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Radiation impact to astronauts depends on the particles’ linear energy transfer (LET) and is dominated by high LET radiation. Radiation risk experienced by astronauts can be determined with the radiation LET spectrum measured and the risk response function obtained from radiobiology. Systematical measurement of the space radiation is an important part for the research on the impact of radiation to astronauts and to make the radiation ALARA (as low as reasonably achievable). For NASA space missions at low Earth orbit (LEO), the active dosimeter used for all LET is the tissue equivalent proportional counter (TEPC) and the passive dosimeters used for the astronauts and for the monitored areas are the combination of CR-39 plastic nuclear track detectors (PNTDs) for high LET and thermoluminescence dosimeters (TLDs) and optically stimulated luminescence dosimeter (OSLDs) for low LET. TEPC, CR-39 PNTDs and TLDs/OSLDs were used to measure the radiation during STS-121 space mission. LET spectra and radiation quantities were obtained with active and passive dosimeters. This paper will introduce the physical principles for TEPC and CR-39 detectors, the LET spectrum method for radiation measurement using CR-39 detectors and TEPC, and will present and compare the radiation LET spectra and quantities measured with TEPC, CR-39 PNTDs and TLDs/OSLDs.

Research indicates that the impact to human tissues from radiation exposure is strongly related to the LET (linear energy transfer) of the particles and particles with high LET ( ⩾ 5 KeV / μ m water) dominate the damage. High LET radiation in LEO (low Earth orbit) is composed mainly of galactic cosmic rays (GCR), solar energetic particles, particles trapped in the SAA (South Atlantic Anomaly), and albedo neutrons and protons scattered from the Earth's atmosphere. So far the active personal dosimeters are not available and the best passive personal dosimeters currently applied to the radiation assessment for astronauts are CR-39 detectors (for the high LET part) in combination with thermoluminescence detectors (TLDs) or optically stimulated luminescence detectors) (OSLDs) (for the low LET part). LET spectra for radiation in LEO were determined with CR-39. This paper introduces the operational principles for CR-39 detectors, describes the method of LET spectrum using CR-39 and presents the results measured with CR-39 and TEPC (tissue equivalent proportional counter) for space mission ISS-Expedition 2, STS-108, STS-112, ISS-7S, STS-114 and STS-121.