Introduction

An important factor restricting future human activities in deep space is radiation exposure from galactic cosmic rays (GCRs) and solar energetic particles (SEPs). Serious radiation damage poses a health hazard to space crews. Current and previous human space activities have been carried out in low-Earth orbit (LEO), where charged particles are partially shielded by geomagnetic fields. Future missions to the moon, the National Aeronautics and Space Administration’s Deep Space Gateway1, Mars, and deep space will face the challenge of high radiation risks because of the small magnetic fields and long mission terms. In addition, space travel for the public will be offered as a commercial product in the future, meaning that long-term space stays will not be exclusively for space crew.

Most GCR particles are protons (~ 87%)2,3. The high linear energy transfer (LET) and biological effects of the high charge and energy (HZE) particles from He to Fe nuclei significantly contribute to the radiation dose e.g., Refs.4,5. Measurements by the Mars Science Laboratory indicated a dose equivalent of ~ 660 mSv or more during an Earth-Mars round trip flight and 0.64 mSv/day during a stay on the Mars surface6,7. Considering the International Commission on Radiological Protection’s (ICRP) public and occupational exposure limits of 1 and 50 mSv/y, respectively8, reliable evaluation of radiation risks and effective radiation shielding is a significant challenge for safe and sustainable manned space development.

To evaluate the radiation risks from HZE particles, the dose equivalent, which is defined by the integral of the absorbed dose and radiation quality factor (QF), is employed9. QF is a relevant parameter to relative biological effect (RBE)10,11. ICRP has released the fluence conversion coefficients for the absorbed dose, mean QF, and dose equivalent to human tissues and organs and their effective average9. Biological effects, such as simple and complex exchanges, gene mutation, and neoplastic transformation, have been investigated in terms of estimating cancer risks e.g., Ref.12,13,14. The QF provided by ICRP (1991)15 is a conventional value of RBEmax under low-dose (< 0.2 Gy) and low-dose rate (~ 0.05 Gy/h) gamma-rays in model biological systems and it is described by a simple function of LET. A reduction in the effective dose equivalent is a good indicator for a reduction in radiation risk. However, the RBEmax produces large uncertainties under various experiments, mainly due to the low yields of the biological effects of the low dose and dose rate gamma-rays5,16,17,18. An approach obtaining RBEγacute values with relatively small uncertainties is to obtain them under acute conditions of high dose (0.5–3 Gy) gamma-rays5,17. The acute gamma-ray model has obtained consistent RBEs by using the linear response to epidemiological data of atomic bomb survivors19,20. Meanwhile, Cucinotta et al.21 suggested that the RBEγacute might be insufficient to assess risk as well as QF due to restricted information on the physics of incident ions; e.g., \({Z}^{*2}/{\beta }^{2}\) in (Eq. 3). After that, Cacao et al.22 obtained the RBEγacute response to particle LET and charge number based on experimental datasets of 16O, 20Ne, 28Si, 48Ti, and 56Fe exposure.

Radiation shielding is a strategy used to reduce radiation exposure risks. Passive shielding is an approach to absorb relatively low energy particles and break up HZE particles into lighter particles in the shielding material, resulting in dose reduction. HZE particle exposure and dose reduction by shielding materials have been studied through calculations and measurements using ground accelerators e.g., Ref.23,24,25,26,27,28,29,30,31 and in space modules e.g., Refs.6,32,33,34,35,36,37,38,39. Several types of shielding materials, such as aluminum (Al), polyethylene (PE, (C2H4)n), and carbon fiber reinforced plastic (CFRP), have been studied. Al is commonly used in spacecraft construction, PE is known to be an effective shielding material, and CFRP is a potential structural material with a relatively high shielding efficiency for spacecraft owing to its high mechanical strength40,41.

In this study, we discuss the effective dose equivalents due to GCR exposure based on two types of biological scale parameters under passive shielding, which will be crucial for evaluating radiation-induced risks during upcoming long-term stays in space.

