Galactic Cosmic Ray Exposure of Humans in Space : influence of galactic cosmic ray models and shielding on dose calculations for low-Earth orbit and near-Earth interplanetary space

Mrigakshi, Alankrita Isha

The radiation environment in space is one of the primary concerns for human spaceflight as it poses potential risk to astronauts’ health. Galactic Cosmic Rays (GCR), consisting of high-energy nuclei, are a major source of radiation exposure in space. As the number of people visiting space in low-Earth orbit is increasing and mankind prepares to go beyond, the issue of radiation protection against GCR thus becomes vitally important. The pre-flight assessment of radiation-related health risks is achieved by performing numerical simulations of the mission scenario to estimate the necessary radiation-dose quantities. This technique requires models describing the radiation spectra, the target and shield configurations, and additionally transport codes to simulate the passage of radiation through matter. The reliability of the calculated dose therefore depends on the accuracy of all these models. During the course of this PhD work, commonly used models describing the GCR spectra are evaluated for their accuracy for various time periods. The model spectra of nuclei, most relevant for space dosimetry, are compared with measurements from high-altitude balloon flights and space missions. The GCR models included in this work are CREME96, CREME2009, Burger-Usoskin, Badhwar-O’Neill2010, Badhwar-O’Neill2011, Matthiä-ACE/OULU and SPENVIS/ISO15390. The influence of using these different GCR models on the dose calculations is studied for a time period ranging over the last four decades. This is achieved by calculating the absorbed dose and dose equivalent rates in a spherical water phantom using the GEANT4 Monte-Carlo framework. Additionally, the influence of aluminium shielding of varying thicknesses (0.3 g/cm2, 10 g/cm2 and 40 g/cm2) on the dose is investigated for a time period ranging from 1997 to 2012. All these investigations are performed for near-Earth interplanetary space and the orbit of the International Space Station (ISS). Apart from examining these parameters the effective dose, being the baseline quantity for radiation-risk assessment, is estimated. The quantity is calculated for the end of the year 2009 when the highest GCR intensity since the dawn of human spaceflight era was observed. Further studies presented in the thesis include the relative contribution of particles with different energies to the total exposure and the comparison of calculated dose with the measurements conducted inside the ISS and in-transit to Mars by the MSL/RAD instrument.

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Mrigakshi, Alankrita: Galactic Cosmic Ray Exposure of Humans in Space. influence of galactic cosmic ray models and shielding on dose calculations for low-Earth orbit and near-Earth interplanetary space. 2013.

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