Award Date

May 2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Interdisciplinary Programs

First Committee Member

Francis Cucinotta

Second Committee Member

Steen Madsen

Third Committee Member

Daniel Young

Fourth Committee Member

Alexander Barzilov

Number of Pages

179

Abstract

The two main sources of primary space radiation, Galactic Cosmic Rays (GCRs) and Solar Particle Events (SPEs), encompass a spectrum of ions, ranging from protons (Z=1) to nickel ions (Z=28), with energies spanning from less than 1 keV to exceeding a few TeV. While SPEs represent sporadic events primarily comprised of a high flux of low- to intermediate-energy protons, GCRs are characterized by continuous low flux of diverse ions with higher energies, peaking in intensity near the solar minimum. During a Mars mission, astronauts will encounter primary particles of high energy from GCR and SPE spectra, capable of penetrating spacecraft and habitat structures, along with secondary particles generated through interactions between primary space radiation and shielding materials and human tissues. While considerable attention has been devoted to primary particles in space missions in prior research, the biomedical consequences of exposure to secondary particles, especially mesons, have received comparatively less scrutiny despite their growing significance in deep space with augmented shielding. This dissertation project fills this gap by investigating the dosimetric quantities of secondary neutrons, pions, and kaons within various human organs for Mars exploration for the first time. Using a realistic computational human male phantom collaboratively developed by the International Commission on Radiological Protection (ICRP) and the International Commission on Radiation Units and Measurements (ICRU), the absorbed dose and dose equivalent in human organs of each baryon, meson, and lepton, as well as their effective dose, have been meticulously evaluated. For the worst-case scenario, exposure to GCR near the solar minimum and two of the most significant SPEs in the space age, events in August 1972 and September 1989, has been considered for the interplanetary cruise phase and the surface phase on Mars with varying thicknesses of aluminum shielding. The simulations have been executed using the PHITS3.27 Monte Carlo simulation toolkit on the Cherry-Creek Cluster at the University of Nevada, Las Vegas (UNLV) National Supercomputing Institute (NSI). Dosimetric quantities have been evaluated based on the ICRP publications and the NASA Space Cancer Risk (NSCR) model. The results indicate that the total effective dose assessed with the NSCR model for GCR exposure in interplanetary space decreases from 43.4 cSv/yr to 39.0 cSv/yr as the aluminum shielding amount increases from 1 g/cm2 to 50 g/cm2. Conversely, the sum of neutron, pion, and kaon effective doses increases from 2.8 cSv/yr to 9.3 cSv/yr. This implies that the contribution of these secondary particles to the total effective dose escalates from 6.5% for 1 g/cm2 aluminum shielding to 23.9% for 50 g/cm2 aluminum shielding. An increase in secondary contribution is also suggested with the ICRP model and SPE exposure. The contribution of secondary particles on the Martian surface is found to be less contingent on aluminum shielding depth, while it surpasses 20% even with thin 1 g/cm2 aluminum shielding. This is attributable to the heightened generation of secondary neutrons in the Martian atmosphere and ground. The comprehensive analysis conducted in this project, pertaining to tissue-specific dosimetric quantities within an advanced human phantom under practical radiation exposure scenarios, furnishes valuable insights into the potential health risks associated with secondary particles during Mars exploration.

Keywords

Galactic Cosmic Rays; Monte Carlo Simulation; Organ Dose Equivalent; Radiation Transport; Solar Particle Events; Space Radiation

Disciplines

Medicine and Health Sciences | Nuclear | Other Physics | Physics

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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