LET Spectra in Low Earth Orbit

The Trapped Ions in Space (TRIS) experiment was flown on the Space Shuttle in October, 1984. It contained a stack of plastic track detectors that recorded linear energy transfer (LET) spectra behind various thicknesses of shielding in a 57 degree inclination orbit. A preliminary analysis of the data shows that behind shielding of at least 40 mils Al equivalent, the LET spectrum is dominated by galactic cosmic rays and the direct ionization of trapped protons and that it is reasonably well predicted by the CREME model. For a minimum shielding of approximately 4 mils Al equivalent, the measured LET flux exceeds the CREME predictions, especially at high LET. This is probably due to contributions from the products of proton-induced nuclear reactions in the detector and helium ions trapped in the inner radiation belt.