6 .Al6 This finishes the qualitative description of the first order sequence of events in stopping cosmic rays. Only the most common decay and interaction modes have been included. The simulation to be done is the same as cosmic ray simulation programs "right here on Earth", and described by Hillas (6.A8). Several programs specifically for radiation shielding are described in references (6.A9) and (6.Al0). As to the value of radiation shielding calculations: in the early 60's the Van Allen belt electrons were a serious worry, so calculations on dosages were done. Generally these were found to be wrong (usually too high) roughly by a factor of 2. Even today (6.A9) (6.Al0) accuracy is not good. Much of this inaccuracy is due to the crude models of men used, such as tissue spheres (6.All), slabs , etc .. Also, the space vehicles themselves are never simple shapes, but have boxes, tanks, and struts which subtend generally inconvenient solid angles. VI.A.5: SIMPLEMINDED CALCULATION OF SHIELDING MASS This analysis presents an estimate of shielding area density to achieve a low dose rate (.5 rem/year) from the galactic background radiation. In the 30's Millikan et al flew ionization chambers in balloons (6.A12). They measured ionization rate and air pressure, and plotted the ionization rate as a function of the shielding thickness of the atmosphere above the ionization chambers (see Figure 6.A6). To use their graph to find a shielding area density for a given dose rate, we first require some suitable conversion factors. Knowing that the average ionization energy of air is 35 eV/ion (6.Al3) and that air density at one atmosphere pressure is .0732 lb/ ft 3 = l.17xl03 g/cm 3 (6.Al4), it is easy to calculate an energy flux per unit mass, E/m. If Xis the ionization rate in i ons/cm 3 -sec-atm from Figure 6.A6, then:
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