TY - JOUR
AU - Ottolenghi,, A
AB - Abstract Personal radiation shielding is likely to play an important role in the strategy for radiation protection of future manned interplanetary missions. There is potential for the successful adoption of wearable shielding devices, readily available in case of accidental exposures or used for emergency operations in low-shielded areas of the habitat, particularly in case of solar particle events (SPEs). Based on optimization of available resources, conceptual models for radiation protection spacesuits have been proposed, with elements made of different materials, and the first prototype of a water-fillable garment was designed and manufactured in the framework of the PERSEO project, funded by the Italian Space Agency, leading to the successful test of such prototype for ease of use and wearability on-board the International Space Station. We present results of Monte Carlo calculations offering a proof-of-principle validation of the shielding efficacy of such prototype in different SPE environments and shielding conditions. INTRODUCTION Personal shielding against space radiation is a promising strategy, complementary to other shielding solutions as habitat shielding, to ensure the safety of the crew of future interplanetary missions(1). As deep-space exploration will likely involve high-complexity operational scenarios, the risk of accidental exposure or need for the crew to carry on emergency interventions in low-shielded areas of the habitat has to be taken into account when evaluating the risk associated to the mission. This is of particular importance in case of occurrence of large solar particle events (SPEs) during the mission(2). Considering a Mars mission scenario(3), SPEs are likely to happen during the cruise to the planet, if this will be planned during solar maximum to take advantage of the lowest background of galactic cosmic rays. Delays in communication to ground have to be considered for the alert system. Now-casting systems can be studied for on-board implementation (e.g. thanks to real-time analysis of SPE precursors), so that mitigation procedures can be activated by astronauts on their own decision. Crew members can take shelter in an area with increased wall thickness, if present, or rearrange shielding elements already on-board, with a configuration possibly optimized based on the results of real-time analysis. Whenever operations are required while the astronauts are in a low-shielding condition, the availability of wearable shielding devices can significantly reduce their exposure levels. Dose levels to more sensitive organs can be kept below thresholds for the onset of acute effects, reducing at the same time the risk of long-term consequences. The possibility to use on-board resources such as water (without water waste) or organic wastes for such wearable devices (and for possible additional shielding elements, in general) should be envisaged, so that no extra volume and mass is added to the payload at launch. In the framework of a feasibility study funded by the European Space Agency we proposed conceptual models for wearable radiation protection spacesuits, with protection elements made of different materials, addressing their shielding efficacy in case of SPEs(4). The PERSEO project (PErsonal Radiation Shielding for intErplanetary missiOns), funded by the Italian Space Agency (ASI), later led to the development of the first prototype of a water-fillable garment that was recently successfully tested on-board the International Space Station (ISS) for practicality of filling/draining (with no water waste) and comfort while wearing(5). This work includes a brief summary of the achievements of the PERSEO project, with a description of the manufactured garment prototype and of the outcome of the experimental session on-board the ISS, and a proof-of-principle validation of the shielding efficacy of the garment prototype with different SPE spectra and shielding conditions. The PERSEO Garment Prototype and Experimental Session on the ISS The garment prototype tested on-board the ISS is a sleeveless garment, with pouches for four embedded containers interconnected via internal circuitry, to be filled with water using a single interface to the on-board water dispenser, and drained back to the water recovery system after use. When filled, all containers have a thickness of 7 cm, their lateral dimensions being tailored to protect the torso of the astronaut selected to perform the experiment on-board: two front containers are protecting the chest and the abdomen, and two are protecting the lower and upper back. The nominal water allowance of the 4-bag system is 22.8-l. The garment was manufactured in compliance with requirements for use by a human subject and to meet a variety of safety requirements established by NASA for its use on the ISS as, e.g. to ensure material compatibility with the space habitat and to avoid the hazard of water leakage. The experimental session on-board the ISS was successful: as reported by the astronaut in a dedicated questionnaire filled on-board, filling and draining operations were judged easy to perform, and the garment was only slightly limiting the freedom of movement. A detailed description of the hardware, including technical solutions adopted for the manufacturing, requirement verification methods and a full report on the experimental session on-board the ISS is given elsewhere(5). Validation of the Shielding Efficacy METHODS The simulation setup was built in a Geant4/GRAS(6, 7) (using the QBBC(8) physics list) environment