GALACTIC COSMIC RAYS Galactic cosmic radiation consists primarily of high-energy nuclei with origins outside our solar system; these particles appear to pervade regions outside our magnetosphere isotropically. Approximately 88 percent are protons, 10 percent are helium nuclei, 1 percent are electrons, and 1 percent are heavy nuclei (Z > 2). Ratios of abundances relative to carbon (Z = 6) are shown for the heavy nuclei as a function of nuclear charge number or Z in Fig. A-2. If each nuclear species has a similarly shaped energy spectrum, the relative absorbed dose in free space in a small sphere of tissue from each species would be the GCR abundance ratios scaled by Z?. Due to differences in the spectral shapes at low energies, there are variations noted from the simple Z? scaling. However, the difference at low energies is important only behind thin shielding. Table A-l presents three calculations of the estimated absorbed dose rate in a small tissue sphere in free space and the relative composition from particles grouped in broad ranges of Z (Curtis, 1974). The first two calculations (Curtis and Wilkinson, 1968; Schaefer, 1968) were obtained by integration of the appropriate energy spectra. The third calculation was based on experimental data from a balloon at high altitude (Anderson, 1968). The daily dose rate of between 30 and 40 mrad/day is the maximum expected, since the spectra used were appropriate for solar minimum when the galactic flux densities are known to be the highest. The daily absorbed dose within the body will be less than this value because, although the dose from the protons will not decrease with depth, there will be a decrease in the contribution from the heavier components (see Section 2). Curtis (1973) calculated that the flux densities of HZE particles with LET > 100 keV/pm decrease from 8 to 3 cm? hr as the shielding is increased from 1 to 10 g/cm? of aluminum. Approximately 65 percent of the particles with charge greater than two have charges in the range of 20-26, and this percentage does not vary significantly with thickness of shielding. Measurements made on Soviet satellites Prognoz 1 and Prognoz 2 over an eight month period in 1972, during quiet radiation conditions outside the magnetosphere, yield an estimated dose-rate of about 24 mrad/ day (Logachev et al., 1974). The estimated dose rate is about 10 mrad/ day at solar maximum. Data from U.S. satellites Mariner II and Mariner IV from late 1962 to 1964 provide estimated dose-rate values of 30 to 45 mrad/day at solar minimum. Pioneer IV measurements correspond to dose rates of about 9.6 and 24 mrad/day, the latter at solar minimum (Janni and Holly, 1969). Pioneer V data yield an estimated dose rate value of about 14.4 mrad/day at solar maximum. These estimates are summarized in Table A-2.
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