Hydrogen- and Methane-Loaded Shielding Materials for Mitigation of Galactic Cosmic Rays and Solar Particle Events

As the United States space program ventures out beyond the low Earth environment into what is known as “free space,” the risks from space radiation will increase substantially. The two sources of deep space radiation are the naturally occurring galactic cosmic radiation (GCR) and solar particle (proton) events (SPEs), both of which vary with the eleven-year solar cycle.

The solar cycle contains periods of high solar activity, known as “solar maximum,” and periods of low solar activity, known as “solar minimum” (Figure 1). During solar maximum, there tends to be more frequent and higher intensity SPEs, with the potential for very large events. The particles that make up these events consist of mostly protons.

Figure 1.

The deep space GCR radiation environment consists of extremely energetic stripped nuclei ranging from hydrogen (proton) to iron with additional elemental nuclei out to uranium. The GCR environment varies with the eleven-year solar cycle such that during solar minimum the GCR fluxes are at a maximum, as seen in Figure 2.

Figure 2.

There have been numerous radiation dose estimation studies (Atwell, 2012; Badavi et al., 2011; Cooper et al., 2008; Rojdev and Atwell, 2011; Simonsen and Nealy, 1991; Tripathi et al., 2006; Wilson et al., 2007) performed over the years for various types of space missions, both crewed and robotic. These missions include low Earth orbit (LEO), geostationary orbit (GEO), lunar, Mars, Jupiter/Europa, and Saturn. Additional mission analyses have also included extravehicular activity (EVA), medium Earth orbit (MEO), geostationary transfer orbit (GTO), and Earth orbit to deep space.

Passive, bulk shielding materials have been used to protect both crew and onboard systems since active shielding (e.g., magnetic and electrostatic) has not been feasible from a safety, weight, and cost standpoint. Evaluated shielding materials have included aluminum, hydrocarbons (e.g., polyethylene), hydrogen, methane, and other heavier materials (e.g., iron, tantalum, tungsten, and stainless steel) (Atwell et al., 2006; Atwell et al., 2013; Barghouty and Thibeault, 2006; Guetersloh et al., 2006; Rojdev et al., 2010; Walker et al., 2010). In general, materials that contain elements of a low atomic number provide better radiation mitigation properties than materials containing elements with a high atomic number. From previous investigations, HDPE has been identified as an efficient shielding material for space radiation applications due to the high hydrogen content. Thus, it is used as a reference material to which all other material results are compared in this work.

Recently, scientists have used certain materials loaded with hydrogen — namely MHs –– to develop fuel cells having various applications, such as the automobile industry (Bowman and Fultz, 2002; Czaja et al., 2009; Iñiguez et al., 2004; Kuppler et al., 2009; Li et al., 2005; Mitrokhin, 2005; Saitoh et al., 2013; Sakintuna et al., 2007; Zhou et al., 2007). Since hydrogen, methane, and other types of hydrocarbons are excellent at shielding against proton radiation, we were curious whether the same materials being investigated for fuel cells could also be used for radiation shielding. In addition, we are most interested in those materials the investigators had difficulty extracting hydrogen that was loaded into the material, suggesting more stable bonding of the hydrogen. Therefore, we have investigated the use of three materials as potential radiation mitigators: hydrogen-loaded MHs, MOFs, and CNTs.

In a preliminary study (Atwell et al., 2014), we investigated 64 hydrogen-loaded materials and simulated their exposure to the series of SPEs that occurred during the 19-24 October 1989 time period. The combined differential and integral proton energy spectra for this series of events are shown in Figure 3. To arrive at this spectrum, the spaceflight data (Figure 4) was fitted using the Band fitting method (Atwell et al., 2010; Tylka et al., 2010). These proton spectra represent three Ground Level Events (GLE) plus one bow shock enhancement, as indicated in Figure 4. A GLE is of high enough energy such that neutron monitors on the surface of the Earth detect the secondary neutron production from the event. The energetic solar particles (ESP) occur when there is a bow shock enhancement of solar protons. We selected this series of events due to the particularly hard spectrum (high fluence of higher energy protons).

