ISSUES IN PROTECTION FROM GALACTIC COSMIC-RAYS

Radiation risks to astronauts depend on the microscopic fluctuations o f energy absorption events in specific tissues. These fluctuations dep end not only on the space environment but also on the modifications of that environment by the shielding provided by structures surrounding the astronauts and the attenuation characteristics of the astronaut's body. The effects of attenuation within the shield and body depends on the tissue biological response to these microscopic fluctuations. In the absence of an accepted method for estimating astronaut risk, we ex amined the attenuation characteristics using conventional linear energ y transfer (LET)-dependent quality factors (as one means of representi ng relative biological effectiveness, RBE) and a track-structure repai r model to fit cell transformation (and inactivation) data in the C3H1 0 T1/2 mouse cell system obtained for various ion beams. Although the usual aluminum spacecraft shield is effective in reducing dose equival ent with increasing shield thickness, cell transformation rates are in creased for thin aluminum shields. Clearly, the exact nature of the bi ological response to LET and track width is critical to evaluation of biological protection factors provided by a shield design. A significa nt fraction of biological injury results from the LET region above 100 keV/mu m. Uncertainty in nuclear cross-sections results in a factor o f 2-3 in the transmitted LET spectrum beyond depths of 15 g/cm(2), but even greater uncertainty is due to the combined effects of uncertaint y in biological response and nuclear parameters. Clearly, these uncert ainties must be reduced before the shield design can be finalised.