Results

GCR fluences behind shielding materials

Figure 1 shows the energy spectra of the major GCR elements in free space and behind the shielding materials: Al, PE, and CFRP. Error bars represent the statistical error of the calculation. Several space modules provide shielding of ~ 20 g/cm2 or more mass thickness on average42,43, therefore, we employed 20 g/cm2 as the typical thickness of spacecraft shielding. The proton flux was increased by shielding because the target and heavier projectile fragmentation reactions produced numerous secondary protons along the pathway of the primary particles in the materials. The number of heavy ions was decreased by the projectile fragmentation reactions, while some light ions at low energies (< 10 MeV/n) were increased by particle energy loss and secondary particle production owing to the target and heavier projectile fragmentations. Secondary particle production in the high energy region was highest in PE by projectile fragmentation, whereas that in the low energy region was highest in Al by target fragmentation. This is explained by the fragmentation cross section per unit mass41 and the particle production rate by fragmentation. PE has the largest cross section of the employed materials, followed by CFRP and Al. The fragmentation of heavier particles produces a larger number of particles e.g., Ref.44.

Figure 1
figure 1

Energy spectra of GCR major elements in free space and with 20 g/cm2 shielding. (a) H, (b) He, (c) C, (d) Ne, (e) Si, and (f) Fe. Red line denotes GCR primary element in free space. Green, blue, and yellow denote GCR element with 20 g/cm2 shielding with Al, PE, and CFRP, respectively. Dotted energy regions (1–10 MeV/n) are not included in calculation primary sources because these low energy primary particles are stopped in a thin layer and do not contribute to the flux behind material.

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The energy spectra of the fluence were converted to LET-dependent fluence spectra in the ICRU four-element tissues45, as shown in Fig. 2. Summed LET spectra were obtained and are shown in Fig. 3. LET peaks for He, C, O, and Fe appeared at approximately 0.9, 7.5, 15, and 150 keV/µm, respectively, in free space. The mean QFICRP and RBEγacute were obtained from these fluence spectra using Eqs. (2) and (3), respectively. Note that our results only include the contributions by the charged particles from protons to Fe ions; the contributions of neutrons, photons, pions, and muons were not considered. The shielding materials significantly reduced the flux of primary particles heavier than He owing to projectile fragmentation. The flux reduction rate increased with heavier nuclei; this was observed for Fe, which is one of the major contributors to the total dose. The shielding of high LET particles should contribute to the efficient reduction of biological effects. The increase in low energy H appeared as an enhancement at LET = 1–30 keV/µm. This was mainly because of the target fragmentations. Similar enhancements appeared in the heavier particles: He (30–100 keV/µm), C (500–1000 keV/µm), and Ne (1000–1500 keV/µm) (Fig. 2).

Figure 2
figure 2

LET spectra of GCR major elements in ICRU four-element tissues in free space and with 20 g/cm2 shielding. (a) H, (b) He, (c) C, (d) Ne, (e) Si, and (f) Fe. Red line denotes GCR primary element in free space. Green, blue, and yellow denote GCR primary element with 20 g/cm2 shielding with Al, PE, and CFRP, respectively.

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Figure 3
figure 3

Summed LET spectra of Fig. 2a–f. Total GCR primary particles in free space (red line) and with 20 g/cm2 shielding with Al (green), PE (blue), and CFRP (yellow).

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Biological scale parameters

The absorbed dose rates and mean QFICRP and RBEγacute of GCR elements in free space are summarized in Table 1. The total mean QFICRP and RBEγacute values were obtained as follows:

Table 1 Absorbed dose rates and mean QFICRP and RBEγacute of GCR elements in free space (with no shielding materials).
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$$\langle QF (or RBE)\rangle =\frac{\sum Q{F}_{i} {D}_{i}}{\sum {D}_{i}},$$
(1)

where subscript \(i\) is an element in the GCR particles (Z = 1–26). The total absorbed dose rate was 162 mGy/year and the QFICRP based effective dose equivalent (HE(ICRP)) was 523 mSv/year. The total mean QFICRP was larger than the total mean RBEγacute by a factor of ~ 2.5. It should be noted that the effective dose equivalents based on QFICRP could not be overestimated by a factor of ~ 2.5. QFICRP is defined based on radiobiological data for conditions of low dose and low dose rate gamma-rays15. The difference due to the reference gamma-ray is compensated by the dose and dose rate effectiveness factor (DDREF)15. The value of the DDREF is a critical factor for obtaining the absolute value of the dose equivalent. The ICRP recommends a DDREF of 28,15. However, the DDREF ranges from 2 to 5 depending on the targets and radiation quality e.g., Refs.46,47,48,49,50,51. The differences between QFICRP levels and RBEγacute (Table 1) were consistent with the DDREF range.

