1 rad = 100 ergs/gram = 0.01 Gy However, the same absorbed dose can result in different degrees of biological effectiveness, depending on the type of radiation. The Linear Energy Transfer (LET) is a measure of the energy loss per length of path. Charged particles with a high energy loss rate are more effective in producing biological effects than particles with low energy loss rates. Therefore, a quality factor, Q, based on the ability of each type of radiation to produce ionizations along its path is included in the term dose equivalent. Q = 1 for X-Rays, Gamma Rays, and Beta Particles. Q = 20 for alpha particles [Vanderploeg, 1992], Dose equivalent is most closely related to biological risk. dose equivalent (sievert, Sv) = Q x absorbed dose (Gy) Dose equivalent is also measured by the rem (roentgen equivalent, man) 1 rem = 100 ergs/gram = 0.01 Sv Radiation Effects on Biological Tissue Radiation can act on the nucleus of a cell to cause a physical cleavage in the cell's chromosomal DNA. This cleavage can lead to cell death, or possibly loss or mutation of a segment of the DNA. In this way, a previously normal cell can be transformed into tumor producing cell or even a malignant, cancerous cell. The most susceptible cells to radiation exposure are those cells of low differentiation and high mitotic rates: blood forming cells of the bone marrow, intestinal cells, skin cells, hair cells etc.. Less susceptible are those more highly differentiated and of low mitotic rate such as muscle or nerve cells. It is becoming apparent that there is no clear threshold value below which radiation exposure is safe [Davies, 1992]. Acute radiation effects generally refer to exposure to a high dose of radiation over a short period of time, manifesting as “radiation sickness”. Dose equivalents of 100 - 200 rem cause nausea and vomiting within hours, usually disappearing within a day or two. 200 - 1000 rem causes nausea, vomiting, diarrhea and after a latent period of two weeks, possible hemorrhaging and hair loss. Doses over 600 rem are generally lethal but can be recoverable with medical attention. Doses of 500 - 1000 rem are possible in severe solar flares as occurred in August, 1972 [Letaw and Clearwater, 1986]. This necessitates a storm shelter of some kind in the space craft. Delayed effects of radiation can occur years after prolonged exposure to high or low doses and include leukemia, cancer of the breast, digestive system and lung as well birth defects in progeny. Space Radiation Protection Protection from dangerous levels of exposure comes from shielding of astronauts in space as well as standards set for the allowable exposure limits. A few pharmacological agents which function as scavengers to the damaging radiation induced free radicals are available but these drugs are often toxic themselves at effective levels. Figure 6.8 [Letaw et al., 1987] shows the dose equivalent per year to the bone marrow from galactic cosmic radiation as a function of aluminum shielding thickness. As it is expensive to add such shielding to a spacecraft, more economical alternatives are to include other materials, such as water or fuel between the outer and inner environment of a spacecraft. In on-site habitats, it would be wise to require that astronauts spend their off duty time in some more shielded area of the craft. Protection from solar flares requires a unique “storm shelter” in the space craft and around the construction area. Protection from GCR is more difficult and little can be done in an orbiting space station. Table 6.6 are the astronaut exposure limits set by NASA. Current NASA regulation limits occupational exposure to 1.25 rem per 3 months and 5 rem per year. These are important values for construction crews of a solar power satellite which may have their mission duration determined by these limits.
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