Equivalent sphere approximations for skin, eye, and blood forming organ dose estimation in space radiation protection studies

Throughout the manned space flight program, protecting astronauts from space radiation has been the subject of continuous and intense study. For interplanetary crews, the two main sources of radiation hazards are solar particle events (SPEs) and galactic cosmic rays (GCR). Solar particle events are composed mainly of high fluxes of protons, neutrons, deuterons, tritons, He-3, and alpha particles which can provide an acute exposure of 200 to 300 cSv from a single event. The galactic cosmic rays are composed of much lower fluxes of high-energy, high-LET, heavy ions which provide stochastic exposures of 12 -15 cGy/yr or 40 - 50 cSv/yr and can total approximately one Sievert for an extended interplanetary mission. For nearly three decades, assessments of crew doses and related shielding requirements have been made using the assumption that body organ exposures are well approximated by exposures at the center of tissue-equivalent spheres. For the skin and blood forming organs (BFO) these spheres have radii of 0 cm and 5 cm respectively. Recent studies indicate that significant overestimation of organ doses occurs if these models are used instead of realistic human geometry models. The use of the latter, however, requires much longer computational times. In this work, we propose preliminary revisions to these equivalent sphere approximations that yield more realistic dose estimates. Calculated doses and dose equivalents for the measured spectra from the February 1956, November 1960, August 1972 and August, September, and October 1989 SPEs along with the GCR solar minimum spectra of 1977 and 1986 and solar maximum spectra of 1970-1971 and 1989 are presented. From these calculated doses and dose equivalents, we recommend a revised equivalent sphere approximation of 9 cm water radius for BFO and 1 cm water radius for skin and ocular lens for SPE. For GCR we recommend a 11 cm radius approximation for BFO and a 3 cm radius approximation for skin and ocular lens.