Figure 4 shows elemental contributions to the effective dose equivalents in free space. The contributions of H, He, C, N, and O to HE(RBEγacute) were larger than their contributions to HE(ICRP). The contribution of Fe to HE(RBEγacute) was smaller than that to HE(ICRP) by a factor of ~ 2.5. The LET dependences of effective dose equivalents in free space and with 20 g/cm2 Al, PE, and CFRP shielding are given in Fig. 5. The vertical axis of Fig. 5b, i.e., HE(RBEγacute), does not consider the DDREF value. Therefore, direct comparisons between (a) and (b) cannot be made. The reduction rates of HE(ICRP) and HE(RBEγacute) were similar for the whole LET range. While the enhancement of the low energy and high LET particles generated in the shielding materials made a small variation in flux for LET < 100 keV/µm, the fragmentations from heavier ions than C contributed much to a dose reduction for LET > 8 keV/um. The mean QFICRP and RBEγacute (Table 1) indicate that the shielding material is efficient at reducing the effective dose equivalent due to the high LET particles of QFICRP > 3.36 or RBEγacute > 2.26. The dose reduction by CFRP was by a factor of ~ 2 at 10 keV/µm, ~ 5 at 100 keV/µm, and ~ 25 at 1000 keV/µm. The variations of relative absorbed dose, mean QFICRP and mean RBEγacute for the different shielding materials are summarized in Table 2.

Figure 4
figure 4

Contribution ratio of GCR particles to the effective dose equivalent. (a) QFICRP and (b) RBEγacute in free space (with no shielding materials).

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Figure 5
figure 5

LET dependencies of the effective dose equivalents. (a) HE(ICRP) and (b) HE(RBEγacute). Red line denotes total GCR primary particles in the free space. Green, blue, and yellow denote total GCR primary particles with 20 g/cm2 shielding with Al, PE, and CFRP, respectively.

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Table 2 Variation of the relative effective dose equivalent, HE, with shielding materials of differing mass thickness. The relative neutron contribution rates (HE(ICRP)neutron) and contributions to the charged particle contributions (HE(ICRP)) are given.
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Discussion

The calculated total absorbed dose and HE(ICRP) in this study were 10–15% and ~ 20% lower than previous interplanetary measurements, respectively6,39. Here, we note that HE(ICRP) was compared with the measured dose equivalents. The effective dose equivalent, which is a protection value, is determined by normalizing dose equivalent of human body, which is an operational value, with tissue or organ weighting factors. Although the effective dose equivalent is not equal to the dose equivalent, their comparison is reasonable considering the differences in irradiation targets (human body for our calculations and detector mediums for measurements). Considering discrepancies between the simple calculation geometry and actual measurement configuration, measurement uncertainties, the differences in the solar modulation factor, and contribution from secondary particles, which were not included in our calculation, these results are almost consistent. The reduction rates of HE(ICRP) by material shielding in this study were 45–55% at 20 g/cm2, which are also similar values to previous calculations considering the above discrepancies38,52. There was an important difference between RBEγacute and QFICRP in the ratio of light and heavy ions (e.g., H vs. Fe): the QFICRP value of Fe was 11–12 times higher than that of H, compared to 3.5 times higher for RBEγacute. This difference implies that the contribution of heavy ions to HE(ICRP) is ~ 3 times higher than that to HE(RBEγacute).

Comparing Figs. 3 and 5 indicates that the low energy edge of H does not contribute to the HE spectra and a small enhancement due to Mg and Si makes a contribution to LET of ~ 30 keV/µm. Because the kinetic energy of the secondary H around the edge is ~ 1 MeV, its energy deposition is not large enough to increase the dose. Meanwhile, heavy ions, such as C and O, for LET ~ 10 keV/µm are relativistic, which increases the dose. The penetrative ability of ions is an important factor when considering the energy deposition in the human body. Therefore, the underestimation of ions with a lower energy of < 1 MeV/n was not significant. Like the H edge, the shielding materials diminished the He edge at 20–100 keV/µm, inducing a small enhancement in Mg and Si.

