Evaluating shielding effectiveness for reducing space radiation cancer risks

Abstract

Abstract:We discuss calculations of probability distribution functions (PDF) representing uncertainties in projecting fatal cancer risk from galactic cosmic rays (GCR) and solar particle events (SPE). The PDF's are used in significance tests of the effectiveness of potential radiation shielding approaches. Uncertainties in risk coefficients determined from epidemiology data, dose and dose-rate reduction factors, quality factors, and physics models of radiation environments are considered in models of cancer risk PDF's. Competing mortality risks and functional correlations in radiation quality factor uncertainties are treated in the calculations. We show that the cancer risk uncertainty, defined as the ratio of the 95% confidence level (CL) to the point estimate is about 4-fold for lunar and Mars mission risk projections. For short-stay lunar missions (<180 d), SPE's present the most significant risk, however one that is mitigated effectively by shielding, especially for carbon composites structures with high hydrogen content. In contrast, for long duration lunar (>180 d) or Mars missions, GCR risks may exceed radiation risk limits, with 95% CL's exceeding 10% fatal risk for males and females on a Mars mission. For reducing GCR cancer risks, shielding materials are marginally effective because of the penetrating nature of GCR and secondary radiation produced in tissue by relativistic particles. At the present time, polyethylene or carbon composite shielding can not be shown to significantly reduce risk compared to aluminum shielding based on a significance test that accounts for radiobiology uncertainties in GCR risk projection.

Heavy ion carcinogenesis and human space exploration

“…Two scaling parameters with large uncertainties are the "radiation quality factor" Q, that estimates the increased effectiveness of HZE nuclei compared to γ-rays for the same dose, and the dose-and dose-rate effectiveness factor (DDREF) that reduces estimates of cancer risk at high dose-and dose-rates when the dose-and dose-rate are low (< 0.05 Gy/hr). Risks for extended missions to the moon and the Mars exploration mission exceed 3% for most Mars mission scenarios with upper 95% confidence levels near 15% risk of death 9 . The scaling of mortality rates for space radiation risks to astronauts to the Atomic bomb survivors introduces many uncertainties 1,2,8 into risk estimates (Figure 1), and there are important questions with regard to the correctness of any scaling approach because of qualitative differences in the biological effects of HZE ions and γ-rays.…”

“…Furthermore, overall uncertainties in space radiation cancer projections are about 5-fold times higher at the 95% confidence level 8,9 than the median risk projection and it is not possible, with the current state of knowledge of cancer biology, to judge if risks are higher or lower than safety standards. These facts further support that improving the understanding of the biology of cancer risks from space radiation exposure is the primary hurdle for a Mars mission.…”

Practical applications of cosmic ray science: Spacecraft, aircraft, ground based computation and control systems, and human health and safety

“…Finally, in-flight crew ionizing radiation dose isn't measured directly but is, rather, calculated using the HZETRN nuclear reaction and transport code (1) with input from calibrated in-flight dosimeters and LET spectrometer measurements. Including the contribution of secondary particle occurring inside the human body is an important aspect of calculating flight crew accurate dose numbers (1,22).…”

“…An important consequence of secondary particle showers in the human body is the realization that much early work on the benefits of low atomic number high hydrogen content materials for spacecraft shielding against galactic cosmic rays has been invalidated (22) as is shown in Figure 6.…”

“…GCRs have higher kinetic energy than solar particle event cosmic rays or trapped radiation (1,22) so that substantially thicker shielding is needed to mitigate space crew GCR dose during long duration missions. As implied by the data shown in Figure 6, the overall programmatic cost of the available passive shielding needed to extend the 180 day limit to 3 years are very likely prohibitive at this time.…”