Issues in risk assessment from solar particle events.

Uncertainties in risk assessment from solar particle events (SPE) include the role of high linear energy transfer (LET) secondary ions, the assessment of dose-rate effects as they relate to acute injury, the risk of cancer mortality and the modification of health effects due to the stress of spaceflight. We discuss several issues where new knowledge is required for improving estimates of radiation risk from SPE's. Secondary particles such as neutrons and low energy and charge ions (LZE) may dominate radiation risk behind a storm shelter and their biological effects are poorly understood, especially at low dose-rate. Dose-rate modulation of radiation response is also related to genetic pre-disposition an important determinant of radiation sensitivity. Molecular pathways that control cell death and tissue response have been elucidated in recent years and should provide new understanding of dose-rate effects for risk assessment. We consider some of these factors and discuss calculations using radiation transport codes, track structure models of energy deposition, and a molecular kinetics approach to model radiation response.