Simulation Study on Radiation Shielding Performance of Aerospace Materials against Solar Cosmic Rays

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1 New Physics: Sae Mulli, Vol. 64, No. 12, December 2014, pp DOI: /NPSM Simulation Study on Radiation Shielding Performance of Aerospace Materials against Solar Cosmic Rays Liu Dong Jewou Ko Jong-Kwan Woo Department of Physics, Jeju National University, Jeju , Korea Se Byeong Lee Proton Therapy Center, National Cancer Center, Goyang , Korea (Received 6 October 2014 : revised 21 October 2014 : accepted 21 October 2014) Solar cosmic rays are some of the biggest problems for space activities due to their damage to the crews and the hardware on spacecraft. Therefore, research of the radiation shielding performance of aerospace materials against solar cosmic rays has become an important concern. In this study, the radiation shielding capabilities of 18 single-element materials and 10 current astronautical structural materials are assessed using the Stopping and Range of Ions in Matter (SRIM) code. The radiation shielding capability of the selected materials are appraised according to the values of the mass stopping power of those materials for solar cosmic-ray particles. This research comprehensively considers the radiation shielding performance and the load problems of various shielding materials. As a result, under the same mass conditions, light-element materials and fiber-reinforced polymers are more suitable for shielding against solar cosmic rays. Finally, light elements and fiber-reinforced polymers should be considered, as much as possible, as structural materials for the manufacture of spacecraft. PACS numbers: Jx, Ev, Tg Keywords: Solar cosmic rays (SCRs), Radiation shielding, Mass stopping power, SRIM I. INTRODUCTION Space radiation mainly contains high energy heavy charged particles that have a high linear energy transfer (LET). They are more dangerous than the kinds of radiation that we experience here on Earth, such as x rays or gamma rays. Moreover, astronauts would face the exposure from space radiation without any natural barrier (Earth s atmosphere and magnetic field) during interplanetary spaceflight. Furthermore, the hardware of spacecraft can be damaged by cosmic rays. For instance, ionization of atoms of spacecraft materials can cause the change of charge distribution thereby leading to undesired results. For the above-mentioned reasons, the assessment of radiation shielding capability of materials that would be used in astronautic field is necessity w00jk@jejunu.ac.kr 1248 for future interplanetary expeditions. Recently years, active shielding methods have been proposed, even the particular design plans are also described, such as electrostatic fields shielding and magnetic fields shielding [1,2]. However, considering the technical problems, in the near future, the passive mass thickness shielding method is still a most feasible method for protection of high energy cosmic rays during manned spaceflight missions. Therefore, the shielding capability of structural materials of spacecraft is one of the important concerns for spaceflight missions. For manned spaceflight missions, there are two main cosmic rays, one is solar cosmic rays (SCRs), and another one is galactic cosmic rays (GCRs). Considering the short mission term of current human spaceflights and the lower fluence of GCRs, the energetic particles from SCRs are most dangerous to crews when a solar particle event (SPE) occurs. A large SPE could subject the This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