using GDML(9). We implemented a simplified software replica of the PERSEO garment, consisting of the four 7-cm thick water protection elements, with 2.5-mm polyurethane for containment. Protection elements were positioned around the torso of the GRAS geometrical phantom, adapting their lateral dimensions. The phantom, with or without the protection elements, was then positioned at the center of a cylindrical Al module (2.25 m radius, 6 m length), to simulate the condition of intra-vehicular activities. Two thickness values were considered for the module walls, 1.5 and 5 cm, respectively, reproducing a low- and average-shielding condition. The module is immersed in an isotropic radiation environment given by protons with emission energies sampled according to three different input spectra: an average spectrum obtained for a mission of 1 year in deep space, with 90% confidence level, using the Emission of Solar Protons (ESP) model(10) on the ESA SPENVIS website(11); two spectra for historical SPE events, namely those occurred in August 1972 and October 1989. For all SPE scenarios and shielding conditions, the dose reduction Dred (in %) achievable when the phantom is wearing the garment is calculated for all organs of interest as: Dred%=Dinmodule−DwithgarmentinmoduleDinmodule (1) Dred values were calculated for tissues/organs subject to the insurgence of short-term non-cancer effects, for which dose limits have been given by NASA, and that can be protected thanks to the use of the garment, namely: blood forming organs (BFO) and the circulatory system. Simplifying assumptions were necessary with the chosen phantom: (i) as done in previous works(4, 5), for the dose to BFO, we calculated the weighted average of doses absorbed by bone structures in the skeletal system of the phantom, with weights equal to the mass of red bone marrow (RBM, actively producing blood cells) in each bone, derived from ICRP 110(12); (ii) for the dose to the circulatory system, we calculated the dose to the heart of the phantom, though NASA recommends the use of an average over heart muscle and adjacent arteries. In both cases, doses entering Eq. (1) were calculated in Gy-Eq(13), applying a proton RBE factor of 1.5(14). Dred values are affected by statistical uncertainties coming from uncertainties in dose values, in turn determined by the statistics for incoming protons (up to several millions) and generally are under a few percent. By construction, dose reduction results are independent on the specific fluence of the SPE event. To compare results in terms of dose rates, information on the time distribution of solar protons is needed: for the ESP spectrum, we used the OMERE worst-hour model(15), as done in previous works(4, 5). For the two historical events, we used for the normalization the integral fluxes for evaluation of short-term effects as given by SPENVIS(8) (in detail, CREME86 for August 1972 and CREME96 for October 1989). Input energy spectra used in this work are plotted in Figure 1. Figure 1. View largeDownload slide Energy spectra for solar protons used in this work for dose calculations: the average spectrum calculated with the ESP model for a 1-year mission, and spectra from historical events (August 1972 and October 1989). Details in the text. Figure 1. View largeDownload slide Energy spectra for solar protons used in this work for dose calculations: the average spectrum calculated with the ESP model for a 1-year mission, and spectra from historical events (August 1972 and October 1989). Details in the text. RESULTS AND DISCUSSION Results for Dred (in %) to BFO and circulatory system are summarized in Figure 2 for the two shielding conditions and all three solar proton spectra considered here. Dose reduction values do not show significant variations for the considered SPEs, with average values of 42% for the BFO and 33% for the circulatory system for the low-shielding condition. In case of thicker habitat shielding, average dose reduction values are reduced (by up to 5%), but still remain significant. The difference in the shielding efficacy of the protection elements for the two shielding conditions is due to the interplay of two effects: on one side, thicker walls stop themselves lower energy incoming protons that would have been stopped by the water containers, thus lowering the efficacy; on the other, they also slow down protons from the external source to an energy range in which they can be stopped in the water thickness offered by protection elements. Figure 2. View largeDownload slide Dose reduction (Eq. (1)) to BFO and circulatory system when the phantom is wearing water protection elements inside the Al module (both for 1.5- and 5-cm thickness for the module walls), and the module is immersed in an isotropic radiation environment of solar protons with different energy spectra: the average spectrum calculated with the ESP model for a 1-year mission, and spectra from historical events (August 1972 and October 1989). Details in the text. Figure 2. View largeDownload slide Dose reduction (Eq. (1)) to BFO and circulatory system when the phantom is wearing water protection elements inside the Al module (both for 1.5- and 5-cm thickness for the module walls), and the module is immersed in an isotropic radiation environment of solar protons with different energy spectra: the average spectrum calculated with the ESP model for a 1-year mission, and spectra from historical events (August 1972 and October 1989). Details in the text. Dose rate values to BFO and circulatory system for the phantom without the suit in both