structural integrity of the system. The galactic cosmic ray dose is not readily attenuated by shielding material. This may be a factor for setting a lower limit on the radiation dose if the bremsstrahlung dose is adequately attenuated. RADIATION DOSE Based on the Reference System model and subject to its limitations and assumptions, it is estimated that an SPS worker in GEO may be exposed to a dose of as much as 40 rem during any one 90-day mission. This estimate is based on the following assumptions: a. That between 87 and 98 percent of the worker-years occur in GEO. For risk estimation, we therefore have assumed that the total mission will occur in GEO. b. That effective shielding of the worker will be equivalent to 8 grams per cm2 aluminum (which includes 5 g per cm2 of body self-shielding). c. That the exposure is represented by the conditions in a parking orbit of 160° west longitude. This is considered to be the worst case. d. That no contribution to the total dose is made by solar particle events. e. That the bulk of the dose is produced by trapped electron bremsstrahlung, and that the small contribution to the total dose made by galactic cosmic rays can be neglected. f. That the time spent in GEO is 90 days per space tour. The uncertainty in these assumptions is of an order that the dose estimate could vary by a factor of two. HEALTH RISK ASSESSMENTS Using U.S. life tables of age and sex, the estimated lifetime risk for cancer is 0.8 to 5.0 excess deaths per 10,000 workers per rad of exposure. Thus, for example, in 10,000 workers who completed ten missions with an exposure of 40 rem per mission, 320 to 2,000 additional deaths, in excess of the 1640 deaths from normally occurring cancer, would be expected. These estimates would indicate a 20 to 120 percent increase in cancer incidence in the worker-population. The wide range in these estimates stems from the choice of the riskprojection model and the dose-response relationship. The choice between a linear and a 1 inear-quadratic dose-response model may alter
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