Radiation dosimetry measurements during U.S. Space Shuttle missions with the RME-III.

Time-resolved radiation dosimetry measurements inside the crew compartment have been made during recent Shuttle missions with the U.S. Air Force Radiation Monitoring Equipment-III (RME-III), a portable battery-powered four-channel tissue equivalent proportional counter. Results from the first six missions are presented and discussed. Half of the missions had orbital inclinations of 28.5 degrees with the remainder at inclinations of 57 degrees or greater; altitudes ranged from 300 to 600 km. The determined dose equivalent rates ranged from 70 to 5300 microSv/day. The RME-III measurements are in good agreement with other dosimetry measurements made aboard the vehicles. Measurements indicate that medium- and high-LET particles contribute less than 2% of the particle fluence for all missions, but up to 50% of the dose equivalent, depending on the spacecraft's altitude and orbital inclination. Isocontours of fluence, dose and dose equivalent rate have been developed from measurements made during the STS-28 mission. The drift rate of the South Atlantic Anomaly is estimated to be 0.49 degrees W/yr and 0.12 degrees N/yr. The calculated trapped proton and GCR dose for the STS-28 mission was significantly lower than the measured values.

Dosimetry measurements of 26, 35 and 45 MeV p-Be and p-Li neutron beams.

Dosimetric properties of neutron beams produced by stopping 26, 35 and 45 MeV protons in beryllium and lithium have been measured. The effects of filtering the p-Be beam with 6 cm of polyethylene have been investigated. The tissue kerma rate in air exhibited an energy dependence of approximately E3 and the rate for p-Be beams was approximately one-fifth of the rate for d-Be beams. The penetrability of the neutrons was significnatly enhanced by the use of the filter, but with a 50% attentuation in tissue kerma rate. The tissue kerma rate for the p-Li beam was nearly the same as that for the p-Be beam.

Composition of A-150 tissue-equivalent plastic.

In recent years, the use of tissue-equivalent materials has become quite common in fast-neutron dosimetry, with the A-150 plastic developed by Shonka et al. probably the most popular. Information on this specific plastic is scantily reported in the literature and as a consequence a preponderance of authors unknowingly reference an article by Shonka describing an early version of a tissue substitute plastic but having a different elemental composition than the present A-150 formulation. We have reviewed the results of 21 chemical analyses which have occurred over a time span of four years on a total of 14 samples of A-150 plastic and based on these data and the formulation of the plastic, have arrived at a suggested composition for A-150 tissue-equivalent plastic. The ambiguities of water absorption by nylon, one of the components of the plastic, and the uncertainty this reflects in the composition of the plastic were evaluated.