Comparative analysis of the effectiveness of natural polymers and conventional space radiation shielding polymers in spacecraft for prolonged space expeditions

This paper investigates the shielding effectiveness of natural polymers, such as natural rubber and cotton, against space radiation. The results are compared with those of conventional shielding materials, such as polyethylene, Kevlar, and polycarbonate. Monte Carlo simulations were performed using a Geant4-based tool, Multi-layered shielding simulation software (MULASSIS). The shielding properties were studied using proton, alpha, and iron ions with energies of 1 GeV n−1. Online Tool for the Assessment of Radiation in Space (OLTARIS) is used for calculating the effective dose equivalent for the GCR spectra. Both studies showed that the natural polymers are just as effective as conventional space radiation shielding materials in terms of dose reduction. Natural rubber is found to be the most effective among the natural polymers. For 50 g cm−2 aluminum with 20 g cm−2 layer of chosen materials configuration, the effective dose equivalent values (mSv/day) for Polyethylene, Kevlar, Polycarbonate, Kapton, Epoxy, Dacron, and Vectran were 0.93, 1.08, 0.995, 1.056, 1.007, 1.031, and 1.042, respectively. The effective dose equivalent values (mSv/day) for natural polymers (natural rubber, cotton, jute, and silk) under the same conditions were 0.95, 1.004, 1.036, and 1.004, respectively. The challenges of utilizing radiation shields made of natural polymers are also briefly covered.


Introduction
Astronauts in long term space missions will experience a variety of health risks as a result of persistent exposure to cosmic radiation, including acute radiation syndrome, cancer, damage to the central nervous system, and others [1,2].Cosmic radiation predominantly consists of charged particles with energy levels within the GeV/n spectrum, originating from a variety of sources dispersed across the cosmos.Beyond the protective shield of Earth's magnetic field, astronauts in space are exposed to radiation primarily deriving from Solar Particle Events (SPE) and Galactic Cosmic Rays (GCR).SPE and GCR differ significantly in their energy ranges [3,4].Coronal Mass Ejections (CMEs) eject highly intense bursts of protons with energy levels ranging from 1 to 100 MeV n −1 , along with small quantities of alpha particles, into the space between planets, thereby initiating Solar Particle Events (SPEs).While SPEs have a connection to solar activity, their occurrence remains inherently uncertain [3,5].SPEs occur infrequently, and their potential harm to a spacecraft depends on its location within the zone of solar-interplanetary magnetic field lines, where charged particles from SPEs are constrained to spread.The danger to astronauts is essentially probabilistic.However, during intense SPEs, astronauts can face significant radiation exposure [3,6,7].The persistent radiation dose imparted by ionizing particles, notably characterized by energies on the order of 1 GeV n −1 , particularly those constituted by highly ionizing, high atomic number (Z) ions, is denominated as Galactic Cosmic Rays (GCR).Regardless of the type of particle, the highest flux of GCR particles typically falls within the energy range of 100 MeV n −1 to 10 GeV n −1 [6,8,9] with a median energy of about 1 GeV n −1 [1].GCRs are composed of a variety of particles, including protons, alpha particles and heavy nuclei.The relative abundance of these particles varies with energy, but protons are the most abundant (89%) GCR particles at all energies.GCRs are thought to be produced by supernovae and other energetic events in space [10].Heavy nuclei, such as Carbon or Iron, make up a small fraction (1%) of GCRs, but they can be very energetic.The interaction of GCR with the spacecraft initiates secondary particle production which produces a greater variety of particles that form the radiation environment inside the spacecraft than outside.Neutrons, produced from this process are among the most concerning from a radiation protection perspective [11].The Earth's magnetic field deflects solar particle events (SPEs) and galactic cosmic rays (GCRs) before they reach the surface, except for during extreme coronal mass ejections (CMEs).As a result, feasible countermeasures are required to protect astronauts from high-charge and energetic particles (HZEs) on deep space expeditions far from Earth [12].
