Low Earth Orbit Radiation Environments and Shield Model Validation for ISS

Abstract

The International Space Station (ISS) provides the provin g ground for future long duration human activities in space. Ionizing radiation measurements in ISS form the ideal tool for the experimental validation of the most recent ionizing radiation environ mental models, nuclear transport code algorithms, and atomic/nuclear reaction processes. Previous studies have demonstrated that computational dosimetry needs environmental models with accurate non-isotropic as well as dynamic behavior, detailed information on rack loadings (thicknes s distribution), and a 6-degree-of-freedom (DOF) description of ISS trajectory and orientation. Th is need arises from the fact that ISS is nearing some form of completion with increasing complexity (12A.1 c onfiguration as of Jan. 2007), resulting in a larger drag coefficient, and hence requiring operation at higher altitudes with higher exposure rates. The current implementation of the ISS environmental model as used in this study is configured for 11A, and uses O’Neill’s Galactic Cosmic Rays (GCR) model (2004), covering charge particles in the 1 Z 28 range. O’Neill’s free space GCR model is coupled wit h the Langley Research Center (LaRC) cutoff model with angular dependency compensation to compute the transmission coefficient at LEO. The trapped proton environment is represented by a LaRC developed time dependent procedure which couples the AP8min/AP8max, Deep River Neutron Monitor (DRNM) and F10.7 radio frequency solar index measurements. The albedo neutron environment is represented by the extrapolation of parametric fit to data gathered by LaRC during the Atmospheric Ionizing Radiation (AIR) measurements studies of the radiation at Supersonic Transport (SST) altitudes in the years 1965 to 1971, covering the rise and decline of solar cycle 20. The charge particle transport calculations are accomplished by using the most recent version (2005) of LaRC’s deterministic ionized particle transport code High charge (Z) and Energy TRaNsport (HZETRN). ISS 11A and LEO radiation environmental model validations are important steps in preparation for the design and validation of the Crew Exploration Vehicle (CEV) under the Constellation program. We describe herein improved ionizing radiation models of GCR, geomagnetic cutoffs, trapped protons and albedo neutrons as applied to the ISS operations. We further compare available measurements of selected target points within ISS 6A, 7A, and 11A during its passage through the Sout h Atlantic Anomaly (SAA) with our simulations, to assess the validity of the current envi ronmental models.