arXiv subjects

Thomas M. Orlando

Publications and source records attributed to Thomas M. Orlando.

7 recordsLinked to original sources

arXiv2026

g4chargeit: Geant4-based kinetic Monte Carlo simulations of charging in dielectric materials

We present g4chargeit, a kinetic Monte Carlo framework built on Geant4 for self-consistent simulation of time-dependent electrostatic charging in dielectric materials. The model explicitly incorporates stochastic particle transport and scattering processes using validated Geant4 cross-sections, while self-consistently evolving the electric potential and field. As a representative application, we simulate the charging of regolith grains under average dayside conditions on the Moon. The surface of the Moon, in addition to other airless planetary bodies, are regularly exposed to solar ultraviolet photons and solar-wind plasma, creating a radiation environment in which electrostatic interactions among regolith grains become significant. Until now, simulations of regolith charging have often relied on analytical approximations that oversimplify grain geometry and interaction mechanisms. Our Geant4-based simulations reveal charge accumulation within intergrain micro-cavities, leading to repulsive electrostatic forces consistent with experimental observations. The framework establishes a multiscale approach that links microscopic scattering events to the continuity equation of surface charge density and to the formation of macroscopic surface charge patches in complex grain geometries. Although demonstrated here for planetary regolith, the method is general and applicable to a broad range of dielectric charging problems. The code is openly available at https://github.com/kgandhi63/g4chargeit.git.

physics.app-ph↗

arXiv2026

Electron-Stimulated Desorption of D Atoms from Gibbsite (Al(OD)3) and D2O Ice: Energy and Temperature Dependence of Translational Energy Distributions

The electron-stimulated desorption (ESD) of neutral D atoms from gibbsite (\ce{Al(OD)3}) nanoplatelets and amorphous \ce{D2O} ice has been investigated using $2+1$ resonance-enhanced multiphoton ionization (REMPI) time-of-flight mass spectroscopy in a high vacuum chamber at temperatures 15 and 300\,K. Electron irradiation at 540, 250, and 150\,eV produces similar translational energy distributions at $\sim$300\,K, with a dominant intermediate-temperature component ($T \sim 1500$--$2100$\,K). Cooling to 15\,K suppresses the D atom yield by approximately 50\% and removes the lowest-temperature (slowest) component. This decrease in yield is consistent with diminished hole mobility and restricted diffusion at cryogenic temperatures. Under identical conditions, \ce{D2O} amorphous solid water ice films produce approximately 20 times greater D atom signal than bare gibbsite, with significantly hotter translational distributions, reflecting the higher deuterium surface density and distinct bonding environments of bulk ice relative to the terminal hydroxyl groups on gibbsite. These results identify hole transport to terminal hydroxyl sites as the rate-limiting step for nonthermal D atom production and provide a mechanistic framework for understanding atomic hydrogen release from aluminum hydroxide phases relevant to radioactive waste storage at the Hanford Site.

physics.chem-ph↗

arXiv2026

Creation of Lunar-Like Rims in Ilmenite using Synthetic Solar Wind

Space weathering of lunar minerals, due to bombardment from solar wind (SW) particles and micrometeoroid impacts, modifies the mineralogy within tens of nanometers of the surface, i.e., the rim. Spectroscopic signatures of these modifications, observed via remote sensing, have long been used to gauge surface exposure times on the Moon. However, the relative contributions of SW and micrometeoroids in the creation of rim features are still debated, particularly for the nanometer-scale clusters known as nanophase iron (npFe0), which commonly form in ferrous minerals. We address this issue in the laboratory, using deuterium ions and low-energy electrons as a synthetic solar wind plasma to irradiate ilmenite (FeTiO3), a common lunar mineral. Characterization by high-resolution scanning transmission electron microscopy and electron energy-loss spectroscopy shows that the SW alone creates rims with all the main characteristics of lunar samples. We conclusively identify npFe0 and quantify its distribution as a function of depth and fluence, allowing us to estimate the SW exposure of Apollo soil 71501. Our results confirm that small npFe0 particles (<10 nm in diameter) form from SW irradiation. Such experiments provide microscopic details of space weathering, improving the link between surface modification processes and macroscopic remote-sensing data.