Figure 3.

Figure 4.

The materials studied were then compared against a typical spacecraft material of aluminum and our standard radiation shielding material of HDPE. The results (Table 1) showed over 60% of the materials studied performed better than aluminum (10 MOFs, 14 CNTs, and 15 MHs) and that these materials may be promising as multifunctional radiation shields. Thus, the work in this paper extends this initial investigation to the GCR environment, as well as compares the hydrogen-loaded materials with methane-loaded counterparts.

In the simulations undertaken in this study, a total of 85 materials were investigated. The material information was theoretically determined for the purposes of this study and these materials were separated into three categories: MOFs, CNTs, and MHs. The nanoporous carbon composites (CNTs) will most likely be made of carbon nanotubes initially when created and tested. The following three tables (Tables 2-4) provide the details of each material that were necessary for the radiation transport material definitions, as well as the radiation environment used for the exposure simulations (GCR or SPE).

Over the years, a number of high energy particle transport/dose codes have been developed. These include FLUKA (Ballarini et al., 2007), GEANT-4 (Bernabeu and Casanova, 2007), PHITS (Sihver et al., 2007), and HZETRN (Wilson et al., 1991; Wilson et al., 1995; Wilson et al., 2006). The first three codes are 3-D Monte Carlo codes and require long run times, as well as enhanced computing capacity. In our analyses we have used the HZETRN code, which is the NASA standard developed at NASA Langley Research Center. It is a one-dimensional, quick-running code that produces results comparable to the Monte Carlo codes.

For this study, we used the 2010 version of HZETRN (Wilson et al., 1991; Wilson et al., 1995; Wilson et al., 2006; Slaba et al., 2010a; Slaba et al., 2010b) to take advantage of the updates in the code. The environments specified for this study were the October 1989 series of SPEs (Figure 3 and Figure 4) and the 1977 solar minimum GCR condition. These environments are of interest because they represent worst-case environments. The October 1989 spectra was fit using the Band fitting method (Tylka and Dietrich, 2009), and the differential spectrum was used as the input SPE environment in HZETRN 2010 (Figure 3). The 1977 solar minimum GCR differential spectrum is pre-coded into the software (Figure 5). Output doses (cGy) in tissue were computed on a laptop at material thicknesses of 1, 5, 10, 20, 30, 50, and 100 g/cm², and a smooth line through these points was displayed.

Figure 5.

There are two parts to this study. One is a continuation of work that was presented at the International Conference on Environmental Systems (Atwell et al., 2014) using the same groups of materials, but exposing them to a GCR environment. These materials are hydrogen-loaded MHs, MOFs, and CNTs.

The second part of the study focuses on methane-loaded materials because of potential concerns with hydrogen. One of the challenges with hydrogen-loaded materials is the stability of the hydrogen — especially in changing environmental conditions. When a vehicle is in space, engineers must design around several challenging environments, which include a changing thermal environment. Exposure of a hydrogen-loaded material to this environment could potentially lead to the hydrogen unbinding from the material and leaking into the spacecraft. Furthermore, hydrogen is known to be flammable and explosive, which could have devastating effects on spacecraft. Thus, given that methane is a radiation mitigator that performs better than polyethylene (Figure 6) and is less flammable than hydrogen, we investigated methane-loading of materials and compared it with the hydrogen-loaded version to determine the difference in dose vs. depth between the two loading methods. For this case, we investigated both the GCR environment and the SPE environment.

Figure 6.

Part 1: H-Loaded Materials Exposed to a Simulated GCR Environment

The first part of the study focused on hydrogen-loaded materials that were exposed to the input GCR environment. The results are presented by material class: MOFs, CNTs, and MHs. The materials are also compared with aluminum (i.e., our typical spacecraft material) and HDPE (i.e., our standard radiation shielding material).