2 Simulation Study on Radiation Shielding Performance of Aerospace Materials Liu Dong et al spacecrafts to as much fluence at energies in the tens of MeV as a year of GCR exposure in a short time period (hours or days). Even a single extra-large SPE could expose a spacecraft to as much fluence at energies above tens of MeV as an entire solar cycle of exposure to GCR [3]. In general, the most significant energy range is from a few tens of MeV to a few hundred MeV. In addition, on average, more than 90 percent of the energetic particles produced in an SPE are protons. For these reasons, protons with energy range from 10 MeV to 1 GeV are the primary concern when evaluating potential SPE radiation hazards [4]. Unlike the shielding design on the Earth, for spaceflight missions, the mass of load is one of the most important points. Thus, it is almost impossible to carry plenty of shielding materials, full attention must be paid to this point. In this scenario, the astronautic materials with a good shielding performance are more suitable to shield high energy protons from SCRs under the same mass conditions for a spacecrafts. II. SIMULATION METHOD The linear rate of energy loss to electrons of atoms along the path of a heavy charged particle in a material is the basic physical quantity to determine the dose that the particle transports in the material. This quantity, designated de/dx is called the electronic stopping power of shielding materials for the incoming particle. Stopping power is often expressed in the units MeV/cm. In fact, there are two kinds of stopping powers for heavy charged particles: electronic stopping power due to interactions of charged particles with atomic orbital electrons and nuclear stopping power due to interactions of charged particles with atomic nuclei. For high energy heavy charged particles, such as cosmic ray particles, nuclear stopping power can be neglected in the calculations. Then, the electronic stopping power can be used to replace total stopping power. In the following contents of this paper, the term stopping power practically refers to electronic stopping power. The Bethe-Bloch formula is used to express the stopping power of a uniform medium for high energy heavy charged particles: de dx = 4πz2 e 4 n mc 2 β 2 [ln 2mc2 β 2 I(1 β 2 ) β2 ] (1) In this relation, z: atomic number of the heavy charged particle, e: magnitude of electron charge, m: electron rest mass, n: electron (number) density, number of electrons per unit volume in the medium, c: the speed of light in vacuum, β: V/c, speed of the charged particle relative to c, I: mean excitation energy of the medium. In addition to stopping power, the mass stopping power, -de/dx, is much more commonly used, because it expresses the rate of energy loss of the charged particle per g/cm 2 of the medium traversed. The corresponding unit for mass stopping power is MeV/g/cm 2. The Bethe- Bloch expression (1) for mass stopping power is: de ρdx = 4πN 0z 2 e 4 Z [ln 2mc2 β 2 mc 2 β 2 A I(1 β 2 ) β2 C Z δ ]. 2 (2) Where Z: atomic number of medium, A: atomic weight of medium, N o : Avogadro number, C/Z: shell effect correction, δ: density effect correction. When the material is a compound or mixture, the mass stopping power can be calculated by using its effective atomic number (Z eff ) and effective excitation energy (I eff ). These two quantities can be calculated by the following formula: where Z i : Z eff = (Z/A) eff = i I eff = exp a i (Z i A i ), (3) [ ] Σi a i ( zi A i )lni i ( Z A ), (4) eff atomic number of element i of compound, A i : atomic weight of element i of compound, a i : weight percentage of element i of compound, I i : mean excitation energy of element i of compound. According to Eq. (2), it can be seen that light elements possess the higher mass stopping power value compare to heavy element (for a specific incident particle, heavy atoms have lower Z/A value and higher I value, in addition, many of their electrons are too tightly bound in the inner shells to participate effectively in the absorption of energy). As a result, we can say that at the same conditions of areal density (g/cm 2 ), the heavy charged particles will loss more energy in the light element materials. Thus, it was suggested that lights elements materials and composites that mainly contain light elements are more suitable to shield high energy cosmic ray particles.

