Impacts on natural populations of invertebrates may be also caused by other mechanisms rather than those affecting gamete production, eggs viability, fecundity, or reproduction delays. For example, the exposure of fourth-instar nymphs to IR from a 137Cs source up to doses of 12 Gy (at a dose rate of 0.25 Gy/min) has shown to affect the acoustic signaling of male crickets (Acheta domesticus) and subsequently their ability to find mates, due to morphological changes in their wings [126]. In fact, few is known about other direct and indirect effects that may affect the fitness individuals, its biotic relationships, and subsequently the dynamics of natural populations and communities at IR contaminated scenarios, rather than those effects identified based on commonly used biomarkers. The complexity of the biotic interactions, as well as the role of dominant abiotic factors determines the type and the impact of the indirect effects on ecosystems, whose responses can be unpredictable [98]. In a birch forest in South Urals, the contamination of litter with 90Sr (doses reaching up 70 Gy) compromised the development of pupae of tachinid flies (Tachinid sp.). This accounted for an increased survival of the host caterpillars of the gypsy moth (Lymantria dispar dispar L.) with increasing IR levels [127] in Geras’kin [98]. Møller et al. [128] also linked the reduction in the set of fruits produced by trees and bushes, at the Chernobyl exclusion zone with the local reduction of pollinator insects. The role of other biotic factors in the radiosensitivity of invertebrates also needs to be investigated, as it may be relevant under specific environmental or industrial scenarios. It was shown that the ability of marine mussels (Mytilus galloprovincialis) to respond to genotoxic induced effects by tritiated water, released by cooling operations of nuclear power plants, was limited by enhanced temperatures [129].


Invertebrates also have a key role in several ecosystem functions, as, for example, the degradation of wood, organic matter, and nutrients recycling. Mousseau et al. [130] conducted a study in forest areas within the Chernobyl exclusion zone, at different distances of the nuclear plant and with levels of background radiation differing by several orders of magnitude (range 0.09–240.25 mSv/h). A significant effect of background radiation in the mass loss of litter bags buried in the surface of the forest soils was registered. The mass loss of litter bags from the sites with high levels of background IR was 40% lower than that recorded at the sites with lower levels of radiation. However, no significant influence of the mesh size of litter bags was found, suggesting that decrease in the decomposition of litter at that site was not only caused by impacts on soil invertebrates’ communities, but also on soil microbiome. Soil invertebrates’ assemblages from pit falls and wood slices from the same area showed that the abundance of taxonomic groups displayed a different relationship with background radiation and with wood contamination with radionuclides, being positively, negatively, or not affected at all [131, 132]. This was consistent with a previous observation of a general loss of diversity in sub-surface and flying invertebrates with increasing concentration activities of 90Sr and 137Cs in the litter of forest sites within the Chernobyl exclusion zone [133], as well as by the apparent decrease in the feeding activity of these organisms measured by the bait-lamina test.9 However, such changes were not followed by changes in total biomass of organisms. These results suggest that chronic environmental exposures to IR may exert their effects on natural communities, through structure and functional diversity simplification, with possible impacts on ecosystem’s functions.


In a first attempt to estimate risk limits for chronic g-radiation exposures, predicted no effect dose rates (PNEDR) of 10 mGy/h (0.24 mGy/day) for freshwater ecosystems and of 67 mGy/h (1.61 mGy/day) for terrestrial ecosystems were obtained, using assessment factors and species sensitivity distribution methods, respectively. The estimated values were found to be highly protective as they were about ×50 to ×100 times higher than the upper bound of the range of natural background concentrations and of the lower dose rates causing effects at contaminated sites [134]. Later, and by applying an assessment factor (AF) of 3 to the HDR510 estimated for invertebrates, a PNEDR of 170 mGy/h for IR was obtained. However, and considering that no sufficient data was available for applying probabilistic methods to estimate PNEDR for specific groups of organisms or for environmental compartments, Garnier-Laplace et al. [135] derived a generic HDR5 from a species sensitivity distribution using data from controlled laboratorial chronic exposures to low dose rates of gamma-radiation, and applied an AF of 5, obtaining a PNEDR of 1.5 mG/h which was considered to be protective for the conditions found at Chernobyl exclusion zone.


Among all the vertebrates, mammals are organisms on which the effects of radiation exposure were most extensively studied in radiobiological experiments. Negative effects on these organisms, due to radiation exposure at high doses (i.e., 10–50 Gy), are primarily due to effects at the hematopoietic system and the gastro-intestinal mucosa [45, 46]. The time needed for death to occur varies widely within species. The dose of radiation needed to cause lethality, due to gastro-intestinal syndrome, to 50% of the exposed organisms (LD50) is approximately as follows for dog—8 Gy, mouse—12 Gy, rat—11 Gy, and rhesus monkey—9 Gy [46]. However, these values were estimated for particular species of these organisms, so there can be wide variations for other species. These variations are normally related with specific intestinal morphologies, which are related to diet (i.e., herbivores, carnivores, and omnivores). Regarding bone marrow damage, the weight of the animals receiving the dose appears to have a significant role in the bone marrow radioresistance, being weight inversely proportional to radiation sensitivity, as LD50 values are greater for smaller mammals (6–10 Gy approximately) than for larger ones (1.2–3.9 Gy). A reduction in life span is also related to the type of radiation to which animals are exposed, being high LET radiation more effective than low LET radiation. Also, acute exposures are substantially more effective by a factor of 7 in causing mortality than chronic exposures [46]. Significant life span shortening occurred in dogs and mice exposed to low LET radiation (gamma radiation) at dose rates between 100 and 1000 mGy/h and the same happened for mice exposed to neutrons (high LET radiation) at the same dose rates [136]. In general, a significant reduction in life span of several mammal species was observed at dose rates higher than 1000 mGy/h [50, 136, 137]. Chronic exposures of less than 100 mGy/h have a low probability of inducing significant effects on most terrestrial organisms [45, 46, 136]. Particularly, a dose rate of less than 40 mGy/h has a low probability of inducing effects on the fertility, fecundity, and the production of viable offspring of a mammalian population [45]. This is true for low LET radiation, however for high LET radiation, this dose rate value is lower, as this type of radiation has a much higher relative biological effectiveness (RBE) [45]. An experiment performed in mice irradiated with neutrons, at dose rates lower than 100 mGy/h for at least 475 days, led to a significant increase of mortality in mice in comparison with the control [136].


Reproduction is a more radiosensitive parameter than mortality, and effects of radiation may appear at radiation levels that apparently do not induce other observable responses. The magnitude of the effects depended also on the developmental stage in which the animal was irradiated [136]. A good example are mice, as the LD50 occurs at a radiation dose approximately between 6 and 10 Gy; however, at a radiation dose of 0.08 Gy, the production of oocytes was reduced to 50% in newborn mice (the most radiosensitive stage in mice) [45, 46]. However, this does not necessarily mean that there will be a decline in fecundity, since mice produce much more oocytes than the amount effectively used for reproduction, but there could be a reduction in the offspring [46]. In adult males, fertility is temporarily impaired after a 10 Gy exposure; however, in young mice (3–5 days old), it can cause permanent sterility. Mice in the second week after birth are also especially sensitive to the detrimental effects of radiation on reproduction [136]. The differences between males and females are mostly a consequence of the differences in the gametogenesis process. There are also differences between species, being mice one of the least radioresistant. Chronic irradiation affects mainly the time needed for oogonial cell division and the size of stem cell pool [46]. In males, the spermatogenic process is maintained, although at lower levels than unexposed organisms [46].


The developing embryo is particularly sensitive to radiation, due to the high number of cells proliferating, reducing fecundity and postnatal survival, potentially influencing population size [46]. Acute radiation exposure, before the implantation of the embryo, causes its early death and can also cause post implantation and postnatal death [46]. This has a good correlation with the occurrence of DNA damage in the form of chromosome aberrations in the blastomeres (cells that result from the cleavage during the early development of the embryo) [46]. Radiosensitivity is strongly influenced by cell cycle stages and mitotic cycle in the very early developmental stages [46]. During organogenesis, the most typical response to acute radiation is the occurrence of malformations (teratogenic effects), which can occur during embryonic and fetal growth and may or may not be fatal. The occurrence of teratogenic effects in a particular organ is related to a high level of cell proliferation in the precursor tissue [46]. Although this has been observed for the several species studied (mouse, hamster, cattle, pig, monkey rabbit, etc.), the responses to specific radiation doses will depend on the species and on its developmental stage at the time of exposure [46]. There are not many studies on the effects of chronic radiation exposure during organogenesis, however a study performed on mice showed doses of 0.01 Gy/day in pregnant mice 6–9 days after conception induce a significant impairment of the offspring’s learning ability [46]. Also, dose rates of 420 mGy/h reduced neonatal brain weight, with unknown effect at the functional and behavioral levels [45].


A direct relationship between DNA damage and radiation dose is expected at high doses of radiation; however below 100 mGy, it is not clear. In reindeer, a tenfold increase in the number of chromosome aberrations was observed at dose rates between 100 and 1000 mGy/h [136]. For rodent species acutely exposed to low LET radiation, mutations in the form of reciprocal translocations (exchange of DNA between homologous chromosomes) occur in stem cell spermatogonia when organisms are exposed to between 0.01 and 0.03 Gy at total doses from 3 Gy [46]. High LET radiation exposure (in the form of alpha particles emitted by 239Pu), delivered at a dose rate of 36 mGy/h significantly increased the occurrence of translocations and acentric fragments (chromosome fragments without a centromere) in spermatogonia and spermatocytes, respectively [46]. In primates, the dose interval is 0.01–0.078 Gy at doses from 1 Gy [46]. Translocations, ring chromosomes (aberrant chromosomes whose ends were broken and then fused together to form a ring) and dicentric chromosomes (the result of two broken chromosomes that fused together) are used for radiation dosimetry in human and non-human biota for a long time, as their frequency increases with radiation dose [138]. In rodents, this is more easily seen at total absorbed doses higher than 0.5 Gy, suggesting that their use as a biomarker of radiation exposure is more effective at high dose exposure than at low doses (below 100 mGy). Regarding carcinogenicity, there is a wide variation in the sensitivity for tumor formation among tissues and species. The induction of cancer, even at high radiation exposure doses (>100 mGy) will also vary according to the age of exposure. Dogs exposed to doses higher than 7 Gy showed soft tissue cancers when exposed in utero but not when exposed as young adults [46]. In rodent species, there were limited carcinogenic effects on animals that were exposed to doses between 0.1 and 1 Gy [138].