One potential approach for mitigating space radiation exposure involves passive shielding, which involves the utilization of materials with the capacity to attenuate particles characterized by high atomic number and energy (HZE).Within these shielding materials, HZE particles dissipate their energy through electronic and nuclear interactions, and they undergo nuclear fragmentation, leading to the formation of lighter nuclei and nucleons.Despite the inherent challenges associated with handling and stability, hydrogen is regarded as the most efficacious element in terms of shielding effectiveness per unit mass density [6,13].Aluminum (Al) Polyethylene (PE), Kevlar, Nextel, Nomex, Lucite have demonstrated shielding capabilities in accelerator beam experiments [7,[14][15][16][17][18][19][20][21][22][23][24][25].Studies have been conducted on the International Space Station (ISS) to investigate the reduction of dose by employing water blocks, polyethylene (PE) blocks, and packages made from water soaked sanitary towels [26][27][28].Hydrides with low density, like Ammonia borane, provide greater reduction in radiation dose compared to polyethylene [17,18,[29][30][31].Kevlar and polyethylene exhibit similar shielding effectiveness [32,33].Lunar soil can protect against space radiation [34].The Martian regolith is also a prospective material for addressing the radiation challenge on the surface of Mars [35][36][37][38][39][40].For prolonged space missions Bond et al has presented the whole body effective dose equivalent results for 59 common aerospace materials [41,42].Materials like Dacron, Polycarbonate and Vectran are used in making inflatable habitat and spacesuits [43].Kapton is another material with low thermal conductivity and radiation shielding capability that has been used in James Webb Space Telescope as the sunshield.While materials such as concrete [44][45][46], alloys [47][48][49][50][51], nanomaterials [52] and glass [53][54][55] have demonstrated gamma and neutron radiation shielding properties, they have not yet been tested for understanding their efficacy in space radiation environments.
Like the conventional space radiation shielding materials, natural polymers like Jute, cotton, natural rubber are lightweight, hydrogenous and flexible.The natural polymers have lower thermal conductivity coefficient.Natural fibers have varying tensile strengths, with some having lower values than synthetic polymers.However, some natural fibers, such as silk, have tensile strengths that are comparable to or even greater than some synthetic polymers.Natural polymers are hydrogenous materials that have the potential to shield against radiation.However, their radiation shielding capabilities have not been widely tested for space radiation shielding.Typically, gamma radiation has been used in order to test the shielding capability of natural polymers.Cotton fabric coated with different materials such as barite (barium sulphate) [56,57], raw jute or composites made with jute [58][59][60], natural rubber with carbon nanotube additions [61] possess the ability to shield gamma radiation.Rare-earth-doped rubber can attenuate cosmic rays by up to 10% compared with conventional aluminum shielding [62].
The objective of the paper to examine the space radiation shielding effectiveness (in terms of dose reduction) of natural polymers and compare to the materials that are already utilized for space radiation shielding.Both the stochastic and deterministic approach were utilized for this study.The necessary parameters, chosen materials, their properties and the simulation setup is discussed in Methods and Materials section (section 2).The results of the study, including absorbed dose and dose equivalent values, are presented in section 3 and the remaining section (section 4) discusses the conclusion of this study.

Absorbed dose, and effective dose equivalent
The absorbed dose (D) refers to the amount of radiation energy (E) deposited in per unit of tissue mass.In other words it represents the concentration of radiation energy absorbed at a particular point within the tissue and it is represented by the following formula Effective dose equivalent (ED) is estimated by first determining the averaged dose equivalent for each organ and tissue [8,63].The weighting factors specified in NCRP-132 are then used to calculate a weighted average of these organ or tissue dose equivalent values using equation (2).
