Organ/Tissue Doses Measured with Solid-State Integrating Dosemeters in a Low-Earth-Orbit Space Misson

INTRODUCTION For future space missions at the International Space Station, health risk to astronauts caused by space radiation needs to be properly quantified and controlled. The radiation field in space is low-dose rate, but the accumulated dose can be high during long-term missions (1). Radiological risk under such a low dose-rate exposure, i.e. stochastic effect, has generally been evaluated using the quantity “effective dose equivalent” (2). Whereas another quantity of “effective dose” has been introduced by ICRP in 1990 (3), this concept is not used in the present study since it is technically difficult to determine the values of radiation weighting factor (wR) for all particle species of isotropic space radiation. The effective dose equivalent can be obtained by summing up the absorbed doses in organs and tissues, weighted by both radiation quality and the relative sensitivity of each organ or tissue. Internal organ and tissue doses in a human body, however, have never been measured in past space missions; absorbed doses in only the brain were measured using a phantom head with thermoluminescent dosemeters (TLD) of LiF (4). Thus, it is desired to directly measure organ and tissue doses using a life-size human phantom at the ISS orbit, not only for evaluation of the effective dose equivalent, but also for verification of the on-going model predictions using anatomical models (5, 6) and transport codes (7, 8). In order to measure the doses in internal organs and tissues in a phantom, we depend on small-scale passive detectors to avoid disturbing a radiation field by the reactions different from those in tissues. Such solid-state detectors have also advantages of long-term stability and simplicity in handling (9). It is known, however, that the responses of TLDs show dependence on LET or particle energy (10-12). In the present study, changes of the TL efficiencies to HZE particles are quantified at particle accelerators before using the TLDs in space.