The effects of radiation exposure in birds are apparently similar to the ones observed in small mammals [45]. The LD50 for wild birds is in the same range as small mammals (5–12 Gy). For poultry, the LD50 determined experimentally for mortality is of 7–11 Gy in 3–4-day-old individuals when irradiation lasts for less than 1 h and of 12–20 Gy when irradiated for 24 h. Egg production is affected in white leghorn chicken at a total absorbed dose of 4–8 Gy and at higher doses, effects are more severe and long lasting [45]. A limited number of experiments performed in artificially incubated chicken embryos showed a LD50 of 12–13 Gy, which apparently indicates a higher radioresistance than adults [46]. In white leghorn chickens, eggs hatchability is affected at a total absorbed dose of 8 Gy, but the progeny is unaffected [48]. The International Commission on Radiation Protection also reported dose ranges for which long-term effects on developing embryos were reported (100–1000 mGy/day), reduced reproductive success (1–10 mGy/day) and increased morbidity (10–100 mGy/day) [139]. Recently, it was reported a decrease in species abundance at a dose range of (from 0.3 to 97 μGy/h) in the Fukushima exclusion zone, which is consistent with the dose ranges reported for increased morbidity and decreased reproductive success [140]. The existing knowledge on DNA damage/alterations on birds exposed to ionizing radiation results from the evaluation of effects of radioactive environmental contamination resulting from the Fukushima and Chernobyl accidents [141].


The information gathered so far for reptiles and amphibians suggest that their radiosensitivity is similar to that of mammals and birds. The LD50 values recorded for frogs, salamanders, turtles and snakes vary between 2 and 24 Gy [46]. The main cause of death identified was damage to the hematopoietic system [46]. In two separate experiments performed on lizards, two very different LD50 doses ranges were obtained (10–12 and 17–22 Gy). The possible reasons for this marked difference are associated with the fact that these values may vary according to radiation type and quality, the dose rate to which the organisms were exposed and their maintenance conditions at the laboratory [46]. An acute exposure to 50 Gy caused temporary sterility in males, but recovery was well in process after 48 days post irradiation and irradiation of gonads in males and females to an absorbed dose of 4.5 Gy leads to a substantial decrease in the production of offspring [46].


Regarding amphibians, different life stages showed different radiosensitivities. For adult toads, the LD50 value is of 24 Gy, for juveniles it is of 10 Gy and for tadpoles it is of 17 Gy [46, 139]. The life stage more sensitive to radiation exposure was the fertilized egg with an LD50/40 (LD50 after 40 days of exposure) of 0.6 Gy [33]. There is evidence that the exposure of male toads to 3–20 Gy caused a reduced survival and increased induction of abnormalities to the offspring [46, 139]. Although these LD50 values for amphibians seem slightly higher than the ones recorded for mammals, time after exposure optimal for the recording of LD50 values seem to be an important factor [33]. Reptiles and amphibians are poikilothermic organisms; therefore, their metabolism is quite variable and different from mammals and birds [33]. A study performed on 4 species of amphibians showed that if the assay period was extended a decrease in the LD50 to values that ranged between 0.8 and 7 Gy would be recorded [33].


Chronic irradiation exposure (5.5 years duration) of common side blotched lizard, western whiptail, long nosed leopard lizard and long nosed lizard showed that at ranges from 285 to 570 μGy/h, radiation exposure caused lack of reproduction, female ovaries regression and some degree of male sterilization [46].


Regarding the induction of DNA damage, it was observed by Ulsh and co-authors [142] that the exposure of turtles from the species Trachemys scripta to 0–8 Gy 137Cs gamma radiation, given at a dose rate of 0.55 Gy/h induced the occurrence of significant levels of chromosome translocations in lymphocytes. Studies on the induction of DNA alterations in amphibians and reptiles have been performed in Fukushima and Chernobyl exclusion zones, as well as in areas contaminated with NORM.


Among non-mammalian aquatic organisms, fish are the most sensitive to the exposure to ionizing radiation [45, 46]. Although these organisms are also poikilothermic (as amphibians and reptiles), and therefore, apparently more radioresistant than mammals, there is a substantial overlap in radiosensitivities [46]. Until now, there is no substantial data on effects of ionizing radiation on marine mammals, however, there is no reason to believe that their radiosensitivity is substantially different from that of terrestrial mammals. Data on acute exposures exist mainly for bony and freshwater fishes, with a small number of studies on cartilaginous and marine and anadromous species.


The LD50 determined for six marine species after 40–50 days of exposure was of 9–23 Gy [46, 139]. Fish developing embryos are, however, more sensitive than adults, as for silver salmon their LD50 after 50 days of exposure is of 0.30 Gy at hatching and 0.16 Gy at a post-hatching larval stage of 90 days [46]. A study performed on sharks (Triakis scyllium and Heterodontus japonicus) exposed to 20 Gy showed that mortality occurred after 20 days of exposure, due to hematopoietic and gastrointestinal damage [33]. This suggests that the radiosensitivity of cartilaginous fish may be similar to that of teleost fish.


Regarding reproduction, an acute exposure to 10 Gy reduced the total number of germ cells at all developmental stages of medaka fish (Oryzias latipes) [46]. A similar radiosensitivity was found in rainbow trout, with an induction of more than 50% sterility in organisms exposed late in embryonic development [46]. This leads to the conclusion that as in mammals, the newly hatched fry and the primordial gonads in fish embryos are more sensitive to the acute radiation exposure than in adult fish [46]. Irradiation of mature medaka fish at acute doses of 5–10 Gy only induced temporary sterility, being completely recovered at 60 days after irradiation [46]. On the other hand, chronic irradiation of males from the fish species Ameca splendens for 5.4 days at a dose rate from 137Cs gamma rays of 7300 mGy/h disrupted spermatogenesis and render the animals sterile at an accumulated dose of 9.7 Gy (8 weeks of exposure) [46]. There was 60–70% recovery, 236 days after irradiation [46]. Another freshwater fish, the guppy (Poecilia reticulata), when exposed to gamma dose rates from 1700 to 13,000 mGy/h showed a significant reduction in fecundity, but no negative effects on survival and sex ratio, as well as no significant higher incidence of abnormalities in the offspring were observed [33, 136]. The marine fishes Pleuronectes platessa and the eelpout (Zoarces viviparus) exposed to 240 and 2000 mGy/h gamma radiation, respectively, showed a significant reduction of testes when compared to the control [136].


There are some findings also on the effects of the exposure to ionizing radiation in the immune system of these organisms. A significant reduction in the humoral immune response in the rainbow trout (Oncorhynchus mykiss) exposed to tritium beta-particles for 20 days at a dose rate as low as 8.3–83 mGy/h during embryogenesis was evidenced through a reduction in antibody titer following a specific challenge [46].


Regarding DNA damage there are very few studies on which some conclusion can be taken on this matter. On a study on medaka fish, at larval stages, there was a significant induction of vertebral anomalies after irradiation at dose rates from 137Cs gamma rays higher than 18,000 mGy/h and also to beta particles from 3H at dose rates higher than 35,000 mGy/h [46]. There is also a report on the occurrence of minor morphological abnormalities in the operculum of salmons exposed to a gamma radiation dose rate of 200 mGy/h that may affect latter survival [136].


Anthropogenic activities of concern related to the environmental release of natural uranium isotopes (mainly 238U and 235U) and other radionuclides from their decay chains, namely 226Ra and 223Ra, 222Rn, and 210Po, include mainly the production of phosphate fertilizers, uranium mining and milling and the incorrect disposal of tailings, uranium conversion and enrichment, the production of uranium fuel, production of coal, oil and gas, extraction of rare earths, extraction and purification of water, extraction of minerals for building materials and the generation of geothermal energy [3, 143]. All of these industrial activities increase the concentration of these elements in all environmental matrices, thereby posing a risk to human and non-human biota as many of them have not been regulated for NORM release [3, 143]. Another important issue is the fact that the contaminated areas that result from these anthropogenic activities do not only present high levels of certain natural radionuclides, like 226Ra, 222Rn, and 210Po but also other important stressors, namely metals like manganese, zinc, iron, aluminum, etc. [143]. These are usually multiple exposure scenarios, which contain several kinds of contaminants that may act synergistically and increase the risk of the occurrence of biological effects on human and non-human biota and even of modifying the susceptibility of cells/organisms to the biological effects of ionizing radiation exposure [144].


The accumulation of small amounts of radionuclides and metal over long periods is translated in chronic exposure to radiation. Naturally contaminated sites harbor a diversity of microbial species that become resistant or tolerant to these contaminants by bioaccumulating radionuclides and metals either by biosorption to their cell surfaces and biomolecules or by internalization into their cells. Briefly, under environmental conditions, chronic IR effects are very complex, particularly when compared to those from laboratory exposures because (1) radiation emitted by the different radionuclides present has different biological effects, (2) radiation from the same location is absorbed differently by different microorganisms, (3) abiotic factors (e.g., temperature, nutrients, pH, other stressors) are present and can interfere with radiation, (4) cooperation/interaction between microbial communities, including diversity and/or abundance can all be modulated by radiation [62]. Regarding uranium, probably the most well studied radionuclide, and for which a lot of information is available, interaction with microbial cells involving solubility by biomineralization (bioprecipitation) depends on all the above factors and also on the presence of affinity groups generated by microorganisms’ cell metabolism, like hydroxides, phosphates, and carbonates. Uranium toxicity is both chemical and radiological. In the environment, uranium exists in its reduced insoluble form U(IV), and/or the oxidized form U(VI), which is soluble and toxic. Microorganisms interact with uranium by changing its redox state, aerobically, through oxidation (biolixiviation), or anaerobically by reduction. In order to do that, microorganisms need to be highly tolerant to uranium and to radiation. Other processes of microbial interaction with metals, involve biosorption, where contaminants passively concentrate through binding to cell structure constituents (e.g., lipopolysaccharides, teichoic acids, peptidoglycan), and biomineralization, which leads to the formation of biominerals using organic phosphate sources and phosphatases.