3 1250 New Physics: Sae Mulli, Vol. 64, No. 12, December 2014 So far, we analyzed the mass stopping power based on the Bethe-Bloch formula and concluded that light elements are ideal structural materials to shield cosmic rays. Next, the mass stopping powers are calculated using Stopping and Range of Ions in Matter (SRIM) code, a widely used to compute a number of parameters relevant to ion beam implantation and ion beam processing of materials [5]. The calculation results of SRIM will show the stopping powers, range, straggling distributions for any ion at a wide energy range in any elemental and complex multi-layer configurations. The input of SRIM program should include the type of incident particle, the energy of incident particles, and the specific definition of target materials. In this study, the energy ranges of incident protons are from 10 MeV to 1 GeV. The target materials include 18 single-element materials that can be used in aerospace field or the radiation protection field and 10 current astronautic structural materials that have potential to be widely employed as major structural materials of spacecraft. The 18 single-element target materials include Hydrogen(H), Helium(He), Lithium(Li), Beryllium(Be), Boron(B), Carbon(C), Nitrogen(N), Oxygen(O), Fluorine(F), Sodium(Na), Magnesium(Mg), Aluminum(Al), Silicon(Si), Potassium(K), Titanium(Ti), Iron(Fe), Zinc(Zn) and Lead(Pb). The 10 current astronautic structural materials include Aluminum alloys, Steel alloys, Titanium alloys, Magnesium alloys, Glass (Quartz glass), Kevlar, Carbon-Fiber-Reinforced Polymer (CFRP), Glass-Fiber-Reinforced Polymer (GFRP), Boron-Fiber-Reinforced Metal-Matrix Composite (BFRM) and Carbon-Fiber-Reinforced Metal-Matrix Composite (CFRM). The main compositions of the 10 current structural materials are assumed to be as follows: Aluminum alloys: 2090 Al-Li alloy (about 95Al 2.7Cu 2.2Li 0.1Zr, wt.%) was selected to represent alumina alloy. Steel alloys: D6AC alloy steel (about 95.6Fe 0.45C 0.8Mn 0.25Si 0.6Ni 1.1Cr 1.1Mo 0.1 V, wt.%) was selected to represent steel. Titanium alloys : 80% Ti-20%Al(mass percent) Magnesium alloys: 90% Mg-10%Li(mass percent) Glass (Quartz glass): SiO 2 Kevlar:[-CO-C 6 H 4 -CO-NH-C 6 H 4 -NH-] n Fig. 1. (Color online) Schematic of the interaction process of incident proton and the shell of spacecraft. Carbon-Fiber-Reinforced Polyethylene was selected to represent CFRP materials, the composite ratio of carbon fiber and polyethylene is 1:1 (mass ratio). Glass-Fiber-Reinforced Polyethylene was selected to represent GFRP materials, the composite ratio of glass fiber and polyethylene is 1:1 (mass ratio). Boron-Fiber-Reinforced Aluminum-Matrix Composite was elected to represent BFRM materials, the composite ratio of boron fiber and Al-Li alloy is 1:1(mass ratio). Carbon-Fiber-Reinforced Aluminum-Matrix Composite as selected to represent CFRM materials, the composite ratio of carbon fiber and Al-Li alloy being 2:3 (mass ratio). Figure 1 shows the interaction process of incident protons in the shell of spacecraft and explained why the shielding capability of structural materials is our concern. First, functional materials are necessity, diversity, and uniqueness. For instance, some materials used for heat-insulating coating, even if they are less efficient in shielding against cosmic rays. Second, the functional materials have a limited usage to overall spacecraft manufacture. On that account, the SCR protons are mainly stopped by structural materials of shell of spacecraft and only the shielding capability of the structural materials should be taken into account in this study. III. RESULTS AND DISCUSSION The calculation results for stopping powers and range for protons at the assignment energy range in selected

4 Simulation Study on Radiation Shielding Performance of Aerospace Materials Liu Dong et al Table 1. The mass stopping power (MeV/g/cm 2 ) of 18 single element materials to proton at 10 MeV and 1 GeV. 10 (MeV) 1000 (GeV) Hydrogen Helium Carbon Nitrogen Oxygen Lithium Boron Beryllium Fluorine Magnesium Silicon Sodium Aluminum Potassium Titanium Iron Zinc Lead Table 2. The mass stopping power (MeV/g/cm 2 ) of 18 single element materials to proton at 10 MeV and 1 GeV. 10 (MeV) 1 (GeV) CFRP GFRP Kevlar Glass CFRM BFRM Magnesium alloys Aluminum alloys Titanium alloys Steel materials could be found from output files of SRIM program. Table 1 shows the value of mass stopping power for 18 single element materials in two typical energy point of incident protons. In the case of incident proton with energy of 10 MeV, the mass stopping power of 18 single element materials from highest to lowest is H, He, C, N, O, Li, B, Be, F, Mg, Si, Na, Al, K, Ti, Fe, Zn and Pb. For high energy point (1 GeV), the rank of mass stopping power of these materials present some slight change, but the overall trend remains stable. In order to analyze the calculation results and compare the shielding capability of various materials better, the whole simulated calculation results are shown in the form of curves (Fig. 2). Figure 2 shows the mass stopping power of protons of SCRs in 18 single-element target materials. To give an analysis of Fig. 2, there are several points to cover. In the sequence of subtitles, it goes like this: 1. For a specific incident protons from SCRs, the radiation shielding effect decreased with the increase of the atomicity of shielding material; generally speaking, it is advantageous to use the low atomic number materials (such as H, Li, Be, B, C and N) as the shielding materials for shielding against protons from SCRs. This Fig. 2. (Color online) The mass stopping power of protons of SCRs in 18 single-element target materials. simulation result is in good agreement with the theoretical analysis that has pointed out that the most effective material per unit mass is provided by light elements. 2. Among the light elements, hydrogen shows an absolute advantage for shielding capability in the whole selected energy range. 3. For a specific target, the shielding capability of the material decreases with the increase of the energy of an incident particle. 4. Among metal materials, Na, Si and Al have relatively better shielding capabilities. 5. Heavy elements, lead for example, while commonly used for x- or γ-ray absorption on earth, are much less efficient than lighter elements for absorbing energetic protons from SCRs. Table 2 shows the value of mass stopping power for 10 astronautic structural materials in two typical energy

