Experimental Assessment of Lithium Hydride's Space Radiation Shielding Performance and Monte Carlo Benchmarking.

The harmful effects of space radiation pose a serious health risk to astronauts participating in future long-term missions. Such radiation effects must be considered in the design phase of space vessels as well as in mission planning. Crew radioprotection during long periods in deep space (e.g., transit to Mars) represents a major challenge, especially because of the strong restrictions on the passive shielding load allowed on-board the vessel. Novel materials with better shielding performance compared to the "gold standard" high-density polyethylene are therefore greatly needed. Because of the high hydrogen content of hydrides, lithium hydride has been selected as a starting point for further studies of promising candidates to be used as passive shielding materials. In the current experimental campaign, the shielding performance of lithium hydride was assessed by measuring normalized dose, primary beam attenuation and neutron ambient dose equivalent using 430 MeV/u 12C, 600 MeV/u 12C and 228 MeV proton beams. The experimental data were then compared to predictions from the Monte Carlo transport codes PHITS and GRAS. The experimental results show an increased shielding effectiveness of lithium hydride compared to reference materials like polyethylene. For instance, the attenuation length for 600 MeV/u 12C primary particles in lithium hydride is approximately 20% shorter compared to polyethylene. Furthermore, the comparison results between both transport codes indicates that the standard Tripathi-based total reaction cross-section model of PHITS cannot accurately reproduce the presented experimental data, whereas GRAS shows reasonable agreement.

Accelerator-Based Tests of Shielding Effectiveness of Different Materials and Multilayers using High-Energy Light and Heavy Ions.

The roadmap for space exploration foresees longer journeys and further excursions outside low-Earth orbit as well as the establishment of permanent outposts on other celestial bodies, such as the Moon or Mars. The design of spacecrafts and habitats depends heavily on the mission scenario and must consider the radiation protection properties of the structural components as well as dedicated shielding. In fact, short- and long-term effects caused by exposure to cosmic radiation are now considered among the main health risks of space travel. One of the current strategies is to find multifunctional materials that combine excellent mechanical properties with a high shielding effectiveness to minimize the overall load. In this work, the shielding effectiveness of a wide variety of single and multilayer materials of interest for different mission scenarios has been characterized. In the experimental campaign, reference and innovative materials, as well as simulants of Moon and Mars in situ resources, were irradiated with 1,000 MeV/u 4He, 430 MeV/u 12C and 962-972 MeV/u 56Fe. The results are presented in terms of Bragg curves and dose reduction per unit area density. To isolate the shielding effectiveness only due to nuclear fragmentation, a correction for the energy loss in the material is also considered. These findings indicate that the best shield is lithium hydride, which performs even better than polyethylene. However, the technical feasibility of shielding needs to be investigated. The classification of all materials in terms of shielding effectiveness is not influenced by the ion species, but the value changes dramatically depending on the beam energy. The output of this investigation represents a useful database for benchmarking Monte Carlo and deterministic transport codes used for space radiation transport calculations. These findings also contribute to recommendations for optimizing the design of space vessels and habitats in different radiation environments.

Characterizing the Potency and Impact of Carbon Ion Therapy in a Primary Mouse Model of Soft Tissue Sarcoma.

Carbon ion therapy (CIT) offers several potential advantages for treating cancers compared with X-ray and proton radiotherapy, including increased biological efficacy and more conformal dosimetry. However, CIT potency has not been characterized in primary tumor animal models. Here, we calculate the relative biological effectiveness (RBE) of carbon ions compared with X-rays in an autochthonous mouse model of soft tissue sarcoma. We used Cre/loxP technology to generate primary sarcomas in KrasLSL-G12D/+; p53fl/fl mice. Primary tumors were irradiated with a single fraction of carbon ions (10 Gy), X-rays (20 Gy, 25 Gy, or 30 Gy), or observed as controls. The RBE was calculated by determining the dose of X-rays that resulted in similar time to posttreatment tumor volume quintupling and exponential growth rate as 10 Gy carbon ions. The median tumor volume quintupling time and exponential growth rate of sarcomas treated with 10 Gy carbon ions and 30 Gy X-rays were similar: 27.3 and 28.1 days and 0.060 and 0.059 mm3/day, respectively. Tumors treated with lower doses of X-rays had faster regrowth. Thus, the RBE of carbon ions in this primary tumor model is 3. When isoeffective treatments of carbon ions and X-rays were compared, we observed significant differences in tumor growth kinetics, proliferative indices, and immune infiltrates. We found that carbon ions were three times as potent as X-rays in this aggressive tumor model and identified unanticipated differences in radiation response that may have clinical implications. Mol Cancer Ther; 17(4); 858-68. ©2018 AACR.

