LET spectra measurements on LDEF: variations with shielding and location.

LET spectra measurements made with passive plastic nuclear track detectors (PNTDs) were found to depend on detector orientation, shielding and experiment location. LET spectra were measured at several locations on LDEF as part of the P0006 LETSME experiment (Benton and Parnell, 1984), the P0004 Seeds in Space experiment (Parks and Alston, 1984), the A00l5 Free Flyer Biostacks and the M0004 Fiber Optics Data Link experiment (Taylor, 1984). Locations included the east, west and Earth sides of the LDEF satellite. The LET spectra measured with PNTDs deviated significantly from calculations, especially for high LET particles (LET infinity H2O > or = 100 keV/micrometer). At high LETs, short-range inelastic secondary particles produced by trapped proton interactions with the nuclei of the detector were found to be the principal contributor to LET spectra. At lower LETs, the spectra appeared to be due to short-range, inelastic and stopping primary protons, with primary GCR particles making a smaller contribution. The dependence of LET spectra on detector orientation and shielding was studied using the four orthogonal stacks in the P0006 experiment. Both measurements of total track density and LET spectra showed a greater number of particles arriving from the direction of space than from Earth. Measurements of LET spectra in CR-39 PNTD on the east (leading) and west (trailing) sides of LDEF showed a higher rate of production at the west side. This was caused by a larger flux of trapped protons on the west side as predicted by the east/west trapped proton anisotropy in the South Atlantic Anomaly (SAA). Track density measured in CR-39 PNTDs increased as a function of shielding depth in the detector stack. A similar measurement made in a thick stack of CR-39 interspersed with layers of Al and exposed to 154 MeV protons at a ground-based accelerator showed a similar result, indicating that a significant fraction of the particle events counted were from secondaries and that the total cross-section for production of proton-induced secondaries increased as the energy of primary protons attenuated. Little change was seen in either total differential or integral LET spectra as a function of shielding depth, indicating that the increase in cross section with decreasing proton energy affected mostly the shorter range secondary components. Similarity in the slopes of LET spectra from ground-based proton exposures and the A00l5 LET spectra showed that modeling of a monoenergetic proton beam transported through a 1-D geometry was a useful first step in modeling the production of secondary particles by trapped protons in the SAA.

Secondary particle contribution to LET spectra on LDEF.

Four experiments utilizing passive detectors (P0006, P0004, A0015, M0004) were flown on LDEF to study the radiation environment. These experiments have been summarized in a companion paper (Benton et al., 1996). One of the experimental goals was to measure LET spectra at different 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 into LEO. The high LET particles detected were mostly short-range (range < 2000 m), indicating that they were secondaries produced locally within the PNTD. The presence of these high LET particle fluences is important for the determination of dose equivalent because of the high Quality Factors (Q) involved. A relatively small fraction of particle fluence can contribute a large fraction of dose equivalent. Short-range, inelastic secondary particles produced by trapped protons in the South Atlantic Anomaly (SAA) were found to be a major contributor to the LET spectra above 100 keV/micrometer. The LET spectra were found to extend beyond the approximately 137 keV/micrometer relativistic GCR Fe peak to over 1000 keV/micrometer. The high LET tail of the LET spectra was measured in CR-39 and polycarbonate PNTDs using different techniques. GCR made a relatively modest contribution to the LET spectra as compared to the contributions from short-range secondary particles and stopping protons. LET spectra intercomparisons were made between LDEF measurements and exposures 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 reduce the range cutoff for detection of the short-range secondaries to approximately 1 micrometer, so that essentially all secondary particles were included in the LET spectra. This has allowed a more realistic assessment of secondary contribution to dose equivalent. Comparisons of measured and calculated LET spectra have been made that demonstrate the need for more accurate modeling of secondary particles in radiation transport codes. Comparisons include preliminary calculations in which attempts have been made to include secondary particles.