The advantages of the spherical geometry setup comparing to planar in GCR dose assessment
Abstract
The Galactic Cosmic Rays (GCR) compose a significant part of the radiation environment in the space. The intensity of the GCR is lower, comparing to Solar Energetic Particles (SEP) but GCR affect astronauts and spacecraft during the entire space mission. For a long-duration mission with the typical shielding of 10 g$\cdot$cm$^{-2}$, the total radiation dose due to GCR would exceed radiation dose due to most of the SEP events. The GCR comprise 86% protons, 12% alpha particles, and heavier nuclei. The energy of the GCR particles is in a broad energy range. Most of the radiation dose inside the shielding is due to particles, which have kinetic energy between 10 and 10$^{5}$ MeV$\cdot$nucleon. High-energy GCR particles induce a lot of secondary particles, which number is higher behind thick shielding. A large part of secondary particles is scattered away from the incoming direction of parent particles.
In this work, we use GEANT4 Monte-Carlo calculations to demonstrate the role of indirectly scattered secondary particles. We use a simple calculation setup, with spherical shielding and spherical water phantom. Instead of a convenient circular unidirectional source, we use a set of unidirectional ring sources. It allows us to calculate the radiation dose from particles, which pass at a certain distance from the central axis of the setup. We demonstrate that a large part of the radiation dose equivalent (up to 50% of the secondary particles dose and up to 90% of the neutron dose in the case of 60 g$\cdot$cm$^{-2}$ shielding (Dobynde and Shprits, 2020)) is associated with primary particles, which propagated initially in the directions away from the phantom. The radiation dose due to secondary particles increases with the increase in the distance between the initial direction of primary particles and the phantom. We attribute this effect to the increase in the amount of shielding along the initial direction of primary particles, which increases with the distance from the shielding center. Since the ``actual'' shielding thickness along particle track is higher compared to ``nominal'' shielding thickness, the number of secondary particles scattered in the direction to the phantom is also higher. We also demonstrated the influence of the value of shielding radius on the radiation dose composition. Summing up, we demonstrate the importance of the geometry factor of the shielding, shielding curvature, and dimensions, for acute GCR dose assessment. We believe that accounting for the geometry factors, even with such a simple geometry, provides a better dose assessment comparing to slab geometry setup, where the geometry factors are excluded. For the same reason, the convenient ray-tracing dose calculation with depth-dose curves is not the optimal choice for high-energy GCR dose assessment. Instead, we suppose to use a ``spherical'' basis for depth-dose dependencies and present first results. Acknowledgments This work used computational and storage services associated with the Hoffman2 Shared Cluster provided by UCLA's Institute for Digital Research and Education's Research Technology Group. References Dobynde, M. I., & Shprits, Y. Y. (2020). Radiation environment created with GCRs inside a spacecraft. Life Sciences in Space Research, 24(August), 116-121. https://doi.org/10.1016/j.lssr.2019.09.001- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E1886D