Neutron dosimetry in low-earth orbit using passive detectors

This paper summarizes neutron dosimetry measurements made by the USF Physic s Research Laboratory aboard US and Russian LEO spacecraft over the past 20 years using two types of passive detector. Thermal/resonance neutron detec tors exploiting the Li-6(n, T)alpha reaction were used to measure neutrons of energies <1 MeV. Fission foil neutron detectors were used to measure neu trons of energies above I MeV. While originally analysed in terms of dose e quivalent using the NCRP-38 definition of quality factor, for the purposes of this paper the measured neutron data have been reanalyzed and are presen ted in terms of ambient dose equivalent. Dose equivalent rate for neutrons < 1 MeV ranged from 0.80 mu Sv/d on the low altitude, low inclination STS-4 1B mission to 22.0 mu Sv/d measured in the Shuttle's cargo bay on the highl y inclined STS-51F Spacelab-2 mission. In one particular instance a detecto r embedded within a large hydrogenous mass on STS-61 tin the ECT experiment ) measured 34.6 mu Sv/d. Dose equivalent rate measurements of neutrons > 1 MeV ranged from 4.5 mu Sv/d on the low altitude STS-3 mission to 172 mu Sv/ d on the similar to6 year LDEF mission. Thermal neutrons (<0.3 eV) were obs erved to make a negligible contribution to neutron dose equivalent in all c ases. The major fraction of neutron dose equivalent was found to be from ne utrons > 1MeV and, on LDEF, neutrons <1 MeV are responsible for over 98% of the total neutron dose equivalent. Estimates of the neutron contribution t o the total dose equivalent are somewhat lower than model estimates, rangin g from 5.7% at a location under low shielding on LDEF to 18.4% on the highl y inclined (82.3 degrees) Biocosmos-2044 mission. (C) 2001 Elsevier Science Ltd. All rights reserved.