Unless disturbance occurs, NORM sites have a characteristic microbiome, which is specific for a given site, but may share common microbial genera and species, regardless of location and/or chemical contamination. It includes nitrate-reducing bacteria that tolerate acidic and low-nutrient conditions, while being highly resistant to metals. Members of the Proteobacteria (Alpha-, Beta-, Delta- and Gamma- proteobacteria), Acidobacteria, Actinobacteria, Bacteroidetes, and Firmicutes are generally associated with uranium transformation and are therefore found in these environments. Most represented bacterial genus include Geobacter, Thiobacillus, Arthrobacter, Bacillus, Actinobacteria, Desulfovibrio, and Microbacterium. Most of the studies are focused on bacteria and bacterial communities. Although little information exists regarding fungi, they are particularly resistant to radiation and thus play a role in the process of detoxification of radionuclides. For instance, an isolate of the genus Paecilomyces, was found to detoxify U(VI) through bioprecipitation of the metal, and the reduction was promoted by phosphate. Also, the yeast S. cerevisiae was able to reduce U(VI) toxicity by biomineralization [60].


Accordingly, the survival, abundance, and maintenance of a given species or community diversity depend on its adaptability to the existing conditions. Furthermore, several studies suggest that in those radionuclide-rich natural sites, resistance to high levels of chronic IR may occur among taxa that tolerate a wide range of environmental conditions and, therefore, have an advantage over other more sensitive species [62].


There have been some studies in aquatic organisms, namely in Daphnia magna, Daphnia longispina, and Moinodaphnia macleayi at NORM sites [145, 146]. When testing several percentages of a uranium mine effluent containing metals and radionuclides from 238U and 235U decay chains, the Antunes et al. [145] study recorded an EC5011 for daphnids immobilization at 50.4% for D. magna and at 28.4% for D. longispina, showing that D. magna was less sensitive than D. longispina. However, regarding fertility, D. magna was more sensitive than D. longispina, as this last species did not show significant effects in the offspring produced at effluent concentrations lower than 30.38%. Regarding M. macleayi, when a natural population of these organisms, living adjacent to a uranium mine in Australia, was challenged with a concentration of uranium ranging from 0 to 700 μg/L, it was shown that this population comparing to other populations tested, was the one that presented the highest sensitivity as it evidenced the lowest NOECs and LOECs.12 It was shown that although this population lived in a water containing already considerable amounts of uranium, there was no tolerance to higher levels of uranium, when compared to the other tested populations. This probably shows that it was an already very stressed population that suffered “genetic erosion” [147] and because of that, it had lower capacity to deal with additional stresses, such as a single high dose of uranium.


When D. magna was exposed to uranium and to a uranium mine effluent [148, 149], significant genotoxic effects (DNA strand breaks) were detected in neonates and <5 days old daphnids after exposure to 55.3 μg/L of uranium and 2% of a uranium mine effluent. Moreover, in this same study, bystander effects, in the form of DNA damage, were detected in unexposed organisms when placed in contact with organisms directly exposed to uranium and to uranium mine effluent. In another paper [149], published by the same authors, on a transgenerational study performed on D. magna exposed to the same concentrations of uranium and uranium mine effluent as the study previously referred, it was observed that DNA damage was transmitted only to the first broods of the exposed organisms. By the third brood, DNA damage was no longer detected. This study showed that although short-term exposure to low concentrations of uranium and uranium mine effluent induces DNA damage to exposed organisms, it seems that it was not enough to significantly affect life history traits of D. magna populations in a long-term scenario. Nevertheless, the interpretation of these results is limited to the response observed for the endpoints here analyzed (DNA strand breaks). As such, other endpoints for genotoxicity assessment (i.e., mutation detection) and also the analyses of the epigenome of these organisms should be performed, as these molecular changes do not reflect a loss of DNA’s structural integrity [149].


As for terrestrial invertebrates, most of the studies conducted so far were on the annelid Eisenia andrei [150–155]. Gene expression alterations were reported in earthworms exposed to sludge from a uranium mine decantation pond. These genes were mainly related with metabolism, oxidoreductase activity, redox homeostasis, and response to chemical stimulus and stress [152]. In these studies, the occurrence of DNA damage in the form of DNA strand breaks and changes in cell’s DNA content in exposed organisms was also detected. Alterations in earthworm’s immune system were also reported, in terms of the frequency of each cell compartment, as it was observed a decrease in the number of effector cells (amebocytes) and an increase of the cells responsible for the maintenance of the organism’s homeostasis (eleocytes) [153, 154]. In parallel with a significant bioaccumulation of metals and radionuclides from uranium’s decay chain (238U, 234U, 235U, 226Ra, 230Th, and 210Pb), it was also observed a significant decrease in earthworms’ biomass, a reproduction inhibition, and significant histological alterations, namely in earthworm’s body wall (epidermis, circular, and longitudinal muscles) and gastrointestinal tract (chloragogenous tissue and intestinal epithelium) [153–155].


Under a real scenario of contamination, all of these effects may explain the lower biodiversity of soils contaminated with NORM, and the subsequent loss of their functions, if the contamination is perceived by the organisms. By using an avoidance assay (a standard ecotoxicological assay), to study earthworms’ behavioral responses to soils collected in a uranium mine area, it was shown that earthworms actively avoided several contaminated soils. Earthworm’s avoidance responses allowed it to discriminate highly to moderately toxic soils. On the other hand, on another study published by the same authors, using the analyses of oxidative stress enzymatic biomarkers (catalase, glutathione peroxidase) and lipid peroxidation biomarkers (through the quantification of thiobarbituric acid reactive substances), in earthworms exposed to soils nearby a uranium mine, showed no response for none of the biomarkers analyzed [150].


Although there have been a wide number of studies performed on the effects of gamma radiation exposure on vertebrates, very few were performed so far for NORM exposure. Regarding aquatic vertebrates, fish have been the most used model organisms. On a study performed in former uranium mines from the Limousin region of France, where Rutilus rutilus specimens were caged on a pond contaminated with NORM and metals, immune, oxidative stress, biotransformation, neurotoxicity, and physiological parameters were measured [156]. The results obtained showed a stimulation of the immune parameters, the occurrence of oxidative stress and a decrease of acetyl choline esterase-AChE in the fish caged in the contaminated pond [156]. Zebrafish (Danio rerio) specimens exposed to uranium mill tailings leaching solution also showed alterations for the oxidative stress biomarkers used (superoxide dismutase—SOD, catalase—CAT, malondialdehyde—MDA and Na+–K+–ATPase) but specially for Na+–K+–ATPase and also evidenced that the organs most susceptible to oxidative stress were the gills [157]. In another study performed on a uranium milling operation in Northern Saskatchewan, Canada, Pimephales promelas specimens (adults and 5-day-old larvae) were exposed to contaminated water and contaminated sediment [158]. Results indicated effects on reproduction (reduced hatching) and larvae development (increase of skeletal deformities) and an increase in metal body burdens. However, the effects detected on the offspring, when considering the increase in egg production, were not significant in the level of deformities between treatments [158]. The effects on reproduction on the same species have already been observed under an exposure to effluent waters also from a uranium mining site in Saskatchewan, Canada. A significant decrease in eggs hatching time and hatching success was registered when early life stages of fathead minnows were exposed [159]. Nevertheless, metals and radionuclides are not the only stressor responsible for the effects caused by effluent waters from NORM sites. Lourenço et al. [160] performed an exposure of zebrafish eggs to a uranium mine effluent, barium chloride-treated mine effluent, and settling ponds sludge elutriates and showed that pH of the mine effluent strongly affected hatching success. After eliminating the effect of pH, this study also showed some teratogenicity associated with the uranium mine effluent, the occurrence of DNA damage, mainly associated with the exposure to treated mine effluent and sludge elutriates and mild effects on growth observed mainly on embryos exposed to the mine effluent and sludge elutriates. This study showed that the use of the Fish Embryo Toxicity Test (FET) test is suitable to test uranium mining wastes to determine and discriminate the risk of discharge. It also showed that the inclusion of the evaluation of genotoxicity endpoints in the FET test prevented the underestimation of risks, when only looking at chemical and radiological benchmark values defined by national and international directives, for the determination of risks, due to the chemical complexity of these wastes.


On what concerns amphibians, there are very few studies on these organisms as well. Marques and co-authors performed very important studies on amphibians, namely Pelophylax perezi exposed to NORM in situ. They have studied both tadpoles and adults and they have analyzed several endpoints, such as growth, survival, oxidative stress biomarkers (catalase, glutathione peroxidase, glutathione reductase, and lipid peroxidation through thiobarbituric acid reactive species (TBARS) quantification), gene expression alterations, histopathological changes, erythrocytic nuclear abnormalities and micronuclei, on organisms exposed to a uranium mine effluent in Portugal [161–165]. A study performed on 2008, on larvae and eggs [165] exposed to a uranium mine effluent, showed a decrease in larvae body length as well as a decrease in stimulus reactions, an increase in pigmentation along with tail deformities and metals bioaccumulation. The in situ exposure of tadpoles of the same species showed decreased survival and growth, a higher glutathione peroxidase activity and an increased lipid peroxidation [164] in organisms exposed in the mine effluent pond, when compared with organisms from a control pond. Although there may have been the influence of NORM and metals exposure, the studies also evidenced the effects of effluent’s acidity (typically seen in metal mining contexts), mainly in the growth and survival parameters and also in metal’s uptake. Another study, performed by the same authors [163], on adults living on the same uranium mine pond, analyzed gene expression changes using a technique called Suppressive Subtractive Hybridization (SSH). Significant changes in the expression levels of genes that play an important role in protecting cells against oxidative stress were shown, evidencing once again that oxidative stress response is very important in protecting cells and in maintaining DNA integrity on organisms exposed to NORMs and metals. Another study performed by this team on Pelophylax perezi adults inhabiting a uranium mining pond [162], showed significant metals bioaccumulation in the liver and the kidneys. Significant histopathological alterations in the liver, the lungs and in the kidneys, mainly in the form of a slight increase in melanomacrophagic centers, a dilatation of the renal tubules, a discrete thickening along with a slight hyperplasia of the alveolar septa and a slight hypoplasia of the goblet cells, were observed. The same animals living in the mine pond also displayed a significantly higher number of erythrocytic abnormalities (micronuclei and notched, kidney and lobed shaped nuclei) as well as a significantly lower frequency of immature erythrocytes. Both observations led to the belief that the removal and replacement of abnormal blood cells might be compromised.


