of ten greater than the Wilson and Denn (1976) calculations. These differences in dose calculations for the 1972 event are still to be resolved. Total 90-Day Dose Equivalent Daily dose equivalents in the three phases of the SPS mission are summarized in Table 1. From the table, the best estimate of dose equivalent from predictable radiation sources at the worst parking orbit is approximately 40 rem for 90 days in GEO, assuming a Q value of unity. This dose equivalent value is derived from the calculations of Seltzer (1980) for a geostationary orbit with an altitude of 35,790 km, an inclination of 0°, and a parking longitude of 160°W, the worst condition for radiation exposure to trapped electrons (Stassinopoulos, 1980). The incident electron spectrum was integrated for the epoch 1979.0 using the AEI7-HI environmental model. This model is based on a recent compilation of trapped electron data yielding conservatively high average values for the flux densities. A total spherical shield of 8 g/cm^ aluminum equivalent material (3 g/cm^ of spherical spacecraft shielding plus 5 g/cm^ effective body selfshielding) is assumed to obtain the above value of the absorbed dose in water. The shape of the dose vs depth curve is such that a 50 percent change in the total shield assumed will affect the volume of the dose by a factor or two. The largest contribution to the dose equivalent is the bremsstrahlung. In addition, there is about a 10 percent probability of an additional 2.5 rem from solar particle events, and a smaller probability that the SPE dose equivalent might be as high as 25 rem. A smaller contribution will be made by GCR. Therefore, the worst case 90-day mission dose equivalents will most likely be within the range of 40 rem (no SPE) to 65 rem (with large SPE). The Committee emphasizes that these are preliminary dose-equivalent estimates with large uncertainties. ASSESSMENT OF RISK FOR CANCER INDUCTION This SPS committee considered radiation-induced cancer the major health risk associated with exposure to ionizing radiations at dose levels most likely to be encountered in the SPS space environment* The risks can be calculated based on the whole-body exposure and a linear-quadratic dose-response model (NAS-BEIR, 1980), and average career dose-equivalent values may be used. As an illustrative example,
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