Pertinent Radiation Terminology III
The radiation from HZE particles (heavy charged particles encountered in space) is high LET radiation, with characteristics much like the radiation from alpha particles, to which we are exposed on earth. We are all regularly exposed to relatively low doses of alpha radiation, from sources in our air, food and water. Cigarette smokers are exposed to considerably higher doses of alpha radiation. Alpha particles have very little penetrating ability, as they expend their energy rapidly. For example, a 5-MeV alpha particle, having a range of 35 microns in tissue, will not penetrate below the cornified layer of the skin, and is therefore is not an external hazard. If inhaled or ingested, however, an alpha particle can become hazardous as it may deposit all of its energy within one or two cells. Thus, the amount of local damage can be very great.
Unlike alpha radiation, the radiation from HZE particles has the ability to penetrate deeply into tissue and cause biologic effects in internal organs. HZE particle radiation is thought to be highly effective at inducing cancer and producing other adverse biological effects, as compared to x-rays. RBEs (RBE = relative biological effectiveness) greater than 20 have been observed for the production of some types of tumors in animals by HZE particles, suggesting that a dose of HZE particle radiation can cause more than 20 times as much cancer in animals as the same dose of low LET radiation.
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This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.