There are a few studies published on the uptake of NORM by mammals that were performed mainly on former uranium mining areas, but very few examined the effects of that exposure. A study performed by Cleveland et al. [166] analyzed NORM uptake and histopathological alterations in liver and kidneys of rodents (Peromyscus maniculatus and P. boylii) inhabiting former uranium mines and observed that rodents bioaccumulated elements from 238U decay chain but without exceeding literature-based effects thresholds for small rodents. The authors also observed that there were some minor lesions in the tissues (liver and kidneys) analyzed that could not, however, be attributed to U mining activities. Lourenço and co-authors [167], captured mice (Apodemus sylvaticus) on the surroundings of a former uranium mining site and on a control area. DNA damage and bioaccumulation of metals and radionuclides were assessed, as well as the expression and the presence of single nucleotide polymorphisms on tumor suppressor genes. Results showed that cadmium and uranium were significantly bioaccumulated by exposed organisms. Organisms living in the former uranium mining area also evidenced significantly higher levels of DNA damage when compared with control organisms and also a higher expression of TP53 tumor suppressor gene and the presence of single nucleotide polymorphisms in Rb tumor suppressor gene. These effects can cause a disturbance in the genetic material of exposed organisms causing genetic instability and changes in the genetic pool of the population, potentially affecting the population’s fitness and stability. However, they cannot be attributed to any of the stressors in particular. It is known that uranium is genotoxic due to its chemical and radiological properties. Nevertheless, other metals present in uranium ore have shown greater genotoxic properties [151].


As for plants, there are a few studies already performed using soil/sludge or plant species collected directly from radium production industry storage sites, uranium rich regions, but mainly uranium mining sites and uranium milling tailings, that showed NORM bioaccumulation [168–180]. However, very few assessed the effects of that bioaccumulation. On a study performed by Evseeva et al. [170], Vicia cracca populations, inhabiting areas contaminated with uranium mill tailings and radium production wastes, were sampled and analyzed for the presence of chromosome aberrations, frequency of embryonic lethal mutations, seed germination and survival rate of seed sprouts. Results showed an increased frequency of embryonic lethal mutations, decreased seed germination, increased chromosome aberration counts and decreased survival rate of seed sprouts. The same authors [171], used Allium cepa specimens to determine the genotoxicity of an effluent from a radium production storage facility, through chromosome aberrations counting. Results showed a significant increase in chromosome aberration counts in the roots of exposed plants. Two studies [168, 179] using soils contaminated with metals and radionuclides from Portuguese former uranium mines were performed using Lactuca sativa and Zea mays as test species to determine the eco (through growth inhibition) and genotoxicity (mutation analysis through the Ames test) of amended and unamended mine soils. Studies showed genotoxicity of the unamended soils containing the highest levels of metals and radionuclides, a significant decrease in Lactuca sativa biomass and also a significant bioaccumulation of these elements. The soil amendment methodology used in these studies significantly decreased the levels of metals and radionuclides in soils leachates and the soil available fraction.


Q1.
What is the relationship between life stage and an organism’s radiosensitivity?
 Q2.
Please indicate which is the most radiosensitive parameter: mortality or reproduction?
 Q3.
Please indicate the most important non-stochastic effects induced by organisms exposure to ionizing radiation at a population level.
 Q4.
Which kind of exposure is more effective in causing organisms mortality?
 Q5.
Regarding radioactive contamination, what information can be retrieved from the omics approaches? What can be the contribution of those studies for future remediation of radiologically contaminated sites?
 Q6.
What are the main traits conferring radioresistance to plants compared to animals?
 Q7.
What does “hormesis” in plants mean?


What is the relationship between life stage and an organism’s radiosensitivity?


Please indicate which is the most radiosensitive parameter: mortality or reproduction?


Please indicate the most important non-stochastic effects induced by organisms exposure to ionizing radiation at a population level.


Which kind of exposure is more effective in causing organisms mortality?


Regarding radioactive contamination, what information can be retrieved from the omics approaches? What can be the contribution of those studies for future remediation of radiologically contaminated sites?


What are the main traits conferring radioresistance to plants compared to animals?


SQ1.
The younger the organisms, the more sensitive they are to the deleterious effects of radiation exposure.
 SQ2.
Reproduction and reproductive capacity is a more sensitive parameter to the effects of radiation exposure both for terrestrial and aquatic invertebrates and vertebrates, than mortality.
 SQ3.
The non-stochastic effects that are most important at a population level are mortality, fertility, and fecundity.
 SQ4.
Acute exposures to high doses of ionizing radiation are more effective in inducing higher injury than chronic exposures to low doses of ionizing radiation. The higher the dose the lower the ability of cells to divide and regenerate the damaged tissue which translates into a higher probability for organisms mortality.
 SQ5.
The application of multiomics approaches, namely genomics, proteomics, metabolomics, and transcriptomics, has gained relevance in many different fields. These high-throughput techniques allow an analysis of the total set of molecules (DNA, proteins, and other metabolites) in a biological sample. Therefore, the integrated data have revolutionized biology and have contributed to advancing our understanding of different biological processes.
Genome sequencing, comparative genomics, and proteomics have allowed the identification of microbial essential genes (key players) that encode biomolecules, mainly proteins, involved in biological processes, including those involved in detoxification of radionuclides and metals. Furthermore, metagenomics approaches directed to the microbial communities of these contaminated environments allow for the identification, and characterization, of microorganisms with relevant functions in the bioremediation/decontamination processes. It is therefore expected that these broader approaches will contribute even more to the identification of microorganisms and to the elucidation of the metabolic pathways and key genes involved in those processes that may be further applied in the bioremediation/decontamination of these sites.
 SQ6.
The elevated radioresistance of plants compared to animals relies on differences in cell structure and metabolism. Plant cells present some traits such as thickened cell walls, cuticles, hairs (pubescence), phenolic compounds, and often polyploidy.
 SQ7.
Low doses of ionizing radiation induce positive outcomes in plants such as increasing growth and production of secondary metabolites engaged in the antioxidant defenses.


The younger the organisms, the more sensitive they are to the deleterious effects of radiation exposure.


Reproduction and reproductive capacity is a more sensitive parameter to the effects of radiation exposure both for terrestrial and aquatic invertebrates and vertebrates, than mortality.


The non-stochastic effects that are most important at a population level are mortality, fertility, and fecundity.


Acute exposures to high doses of ionizing radiation are more effective in inducing higher injury than chronic exposures to low doses of ionizing radiation. The higher the dose the lower the ability of cells to divide and regenerate the damaged tissue which translates into a higher probability for organisms mortality.


The application of multiomics approaches, namely genomics, proteomics, metabolomics, and transcriptomics, has gained relevance in many different fields. These high-throughput techniques allow an analysis of the total set of molecules (DNA, proteins, and other metabolites) in a biological sample. Therefore, the integrated data have revolutionized biology and have contributed to advancing our understanding of different biological processes.


Genome sequencing, comparative genomics, and proteomics have allowed the identification of microbial essential genes (key players) that encode biomolecules, mainly proteins, involved in biological processes, including those involved in detoxification of radionuclides and metals. Furthermore, metagenomics approaches directed to the microbial communities of these contaminated environments allow for the identification, and characterization, of microorganisms with relevant functions in the bioremediation/decontamination processes. It is therefore expected that these broader approaches will contribute even more to the identification of microorganisms and to the elucidation of the metabolic pathways and key genes involved in those processes that may be further applied in the bioremediation/decontamination of these sites.


The elevated radioresistance of plants compared to animals relies on differences in cell structure and metabolism. Plant cells present some traits such as thickened cell walls, cuticles, hairs (pubescence), phenolic compounds, and often polyploidy.


Low doses of ionizing radiation induce positive outcomes in plants such as increasing growth and production of secondary metabolites engaged in the antioxidant defenses.


Experiments in space provided us with new insights into radiation biology. This chapter is organized as follows. First, we present a historical overview of the field that can be traced to the first experiments at the Eiffel tower. Then, we overview the space radiation environment and mathematical models used to describe it. Later in this chapter, we present a macroscopic picture of health effects in humans (observed or anticipated in the space environment). Afterward, we turn to a microscopic level and describe biomolecular changes introduced by space radiation. Then, we describe experimental evidence obtained from models—small animals, plants, eukaryotic cells, and extremophiles (organisms living under conditions extreme from a human point of view). Finally, we present an overview of ground-based facilities mimicking the space environment.


Human space travels were very early the concerns of a number of scientists like Johannes Kepler who warned that extraterrestrial trips would require ships fit to withstand the breezes of heaven [1]. The development of electroscopes manufactured by Pierre Curie made the assessment of local currents possible due to any particle crossing the two metallic plates [2]. The Italian physicist Domenico Pacini suggested in 1910 that the background noise measured with the Curie electroscopes was caused by radiation emitted from the Earth ground [3]. By performing some experiments with the Curie electroscope at the Eiffel tower, Theodor Wulf, a Jesuit priest, demonstrated that half of the radiation emitted by the Earth ground disappears at a height of 300 m. When the technology of balloons was safe, Victor Hess observed that the ionization density of the atmosphere progressively decreases up to 1000 m, but increases above 1800 m, suggesting the existence of two components of natural radiation: one from the Earth ground, the other from space, “cosmic rays” [4]. In 1936, Hess was awarded the Nobel Prize in physics for his discoveries [5].


Between the 1930s and the 1940s, there were considerable advances in the technology of particle counters and in the knowledge of particle physics; thanks to balloon experiments, new clues were brought to support that cosmic rays consist of very energetic (108–1020 eV) particles. Furthermore, data hinted an unexpectedly high proportion of the iron-associated elements in the galactic cosmic radiation (GCR). The latter observation led to the hypothesis of the nucleosynthesis origin of cosmic rays [3]. Figure 10.1 shows the advances in space radiation biology since that period.

A timeline from the 1930s to 2010s depicts significant milestones achieved in space radiobiology, advances in radiobiology, main human missions, and advances in physics.