shielding conditions are summarized in Tables 1 and 2. It is interesting to recall that dose limits for the BFO and for the circulatory system are set by NASA standards to 0.25 Gy-Eq for a 30-day period (regulation exists for low Earth orbit (LEO) missions only(14)). This means that the limiting dose to BFO could be reached by astronauts in a time interval from ~2.5 h to ~4 h for exposure to the considered solar proton events in low-shielding conditions, without additional personal shielding. As well known, protecting the BFO in particular is of major importance, and the ‘safe-time’ interval for accidental exposure or emergency operations can be significantly increased if water-filled protection elements are worn. Table 1. Dose rate values to BFO for the phantom without protection elements. Dose ratea (Gy-Eq/h) ESP August 1972 October 1989 Module 1.5 cm 0.10 0.06 0.09 Module 5 cm 0.04 0.01 0.03 Dose ratea (Gy-Eq/h) ESP August 1972 October 1989 Module 1.5 cm 0.10 0.06 0.09 Module 5 cm 0.04 0.01 0.03 aStatistical uncertainties are lower than a few percent. Table 1. Dose rate values to BFO for the phantom without protection elements. Dose ratea (Gy-Eq/h) ESP August 1972 October 1989 Module 1.5 cm 0.10 0.06 0.09 Module 5 cm 0.04 0.01 0.03 Dose ratea (Gy-Eq/h) ESP August 1972 October 1989 Module 1.5 cm 0.10 0.06 0.09 Module 5 cm 0.04 0.01 0.03 aStatistical uncertainties are lower than a few percent. Table 2. Dose rate values to the circulatory system for the phantom without protection elements. Dose ratea (Gy-Eq/h) ESP August 1972 October 1989 Module 1.5 cm 0.06 0.03 0.05 Module 5 cm 0.03 0.01 0.02 Dose ratea (Gy-Eq/h) ESP August 1972 October 1989 Module 1.5 cm 0.06 0.03 0.05 Module 5 cm 0.03 0.01 0.02 aStatistical uncertainties are lower than a few percent. Table 2. Dose rate values to the circulatory system for the phantom without protection elements. Dose ratea (Gy-Eq/h) ESP August 1972 October 1989 Module 1.5 cm 0.06 0.03 0.05 Module 5 cm 0.03 0.01 0.02 Dose ratea (Gy-Eq/h) ESP August 1972 October 1989 Module 1.5 cm 0.06 0.03 0.05 Module 5 cm 0.03 0.01 0.02 aStatistical uncertainties are lower than a few percent. Dose reduction to other organs subject to short-term effects were not calculated in this work for various reasons: the skin is only partially protected, no actual dose limit is given for the gastrointestinal tract, and no protection to lenses or to the central nervous system is offered with the garment model under consideration. CONCLUSIONS Personal radiation protection with wearable devices is a promising strategy to offer complementary shielding solutions for astronauts during future deep-space missions, particularly in case of occurrence of SPEs. Taking as driver the optimization of resources available in the space habitat, we propose the use of water-fillable garments, that can be filled and drained on-board, without water waste. The PERSEO project, funded by ASI, led to the design and manufacturing of a first prototype of a garment of this kind, that was successfully tested for ease of use and wearability on-board the ISS. In this work, besides a brief description of the garment, we focus on the validation of its shielding efficacy with Monte Carlo radiation transport calculations in the Geant4/GRAS environment. We present results of dose reduction to organs subject to the occurrence of short-term non-cancer effects and protected by the garment: BFO and circulatory system, for which short-term exposure limits have been set by NASA (though for LEO only). Even if obtained with simplified phantom, garment and habitat geometries, our results offer a proof-of-principle validation of the shielding strategy, and indicate that a significant dose reduction is achievable for all different considered SPE spectra (on average, 42% for BFO, as the main critical target in low-shielding conditions). This translates into an increase of the time interval the astronauts can be exposed, either accidentally or because obliged to work outside a radiation shelter, before the dose threshold for the onset of short-term effects is reached. The decrease in absorbed dose also translates into an overall decrease of detrimental long-term effects, though this is not explicitly modeled here. Taken altogether, the successful outcome of the PERSEO experiment on-board the ISS and the preliminary validation of the shielding efficacy of a water-fillable garment indicate that research and development into personal shielding devices of the kind here presented deserve great efforts, as they can represent an important breakthrough in radiation protection for the crew of future interplanetary journeys. ACKNOWLEDGEMENTS This work was partially funded by the Italian Space Agency (ASI), contract no. 2016-3-U.0. Thanks are due to all members of the PERSEO collaboration and all project partners and contributors: University of Pavia, Physics Department; Thales Alenia Space—Italy, Società Metropolitana Acque Torino S.p.A.; AVIOTEC S.p.A.; ALTEC S.p.A.; University of Rome Tor Vergata, Physics Department; Kayser Italia S.r.l; ARESCOSMO S.p.A. REFERENCES 1 Wilson , J. W. , Anderson , B. M. , Cucinotta , F. A. , Ware , J. and Zeitlin , C. J. Spacesuit radiation shield design methods. 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TI - INNOVATIVE SOLUTIONS FOR PERSONAL RADIATION SHIELDING IN SPACE
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncy216
DA - 2019-05-01
UR - https://www.deepdyve.com/lp/oxford-university-press/innovative-solutions-for-personal-radiation-shielding-in-space-mdaykwphvT
SP - 228
VL - 183
IS - 1-2
DP - DeepDyve
ER -