6.All The rbe is a quadratic function of the LET, or ~inear ~nergy !ransfer by ionization in material. The LET is simply dE/dx, where Eis the kinetic energy of the incident particle and xis path length. For charged particles, dE/dx goes through an extremum near the end of their range. This gives rise to the increase in rbe for intermediate energy particles in the rbe vs. E curves on Figure 6.A5. For high rates of energy transfer the concept breaks down because the particle can do no more damage than to kill every cell that it traverses. For this reason the rbe saturates at 20 when the LET ~ 200 keV/micron. There are several cases where the rbe = 1. This is for all energies of photons and all Z = 1 charged particles (Z is the atomic number) with kinetic energies greater than 10 9 ev, as well as neutrons with E ~ 10 9 eV. Also, for electrons of all energies rbe = 1. Even though the basic aspects of the problem are well understood, there is still controversy over how many rem should be considered an allowable dose for given conditions. Table 6.A4 states dose rates for a variety of cases. In addition to the symptoms listed there, there may be delayed effects such as: 1. Depilation 2. Chronic skin damage 3. Cataracts 4. Leukemia 5. Bone Cancer 6. Premature aging Note the distinction between skin dose and whole body dose. The galactic background radiation is quite penetrating and would give whole body doses, whereas the doses from solar partiGle events and the Van Allen belts may give substantial skin doses. The most stringent standard is that for pregnant women and young children, .5 rem/year. This is close to the natural background radiation on Earth, so it is a rather conservative standard. For purposes of this study this is considered the target dose rate that any radiation shielding scheme must be capable of beating.
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