Short-range High-LET Secondaries Produced by Trapped Protons:
     Contribution to the LET Spectra, in Low Earth Orbit

E.V. Benton, A.L. Fank, E.R. Benton, R.P. Keegan, D. Sanner, L.A. Frigo 
Department of Physics
University of San Francisco 
San Francisco, CA 94117-1080


    Trapped proton-induced secondary particles due to their high LETs, are likely to be of significant radiobiological importance. Measurements in CR-39 plastic nuclear track detectors (PNTDs) flown on LDEF (Long Duration Exposure Facility, 28" inclination) show the LET Spectra extending to higher LETs than the cut-off LET imposed on galactic cosmic radiation (GCR) by the geomagnetic field. This high-LET tail (2150 keV/um) is believed to be the result of short-range (>2 um, minimum particle range imposed by CR-39 track detector technique) secondary particles produced by trapped protons. LET spectra measurements made with CR-39 PNTDs and with the JSC-TEPC on STS-57 (280 inclination) are consistent with LDEF results in that they extend to higher LETs than do modelpredictions. LET spectra measured on STS-60 (570 ·inclination) in CR-39 PNTDs and TEPC show close agreement with model calculations at high LETs than do LET Spectra measurements on the lower inclination, higher altitude STS-57 and LDEF missions. This supports the hypothesisthat the majority of high-LET secondaries are produced in the South Atlantic Anomaly(SAA) in interactions between trapped protons and the nuclei of the stopping material.

    To simulate exposure to the trapped proton environment in space, LET spectra measurements were carried out in CR-39 PNTDs exposed to accelerator beams of monoenergetic protons in the same regime as trapped proton energies (80, 110 and 154 MeV·at the Harvard Cyclotron Laboratory; 1.8 and 4.9 GeV at the LBL Bevatron; 80, 200 and 250 MeV at the Loma Linda Proton Synchrotron). Due to the minimum LET threshold for track registration in CR-39 (~5 keV/um), no primary protons could register and ail measured signal was produced by secondary particles. Since the accelerator protons were monoenergetic, interpretation of the LET spectra was easier than for space-based measurements. It appears thatat high LETs (>150 keV/um), the LET spectrum is dominated by inelastic secondaries of Oxygen and Carbon. The lower LET region may consist of secondary protons produced in inelastic interactions between primary protons and the O and C nuclei of the stopping medium .

     To date, radiation transport codes (HETC, HZETRN) have not adequately modeled the contribution from short-range, high-LET secondaries produced by trapped protons to the LET spectra. Preliminary results of model calculations of high-LET inelastic secondaries are presented and compared with measurements.