SECONDARY PARTICLE CONTRIBUTION TO LET SPECTRA ON LDEF

Four experiments utilizing passive detectors (P0006, P0004, A0015, M00 04) were flown on LDEF to study the radiation environment. These exper iments have been summarized in a companion paper (Benton et al., 1996) . One of the experimental goals was to measure LET spectra at differen t locations and shielding depths with plastic nuclear track detectors (PNTD). It was found that the LET spectra extended well above the LET cutoff imposed by the geomagnetic field on GCR particle penetration in to LEG. The high LET particles detected were mostly short-range(range < 2000 Irm), indicating that they were secondaries produced locally wi thin the PNTD. The presence of these high LET particle fluences is imp ortant for the determination of dose equivalent because of the high Qu ality Factors (Q) involved. A relatively small fraction of particle fl uence can contribute a large fraction of dose equivalent. Short-range, inelastic secondary particles produced by trapped protons in the Sout h Atlantic Anomaly (SAA) were found to be a major contributor to the L ET spectra above 100 keV/mu m. The LET spectra were found to extend be yond the similar to 137 keV/mu m relativistic GCR Fe peak to over 1000 keV/mu m. The high LET tail of the LET spectra was measured in CR-39 and polycarbonate PNTDs using different techniques. GCR made a relativ ely modest contribution to the LET spectra as compared to the contribu tions from short-range secondary particles and stopping protons. LET s pectra intercomparisons were made between LDEF measurements and exposu res to 154 MeV accelerated proton beams. The similarities support the role of nuclear interactions by trapped protons as the major source of secondary particles in the PNTDs. Also techniques were employed to re duce the range cutoff for detection of the short-range secondaries to similar to 1 mu m, so that essentially all secondary particles were in cluded in the LET spectra. This has allowed a more realistic assessmen t of secondary contribution to dose equivalent. Comparisons of measure d and calculated LET spectra have been made that demonstrate the need for more accurate modeling of secondary particles in radiation transpo rt codes. Comparisons include preliminary calculations in which attemp ts have been made to include secondary particles. Copyright (C) 1996 E lsevier Science Ltd