Impaired Attentional Set-Shifting Performance after Exposure to 5 cGy of 600 MeV/n 28Si Particles.

Astronauts on deep space missions will be required to work more autonomously than on previous missions, and thus their ability to perform executive functions could be critical to mission success. One of the most common measures of executive function in humans is the ability to perform attentional set shifting, which requires contributions from working memory, discrimination, reversal learning, attentional set shifting and attention. Rodent attentional set shifting assays require rats to form an association between the presence of the food reward and an associative cue, which is either the digging media or the scent that is placed in the bowl; by altering the combination of scent and digging media, progressively more complex cognitive processes can be tested. In this study, we have determined the effect that exposure to 5-20 cGy of 600 MeV/n 28Si particles has on the ability of male retired breeder Wistar rats to perform attentional set shifting at three months postirradiation. All doses of Si resulted in a significant impairment in the ability of the rats to perform the first and most simple step of the ATSET assay, the simple discrimination (SD) task. If astronauts were to experience HZE-induced SD impairments, they would be unable to identify key factors to successfully resolve a situation. Performance in at least one other component of the ATSET test was impaired at all doses studied, however, these varied according to the dose. Compared with our previous studies using 1 GeV/n 56Fe and 48Ti particles, 600 MeV/n 28Si ions impaired attentional set-shifting performance at lower doses than the heavier ions. However, when the effect of isofluences of the three HZE ions were compared, there were no significant differences in the severity of the impaired performance; there were, however, ion-specific decrements in the ability of rats to perform within the various stages of the test. This study further supports the notion that "mission-relevant" doses of HZE particles (<20 cGy) can impair certain aspects of attentional set-shifting performance in retired breeder rats, but there may be some ion-specific changes in the specific cognitive domains impaired.

Benchmarking Geant4 hadronic models for prompt-γ monitoring in carbon ion therapy.

PURPOSE: The real-time monitoring of the spread-out Bragg peak would allow the planned dose delivered during treatment to be directly verified, but this poses a major challenge in modern ion beam therapy. A possible method to achieve this goal is to exploit the production of secondary particles by the nuclear reactions of the beam with the patient and correlate their emission profile to the planned target volume position. In this study, we present both the production rate and energy spectra of the prompt-γ produced by the interactions of the 12 C ion beam with a polymethyl methacrylate (PMMA) target. We also assess three different Monte Carlo models for prompt-γ simulation based on our experimental data. METHODS: The experiment was carried out at the GSI Helmholtz Centre for Heavy Ion Research, Darmstadt, Germany with a 220 MeV/u 12 C ions beam impinging on a 5× 5× 20 cm3 polymethyl methacrylate beam stopping target, with the prompt-γ being detected by a hexagonally-shaped barium fluoride scintillator with a circumscribed radius of 5.4 cm and a length of 14 cm, placed at 60° and 90° with respect to the beam direction. Monte Carlo simulations were carried out with three different hadronic models from the Geant4 code: binary ion cascade (BIC), quantum molecular dynamics (QMD), and Liege intranuclear cascade (INCL++ ). RESULTS: An experimental prompt-γ yield of 1.06 × 10-2  sr-1 was measured at 90°. A good agreement was observed between the shapes of the experimental and simulated energy spectra, especially with the INCL++ physics list. The prompt-γ yield obtained with this physics list was compatible with the measurement within 2σ, with a relative difference of 26% on average. BIC and QMD physics lists proved to be less accurate than INCL++ , with the difference between the measured and simulated yields exceeding 100%. The differences between the three physics lists were ascribed to important discrepancies between the models of the physical processes producing prompt-γ emissions. CONCLUSION: In conclusion, this study provides prompt-γ yield values in agreement with previously published results for different carbon ions energies. This work demonstrates that the INCL++ physics list from Geant4 is more accurate than BIC and QMD to reproduce prompt-γ emission properties.