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The quality factor (QF) as defined in International Commission on Radiological Protection report no. 26 (ICRP 26, ref. 1) or in International Commission on Radiation Units and Measurements report no. 40 (ICRU 40, ref. 2) is not expected to be a valid method for assessing the biological risk for deep space missions where the high-energy heavy ion (HZE) particles of the galactic cosmic rays (GCR) are of major concern. No human data for cancer induction from the HZE particles exist, and information on biological effectiveness is expected to be taken from experiments with animals and cultured cells (ref. 3). Experiments with cultured cells (refs. 4-6) indicate that the relative biological effectiveness (RBE) of the HZE particles is dependent on particle type, energy, and the level of fluence. Use of a single parameter, such as linear energy transfer (LET) or lineal energy (see ref. 2), to determine radiation quality will therefore represent an extreme oversimplification for GCR risk assessment.
Katz has presented a theoretical model (refs. 7 and 8) that predicts the correct RBE behavior as observed in recent experimental studies using track-segment irradiations with heavy ions on cultured mammalian cells. Cells at risk in deep space will be subject to a complicated mixture of particles varying in composition with the amount and type of shielding surrounding them. The fluence levels in space are such that a single cell will likely be exposed to only one ion encounter over an extended period. Katz has developed the ion-kill mode of cell death or transformation that corresponds to low-fluence exposures. The delta-ray (energetic electrons produced in ion collisions) radial dose distribution surrounding the ion path is assumed to initiate the biological damage, and the cell response to the radiation field is parameterized using target theory and results from gamma-ray and track-segment irradiations. The level of damage for a mixed-radiation field is determined by the cellular response parameters and the local flux of particles. The Langley Research Center has developed a deterministic transport code for calculating the differential flux of ions behind natural and protective radiation shielding exposed to the GCR spectrum (refs. 9 13). In this paper we consider the biological damage to mammalian cell cultures expected for 1 year in deep space at solar minimum behind various depths of aluminum shielding using the Katz cellular damage model and the Langley GCR code. Cell death and neoplastic transformations for C3H10T1/2 cells (mouse embryo cells) are considered for typical levels of spacecraft shielding. The results of this study must be considered preliminary in that the transport code is in an early stage of development (ref. 13).