Deep-Space Radiation Exposure Analysis for Solar Cycle XXI (1975-1986)

Ionizing radiation exposures and associated dosimetric quantities are evaluated for the 11-year solar cycle ending in 1986. Solar flare fluences for the 55 largest flares occurring during the cycle are superimposed on the galactic cosmic ray flux. Published summaries of flare data from the Interplanetary Monitoring Platform (IMP)-7 and IMP-8 satellites are used and include flares whose integrated fluences are greater than 10⁷ protons/cm² for energies in excess of 10 MeV. A standard cosmic ray environment model for ion flux values at solar minimum and maximum is invoked with an assumed sinusoidal variation between the lower and upper limits. The radiation shielding analysis is carried out for equivalent water-shield thicknesses between 2 and 15 g/cm². Results are expressed in terms of cumulative incurred dose equivalents for deep-space missions lasting between 3 months and 3 years. Since no predominantly large proton flares occurred during this period, this analysis should provide an approximate quantification of doses to be normally expected in the course of a long-duration interplanetary mission. It was found that medium-to-large flare contributions are of greatest importance for the shorter term missions, while the galactic component dominates for the longer duration missions. Predicted dose equivalents are presented for various mission durations within any portion of the solar cycle.