Fig. 10.1
Synopsis of advances in space radiation biology. The continuous increase of space mission duration up to 400 days is illustrated by the grey line on the left. For Mercury, Gemini, Apollo, Salyut, Skylab, and Mir missions, the maximum dose values are given as red dots [6]. ALTEA anomalous long-term effects on astronauts, FISH fluorescence in situ hybridization, HPRT hypoxanthine guanine phosphoribosyl transferase, ISS International Space Station, LNT linear no-threshold, NLT nonlinear threshold, PCC premature chromosome condensation, SilEye silicon eye. (Reprinted with permission from Maalouf  et al. [6])


A timeline from the 1930s to 2010s depicts significant milestones achieved in space radiobiology, advances in radiobiology, main human missions, and advances in physics.


The pioneer works of William Gilbert, Carl Friedrich Gauss, and Henri Poincaré about magnetism suggested that charged particles may be influenced by the Earth’s magnetism and that a ring current should exist around the Earth. At the end of the 1950s and overall in the 1960s, the number of artificial satellites increased drastically and permitted to verify these hypotheses. In 1958, James Van Allen and Louis A. Frank pointed out the existence of the Earth’s radiation belt, based on data collected by the Explorer I and Pioneer IV satellites. Protons and electrons were found to be the major constituents of the Van Allen belt [7, 8]. In the same decade, Mariner II provided important data about the solar wind that permitted to document our knowledge on the radiation component from our Sun [9].


During short-term (less than 2 weeks) missions in the 1960s at low Earth orbit (LEO), astronauts were exposed to several mGy at an average dose rate of about 0.17 μGy/min (245 μGy/mission day). This dose was delivered discontinuously as (1) the inner and outer zones of the Van Allen radiation belt contain protons and electrons of differing energy spectra that result in different secondary particles at dose rates different and energies; (2) the South Atlantic Anomaly (SAA), the area where the inner Van Allen radiation belt is the closest to the Earth surface, leading to an impressive flux of protons and electrons is passed about 15 times a day; here the dose rate can increase sixfold resulting in a significant contribution to the radiation exposure; and (3) unpredicted solar particle events (SPE) can increase the total dose, while the protection in LEO is still sufficient to prevent life-threatening acute radiation syndrome (see Sect. 10.6.2.1).


The characterization of individual heavy cosmic particles of high-energy and high atomic number—Z—(HZE) was performed with different physical radiation detectors (nuclear emulsions, plastics, silver chloride (AgCl) crystals, and lithium fluoride (LiF) thermoluminescence dosimeters) for the first time in space in the Biostack experiments flown aboard Apollo 16 and 17 (see Sect. 10.5). In parallel, their biological effects were examined in different biological objects such as bacterial spores, protozoa cysts, plant seeds, shrimp eggs, and insect eggs investigating various radiobiological end-points [10].


Examples of short-term experiments of up to 2 weeks in LEO combining radiation dosimetry and biological investigations were loaded on Space Shuttles (e.g., STS 9, 42, 45, 65), on free-flying satellites (e.g., LDEF, EURECA, BIOPAN 1–6) and on the MIR space station (Perseus mission). Later on, similar long-term experiments were performed on the International Space Station (ISS) (EXPOSE-E, -R, -R2) [11, 12].


“Cytogenetics observations revealed for the first time the major biological consequences of an exposure to space radiation: the yield of chromosome breaks seemed to increase after flight, but statistical significance was still needed (see Sect. 10.4.2.2). Data from eye flashes and helmets (see Sect. 10.4.2.3) suggested the existence of a certain “hidden part” of the heavy ions’ component, probably due to secondary particles generated by the interaction of very high-energy particles with metallic materials. The contribution of these heavy ions to the total dose of radiation remained unknown at the end of the 1960s” [6].


Space experiments in combination with ground-based research (see Sect. 10.10) enabled a better understanding of the effects of space radiation and microgravity on human cells, microbes, and other biological models such as the roles of different complementary DNA repair mechanisms, the reactive oxygen species detoxification system and the intracellular accumulation of compatible solutes summarized, e.g., in Senatore et al. [13]. The modern picture of the space radiation dosimetry and its effects on human cells may be summarized as following [6]:1.
The energy spectrum of space particles and the dose spacecraft crews are exposed to can be quantified precisely by active and passive dosimetry. The dose delivered by secondary particles and countermeasures to reduce it require further investigations into the interaction of space radiation with a diversity of materials and in a complex spacecraft geometry.
 2.
Epidemiological studies for estimating hazards due to space radiation exposure are hampered by the small astronaut population, the individual radiation susceptibility, and radiation exposure history of each astronaut. International collaboration integrating different astronaut cohorts may help in overcoming these restrictions.
 3.
“Cytogenetic data undoubtedly revealed that space radiation exposure produces significant damage in cells. However, our knowledge of the basic mechanisms specific to heavy ion and low-dose and repeated exposures, and of adaptive responses is still incomplete. Furthermore, experiments about genomic instability and delayed mutagenesis may help in quantifying the risk of potential space radiation-induced cancer. The application of new radiobiological techniques may help in progressing in this field.”


The energy spectrum of space particles and the dose spacecraft crews are exposed to can be quantified precisely by active and passive dosimetry. The dose delivered by secondary particles and countermeasures to reduce it require further investigations into the interaction of space radiation with a diversity of materials and in a complex spacecraft geometry.


Epidemiological studies for estimating hazards due to space radiation exposure are hampered by the small astronaut population, the individual radiation susceptibility, and radiation exposure history of each astronaut. International collaboration integrating different astronaut cohorts may help in overcoming these restrictions.


“Cytogenetic data undoubtedly revealed that space radiation exposure produces significant damage in cells. However, our knowledge of the basic mechanisms specific to heavy ion and low-dose and repeated exposures, and of adaptive responses is still incomplete. Furthermore, experiments about genomic instability and delayed mutagenesis may help in quantifying the risk of potential space radiation-induced cancer. The application of new radiobiological techniques may help in progressing in this field.”


Space is permeated with radiation, both electromagnetic radiation and particles with mass. Electromagnetic radiation in space spans many wavelengths, from long wavelength radio waves to very short-wavelength gamma rays. Gamma rays, X-rays, and some far/extreme ultraviolet (UV) waves, which can be generated for example during some transient events on the Sun [14], are actually ionizing radiation. The wavelengths of UV, X-, and gamma rays are all shorter than those of visible light. The majority of these rays are absorbed by the Earth’s atmosphere. The extraterrestrial solar UV radiation ranges from vacuum UV (wavelength <200 nm) to UVA (320–400 nm). The ozone layer absorbs some, but not all, of these types of UV radiation: UVA is not absorbed by the ozone layer, UVB (wavelength: 290–320 nm) is mostly absorbed by the ozone layer, but some does reach the Earth’s surface, while UVC (wavelength: 100–290 nm) is completely absorbed by the ozone layer and atmosphere. Overall, the electromagnetic radiation reaching the Earth’s surface encompasses radio waves, some microwaves, some infrared light, UVB and UVA radiation, and visible light. Of the light that reaches Earth’s surface, infrared radiation makes up 49.4% while visible light provides 42.3%. UV radiation makes up just over 8% of the total solar radiation. UVA and UVB radiation contribute not only to premature aging of the skin but also to some serious health effects such as skin cancer, cataracts, and suppression of the immune system.


Generally however the expression “space radiation” mainly refers to radiation consisting of particles with a mass. There are three main radiation populations in space: galactic cosmic rays (intra- and extragalactic; GCR), solar radiation (including both the Solar Energetic Particles, SEP, and solar wind), and trapped radiation. A schematic representation of these radiation types and the environment which they can influence is given in Fig. 10.2.

An illustration depicts the effect of solar energetic particles, galactic cosmic rays, solar wind, magnetosphere, Van Allen belt, trapped and secondary particles on a spacecraft and an astronaut.

Fig. 10.2
Radiation environment during a space mission. (Image courtesy by ESA and reprinted from Chancellor et al. [15] with permission under Creative Commons Attribution-NonCommercial-NoDerivatives License: http://creativecommons.org/licenses/by-nc-nd/4.0/)


An illustration depicts the effect of solar energetic particles, galactic cosmic rays, solar wind, magnetosphere, Van Allen belt, trapped and secondary particles on a spacecraft and an astronaut.


GCRs are constantly present highly energetic radiation in space. Their intensity is slowly varying and low with a few particles per second traversing an area of a cm2 to a m2 or more. They are nearly isotropic, meaning that they impinge from all directions. They originate from outside the heliosphere, most likely from deep space high-energy phenomena [16], such as supernova shock waves throughout the Galaxy, and also possibly from stellar wind termination shocks, pulsars, or other more exotic objects. They are composed of 98% baryons, of which 87% protons (hydrogen ions), 12% helium ions (α-particles), and 1–2% high-energy and highly charged ions, called High charge Z and Energy (HZE) particles, and 1% electrons and positrons [17]. HZE comprises ions from Z = 3 (Li) to Z = 28 (Ni). The most common elements are C, O, Mg, Si, and Fe ions (Fig. 10.3). Ions heavier than Ni can be encountered, yet these are very rare.

A double line graph of relative abundances versus atomic number Z plots the curves of solar system and G C R for the following elements. H, H e, B e, C, N e, S i, A r, T i, F e, and Z n.

Fig. 10.3
GCR composition, as based on data from NASA’s Advanced Composition Explorer (ACE) spacecraft. (Reprinted with permission from http://www.srl.caltech.edu/ACE/ACENews/ACENews83.html)


A double line graph of relative abundances versus atomic number Z plots the curves of solar system and G C R for the following elements. H, H e, B e, C, N e, S i, A r, T i, F e, and Z n.


The spectrum of the GCRs is influenced by periodical, long-term, and short-term effects. Also, the Sun’s behavior is periodical and follows an 11-year cycle which affects the interplanetary medium. The increased solar and heliospheric magnetic fields during the maximum phase of the solar cycle partially shield the solar system and decrease the low-energy portion of the GCRs flux, by preventing it from entering the inner heliosphere [18], while at solar minimum the reduced interplanetary magnetic field strength implies a more intense GCRs population [19]. The GCR flux is thus inversely proportional to the solar activity and decreases by a factor of 2–4 when moving from solar minimum to solar maximum, depending on the depth of the solar minimum and the intensity of the solar maximum [20, 21].


