. In this work, two deuterated stilbene crystals were characterized with neutron energies up to 35 MeV at the Los Alamos Neutron Science Center. For one of the crystals, the response is characterized along the a, b, and c ' axes. This characterization shows a distinct anisotropy along the axes in deuterated stilbene, which is very similar to that found in regular stilbene, such that the a axis is the brightest, while the b and c ' axes are approximately 3% and 20%-35% lower relative to the a axis.

Neutrons are encountered in many different fields, including condensed matter physics, space exploration, nuclear power, and healthcare. Neutrons interacting with a biological target produce secondary charged particles that are damaging to human health. The most effective way to shield neutrons is to slow them to thermal energies and then capture the thermalized neutrons. These factors lead us to consider potential materials solutions for neutron shields that maximize the protection of humans while minimizing the shield mass and adapt well to modern additive manufacturing techniques. Using hexagonal boron nitride (hBN) as a capture medium and high-density polyethylene (HDPE) as a thermalization medium, we aim to design the optimal internal structure of h(10)BN/HDPE composites by minimizing the effective dose, which is a measure of the estimated radiation damage exposure for a human. Through Monte Carlo simulations in Geant4, we find that the optimal structure reduces the effective dose up to a factor of 72 over aluminum (Al) and up to a factor of 4 over HDPE; this is a significant improvement in shielding effectiveness that could dramatically reduce the radiation exposure of occupational workers.

The experiment for space radiation analysis is an upcoming project for Los Alamos National Laboratory. This 12U CubeSat will be placed in a geosynchronous transfer orbit to make measurements of the charged particle populations in the radiation belts. Currently, there are no data from CubeSats operating in the radiation belts, and although the GTOsat will soon be operating in the radiation belts, this is an active space for CubeSat development. The energetic charged particle sensor on board the experiment for space radiation analysis is a particle telescope which will test and mature new technologies for use in space missions. The energy range of interest for the energetic charged particle sensor is from 100 keV to 1000 MeV for protons, and from 100 keV to 20 MeV for electrons.

The observation of secondary gamma-rays provides an alternative method of measuring cross sections that populate excited final states in nuclear reactions. The angular distributions of these gamma-rays also provide information on the underlying reaction mechanism. Despite the large number of data of this type in the literature, publicly available R-matrix codes do not have the ability to calculate these types of angular distributions. In this paper, the mathematical formalism derived by C. R. Brune and R. J. deBoer [Phys. Rev. C 102, 024628 (2020)] is implemented in the R-matrix code AZURE2 and calculations are compared with previous data from the literature for the N-15(p, alpha(1 gamma))C-12* reaction. In addition, new measurements, made at the University of Notre Dame Nuclear Science Laboratory using the Hybrid Array of Gamma Ray Detectors (HAGRiD), are reported that span the energy range from E-p = 0.88 MeV to E-p = 4.0 MeV. Excellent agreement between the data and the phenomenological fit is obtained up to the limit of the previous fit at E-p = 2.0 MeV and the R-matrix fit is extended from E-x approximate to 13.5MeVup to E-x approximate to 15.3MeV, where N-15+p and C-12+alpha reactions are fit simultaneously for the first time. An excellent reproduction of the N-15(p, alpha(1)gamma)C-12* and C-12(alpha, alpha)C-12 data is achieved, but inconsistencies and difficulty in fitting other data are encountered and discussed.