ESTIMATED ABSORBED DOSES AND DOSE EQUIVALENTS IN SPS WORKERS Low Earth Orbit Dose estimates are most accurate for the LEO phase of the mission. Protons in the South Atlantic Anomaly pose the only major source of external radiation. The dose equivalent in LEO will vary by about a factor of two between solar minimum and solar maximum (Stassinopoulos, 1979). The dose rate estimates at solar minimum, when the doses are higher, range from 0.15 rad per day (Hardy, 1979) to 0.3 rad per day, the latter value corrected from a semi-infinite slab calculation (Stassinopoulos, 1979) to spherical shielding (Seltzer, 1979). Since Q for this radiation is close to unity, these values are good estimates of the dose equivalent rate in rem per day. There will be a negligible contribution to the dose from galactic cosmic rays and solar particle events, due to the large amount of geomagnetic shielding available in the LEO trajectory. The total dose equivalents for a 90-day mission in LEO are therefore estimated to be between 14 and 28 rem at solar minimum and between 7 and 14 rem at solar maximum. Transfer Ellipse Dose calculations have been made for the 5.25 hour transfer ellipse from LEO to GEO. These results vary between 1.0 rad (Hardy, 1979) due primarily to protons, and 0.018 rad (Stassinopoulos, 1979) due primarily to bremsstrahlung. As is the case for LEO, these are the estimates for the dose equivalents in rem as well. The large difference is the result of different assumptions in the trajectory made for the calculations. Geosynchronous Orbit In GEO, a majority of the absorbed dose is due to bremsstrahlung produced by the trapped electrons. An estimate of the dose equivalent for a worst case exposure from predictable radiation is 0.43 rem per day inside an aluminum sphere with radius 8 g/cm^ (Seltzer, 1980). The contribution to the dose equivalent by the galactic cosmic rays (GCR), particularly from the heavy charged particle component, may be important. The fragmentation characteristics of the heavy particle component must be considered for an accurate estimation of the absorbed dose from GCR (Wilkinson and Curtis, 1972). The quality factors (Q) necessary to convert absorbed dose to dose-equivalent are generally unknown for these radiations. Using a Q of 3 for the GCR, independent of depth, yields a very rough estimation of the dose equivalent as a function of depth. Use of an average value for Q of 3 for the galactic cosmic ray contribution is consistent with current recommendations (ICRP 26, 1977). Q values are under continual reassessment, and it is
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