The modulation of the GCR flux for different ions is reported for the period of solar maximum and minimum. In the short term, GCRs can also be reduced by intense release of high-energy particles (mostly protons) during transient solar eruptions [22] (see Sect. 10.3.1.2). The energy spectrum of GCRs covers a huge range of energies: it commences at about 107–108 electron volt (eV) (10–100 MeV), and the most energetic cosmic rays reach up to 1020 eV (Fig. 10.4). A prominent feature is a so-called knee, with an energy of about 2.7–3.1 PeV (PeV = 1015 eV). This energy originated from the diffusive shock acceleration from the Galactic supernova remnants. The so-called anide or ankle, with an energy of about 5 × 1018 eV, is another characteristic of the energy spectrum. It is believed to mark the lower end of the energy of ultra-high energy GCRs, those that originate from extragalactic sources [24].

A graph of flux versus energy plots the following data points. 1 particle per meter squared per second. Knee, 1 particle per meter squared per year. Anide, 1 particle per kilometer squared per year.

Fig. 10.4
GCR overall average fluxes versus energy. (Data from Beatty et al. [23])


A graph of flux versus energy plots the following data points. 1 particle per meter squared per second. Knee, 1 particle per meter squared per year. Anide, 1 particle per kilometer squared per year.


When traversing Earth’s atmosphere, GCRs induce nuclear-electromagnetic-muon cascade reactions resulting in ionization of atmospheric molecules and generation of secondary particles [25, 26]. A small fraction of the initial primary particles, together with secondary particles of sufficient energy, reaches the ground. The maximum in secondary particle energy release (Pfötzer maximum) occurs at altitudes of 15–26 km depending on latitude and solar activity level. The radiation reaching the Earth’s surface has levels similar to other low levels of radiation that humans are frequently exposed to. The average yearly exposure of a person is around 3.5 millisieverts (mSv). About half of this dose can be attributed to artificial sources (X-ray, computer tomography (CT) scan, mammography), while the other half originates from natural sources, including around 10% from cosmic radiation.


SEPs originate from transient events on the Sun and come as massive injections of mostly protons and electrons (and to lesser extent helium (~4%) and heavier ions), with typical energies from ten to hundreds of MeV [27]. These transient events are Sun eruptions such as flares and Coronal Mass Ejections (CMEs). Characteristically, a flare lasts only minutes to hours and is the result of an explosive energy release from the Sun’s coronal magnetic field. Also, the electromagnetic flux increases, particularly in the short-wavelength (Extreme ultraviolet—XUV, gamma ray) range, and also in the radio regime. Usually originating in active regions, CMEs are large-scale plasma-magnetic structures with high speeds (up to thousands of km/s) associated with prominence eruptions and flares.


The likelihood of CME events increases with the power and size of the related flare event, although not all CMEs are associated with flares. Such events extend from several hours to a few days, and they have a higher likelihood of occurring during solar maximum. The level of the Sun’s activity is fairly described by the number of sunspots, which provides an indication of the phases of the cycle. The number of spots increases toward the solar maximum, while during solar minimum the Sun’s surface is almost spotless. Nevertheless, such SEPs events are hardly predictable and can also occur during solar minimum. Examples of an active region, an initial flare, and then a prominent eruption initiating a CME is shown for the 28/10/2003 event as part of the “Halloween Storms of 2003” in Fig. 10.5, with the related sudden increase in proton flux as detected by the Geostationary Operational Environmental Satellite (GOES) satellite (Fig. 10.6a). Examples of fluences (integral of the flux over the period of the event) related to major SEP events are shown in Fig. 10.6b.

Four photographs taken by SOHO satellite on 28 10 2003 depict Halloween Storms of 2003. Agile areas, sun spot, huge bubbles of coronal plasma threaded by magnetic field lines ejected from the sun.

Fig. 10.5
The active regions (upper left), solar flare (upper right), and coronal mass ejections (CME, lower left and right) of the 28/10/2003 event captured by the Solar and Heliospheric Observatory (SOHO) satellite. The CME was imaged by the Large Angle and Spectrometric COronagraph (LASCO) instrument by blocking the light from the solar disk. (Courtesy of SOHO/EIT and SOHO/LASCO consortium. SOHO is a project of international cooperation between ESA and NASA)

Line graph a of particles versus universal time plots five minute data of G O E S proton flux. Line graph b of integral fluence versus proton kinetic energy plots the curves for 1956 to 1989.

Fig. 10.6
(a) Proton flux between 28 and 31 October 2003. The 5-min averaged integral proton flux for energy thresholds of >10 (red line), >50 (blue line), and >100 MeV (green line) was measured by the primary Geostationary Operational Environmental Satellite (GOES) satellite of the Space Weather Prediction Center (SWPC). CO Colorado, MeV Mega electron volt, NOAA National Oceanic and Atmospheric Administration, s second, sr steradian, UTC Coordinated Universal Time. Reprinted with permission under terms of the Creative Commons Attribution License [28]. (b) Distribution in the energy of proton fluences for major past SEPs events (free space). (Reprinted with permission from: The space radiation environment: an introduction. Schimmerling W. https://three-jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf. Date posted: 2-5-2011)


Four photographs taken by SOHO satellite on 28 10 2003 depict Halloween Storms of 2003. Agile areas, sun spot, huge bubbles of coronal plasma threaded by magnetic field lines ejected from the sun.


Line graph a of particles versus universal time plots five minute data of G O E S proton flux. Line graph b of integral fluence versus proton kinetic energy plots the curves for 1956 to 1989.


A classification exists between Impulsive SEP events, which are short (≤1 day), numerous (~1000/year in periods of high activity), and of low intensity, and gradual events, which are long (several days at energies of a few MeV/nucleon), rather rare (a few tens per year), characterized by orders of magnitude higher protons fluences than impulsive events and ascribed to acceleration by CME-driven shocks as they propagate through the heliosphere. There is however some debate about the role played by “flare acceleration” in these events [29, 30].


Contrary to GCRs, SEP events can be considered mostly inducing deterministic effects (Sects. 10.4.2 and 10.6.2). Deterministic effects are those certainly occurring once a specific threshold dose has been overpassed. The high-intensity SEP flux can significantly increase the absorbed dose to astronauts, for example, during extravehicular activities (EVA) at the ISS, or eventually, if the event is characterized by a “hard” spectrum with a strong high-energy component, also during both interplanetary mission or missions on thin atmosphere such as Mars. Acute radiation syndrome (ARS), sickness, or, in extreme cases, death after a lethal dose can occur [31].


A comparison between GCR and SPE can be found in Table 10.1 (adapted from NASA Space Flight Human System Standards—NASA Standard 3001).Table 10.1
Comparison of GCR and SPE

GCR

SPE

Spatial distribution

Isotropic beyond terrestrial influence (no preferred direction of arrival)

Non-isotropic at onset, later becoming diffused through the solar system

Composition

Protons (~87%) and helium ions (~12%) with the remainder consisting of HZE (1–2%)

Mostly protons

Temporal variations

Chronic

Acute

Energy

Extending to at least 1017 eV in some cases (much greater maximum than solar particles)

About 1010 eV highest recorded

Origin

Theories only; supernova explosions, neutron stars, pulsars, or other sources

Active regions of flares on the Sun, CMEs

Flux density

Relatively low: about 2 particles/cm2/s of all energies

Very high: may be as high as 106 particles/cm2/s

Biological effects

Primarily genotoxic and mutagenic with some vital cell destruction

Primarily acute damages, possible sudden illness, incapacitation, or death

Adapted from https://msis.jsc.nasa.gov/sections/section05.htm#_5.7_RADIATION


The solar wind is a continuous flow of plasma from the Sun’s corona, mainly consisting of protons, electrons with a small percentage of He ions, with kinetic energies between 0.5 and 10 keV. There are also some trace amounts of heavy ions and atomic nuclei such as C, N, O, Ne, Mg, Si, S, and Fe. Their energy results from the high temperature of the Sun’s corona and allows them to escape the Sun’s gravity. The flux of the solar wind varies over time, solar longitude and latitude, together with its temperature, density, and speed. At distances of more than a few solar radii from the Sun, the solar wind reaches supersonic speeds of 250–750 km/s [32]. At much greater distances, about 75–90 astronomical units (1 au is the distance Sun-Earth), the so-called “termination shock,” interactions of the local interstellar medium with the solar wind slow it down to subsonic speed.


There are different classes of solar wind [30]:(a)
The long-lived solar wind high-speed streams, representatives of the inactive or “quiet” Sun. Sources for such streams are coronal holes usually located above inactive parts of the Sun, where “open” magnetic field lines prevail, e.g., around activity minima at the polar caps;
 (b)
A slow wind stream from more active near-equatorial regions on the Sun, often associated with “closed” magnetic structures. Sharp boundaries exist between these two solar wind streams (in longitude as well as in latitude), and their main properties differ significantly according to the location and magnetic properties at the source;
 (c)
Another slow solar wind stream emerging during high solar activity, from active regions distributed over large parts of the Sun, in a highly turbulent state. It is highly variable and usually contains a significant fraction (about 4%) of alpha particles;
 (d)
The solar wind disturbances superimposed on the ambient solar wind in case of CMEs. They exhibit unusually high percentages of alpha particles (up to about 30%).


The long-lived solar wind high-speed streams, representatives of the inactive or “quiet” Sun. Sources for such streams are coronal holes usually located above inactive parts of the Sun, where “open” magnetic field lines prevail, e.g., around activity minima at the polar caps;


A slow wind stream from more active near-equatorial regions on the Sun, often associated with “closed” magnetic structures. Sharp boundaries exist between these two solar wind streams (in longitude as well as in latitude), and their main properties differ significantly according to the location and magnetic properties at the source;


Another slow solar wind stream emerging during high solar activity, from active regions distributed over large parts of the Sun, in a highly turbulent state. It is highly variable and usually contains a significant fraction (about 4%) of alpha particles;


The solar wind disturbances superimposed on the ambient solar wind in case of CMEs. They exhibit unusually high percentages of alpha particles (up to about 30%).


The Earth’s magnetosphere deflects the solar wind, causing most of the solar wind to flow around and beyond us. Nevertheless, a small number of particles from the solar wind reach the upper atmosphere and ionosphere. This may produce phenomena such as aurora and geomagnetic storms, the latter occurring when large inflation of the magnetosphere, due to an increased pressure of the contained plasma, distorts the geomagnetic field.


