minimum and maximum activity time periods and, in the preliminary study, are restricted to attenuation through spherical aluminum shells of varying thicknesses with a conversion to dose in water at the center of the sphere. The 6-month parametric data of dose versus aluminum thickness span a range of dose levels that would encompass specific dose criteria set for SPS missions. Figures 4 and 5 present the radiation attenuation data for 500 km, 30 degree circular LEO during solar minimum and maximum activity time periods respectively. It is noted that the most damaging radiation affecting the occupational health for this orbit is due to trapped protons, and in particular during the solar minimum time period. This is due to the fact that during solar maximum period the lower Van Allen Belt has fewer trapped protons due to atmospheric interaction with the low part of the belt at this time compared to the solar minimum period. For GEO, the radiation attenuation data for both solar maximum and minimum activity periods are presented in Figure 6. It is noted that for thin-walled space vehicles the most deleterious radiation would be from trapped electrons. However, on incidence of a solar flare, the flare proton dose predominates for wall thicknesses greater than 2 1 gm/cm of equivalent aluminum. The solar flare protons are reported for both the solar minimum and maximum activity periods (Ref. 10). The 90% confidence level reported represents solar cosmic protons received from a statistical sampling of many larger flares of the past solar cycle and means that at the 90% confidence level no greater fluence of protons, with the spectral rigidity of the sample, would be received in the defined mission time. Other confidence levels could be used for the design criteria (e.g., 99% or 99.9%) but with a significant increase in proton fluence, hence dose, of approximately an order of magnitude (factor of 10). However, since the SPS operations are Earth orbiting and not deep space manned missions with limited alternative evasive moves possible, it is reasonable to use the lower total proton fluences (90% confidence level) with the potential of replacing the crew when and if they have received several large flares, thus shortening their mission life in space.
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