possible the Q value could be increased for the HZE particle contributions. This Committee believes that an average Q of 3 may not be conservative for carcinogenic or mutagenic effects produced at low doses and low dose rates over long periods. This is a simplified model and more must be learned about the variation of Q with dose and depth for the GCR before a more accurate value for the dose equivalent can be obtained. The shielding of manned space vehicles against the proton and helium ion component of the GCR has been examined (Santoro et al., 1973). The absorbed dose and dose equivalent due to primary particles and to secondary particles arising from the self-shielding of the tissue sphere are little affected by increasing the sphere radius from 5 to 20 g/cm? of aluminum. Protons and helium ions and their associated secondary radiations contribute about 55 to 65 percent to the total galactic cosmic ray dose and a smaller percent to the total dose equivalent. The dose and dose-equivalent rates for the combined proton and helium ion components average 31 mrad per day and 73 mrem per day, respectively, yielding an average Q of 2.4. This value of Q is a lower limit when the HZE particle contribution is very small. If the Q of 2.4 is the value for the proton and helium ion components, the average for all components may be higher than the Q value of 3 assumed above. Solar particle events (SPE) will be a major hazard in GEO; special shielding—a storm cellar—is needed for space workers during a solar particle event. The dose received from a given SPE will depend upon the size of the SPE, the length of time of the warning before the particle buildup, the time required for the workers to get inside the storm cellar and the storm cellar shielding thickness. The size and time of occurrence of an SPE are not currently predictable. It is known that a correlation exists with the sunspot number and that the event frequency has an 11-year cycle. Within this cycle, there is a three- to five-year period that is almost event free. During the remaining six to eight years of the period, there is about a 40 percent probability of a large SPE each year. The total dose from the solar particles within the 11-year cycle is generally dominated by the contribution of the largest event within the cycle. This makes the accuracy of the prediction of size of an event that is about to occur, or is just starting, very important because of the special precautions which must be taken. For a 30 g/cm^ tissue sphere, the Wilson and Denn (1976) calculations provide a dose equivalent of an additional 2.5 rem for an SPE with the size and energy spectrum of the August 1972 event and 25 rem for the February 1956 event (Fig. 1). Webber (1963) made a similar estimate for the 1956 event. Rossi and Stauber (1977) estimated the dose equivalent behind 40 g/cm^ of aluminum (equivalent to about 30 g/cm^ of tissue) to be 25 rem for the August 1972 event, a factor
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