In space missions, the solar wind has no impact on astronauts, as it is efficiently stopped by the spacecraft shielding and also by appropriate astronaut suits, because of the small range in a matter of the low speed-solar wind particles. However, if not appropriately shielded, the solar wind particles may affect the human body during eventual EVAs in deep space or on the surface of airless bodies, such as the Moon.


Trapped radiation particles are produced mainly by the interaction of GCRs and SEPs with the Earth’s atmosphere and are trapped by its magnetic field into the Van Allen radiation belts. These comprise:(a)
A stable inner belt of trapped protons and electrons with energies between some keV and 100 MeV that is centered at a height between 300 and 1000 km above the Earth and reaches up to a height of around 10,000 km.
 (b)
A less stable outer electron belt, comprising mainly high-energy (0.1–10 MeV) electrons and which extends from an altitude of about 10,000–40,000 km (see Fig. 10.7 for a schematic representation).

A schematic diagram depicts the earth’s vertical rotational axis, angled magnetic axis, inner radiation belt, and outer radiation belt.

Fig. 10.7
Radiation belts of the Earth. (Figure from Van Allen radiation belt. Reprinted with permission from Wikipedia. Author Booyabazooka at English Wikipedia, https://commons.wikimedia.org/wiki/File:Van_Allen_radiation_belt.svg)


A stable inner belt of trapped protons and electrons with energies between some keV and 100 MeV that is centered at a height between 300 and 1000 km above the Earth and reaches up to a height of around 10,000 km.


A less stable outer electron belt, comprising mainly high-energy (0.1–10 MeV) electrons and which extends from an altitude of about 10,000–40,000 km (see Fig. 10.7 for a schematic representation).


A schematic diagram depicts the earth’s vertical rotational axis, angled magnetic axis, inner radiation belt, and outer radiation belt.


In the radiation belts, the energetic particles move along Earth’s magnetic field lines, via the combination of three types of motion: a fast rotation (or “gyration”) around magnetic field lines, typically thousands of times each second; a back-and-forth bouncing along the stronger magnetic fields in the northern and southern hemispheres, typically lasting 1/10 s; a slow drift around the magnetic axis of the Earth (the drift is eastward for electrons and westwards for ions), such drift is from the current field line to its neighbor, with the particle keeping roughly the same distance from the axis. A typical time to complete a full circle around the Earth is a few minutes.


In the area above the southeastern part of South America and the South Atlantic, the inner radiation belt approaches the surface of Earth down to a few hundreds of kilometers (South Atlantic Anomaly, SAA). This is caused by the tilt and shift of the axis of the dipole-like magnetic field of the Earth with respect to its axis of rotation [33]. The dip of magnetic lines leads to an increased particle flux within this region.


The dose rate experienced by the astronauts on the ISS has a considerable contribution from trapped protons in the inner Van Allen belt because the ISS orbit with an altitude of about 400 km passes through this belt at the SAA (roughly 50% of the total dose rate) [34].


A low Earth orbit (LEO) is an Earth-centered orbit close to our planet with an altitude ranging from 160 to 2000 km. Thus, the ISS, which flies at an altitude of around 400 km, is also in such an orbit, with an orbital inclination (the tilt of the orbital plane with respect to the equatorial plane, which helps to understand an orbit’s orientation with respect to the equator) of 51.6° and an orbiting period of 90–93 min. Consequently, in 24 h the ISS makes 16 orbits of Earth and travels through 16 sunrises and sunsets. The environment of these altitudes is extreme and characterized by microgravity, high vacuum, meteoroids, extremes of temperature, ionospheric plasma, space debris, and UV as well as ionizing radiations.


The radiation sources are GCR, trapped radiation, and SEP events. The GCR environment accounts for about 50% of the total dose rate, the other 50% being induced by trapped protons of the inner belt, the only component of the inner belts that reaches energies and intensities to be important for effects on astronauts inside the ISS [35]. Other orbits, such as Medium Earth Orbits (2000–35,786 km), Geostationary orbits (35,786 km), and High Earth Orbits (over 35.786 km), are exposed to different sub-components of the trapped radiation, some may not pose any danger. On board the ISS, astronauts encounter SPE events as a transient increase in dose rates. As mentioned above, the GCR flux is modulated by the solar cycle. At the ISS altitude, the GCR flux is also modified by the geomagnetic field, besides the modulation due to the solar activity. This field removes particles with lower energies (~few GeV/nucleon), but particles of higher energies are unaffected [36]. At low altitudes, the trapped radiation is also modulated by solar activity: at solar maximum, because of the increase in UV radiation, the upper atmosphere expands, leading to the loss of trapped protons at low altitudes. Furthermore, the inner radiation belt is mainly filled by decaying neutrons created by incoming GCR particles and the GCR flux is inversely proportional to solar activity [37]. Therefore, at solar maximum, a lower proton flux is present, leading to a smaller radiation hazard compared to the solar minimum [36, 38].


The interaction of energetic protons and HZE nuclei with spacecraft structures produces an additional intravehicular radiation field. This secondary radiation includes mainly, protons, neutrons, photons (X-rays and gamma rays), leptons (e.g., electrons and positrons), mesons (e.g., charged pions) and a great number of lighter and heavier nuclear isotopes (ions) [39, 40]. This happens in LEO and is of high concern in particular for the deep Space phase of a mission (see below), as the spacecraft would not be protected by the Earth’s atmosphere and magnetic field.


Radiation challenges for astronautic missions beyond LEO, such as travel to the Moon or Mars, come from SEP events, GCR and intravehicular secondary radiation (Fig. 10.2). The solar wind particles, also constantly present in deep Space, do not contribute to the radiation dose induced in crews inside a spacecraft, as they are efficiently stopped even by thin shielding thicknesses.


Similar to the case of the LEO scenario, most GCRs are not efficiently stopped by regular depths of spacecraft shielding. The intravehicular radiation field is constituted by the ensemble of secondary radiation mentioned above. Adding more shielding would increase to a considerable extent the weight at launch and would not reduce the GCR-induced absorbed dose to zero. As the only modulation of GCR in deep space is provided by the shielding of the heliospheric field during solar maximum, the idea of carrying out missions to Mars during solar maximum has been considered a viable option. If one considers that during a 180-day trip at the solar maximum peak a crew would also likely receive a total SEP-contributed dose equivalent, a round trip to Mars would result in a total dose equivalent of 560 ±180 mSv,1 higher than the estimation based on the data from the Radiation Assessment Detector (RAD) onboard the Mars Science Laboratory NASA mission [41] which was on cruise during solar minimum [42]. The above estimate for the radiation exposure is substantially lower than the accepted safe upper limit for 30–60-years old nonsmoking females and males (above 1500 mSv—see Fig. 10.8). However, inaccuracy and limitations of the models and unpredictability of SEP events must be considered.

A scale of radiation in millisievert for different medical procedures, space missions, celestial body, general population, and NASA astronaut limits.

Fig. 10.8
Relative radiation exposure of varying duration during medical procedures (green), specific space missions (purple), and on various celestial bodies (blue). The astronaut yearly and career limits are given in red boxes. For comparison, some facts on radiation exposure of the general population and occupational exposure limits (US) are indicated (gold). (Reprinted with permission from Iosim et al. [43])


A scale of radiation in millisievert for different medical procedures, space missions, celestial body, general population, and NASA astronaut limits.


The Moon is about 380,000 km away from Earth and is the next endeavor for space missions beyond LEO. Although some areas of the Moon have a weak magnetic field, the Moon does not have a global magnetic field like on Earth and no atmosphere. Consequently, its surface is not shielded from radiation. The solar wind particles get stopped in the first millimeters or, maximally, centimeters of the lunar regolith, while GCR and SEP can impact the lunar surface also resulting in the production of backscattered secondary particles. The total amount of radiation that astronauts will be exposed to is influenced strongly by solar activity, their whereabouts on the Moon surface with respect to local magnetic fields, and the type and amount of radiation shielding used in spacecraft, Moon vehicles, and habitats. Recently, the Lunar Lander Neutrons and Dosimetry experiment aboard China’s Chang’E 4 lander revealed that radiation levels on the Moon’s surface are 200–1000 times more than that on Earth’s surface and 2.6 times more than what astronauts onboard the ISS are exposed to Zhang et al. [44]. Efficiency of the radiation shielding by lava tubes on the Moon appears promising to reduce the dose rates considerably [45].


Mars does not possess a global magnetic field, and it has only a thin atmosphere with its surface pressure less than 1% of that at Earth’s surface. Therefore, high-energy GCRs can reach the surface, although still a considerable portion of them will induce hadronic-electromagnetic-muon cascades in the atmosphere, causing fragmentation/spallation and ionization showers of downward secondaries. All these particles can then induce further reactions in the planet’s regolith, which generate a backscattered, albedo radiation component, giving overall complex spectra including both primaries and (downward and upward) secondaries at the surface [46–48].


SEP events can increase the dose rate and dose equivalent at the Martian surface and constitute a danger for EVA on Mars. Only protons impinging the top of the atmosphere with energy above ~200 MeV do actually reach the ground, and thus SEPs events with high flux contribution at high energy constitute the biggest hazard for explorers on Mars if they are not in a habitat or otherwise sufficiently shielded. For the solar wind, despite the thin character of Mars’ atmosphere, the upper layers of the latter are able to stop such radiation. Underground solutions for Mars habitats, shielded from the radiation by the regolith, are being investigated [49].


Overall, to contextualize radiation doses in space, a comparison of these doses to doses received during medical interventions is shown in Fig. 10.8. It is important to emphasize that being exposed to a hefty radiation dose within a short time (minutes to hours) will be more health-threatening than the same dosage over a longer duration of months of years. Yet, although the health effects of acute radiation exposure are well studied, less is known about the effects of chronic exposure.