5 1252 New Physics: Sae Mulli, Vol. 64, No. 12, December 2014 Fig. 3. (Color online) The mass stopping power of protons of SCRs in 10 current astronautic structural materials. Fig. 4. (Color online) The mass range of protons of SCRs in 18 single-element target materials. point of incident protons. In the case of incident proton with same energy, the mass stopping power of 10 astronautic structural materials from highest to lowest is CFRP, GFRP, Kevlar, Glass, CFRM, BFRM, Magnesium alloys, Aluminum alloys, Titanium alloys and Steel. Similar, Fig. 3 shows the mass stopping power of protons in 10 current astronautic structural materials using the whole calculation date. Based on the careful analysis of Fig. 3, we can draw the following general conclusions: 1. By and large, the mass stopping of composites and nonmetal materials are better than metal alloys. 2. For metal alloys, the shielding performance of aluminum alloys is similar to that of magnesium alloys, and their specific shielding capabilities depend on specific alloy compositions. The shielding performance of titanium alloys is lower than that of aluminum and magnesium alloys. The shielding effect of steel is lowest compared to that of other selected metal alloys. 3. For two nonmetal materials, Kevlar is more suitable than glass in radiation shielding. 4. For four fiber-reinforced materials, in a word, fiber-reinforced polymer is more advanced than fiberreinforced metal-matrix composites. Among the 4 reinforced materials, the shielding efficiency of CFRP is the best. 5. The composites with high shielding capability, such as fiber-reinforced polymer, are mainly composed of light elementary materials. Fig. 5. (Color online) The mass range of protons of SCRs in 10 current astronautic structural materials. In order to confirm the correctness of above conclusions, the Mass Range of protons with energy range from 10 MeV to 1 GeV is also calculated by SRIM code, and the results are shown in the form of curves (in Fig. 4 and 5). The mass range is expressed in g cm 2 ; that is, the range in cm multiplied by the density of target materials. Through analyze the Fig. 4 and 5, it was clear that light element materials and advance astronautic composite (such as CFRP and GFRP) have lower mass range values to proton. This means that under the same mass conditions of materials, proton has shorter range in these materials. In another word, these materials are more suitable to shield SCR protons. This

6 Simulation Study on Radiation Shielding Performance of Aerospace Materials Liu Dong et al shows a good agreement with the conclusions based on the analysis of mass stopping power of materials. IV. CONCLUSION Through the analysis of the shielding capability of 18 single-element materials and 10 current astronautic structural materials, the following conclusion can be made. Under the same mass conditions, from a radiation protection point of view, it is advantageous to use the low atomic number materials (such as H, Li, Be, B, C and N) and a fiber-reinforced polymers (such as CFRP and GFRP) as the structural materials to stop and slow down high energy SCR protons. As a result, for the design and manufacture of spacecraft, on the premise of ensuring aerospace requirements, low atomic number materials and fiber-reinforced polymers should be employed as the structural materials as much as possible. ACKNOWLEDGEMENTS This R&D was supported by the NRF grant funded by MEST (Center for Korean J-PARC Users, Grant No. NRF-2013K1A3A7A ). REFERENCES [1] P. Spillantini, Adv. Space Res. 45, 7 (2010). [2] R. K. Tripathi, J. W. Wilson and R. C. Youngquist, Adv. Space Res. 42, 6 (2008). [3] NCRP Report No.153, Information Needed to Make Radiation Protection Recommendations for Space Missions Beyond Low-Earth Orbit, [4] Managing Space Radiation Risk in the New Era of Space Exploration (The National Academic, Washington, D.C., 2008), pp [5] J. F. Ziegler, M. D. Ziegler and J. P. Biersak, Nucl. Instrum. Methods Phys. Res., Sect. B 268, 11 (2010).

Interactions of particles and radiation with matter

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