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.
Kinetic Monte Carlo (KMC) simulations of dielectric materials are of broad interest across multiple fields, including the modeling of photovoltaic devices [1], electrostatic forces in DNA packing [2,3], radiation transport in semiconductors [4,5], and dielectric breakdown in space equipment [6,7]. Radiation transport is commonly simulated using Monte Carlo (MC) codes because of their generalizability and computational efficiency, employing random walks to sample complex transport dynamics. However, such MC codes often lack the capability to selfconsistently model time-evolving phenomena, such as charge accumulation [5,8]. As a result, these simulations frequently require multiple interdependent simulation toolkits, leading to hybridized computational models [6,7,9,10].
A sought-after application of KMC simulations is modeling the charge evolution of regolith dust on airless bodies, which is critical both for advancing our understanding of fundamental planetary surface processes and for addressing practical challenges in space exploration [11]. Airless bodies possess virtually no atmosphere and are therefore exposed to solar ultraviolet (UV) radiation, which produces photoelectrons (PEs), as well as to solar-wind (SW) plasma. For example, regolith grains on the Moon’s surface become electrically charged [12,13,14]. The behavior of these charged grains influences several key processes, including modification of the local plasma environment, which affects dust transport and adhesion and, in turn, impacts the performance and longevity of equipment such as solar panels, spacesuits, and optical instruments [15,16,17,18]. Regolith-grain mobility also has important implications for in situ resource utilization, instrument degradation, and habitat contamination. The charging of regolith grains further affects the performance of electrodynamic dust shields and filtration devices, and electrostatic adhesion to lunar exploration systems [19,20].
The accumulation of charge is primarily driven by the reabsorption of emitted PEs, especially within micro-cavities formed between neighboring regolith grains, leading to heterogeneous and spatially patchy charge distributions. This localized charge buildup can trigger dust lofting or even dielectric breakdown [21,22,23,24,25]. To quantify these effects, Wang et al. [24] introduced the patched-charge model, which emphasizes the role of micro-cavities in facilitating localized charge accumulation on airless planetary bodies such as the Moon. The patched-charge model considers a portion of a grain within the regolith bed that emits PEs and/or secondary electrons into a cavity, resulting in repulsive forces between surrounding grains [24]. Complementary to this work, Zimmerman et al. [26] developed a purely analytical model for charge buildup on hexagonally packed spherical grains and showed that electric fields can reach strengths of MV/m in less than one lunar day-sufficient to induce dielectric breakdown [26]. However, these analytical models neglect the effects of non-spherical and asymmetrical grain geometries and lack the capability to explore dependencies on grain composition.
In this article, we model charge accumulation within micro-cavities of regolith grains on airless bodies to demonstrate the capabilities of our KMC framework built in Geant4 (GEometry ANd Tracking) [27,28,29]. By leveraging the capabilities of Geant4, we enable simulations with arbitrary geometries and material compositions while capturing the stochastic nature of electron emission, reabsorption, and SW interactions on a grain-by-grain basis and evolving the resulting electric field in situ. We employ a self-consistent MC architecture that links microscopic scattering events to the continuity equation of surface charge density, enabling accurate simulation of regolith grain charging dynamics. We benchmark our Geant4-based code, called g4chargeit, against a simple stacking of spheres [26] and the patched-charge model [24]. This work provides an open-source, all-in-one software package for the broader scientific community.
The remainder of the paper is organized as follows. Section 2 describes the simulation framework, and Section 3 details the implementation and execution. Section 4 presents grain stacking configurations of increasing complexity and demonstrates the capabilities of the code. The paper concludes with a discussion of limitations and potential applications beyond space science.
Geant4 is an open-source, C++-based simulation toolkit used to model particle scattering and transport through matter, with applications in high-energy, nuclear, medical, and space physics [27,28,29]. Our MC framework for time-dependent simulations of electrostatic fields is built in Geant4 (version 11.3.0). The following extensions to Geant4 are incorporated into g4chargeit:
(i) Geometry Description Markup Language (GDML) for importing complex geometries and computer-aided design (CAD)-based structures [30],
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