2. REFERENCE SYSTEM EVALUATION In determining potential radiation risks to the health of SPS workers in the space environment, an estimate must first be made of the radiation environment outside the space vehicle. This environment consists of trapped electrons and protons, galactic cosmic rays (GCR), ahd sporadic solar particle event (SPE). The average values of the flux densities of the primary radiation are presently known only to within a factor of two. The spacecraft and the space workers comprise a complex distribution of shielding materials which attenuates the primary radiation and which is also a source of secondary radiations. The transport of the radiation through the shield is fairly well understood, but there is uncertainty in the calculations due to assumptions of shield geometry and composition. Dose estimates for previous space missions have been based on three-dimensional distribution of the shielding materials in the spacecraft and in the astronauts. Other calculations have been based upon solid-angle sectoring of the available shielding. All calculations for an SPS mission thus far have been made assuming simple geometries. Based on a recent analysis (Selzer, 1979), the absorbed dose calculated inside a spherical shell at a given radius is three to four times that at the same depth within a semi-infinite slab with isotropic radiation impinging on it from one hemisphere. For the purposes of this report, spherical shield geometry is therefore used to provide a worst case estimate of dose. However, this SPS Committee recognizes that until calculations for SPS radiation environments are based on more realistic shielding configurations, these calculations remain inadequate for detailed, accurate estimation of dose equivalent. In this report, the shielding assumed is 3 g/cm^ of aluminum for the habitat and work stations and 20-30 g/cm? for the storm cellar to be used during solar particle events (SPE). The assumptions are based on the Boeing contribution to the Reference System (DOE/NASA, 1978). In addition, an average of 5 g/cm^ of aluminum equivalent shielding is assumed for the body self-shielding in the present analysis. Thus, the estimates made below can be considered representative average doses at the center of a sphere of radius 8 g/cm^ of aluminum equivalent material. Because the relative biological effectiveness of the different radiations is variable, a quality factor (Q) is assigned to each radiation, permitting calculations of a total dose equivalent (rem). The use of Q is reasonable for biological endpoints of principal concern. However, there is the possibility that other health effects are caused only by high energy heavy ions (HZE particles). Therefore, at the present time, the use of a Q for the HZE particles does not provide a complete assessment of the risk to health of SPS workers.
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