MEASUREMENTS ON THE SHUTTLE OF THE LET SPECTRA OF GALACTIC COSMIC-RADIATION AND COMPARISON WITH THE RADIATION TRANSPORT MODEL

A new class of tissue-equivalent proportional counters has been flown on two space shuttle flights. These detectors and their associated electronics cover a lineal energy range from 0.4 to 1250 keV/mu m with a multichannel analyzer resolution of 0.1 keV/mu m from 0.4 to 20 keV/mu m and 5 keV/mu m from 20 to 1250 keV/mu m. These detectors provide the most complete dynamic range and highest resolution of any technique currently in use. On one mission, one detector was mounted in the Shuttle payload bay and another older model in the mid-deck, thus providing information on the depth dependence of the lineal energy spectrum. A detailed comparison of the observed lineal energy and calculated LET spectra for galactic cosmic radiation shows that, although the radiation transport models provide a rather accurate description of the dose (+/-15%) and equivalent dose (+/-15%), the calculations significantly underestimate the frequency of events below about 100 keV/mu m. This difference cannot be explained by the inclusion of the contribution of splash protons. The contribution of the secondary pions, kaons and electrons produced in the Shuttle shielding, if included in the radiation transport model, may explain these differences. There are also significant differences between the model predictions and observations above 140 keV/mu m, particularly for 28.5 degrees inclination orbit.