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Contribution of cosmic alphas to generation of albedo neutrons and gammas in lunar regolith: A computational study based on GEANT4 simulations

This study utilizes GEANT4 Monte Carlo simulations to quantify the significant contribution of Galactic Cosmic Ray alpha particles to the generation of albedo neutrons and gamma rays in the lunar regolith, revealing that these secondary particles are predominantly produced in the near-surface region and are primarily low-energy.

Original authors: Harshala Gaonkar, A. Ilker Topuz

Published 2026-07-29

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CC BY 4.0

Original authors: Harshala Gaonkar, A. Ilker Topuz

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Moon as a lonely, airless rock floating in a cosmic shooting gallery. Unlike Earth, which wears a thick blanket of air and a giant magnetic shield to deflect dangerous space bullets, the Moon has nothing to hide behind. Every day, it gets pelted by a relentless rain of high-speed particles from deep space, known as Galactic Cosmic Rays. Most of these bullets are tiny protons, but a significant chunk—about 10 to 15 percent—are alpha particles, which are essentially helium nuclei (two protons and two neutrons stuck together) zooming through the void. When these cosmic bullets hit the Moon's dusty surface, called regolith, they don't just bounce off; they smash into the dirt, triggering a chaotic chain reaction. This collision creates a shower of new, secondary particles, including neutrons and gamma rays, that bounce back up into space. Scientists call this "albedo," a fancy word for the reflection of particles. Understanding this invisible rain of reflected radiation is crucial because it tells us what the radiation environment is really like on the Moon, which is vital for keeping future astronauts safe and helping them understand the Moon's composition.

This paper is a deep dive into one specific part of that cosmic shooting gallery: the role of those alpha particles. While we know protons do a lot of the heavy lifting, the authors wanted to see exactly how much the alpha particles contribute to the mix of reflected neutrons and gamma rays. To do this, they didn't go to the Moon; they built a virtual one inside a supercomputer using a toolkit called GEANT4. They fed the simulation with real-world data on how many alpha particles are out there and how fast they are moving, based on measurements from the PAMELA spectrometer, covering speeds from 0.14 to 52 GeV. They then watched, in slow motion, what happened when these virtual alpha particles hit a block of simulated lunar dirt.

The results of this digital experiment paint a clear picture. The simulation shows that when alpha particles hit the Moon, they don't travel very deep into the soil. Most of them stop within the top layers, like a bullet that loses its energy quickly in a thick wall. Because they stop so early, the explosion of new particles they create is also concentrated right near the surface. As you go deeper into the regolith, the number of new particles generated drops off sharply. Furthermore, the "debris" that flies back up—both the neutrons and the gamma rays—tends to be slow and low-energy. The simulation reveals that while there are a few high-speed, high-energy particles, the vast majority of the reflected radiation is in the low-energy range.

The authors found that even though alpha particles are fewer in number than protons, they are still a major player in creating this secondary radiation environment. The study suggests that ignoring them would give us an incomplete picture of the Moon's radiation landscape. By isolating the alpha particles in their simulation, the researchers demonstrated that these particles are responsible for a quantifiable amount of the albedo neutrons and gamma rays. This isn't a discovery made by measuring the Moon directly in this specific study, but rather a robust computational finding that fills a gap in our understanding. It confirms that the Moon's surface is a busy factory of secondary radiation, driven by both protons and alphas, and that this factory operates mostly in the shallow topsoil, churning out a flood of low-energy particles that bounce back into the void.

Technical Summary: Contribution of Cosmic Alphas to Albedo Neutrons and Gammas in Lunar Regolith

Problem Statement
The lunar surface lacks a substantial atmosphere and global magnetic field, leaving it directly exposed to Galactic Cosmic Rays (GCRs). While GCRs are predominantly composed of protons, alpha particles constitute approximately 10–15% of the total flux. These high-energy alpha particles interact with lunar regolith via spallation reactions and low-energy ( $\alpha$ , n) processes, generating secondary particles including neutrons and gamma rays. A portion of these secondary particles escapes the surface, forming the "lunar particle albedo." While previous research has addressed the influence of protons and hydrogen on lunar albedo neutrons, the specific quantitative contribution of cosmic alpha particles to the generation of albedo neutrons and gamma rays remains a critical area for characterization. Understanding these interactions is essential for accurately modeling the lunar radiation environment, surface composition, and radiation transport for future exploration missions.

Methodology
This study employs Monte Carlo simulations using the GEANT4 toolkit to model the irradiation of lunar regolith by cosmic alpha particles.

Geometry and Material: The simulation geometry consists of a 3×2×3 m³ lunar regolith volume with a depth of 200 cm and a density of 1.93 g/cm³. The regolith composition is defined by nine elemental constituents (Oxygen, Silicon, Iron, Aluminum, Calcium, Magnesium, Titanium, Sodium, and Manganese) based on mass fractions extracted from prior literature.
Primary Source: Incident alpha particles are modeled as a probability-based primary source directed vertically toward the regolith. The energy spectrum is derived from experimental measurements by the PAMELA spectrometer, covering a kinetic energy interval from 0.14 to 52 GeV. The spectrum is discretized into 68 intervals to sample particle energies according to their measured probability distribution.
Physics Configuration: The FTFP_BERT_HP reference physics list is utilized to model particle-matter interactions and transport, ensuring high precision in low-energy neutron transport across a broad energy range.
Detection and Analysis: A thin surface detector (scoring volume) is positioned immediately above the regolith to record escaping secondary neutrons and gamma rays. Particle data, including kinetic energy, spatial coordinates, and transport processes, are recorded during the stepping action. Post-simulation analysis is conducted using Python to aggregate data into energy histograms and determine generation depths.

Key Results

Penetration Range: The penetration range of cosmic alpha particles within the regolith is highly energy-dependent. The majority of incident alpha particles travel short distances, with the population of particles reaching deeper regions decreasing significantly as depth increases. The simulation tracks ranges up to approximately 200 cm.
Albedo Neutron Spectrum: The energy spectrum of albedo neutrons escaping the surface extends over a broad kinetic-energy interval but exhibits a strong decreasing trend on a logarithmic scale. The population is predominantly concentrated in the low-energy region, with a significant reduction in counts as energy increases toward the MeV range. This distribution results from the initial production mechanisms and subsequent transport modifications (scattering and nuclear reactions) within the regolith.
Albedo Gamma Spectrum: Similarly, the albedo gamma-ray spectrum shows a strong energy dependence, with the highest population at lower energies and a quantifiable reduction as energy increases. While the majority of escaping gamma rays are low-energy, a sparse high-energy tail extends into and beyond the MeV region, originating from nuclear de-excitation and inelastic scattering processes within the alpha-induced cascade.
Depth Dependence: The generation of secondary neutrons and gamma rays is primarily concentrated in the near-surface region of the regolith. The production rate progressively diminishes with increasing depth, correlating with the rapid attenuation of the incident alpha particle flux.

Significance and Claims
The study demonstrates that cosmic alpha particles, despite constituting a smaller fraction of the GCR flux compared to protons, make a quantifiable and prominent contribution to the generation of secondary radiation (albedo neutrons and gamma rays) at the lunar surface. The research establishes a computational framework using GEANT4 to evaluate the lunar albedo, radiation transport, and surface composition specifically regarding alpha-induced interactions. These findings complement existing knowledge on proton- and hydrogen-related contributions, offering a more comprehensive understanding of the lunar radiation environment necessary for characterizing surface conditions for future lunar exploration missions.

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