for periods of days after the sun has been active. Consequently, the integral particle fluxes will be on the order of lO1^ particles/cm2 for a five-year period. Fortunately, the Van Allen belt particles at synchronous Earth orbit are rather easily stopped, the protons by £10“^ gm/cm^, the electrons by £2 gm/cm^, essentially independent of material. However, while these low- energy protons do not produce secondary nuclear radiation, the electrons produce secondary X rays (bremsstrahlung). The production of the bremsstrahlung is linearly dependent upon the atomic number (2) of the material in which the electrons are stopped. Thus, beryllium (2 = 4) produces only 4/82 as bremsstrahlung as lead (2 = 82). Aluminum (2 = 13) is a standard spacecraft material and was used for these bremsstrahlung calculations. Because the bremsstrahlung is difficult to shield against, it is the Van Allen belt produced radiation present for aluminum shields £2 gm/cm^. The total Van Allen particle dose is the sum of the electron dose (£2 gm/cm^) and the bremsstrahlung dose (£2 gm/cm^). Because electrons and bremsstrahlung have an RBE (relative biological effectiveness) or QF (quality factor) of unity, the rad dose is equal to the rem dose (rem = Roentgen equivalent man). The solar flare particle dose at synchronous earth orbit (Figure 6.1-2) is essentially the same as that for free space. While the quiet sun geomagnetic cutoff energy for protons at this altitude is ^30 Mev (protons less than this energy cannot reach the earth from the outside because of the earth's magnetic field), this cutoff energy decreases considerably when the sun is active. . Since 30-Mev protons can only penetrate ^1 gm/cm2, even the quiet sun geomagnetic field will not reduce the solar flare particle doses below the free space value for shields £1 gm/cm2 (and the active sun geomagnetic field has even less effect). The solar flare model used here is that from NASA, TM-x-64627, and has an integral proton flux of ^10^ protons/cm2 above 30 Mev. This is a large model, exceeding the total solar flare particle fluxes for the five active years of 1956 through 1960. However, in August, 1972, there was an extremely large solar flare, which produced particle fluxes of £10^0 cm“^ above 30 Mev at the earth. Therefore, this NASA solar flare model appears to be reasonable. Based on this model, the tissue dose as a function of shield thickness was calculated in rads and in rem. The rem dose is higher because the RBE (or QF) are >1 for protons. The galactic (cosmic ray) particles (Figure 6.1-3) have such high energies (£1,000 Mev) that they are virtually unshieldable as far as spacecraft are concerned. Fortunately, their flux is low (£4 particles/cm2 - sec), so their dose rate is £15 rad/yr. Because of the screening effect of the sun's magnetic field (carried by the solar wind), the galactic particle flux and dose rate decrease when the sun is active to ^6 rad/yr. The earth's magnetic field has essentially no effect on these particles at synchronous orbit. Because of the high energies of galactic particles, the first ^100 gm/cm of shielding actually increase the dose rate because of the secondary radiations they produce (cascade and evaporation nucleons). Adding the Van Allen, solar flare, and galactic particle dose rates yields a dose versus shield thickness curve (Figure 6.1-4) which decreases from ^10° rems/5 years (at ^0 gm/cm^) to <10^ rem/5 years (for 2 gm/cm2)f but is
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