( ) Where W T represents NCRP-132 tissue weighting factors and H T signifies the organ or tissue averaged dose equivalent.The process of determining the organ or tissue-averaged dose equivalent involves initially computing the dose equivalent at a sufficient number of specific target sites within the organ or tissue to provide a precise characterization of that specific organ or tissue.Subsequently, these individual values are combined through averaging.This is accomplished by using anatomical models such as Computerized Anatomical Man (CAM) [64], Female adult voxel (FAX) [65,66], among others.On-Line Tool for the Assessment of Radiation in Space (OLTARIS, Version 5.0) is a space radiation analysis tool [66] based on High Charge and Energy Transport (HZETRN) [9].OLTARIS can be used to analyze the effects of space radiation on both humans and electronic systems across a range of spacecraft and mission situations.In this study, it was employed to determine the effective dose equivalent for an average-sized male human using the Male Adult Voxel model (MAX) [67].The MAX phantom is a computational model developed to faithfully replicate the anatomical characteristics of a standard adult male as defined by the International Commission on Radiological Protection (ICRP).This model comprises segmented images of the head, upper body, arms, and lower limbs, encompassing approximately 40 organs and tissues in the trunk, arms, and legs, and roughly 56 organs and tissues in the head region.The torso portion contains organs such as the lungs, liver, spleen, kidneys, pancreas, stomach, colon, small intestine, and other internal parts.The head section comprises components like the brain, eyes, ears, nose, mouth, and other anatomical elements.MAX comprises a skin dosimetric model, a model for assessing skeletal radiation exposure, and a computational exposure model.

Materials
The properties of the materials are given in table 1.In this study, we have selected four natural polymers that are hydrogenous and flexible, namely natural rubber, cotton, jute, and silk.To compare their radiation shielding capability with commonly used materials for space radiation shielding, we have chosen Polyethylene (PE), Kevlar (poly-para-phenylene terephthalamide), Polycarbonate, Kapton (poly (4,4'-oxydiphenylenepyromellitimide)), Epoxy (polyepoxides), Dacron (Polyethylene terephthalate), Vectran (Liquid crystal polymer).In section I, it is briefly discussed how these materials are utilized.There is even more hydrogen mass fraction in natural rubber, jute, silk and cotton than in some of the polymers, such as Kevlar and Vectran, which are already being tested for shielding against space radiation.

Simulation setup
To assess the effectiveness of the materials in reducing radiation dose, both stochastic and deterministic approaches were used.Slab and sphere geometries were used to see how changes to the setup affected the results.These geometries allowed for a complete understanding of how the materials performed under different conditions and configurations.We conducted Monte Carlo simulations using Multi-Layered Shielding Simulation Software (MULASSIS, version 1.23) [68,69], a GEANT4 [70] based tool for dose and particle fluence analysis associated with the use of radiation shields.It's incorporated into the Space Environment Information System (SPENVIS) package, and MULASSIS demonstrates strong performance in assessing the radiation shielding efficiency of both homogeneous and composite materials [71].We set a slab of 50 g cm −2 thickness of Aluminum followed by a layer of selected materials of variable thicknesses (5-20 g cm −2 ), and a 5 g cm −2 of soft tissue region.Figure 1 is representing the simulation setup in MULASSIS.In figure 1(a), the cyan region (1) designates the Aluminum (50 g cm −2 ).The adjacent red area (2) specifies another layer where we have applied the chosen materials with varying thicknesses ranging from 5 to 20 g cm −2 .The remaining portion (Grey) indicates the soft tissue region (4) of 5 g cm −2 thickness.Monoenergetic particle option (parallel beams) was chosen for the proton, alpha, and iron particles with energy 1 GeV n −1 .Additionally, the values of the absorbed dose without the additional layer after the aluminum were recorded with the same particles with same energies to examine the effectiveness of the materials selected, shown in figure 1(b).In this study, in OLTARIS, we employed the Sphere Geometry option to create the necessary configuration.The Badhwar' O'Neil 2020 [10] GCR Model (within 1 Astronomical Unit) for the GCR spectrum with 20 d mission duration for the event 2010 solar minimum was utilized.The effective dose equivalent values were determined for a Male adult voxel (MAX) phantom (with 'Never Smoker Population' weighted tissue).In      3(a) shows for 1 GeV n −1 proton.Figure 3(b) shows for 1 GeV n −1 alpha.Figure 3(c) shows for 1 GeV n −1 iron.