Metal organic frameworks

The results of the MOF materials are shown in Figure 7 and Figure 8. In these results, we see all MOF materials perform better than the typical spacecraft material of aluminum. In all cases, the hydrogen-loaded versions perform better than the non-hydrogen-loaded versions. Additionally, none of the MOFs are better mitigators than HDPE. However, there are two hydrogen-loaded MOFs that perform similarly to HDPE, namely C₂₀₀H₃₂₅ and C₄₃₂H₁₁₂₀Be₄₈O₁₄₄.

Figure 7.

Figure 8.

Nanoporous carbon composite

The results for the CNTs are shown in Figure 9 and Figure 10. The non-loaded versions (Figure 9) are better radiation mitigators than aluminum. While the non-loaded CNTs do not outperform HDPE as a radiation mitigator, they are very similar. The hydrogen-loaded versions of these CNTs (Figure 10) outperform aluminum again and just slightly outperform HDPE.

Figure 9.

Figure 10.

Metal hydrides

The results for the various MHs investigated are shown in the following figures: Figure 11 shows a series of lithium MHs with increasing hydrogen content and decreasing lithium content, Figure 12 shows additional MHs, and Figure 13 shows two lithium-based materials and their hydrogen-loaded counterparts.

Figure 11.

Figure 12.

Figure 13.

All of the lithium-containing materials consistently outperform aluminum and the lithium MHs either outperform both aluminum and HDPE, or are in line with HDPE. The other investigated MHs only surpass aluminum in their radiation mitigation traits.

There were also 25 additional materials investigated (not shown in the graphs) that performed worse than aluminum.

Part 2: CH₄-Loaded Exposed to Simulated GCR and SPE Environments

In the second part of the study, we investigated several MOFs and CNTs. We compared the unloaded versions to the hydrogen-loaded and methane-loaded versions, as well as to our typical spacecraft material of aluminum and our standard shielding material of HDPE. For these materials, we investigated both the SPE and the GCR environments.

SPE

The following graphs show the results of various MOF materials exposed to the October 1989 series of SPEs. Figure 14 and Figure 15 show five materials and their respective hydrogen-loaded and methane-loaded versions. In each of the cases, the hydrogen-loaded version outperforms the base and the methane-loaded versions. However, the methane-loaded version is quite comparable to the hydrogen-loaded version. Additionally, all of the MOF materials outperformed aluminum in their radiation mitigation qualities.

Figure 14.

Figure 15.

Figure 16, Figure 17, and Figure 18 are the results of the CNTs exposed to the same October 1989 series of SPEs. For these materials, all three cases (base material, hydrogen-loaded, and methane-loaded) are very similar. All the materials perform better than aluminum and only the hydrogen-loaded versions outperform HDPE. However, they are all relatively close to the performance of HDPE.

Figure 16.

Figure 17.

Figure 18.

GCR

Figure 19 and Figure 20 show the results of the MOF materials exposed to a GCR environment. In these results there are a few of the base materials that perform worse than aluminum. However, the hydrogen- and methane-loaded counterparts surpass the radiation mitigation qualities of aluminum. Only one hydrogen-loaded MOF outperforms HDPE, namely Zn₂₁₆C₃₁₃₂O₇₀₂H₁₄₈₁₄.

Figure 19.

Figure 20.

Figure 21, Figure 22, and Figure 23 show the results of the CNTs exposed to the GCR radiation environment. The CNT materials again behave similarly to HDPE — as was shown with the SPE environment—and thus, outperform aluminum as a radiation mitigating material. There are a few hydrogen-loaded CNT materials that also outperform HDPE.

Figure 21.

Figure 22.

Figure 23.

There are two objectives in this study. The first objective is to evaluate these types of materials against our typical spacecraft and shielding materials to determine whether they may be viable as multifunctional materials that also protect against space radiation. The second objective is to determine whether the methane-loaded versions of these materials are comparable to the hydrogen-loaded versions to remove some of the concerns in working with hydrogen.

