Physics and biophysics problems in cosmic ray exposures and risk assessment

The understanding of the risk from exposure to cosmic rays to personnel and electronic equipment in spaceflight and in the upper atmosphere is confounded by the high energy nuclear interactions and the track structure of fast ions. The Langley Research Center has developed several radiation transport codes and associated nuclear interaction data bases for describing the passage of high energy protons, neutrons, and light and heavy ions through materials and tissue. We present and overview these transport codes and databases including comparisons to laboratory and spaceflight experiments. Several risk assessment models are considered in order to study the attenuation properties of selected shielding candidates. These comparisons show that in contrast to conventional risk models based on linear energy transfer (LET), track structure models indicate that only hydrogen rich materials are effective in reducing risk. The contrasting conclusions of the conventional and track structure based approaches are shown to be due to differences in the assessment of the effectiveness of secondary ions such as neutrons, protons, alphas, and heavy ion target fragments. Microdosimetric measurements with proton and heavy ion beams are also discussed and as reported earlier in this session by Dicello, reach the same conclusions as the track models on the importance of target fragmentation.