2. Short-term enhancements—Electron flux densities can increase markedly due to intermittent changes in the solar plasma associated with small substorms. Flux densities may rise two to three orders of magnitude in several hours followed by a decay lasting several days (Stassinopoulos, 1980). BREMSSTRAHLUNG In general, the absorbed dose from bremsstrahlung is not dominant behind thin shielding (e.g., <3 g/cm? aluminum) because of the overriding preponderance of primary electrons. However, at greater depths, the dose from electrons drops sharply, dependent largely on the shape of the incident electron energy spectrum. This causes the bremsstrahlung to dominate the absorbed dose at large shielding thicknesses, (e.g., >3 g/cm^ aluminum). For inner zone electrons at LEO, the bremsstrahlung dose is completely dominated at all thicknesses by the dose from trapped protons (see below). Here, the bremsstrahlung dose is sufficiently small to be negligible. For outer zone electrons at GEO, on the other hand, the bremsstrahlung dose dominates behind shielding of 3 g/cm? aluminum or greater thickness (Stassinopoulos, 1980). Thus, the bremsstrahlung dose is an important component of the radiation environment in GEO and in the transfer ellipse between LEO and GEO, taking into account certain assumptions of trajectory and vehicle speed. TRAPPED PROTONS Protons are also trapped in large numbers by the geomagnetic field. A region of the trapped proton zone dips close to tne earth in the southern Atlantic Ocean, southeast of the Brazilian coast. This region, called the South Atlantic Anomaly, is the most important contributor to the radiation environment for space workers in LEO. A spacecraft in LEO, however, will pass through this region on only 60 percent of its revolutions. Stassinopoulos (1979) has estimated that 86 percent of the time in LEO is flux-free, i.e., in a radiation environment of less than one particle/cm2-sec of electrons with energies greater than 0.5 MeV and protons with energies greater than 5 MeV. Each day, for a maximum duration of ten hours, there are about six consecutive flux-free revolutions without danger of increased radiation exposure. Trapped protons may be important in the TE between LEO and GEO, depending on the trajectory and vehicle speed selected. Two different calculations of the proton environment encountered by a transfer vehicle have been made, assuming proton environments varying by several orders of magnitude (see "Transfer Ellipse," p. 8). In GEO, the trapped protons are of such low energy that they will not present a health risk to workers at nominal shielding thicknesses.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==