The Low Earth Orbit validation of a dynamic and anisotropic trapped radiation model through ISS measurements

摘要

The International Space Station (ISS) provides the proving ground for future long duration human activities in space. Ionizing radiation measurements in ISS form the ideal tool for the experimental validation of radiation environmental models, nuclear transport code algorithms and nuclear reaction cross sections. Indeed, prior measurements on the Space Transportation System (STS; Shuttle) have provided vital information impacting both the environmental models and the nuclear transport code development by requiring dynamic models of the Low Earth Orbit (LEO) environment. Previous studies using Computer Aided Design (CAD) models of the evolving ISS configurations with Thermo-Luminescent Detector (TLD) area monitors, demonstrated that computational dosimetry requires environmental models with accurate non-isotropic as well as dynamic behavior, detailed information on rack loading, and an accurate six degree of freedom (DOF) description of ISS trajectory and orientation. It is imperative that we understand ISS exposures dynamically for crew career planning, and insure that the regulatory requirements of keeping exposure as low as reasonably achievable (ALARA) are adequately implemented. This is especially true as ISS nears some form of completion with increasing complexity, resulting in a larger drag coefficient, and requiring operation at higher altitudes with increased exposure rates. In this paper ISS environmental model is configured for 11A (circa mid 2005), and uses non-isotropic and dynamic geomagnetic transmission and trapped proton models. ISS 11A and LEO model validations are important steps in preparation for the design and validation for the next generation manned vehicles. While the described cutoff rigidity, trapped proton and electron formalisms as coded in a package named GEORAD (GEOmagnetic RADiation) and a web interface named OLTARIS (On-line Tool for the Assessment of Radiation in Space) are applicable to the LEO, Medium Earth Orbit (MEO) and Geosynchronous Earth Orbit (GEO) at quiet solar periods, in this report, the validation of the models using available measurements are limited to STS and ISS nominal operational altitudes (300-400 km) range at LEO where the dominant fields within the vehicle are the trapped proton and attenuated Galactic Cosmic Ray (GCR) ions. The described formalism applies to trapped electron at LEO, MEO and GEO as well. Due to the scarcity of available electron measurements, the trapped electron capabilities of the GEORAD are not discussed in this report, but are accessible through OLTARIS web interface. GEORAD and OLTARIS interests are in the study of long term effects (i.e. a meaningful portion of solar cycle). Therefore, GEORAD does not incorporate any short term external field contribution due to solar activity. Finally, we apply these environmental models to selected target points within ISS 6A (circa early 2001), 7A (circa late 2001), and 11A during its passage through the South Atlantic Anomaly (SAA) to assess the validity of the environmental models at ISS altitudes.