Ionizing radiation exposure is one of the most critical health risks for astronauts. Inside the ISS, astronauts are exposed to an effective dose rate of the order of 20 μSv/h, which is about 100 times higher than on the Earth’s surface. Beyond LEO in deep space, the protection of the Earth’s atmosphere and magnetic field disappears, leading to an effective dose rate of the order of 75 μSv/h. Also, on the surface of the Moon or Mars, there is only limited protection and astronauts are exposed to respectively about 30 and 25 μSv/h. It is estimated that astronauts will accumulate during a Mars mission a total effective dose of the order of 1 Sv, leading to an extra risk for cancer of the order of a few percent up to more than 10% depending on sex and age [50]. Furthermore, on their way through deep space or on the surface of the Moon or Mars, astronauts can receive such high doses during intense solar storms that immediate health effects or even a deadly outcome are possible (see Sect. 10.4.2). Therefore, it is clear that astronauts need to be protected against ionizing radiation in space.


The only technology that can currently be used in practice to reduce the radiation level in spacecraft is to use shielding materials for stopping part of the radiation. The heavy ion impinging on the shielding material is the projectile, and the shielding material is the target. A multitude of interactions can occur when the projectile hits the target, including fragmentation of the projectile or target. For comparison of different materials, the area density as mass per unit area in g/cm2 is used (for example, an 1 cm thick plate of Al with the density of 2.7 g/cm3 has an area density of 2.7 g/cm2). In current spacecraft, one makes most use of constructive materials such as aluminum. Unfortunately, such materials are not the most efficient for radiation shielding in space (see Chap. 4). The interaction of energetic GCRs with heavier elements such as aluminum results in the breakup of these heavier elements and the creation of secondary cosmic radiation such as energetic heavy ions and neutrons. Therefore, when using aluminum for shielding, the effective dose rate first increases as function of the shielding thickness before it starts to decrease and this decrease is quite flat as attenuation of heavy ions is nearly in balance with the build-up of light particles (Fig. 10.9 Left).

Two line graphs of dose equivalent rate versus depth plot G C R and Van Allen trapped protons, for aluminum, water, polyethylene, and liquid hydrogen.

Fig. 10.9
Calculated dose equivalent rate in LEO (51.6° inclination, 390 km altitude) as a function of shielding thickness given as area density for different shielding materials: (left) GCR, (right) Van Allen trapped protons. (Data used with permission from Dietze et al. [37])


Two line graphs of dose equivalent rate versus depth plot G C R and Van Allen trapped protons, for aluminum, water, polyethylene, and liquid hydrogen.


Materials consisting of lighter elements such as hydrogen have a higher stopping power per unit of mass for charged radiation particles as they attenuate their fluence via projectile fragmentation. They also minimize the build-up of neutrons and other target fragments. Radiation protection of astronauts can thus be further optimized by making use of lighter shielding materials or, for instance, also by making strategic use of the necessary stock of water as additional shielding. Figure 10.9 shows the calculated dose equivalent rate for a LEO orbit similar to that of ISS (51.6° inclination, 390 km altitude) as a function of shielding thickness for different shielding materials. At standard temperature and pressure, based on a density of 1000 kg/m3, the water column required to reach an area density of 20 g/cm2 would have a height of 20 cm. For 20 g/cm2 aluminum, a material thickness of 7.4 cm is derived from the density at room temperature of 2.7 g/cm3. At the same area density of 20 g/cm2, the shielding effect of water is much more pronounced than the one of aluminum. The thickness of the two materials is different, but they would contribute to the same extent to the mass budget of the spacecraft which is critical for leaving the Earth surface during launch. The left and right plots show the results for respectively GCRs and Van Allen protons. These plots clearly show that hydrogenous materials are much more efficient for radiation shielding in space.


In spacecraft it is unfortunately not possible to reduce the effective dose rate to the dose rate on Earth’s surface. With limited shielding, a large part of the energetic protons and electrons from SEPs and the Van Allen protons can be stopped. However, GCRs have such high energies that about 1000 g/cm2 of shielding is required to reduce the effective dose rate to the level on Earth’s surface. Due to mass constraints in spacecraft, only shielding of the order of a few 10 g/cm2 is possible. In spacecraft, astronauts can thus be protected against sudden very high and potentially deadly doses from solar storms, but they will be unavoidably chronically exposed to the ever-present GCRs leading to an increased risk for late effects.


It is clear that with current technology additional radiation exposure in spacecraft is unavoidable. However, for future manned missions to the Moon or Mars during which astronauts will stay on the surface for a longer time it will be necessary to strongly reduce their radiation exposure during their stay. This is possible because on the surface of the Moon or Mars, we can make use of the present soil material to provide adequate shielding. A few meters of soil material should suffice to reduce the effective dose rate level to similar levels as on Earth’s surface. This can be done by building igloos or by living in caves or lava tubes.


Besides shielding by using materials to block the radiation, it is in principle also possible to make use of strong electromagnetic field for shielding. Several research groups are investigating this possibility. However, the required mass and energy consumption of such systems makes the concept practically impossible with current technology (Box 10.1).


(a)
GCRs are the constantly present highly energetic radiation in space, they are mostly constituted by protons, with a smaller contribution from alpha particles and HZE particles. They generate particle showers in the atmosphere, although a small portion of direct GCRs can eventually reach the ground.
 (b)
SEP events are more probable during solar maximum, but they can actually also occur during solar minimum.
 (c)
Trapped radiation is constituted by GCRs and solar protons trapped in the Van Allen belts. Trapped radiation is a concern for ISS-like missions, especially because of the flux accumulated during different orbits in the SAA, or also missions on other orbits crossing one or the other belt.


GCRs are the constantly present highly energetic radiation in space, they are mostly constituted by protons, with a smaller contribution from alpha particles and HZE particles. They generate particle showers in the atmosphere, although a small portion of direct GCRs can eventually reach the ground.


SEP events are more probable during solar maximum, but they can actually also occur during solar minimum.


Trapped radiation is constituted by GCRs and solar protons trapped in the Van Allen belts. Trapped radiation is a concern for ISS-like missions, especially because of the flux accumulated during different orbits in the SAA, or also missions on other orbits crossing one or the other belt.


The modeling of the radiation environment at or inside a spacecraft, at different altitudes in the atmosphere or at the surface/subsurface of a planet, a moon, or a small body allows to obtain the relevant dosimetric quantities for the assessment of the health risks incurred by humans due to radiation [51–53], as well as to estimate the half-lives of biomolecules in search-for-life studies [54, 55].


The transport of radiation through matter is described by the time-independent Linear Boltzmann Transport Equation, which allows to treat atomic and nuclear collisions. The Boltzmann transport equation (10.1) describes the flux ni(r, E, Ω, t) of several types of particles i, possessing different energies E, and moving in different directions Ω by considering the particle balance in a small volume V. It thus gives the average space-time distribution of the expected energy-momentum behavior of the particle beam, transported and scattered across the target, where each interaction is characterized by its own differential cross-section

. The Boltzmann equation reads as follows:

 (10.1)


In this equation:
the first term is the time-dependent flux change, due to particles escaping from the system boundaries, or disappearing by an absorption reaction or radioactive decay;

on the right-hand side, the unscattered term represents the flux change due to translation without change of energy and direction (free flight);

the particles scattered out are those exiting a “cell” (a unit volume in the phase space, the latter comprising both space and time variables);

the particles scattered in are those entering a “cell” from a “cell” at a previous point in the phase space;

the production of secondaries represents the effect of collisions;

the source term can be external (e.g., a particle beam irradiating the target volume), or internal (e.g., neutrons from fission reactions in the volume).


the first term is the time-dependent flux change, due to particles escaping from the system boundaries, or disappearing by an absorption reaction or radioactive decay;


on the right-hand side, the unscattered term represents the flux change due to translation without change of energy and direction (free flight);


the particles scattered out are those exiting a “cell” (a unit volume in the phase space, the latter comprising both space and time variables);


the particles scattered in are those entering a “cell” from a “cell” at a previous point in the phase space;


the source term can be external (e.g., a particle beam irradiating the target volume), or internal (e.g., neutrons from fission reactions in the volume).


In particular for high-energy particles, the number of interactions that must be described in order to find the solution to this equation is daunting, including ionization, excitation, spallation/fission/fragmentation, production of positron-emitting nuclei, and de-excitation through gamma rays. A solution to the problem can be attained via two different approaches:1.
Deterministic methods. These are deterministic approaches based on approximations to the Boltzmann equation and often on the reduction to a 1D problem via the use of the straight-ahead approximation, according to which the secondary particles from nucleon-nucleus collisions are emitted in the direction of the incident nucleon [37]. They rely on models for the relevant quantities in the transport calculation and use the continuous slowing down approximation (CSDA). Deterministic codes such as NASA’s HZETRN [56] and BRYNTRN [57] follow such an approach and require relatively low computational resources to perform calculations and the calculation time is relatively short. This is due to the fact that deterministic codes do not consider all products of reactions and neglect their correlation, e.g., the coefficients used in the Boltzmann equation are related to relatively simple one-particle quantities. Thus, correlations on event-by-event basis are not considered and particle scattering at an angle is ignored [58]. Last, such methods can only be applied to restricted geometries and restricted interaction models.
 2.
Monte Carlo method. Monte Carlo (MC) is a stochastic method, exploiting random numbers to (a) “generate” an initial particles’ “cocktail”; (b) track them in arbitrary geometries; (c) accumulate the contribution of each track to a statistical estimator of the desired physical observables [59]. Step-by-step particles’ transport is simulated according to the statistical model of their interactions. Quantities (such as step lengths, event type, energy losses, and deflections) are sampled via generation of random values according to a given probability distribution. Indeed, in MC codes, the MC method deals with sampling from suitable stochastic distributions, with large samplings allowing to solve the integrations of multidimensional integrals.


Deterministic methods. These are deterministic approaches based on approximations to the Boltzmann equation and often on the reduction to a 1D problem via the use of the straight-ahead approximation, according to which the secondary particles from nucleon-nucleus collisions are emitted in the direction of the incident nucleon [37]. They rely on models for the relevant quantities in the transport calculation and use the continuous slowing down approximation (CSDA). Deterministic codes such as NASA’s HZETRN [56] and BRYNTRN [57] follow such an approach and require relatively low computational resources to perform calculations and the calculation time is relatively short. This is due to the fact that deterministic codes do not consider all products of reactions and neglect their correlation, e.g., the coefficients used in the Boltzmann equation are related to relatively simple one-particle quantities. Thus, correlations on event-by-event basis are not considered and particle scattering at an angle is ignored [58]. Last, such methods can only be applied to restricted geometries and restricted interaction models.