SOLAR PARTICLE EVENTS Giant solar particle events (SPE) are caused by large upheavals on the solar surface which accelerate protons and, to a lesser extent, heavier nuclei to high energies. These particles are then transported through the solar magnetic field and can increase radiation levels to high values for several hours or days in the vicinity of the earth outside our magnetosphere. GEO, at 6.6 earth radii, is in such minimal geomagnetic shielding that particles down to very low energy arriving in the vicinity of the earth will be able to reach it. Large events occur with highest probability during the rising or falling portions of the eleven-year solar activity cycle. A very small number of events have dominated the total fluence of particles arriving in an eleven-year period. Figure A-3 shows that the events occurring in August 1972 dominated the total fluence arriving in cycle 20 and compares these events with others which occurred in 1967, 1969, and 1971. In Figure A-4, two different estimates of the dose equivalent at the center of a sphere of radius r from the August 1972 event are plotted as a function of the radius; a discrepancy exists between the two calculations, and this is still to be resolved. The figure shows, however, that the sphere radius must be at least 10 g/cm^ of tissue before the dose equivalent drops below 100 rem. The occurrence of such large events, therefore, results in high dose-rate exposures which will be a considerable radiation hazard to SPS workers in GEO. For workers in LEO, on the other hand, enough shielding is provided by the geomagnetic field to make dose equivalents from solar events negligible. DIFFERENT EFFECTS OF HIGH- AND LOW-LET RADIATIONS In the evaluation of the potential health effects caused by space radiations, the radiations may be divided into general categories based on linear energy transfer (LET) or collision stopping power. Low-LET radiations, such as nigh energy electrons and protons, have ionizations which are relatively far apart, with only a small probability of interaction between the ionization products created by a single ionizing particle. Those interactions which do occur are primarily the result of multiple particle tracks. High—LET radiations, such as heavy charged particles, are characterized by ionizations which are normally more closely spaced, and there is therefore a correspondingly greater probability of interactions between the ionization products created by the passage of a single particle. This difference in the microscopic dose distribution generally causes the higher LET radiations to have a greater biological effectiveness. QUALITY FACTOR A quality factor, Q, has been defined to account for the varying degrees of potential adverse health effects in man caused by different
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