Masters Theses

Date of Award

12-1997

Degree Type

Thesis

Degree Name

Master of Science

Major

Nuclear Engineering

Major Professor

Lawrence W. Townsend

Abstract

Since the advent of space flight, the understanding of the space radiation environment has been an important subject of study. With the advent of manned space flight, and the possible extension of these missions into interplanetary space, this understanding has taken on new importance. For these longer missions which require travel outside the Earth's magnetosphere, the two main sources of exposure are solar particle events (SPEs), and the galactic cosmic rays (OCRs). Solar particle events are periods during which the Sun emits large fluxes of charged particles over a wide energy range into the surrounding interplanetary medium. Galactic cosmic rays in space constitute an ever-present, isotropic radiation field of low flux and relatively high energy. It is common knowledge that both SPEs and OCRs can yield significant health risks to crews on deep-space missions. OCR flux levels are well documented, and even in the absence of shielding do not constitute a danger from acute exposure, with the hazard being increased lifetime risk from prolonged exposures at levels typically around 40-50 cSv/yr during solar minimum. SPEs are not a well-understood phenomenon, nor are they currently predictable. This is the major source of concern in mission planning and crew protection, since acute doses received by an astronaut behind shielding equivalent to a spacesuit during a large, energetic SPE can range as high as ~500 cGy. In this work, we propose a possible approach to the study of the SPE phenomenon that holds promise of providing in flight, real-time radiation exposure warning to crews of deep space missions. Calculations of total dose and dose-equivalent as functions of time, as well as dose-rate and dose-equivalent-rate since event start are presented for twenty-five of the larger solar particle events that occurred during the period between November 1987 and August 1991. The doses, dose-equivalents, and rates presented are for exposures to the skin, ocular lens, and bone marrow behind a thickness of Aluminum shielding which provides protection comparable to that of a spacesuit. The calculated dose vs. time profiles are parameterized using a Weibull cumulative distribution as the fitting function. Parameters are determined using least-squares techniques. These fitted dose vs. time profiles are then differentiated to produce smoothed dose-rate curves for each of the events. These results provide a useful starting point for the development of methods to predict the cumulative doses and times to reach various dose limits from a limited number of dosimeter measurements early in the evolution of a solar particle event.

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