The reduction in the absorbed dose mainly came from the fragmentation of HZE particles. Around the mean energy of the GCR particles (~ 1 GeV/n), the energy loss in the shielding materials, which depended on the stopping power, was not significant. The mean QFICRP and RBEγacute values decreased as a function of shielding thickness. The reduction rate of HZE particles by fragmentation in the shielding materials was larger than the rate of increase in protons. At a typical mass thickness of 20 g/cm2, the mean QFICRP and RBEγacute decreased to 30% and 15% for Al, 40% and 20% for PE, and 37% and 18% for CFRP, respectively. The reduction rates of mean RBEγacute were lower than those of mean QFICRP because the heavy ion contributions to mean QFICRP were larger than those to mean RBEγacute. The relative variations in HE values among the shielding materials were obtained from the relative absorbed doses and relative mean QFICRP or RBEγacute (Eq. 3). At a thickness of 20 g/cm2, the reduction rates of HE(ICRP) and HE(RBEγacute) were 45% and 32% for Al, 55% and 40% for PE, and 52% and 38% for CFRP, respectively. PE achieved ~ 24% and ~ 5% higher reduction rates than Al and CFRP, respectively. An idea for deep space missions is to construct some spacecraft parts made of CFRP, which has a better shielding capability than conventional Al based materials. Our previous evaluation implied that the material switching from Al to CFRP at the same actual thickness provided a similar dose reduction despite that the total mass of CFRP module was much smaller than that of Al by a factor of their density ratio41. The actual shielding materials are not only spacecraft materials but also fuel, water and other supplies. The materials to construct a spacecraft will be selected by not only radiation shielding performance but also many requirements such as thermal property, ultraviolet resistance, moisture absorption resistance and so on. If the complete material switching from Al to CFRP is attained by the same mass thickness (g/cm2), CFRP will give a benefit for the protection of crews from GCRs.

The results were discussed for charged particles from protons to Fe ions as mentioned in the results section; the contributions of neutrons, photons, pions, and muons were not considered. In particular, the neutron contribution to the dose equivalents may not be small among secondary radiation particles (e.g., Refs.4,5). The neutron contributions obtained from the ICRP conversion coefficients, HE(ICRP)neutron, are also listed in Table 2. The neutron contribution rate was much lower than the charged particle contributions (5.9% of the HE(ICRP) at most). The fragmentation reactions of the primary particles, which produce neutrons, occur effectively in light shielding materials. The higher neutron contribution in Al indicates a high thermalization with hydrogen atoms in PE and CFRP. RBEγacute for neutrons is one of issues to be addressed in future.

The difference between QFICRP and RBEγacute, except for the reference gamma-ray, is in the models: QFICRP depends on the energy deposition (i.e., LET) while RBEγacute depends on the charge and energy of the incident particles. The energy of the secondary particles is dependent on that of the primary particles. One possible explanation for the larger QFICRP peak than RBEγacute peaks in their LET dependencies (Fig. 6) is the difference in targeted radiation for dose assessment. Heavy ions at their peaks have relativistic energy, which produces relativistic secondary particles. The fact that the RBEγacute peaks are lower than the QFICRP peak reflects the significant contribution of low energy secondary particles because electrons are biologically effective at low energies (< 10 keV)10.

The advantages and disadvantages of QFICRP and RBEγacute are summarized in Table 3. The RBEγacute evaluates relative biological effectiveness with a smaller uncertainty than RBEmax relevant to QFICRP, as mentioned in the introduction. One of the difficulties in using RBEγacute is the selection of the parameters. The selected parameters, and thus RBEγacute, depend on the targeted radiation field and biological effects. Thus, the obtained dose equivalent was not comparable in different radiation environments. Meanwhile, QFICRP, which is determined by LET, offers good usability. However, the effective dose equivalents and QFICRP have been established for general dose assessment on the ground and have been replaced with the effective dose15. The radiation environment on the ground is primarily composed of photons from natural radioisotopes. Other contributors to this environment are protons, alpha particles, and neutrons in specific radiation fields, such as medical accelerators and nuclear power plants. HZE particles were not the main target of QFICRP on the ground. QFICRP has not been updated since the 1990 ICRP recommendation15. The establishment of a standard parameter specified for a mixed radiation environment occupied by protons and heavy ions is necessary for practical dose assessment in deep space.

Figure 6
figure 6

LET dependencies of biological scale parameters. RBEγacute (left axis) for simple exchanges to human peripheral blood lymphocytes22 and QFICRP (right axis)15.