astro-ph.EP↗

arXiv2025

Micrometeoroid Impacts: Dual Pathways for Iron Reduction and Oxidation on Lunar and Asteroidal Surfaces

Nanophase metallic iron ( $\mathrm{npFe}^0$ ) is a key indicator of space weathering on the lunar surface, primarily attributed to solar wind irradiation and micrometeoroid impacts. Recent discoveries of hematite ( $\mathrm{Fe}_2 \mathrm{O}_3$ ), a highly oxidized form of iron, in the lunar polar regions challenge the prevailing understanding of the Moon's reducing environment. This study, using ReaxFF molecular dynamics simulations of micrometeoroid impacts on fayalite ( $\mathrm{Fe}_2 \mathrm{SiO}_4$ ), investigates the atomistic mechanisms leading to both reduced and oxidized iron species. Our simulations reveals that the high-temperature and pressure conditions at the impact crater surface produces a reduced iron environment while providing a transient oxygen-rich environment in the expanding plume. Our findings bridge previously disparate observations-linking impact-driven $\mathrm{npFe}^0$ formation to the puzzling presence of oxidized iron phases on the Moon, completing the observed strong dichotomous distribution of hematite between the nearside and farside of the Moon. These findings highlight that micrometeoroid impacts, by simultaneously generating spatially distinct redox environments, provide a formation mechanism that reconciles the ubiquitous identification of nanophase metallic iron ( $\mathrm{npFe}^0$ ) in returned lunar samples with $\mathrm{Fe}^{3+}$ signatures detected by remote sensing. This underscores the dynamic nature of space weathering processes. For a more nuanced understanding of regolith evolution, we should also consider the presence of different generations or types of $\mathrm{npFe}{ }^0$, such as those formed from solar wind reduction versus impact disproportionation.

astro-ph.EP↗

arXiv2023

Designing a boron nitride polyethylene composite for shielding neutrons

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 which 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 72x over aluminum (Al) and 4x over HDPE; this is a significant improvement in shielding effectiveness that could dramatically reduce the radiation exposure of occupational workers.

physics.app-ph↗

arXiv2022

Performance of chemically modified reduced graphene oxide (CMrGO) in electrodynamic dust shield (EDS) applications

Electrodynamic Dust Shield (EDS) technology is a dust mitigation strategy that is commonly studied for applications such as photovoltaics or thermal radiators where soiling of the surfaces can reduce performance. The goal of the current work was to test the performance of a patterned nanocomposite EDS system produced through spray-coating and melt infiltration of chemically modified reduced graphene oxide (CMrGO) traces with thermoplastic high-density polyethylene (HDPE). The EDS performance was tested for a dusting of lunar regolith simulant under high vacuum conditions (~10-6 Torr) using both 2-phase and 3-phase configurations. Uncapped (bare) devices showed efficient dust removal at moderate voltages (1000 V) for both 2-phase and 3-phase designs, but the performance of the devices degraded after several sequential tests due to erosion of the traces caused by electric discharges. Further tests carried out while illuminating the dust surface with a UV excimer lamp showed that the EDS voltage needed to reach the maximum cleanliness was reduced by almost 50% for the 2-phase devices (500 V minimum for rough and 1000 V for smooth), while the 3-phase devices were unaffected by the application of UV. Capping the CMrGO traces with low-density polyethylene (LDPE) eliminated breakdown of the materials and device degradation, but larger voltages (3000 V) coupled with UV illumination were required to remove the grains from the capped devices.

physics.app-ph↗

arXiv2020

Lunar Volatiles and Solar System Science

Understanding the origin and evolution of the lunar volatile system is not only compelling lunar science, but also fundamental Solar System science. This white paper (submitted to the US National Academies' Decadal Survey in Planetary Science and Astrobiology 2023-2032) summarizes recent advances in our understanding of lunar volatiles, identifies outstanding questions for the next decade, and discusses key steps required to address these questions.

astro-ph.IM↗