LET distribution measurement with a new real-time radiation monitoring device-III onboard the Space Shuttle STS-84

A new type of Real-time Radiation Monitoring Device, RRMD-III, consisting of three double-sided silicon strip detectors (DSSDs) has been constructed and used onboard the Space Shuttle mission STS-84. The Space Shuttle cruised at an altitude of 300–400 km and an inclination angle of 51.6° for 221.3 h. RRMD-III succeeded in measuring the linear energy transfer (LET) distribution over the range of 0.2–600 keV/μm for 178 h. The obtained LET distribution of particles was investigated in detail by classifying it into galactic cosmic ray (GCR) particles and trapped protons in the South Atlantic Anomaly (SAA) region. The result shows that GCR particles contribute 60% to the total dose equivalent. The total absorbed dose rate during the mission was 0.516 mGy/day, the effective quality factor was 1.81 by ICRP-Pub.26, and the dose equivalent rate was 0.935 mSv/day. The average absorbed dose rates are 0.120 μGy/min for GCR particles and 4.80 μGy/min for trapped protons. The effective quality factors are 3.16 for GCR particles and 1.19 for trapped protons. RRMD-III data were also compared with the data of the tissue equivalent proportional counter (TEPC), proving that RRMD-III is a reliable device for deriving the true-LET distribution in real time for evaluating space radiation.