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Table 3 Advantages and disadvantages for QFICRP and RBEγacute.
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Conclusion

We investigated the radiation shielding performance and effective dose equivalents based on the ICRP radiation QF and the plausible RBE for three shielding materials, Al, PE, and CFRP, using the Geant4 Monte Carlo simulation. The QFICRP values of Fe were 11–12 times larger than those of H, compared to ~ 3.5 times larger for RBEγacute. Therefore, the contribution of heavy ions to HE(ICRP) was larger by a factor of ~ 3 compared to that to HE(RBEγacute). The shielding materials reduced the flux of primary particles heavier than He due to projectile fragmentation. The flux reduction rate increased with successively heavier nuclei and the increase was particularly large for heavier ions. The shielding materials efficiently reduced the effective dose equivalent due to ions with QFICRP > 3.36 and RBEγacute > 2.26. The reduction rates of HE(ICRP) were higher than those of HE(RBEγacute) because of the large contribution of heavy ions. The expected radiation exposure risk was reduced by 50% for QFICRP and 38% for RBEγacute when using 20 g/cm2 CFRP. Therefore, a spacecraft made of CFRP could improve the radiation shielding performance compared to conventional Al-based spacecraft and help mitigate space radiation hazards in future space missions. The discrepancy between QFICRP and RBEγacute highlighted the necessity of a new standard for mixed radiation environments occupied with protons and heavy ions in deep space.

Methods

We calculated the fluences of GCR protons and heavy ions up to Fe (Z = 1–26) with energies ranging from 1 MeV/n to 100 GeV/n in free space and with shielding materials using Monte Carlo simulation with Geant4 ver. 10.04.0253,54,55. The primary particles in the energy region of 1–10 MeV/n (Fig. 1 dotted lines) did not contribute to the flux behind the materials because these low energy particles were stopped within a thin shielding layer of only a few micrometers. Primary particles in this energy range were excluded from the projectile to reduce the calculation cost. The mean QFICRP and RBEγacute values were obtained, including the particles in this energy range. Al, PE, and CFRP with mass thicknesses of 5, 10, 15, and 20 g/cm2 were employed as shielding materials. The GCR source was derived from the DLR model during the solar minimum phase56. The solar minimum assumption provides the worst case for radiation exposure. The number of each primary ion was fixed at 106 to obtain the total dose rate by merging all the dose rates. The elemental composition of the CFRP was assumed to be that of a commercial composite material, CF/PEEK (Toray Cetex TC1200, Toray Advanced Composites, USA). The dose and LET changes due to nuclear fragmentation simulated by Geant4 have been experimentally validated in our previous studies40,41.

The absorbed dose (D) for the whole human body, due to each particle in free space and behind the target materials, was obtained from the particle fluences and ICRP conversion coefficients for isotropic exposure9. The ICRP defines the QFICRP as the following15:

$$Q{F}_{ICRP}=\left\{\begin{array}{c}1\\ 0.32 LET-2.2\\ 300/\sqrt{LET}\end{array}\right. \begin{array}{c}(LET<10)\\ (10<LET<100)\\ \left(LET>100\right).\end{array}$$
(2)

The mean RBEγacute was obtained through the charge and energy dependences for the targeted effects model as follows21,22:

$$\begin{array}{c}RB{E}_{\gamma acute}=(1-P)+6.24{\sigma }_{0}/{\alpha }_{\gamma }LET\\ P={(1-exp(-{Z}^{*2}/\kappa {\beta }^{2}))}^{m},\end{array}.$$
(3)

where \({Z}^{*}\) and \(\beta\) are the effective charge number of the particle and particle velocity relative to light, respectively; parameters \({\sigma }_{0}\), m, and \(\kappa\) are constants based on radiobiological experiments; and \({\alpha }_{\gamma }\) is the linear regression coefficient for the acute dose of gamma-rays at the same endpoint. We employed these parameters for simple exchanges with human peripheral blood lymphocytes from Cacao et al.22 since blood is uniformly distributed in the whole body. Here, the dose rate in space is of the order of hundreds of micro grays per day e.g., Refs.6,33,34,37,43,57. Non-targeted effects are expected to be negligible at sufficiently low doses of < 1 mGy e.g., Refs.58,59,60,61 and a large number of DNA double-strand breaks are required for complex exchanges. Therefore, we selected a targeted effects model and assumed a simple exchange with human lymphocytes. Although the parameters studied in previous works17,21 provided 2–5 times larger RBEγacute values, these studies targeted solid cancer or leukemia, which are also relevant to nontargeted effects and complex exchanges. Other parameters must be used to target these critical radiation hazards. The RBEγacute and QFICRP values of major GCR particles as a function of LET are presented in Fig. 615,22. The LET range of each particle corresponded to an energy range of 1 MeV/n–10 GeV/n. The dose equivalent HE was obtained by the products of the absorbed dose and the QFICRP or RBEγacute:

$${H}_{E}= \Sigma {D}_{i} Q{F}_{ICRP i} \left(or RB{E}_{\gamma acute i}\right),$$
(4)

where \(i\) indicates the GCR particle (Z = 1–26).