MEASUREMENT OF LET DISTRIBUTION AND DOSE-EQUIVALENT ON BOARD THE SPACE-SHUTTLE STS-65

Space radiation dosimetry measurements have been made on board the Spa ce Shuttle STS-65 in the Second International Microgravity Laboratory (IML-2). In these measurements, three kinds of detectors were used; on e is a newly developed active detector telescope called ''Real-time Ra diation Monitoring Device (RRMD)'' utilizing silicon semi-conductor de tectors and others are conventional detectors of thermoluminescence do simeters (TLDs) and CR-39 plastic track detectors. Using the RRMD dete ctor, the first attempt of real-time monitoring of space radiation has been achieved successfully for a continuous period of 251.3 h, giving the temporal variations of LET distribution, particle count rates, an d rates of absorbed dose and dose equivalent. The RRMD results indicat e that a clear enhancement of the number of trapped particles is seen at the South Atlantic Anomaly (SAA) without clear enhancement of dose equivalent, while some daily periodic enhancements of dose equivalent due to high LET particles are seen at the lower geomagnetic cutoff reg ions for galactic cosmic ray particles (GCRs). Therefore, the main con tribution to dose equivalent is seen to be due to GCRs in this low alt itude mission (300 km). Also, the dose equivalent rates obtained by TL Ds and CR-39 ranged from 146.9 to 165.2 mu Sv/day and the average qual ity factors from 1.45 to 1.57 depending on the locations and direction s of detectors inside the Space-lab at this highly protected orbit fdr space radiation with a small inclination (28.5 degrees) and a low alt itude (300 km). The LET distributions obtained by two different detect ors, RRMD and CR-39, are in good agreement in the region of 15-200 keV /mm and difference of these distributions in the regions of LET < 15 k eV/mm and LET > 200 keV/mm can be explained by considering characteris tics of CR-39 etched track formation especially for the low LET tracks . Copyright (C) 1996 Elsevier Science Ltd