To determine how these materials fare against our typical spacecraft materials, the dose results have been compared with aluminum (the typical spacecraft shell) and HDPE (our standard radiation shield). We have aggregated the data of the 86 materials investigated and separated them out by the following categories: materials that perform better than HDPE, materials that perform better than aluminum but not better than HDPE, and materials that do not perform better than aluminum. These comparisons were made at each of the thicknesses investigated. The aggregated data are shown below in Table 5 and Table 6.

From these tables, it is quite clear a majority of these materials do outperform aluminum as a shielding material, and a few also outperform HDPE. Of particular interest are the lithium-based MHs that outperformed HDPE, as shown in Figure 11, Figure 12, and Figure 13. Further work will be needed to determine why this occurs.

The other group of materials that performed particularly well are the CNTs. Both the non-loaded and the methane-loaded versions of the materials performed better than aluminum, and while they did not perform better than HDPE, they are comparable (Figure 9, Figure 16, and Figure 23). The hydrogen-loaded versions of the CNTs outperformed HDPE, but only marginally. The reason the results of the CNTs are so similar to HDPE is their chemical makeup is very similar and thus, they have similar radiation shielding characteristics.

There were also several MHs that did not outperform aluminum. These materials tended to have elements with higher atomic numbers — such as lanthanum, barium, calcium, etc. It is well known from previous radiation shielding studies (Atwell et al., 2006; Atwell et al., 2013; Barghouty and Thibeault, 2006; Guetersloh et al., 2006; Walker et al., 2010; Wilson et al., 1997) that better radiation mitigators will have a smaller atomic number. One caveat is certain materials —such as lithium and boron — are known to be good neutron absorbers while also containing a higher atomic number than hydrogen. Thus, optimizing a material to reduce the primary radiation energy enough to stop the particle, as well as providing some neutron absorption from secondary radiation production, may be the better option for a radiation shield.

In addition to determining the radiation shielding capability of these materials relative to our typical spacecraft materials, we wanted to determine whether the methane-loaded versions were comparable to the hydrogen-loaded versions. Therefore, we calculated the percent difference in dose between the hydrogen-loaded versions and the methane-loaded versions for a thickness of 30 g/cm² using Equation (1). In the equation, X_CH4 is the dose for the methane-loaded version and X_H is the corresponding dose for the hydrogen-loaded version. The differences are shown in Table 7 and Table 8. Equation (1) % increase in dose =xCH4−XHxH×100 \% \text { increase in dose }=\frac{x_{C H_{4}}-X_{H}}{x_{H}} \times 100

In examining these tables, it is quite clear there is minimal difference in the dose between the methane-loaded and hydrogen-loaded versions of these materials, with the exception of Zn₂₁₆C₃₁₃₂O₇₀₂H₁₂₄₂ during the SPE exposure. It is unclear at this time why there is such a departure between the hydrogen-loaded and methane-loaded version of this material, and future study will be needed. Furthermore, in the GCR cases there tends to be even less of a difference between the hydrogen- and methane-loaded versions.

This study examined several hydrogen- and methane-loaded materials to determine whether they would be feasible multifunctional materials that could serve as radiation shields in space. Furthermore, there are general safety concerns about using hydrogen due to its flammability and instability in changing environments. Thus, we considered methane as an alternative loading material and compared its radiation shielding performance against the hydrogen-loaded counterpart.

Overall, the results showed several MOFs, CNTs, and MHs that performed very well when compared with our typical spacecraft material of aluminum and our standard shielding material of HDPE. Of particular interest are the lithium MHs that outperformed HDPE. It is recommended future studies investigate additional lithium-based materials and perform deeper investigation to determine why these materials perform so well. Additionally, CNTs have similar chemical composition to HDPE and thus provide similar radiation protection. Since HDPE tends to be a parasitic shielding material due to its poor structural performance and flammability concerns, it is recommended future studies also include CNTs that may have multifunctional structural uses as well. Future work will also need to consider dose equivalent and other exposure quantities of interest for human spaceflight.

Finally, this study showed there is little difference in the shielding effectiveness between hydrogen-loaded and methane-loaded materials of the same base chemistry. Thus, future studies should focus on methane-loaded materials to remove some of the safety concerns in using hydrogen.