Results
The results are shown in figures 2-4.In both the slab and sphere geometries, similar findings regarding dose reduction have been observed.Figures 2 and 3 are obtained from the Monte Carlo simulation in MULASSIS/ Geant4.Figure 2(a)-(c) is showing the results for the absorbed dose from MULASSIS/Geant4 for proton, alpha, and Fe ions with energies of 1 GeV/n.With the increase of thickness the absorbed dose reduces and maximum reduction is observed at 50 g cm −2 Aluminum with 20 g cm −2 of the chosen materials.For convenience, for showing the variation of absorbed dose with thickness, the solid lines are for conventional space radiation shielding materials and the dotted lines are for natural polymers (in figures 2(a), (b) and (c)).The effectiveness of natural polymers in reducing radiation dose is comparable to that of conventional materials used for space radiation shielding.
Figure 3 is showing the normalized absorbed dose for the 50 g cm −2 Aluminum and 20 g cm −2 of the chosen materials configurations.These figures (figure 3(a, b, c)) demonstrate the effectiveness of utilizing the additional shielding materials of 20 g cm −2 .The effectiveness is relatively lower for proton of 1 GeV n −1 but for alpha and Iron ion of 1 GeV n −1 , the effectiveness is comparatively greater.In figure 3(a), for proton of 1 GeV n −1 .20 g cm −2 Polyethylene with 50 g cm −2 outperforms all the materials in terms of dose reduction.The addition of a 20 g cm −2 polyethylene layer to a 50 g cm −2 aluminum (Al) shield reduces the absorbed dose by approximately 4%, as compared to a Aluminum shield without the polyethylene layer (figure 3(a)).For Kevlar, polycarbonate, Kapton, Epoxy, Dacron and Vectran layer the dose reductions are 2.36%, 1.87%, 2.41%, 0.62%, 2.04% and 1.40% respectively.In addition to conventional materials, natural polymers also exhibit comparable dose reduction percentages.A 20 g cm −2 layer of natural rubber, cotton, jute, or silk, when combined with a 50 g cm −2 aluminum shield, reduces the absorbed dose by 2.10%, 2.86%, 2.30%, and 2.14%, respectively, when compared to a 50 g cm −2 aluminum shield without any additional layers.Interestingly, for 1 GeV n −1 alpha and Iron ion the effectiveness of Natural rubber is higher than all other materials.Other natural polymers' effectiveness is comparable to or even superior than that of conventional materials.For 1 GeV n −1 Alpha dose  reduction for polyethylene, Kevlar, polycarbonate, Kapton, Epoxy, Dacron and Vectran are 59.9%, 58.10%, 58.9%, 57.76%, 49.08%, 58.8%, 59.42% (compared to 50 g cm −2 Aluminum without the 20 g cm −2 of the chosen material figure 3(b)) where for natural polymers the dose reduction are 62.2%, 58.96%, 56.99% and 59.75 for natural rubber, cotton, jute and silk respectively.For 1 GeV n −1 Iron ion dose reduction for polyethylene, Kevlar, polycarbonate, Kapton, Epoxy, Dacron and Vectran are 68.41%,59.10%, 60.9%, 54.89%, 58.33%, 58.16%, 54.58% (compared to 50 g cm −2 Aluminum without the 20 g cm −2 of the chosen material figure 3(c)) where for natural polymers the dose reduction are 70.6%,60.98%, 54.80% and 58.49 for natural rubber, cotton, jute and silk respectively.
Figures 4(a), (b) shows the effective dose equivalent results from the OLTARIS study using GCR spectra within 1 Astronomical Unit (Badhwar-O'neil GCR Model, Event: 2010 solar minimum, 20 d mission duration with MAX phantom).The results are comparable to the Monte-Carlo simulation with MULASSIS/Geant4, with the maximum dose reduction occurring at 50 g cm −2 Aluminum with 20 gm cm −2 of selected materials.In terms of dosage reduction, Polyethylene outperformed all other materials.Natural rubber is next to it.The remaining natural polymers provide a similar dose reduction (denoted by dotted lines in figure 4(a)) to conventional space radiation shielding materials (denoted by solid lines in figure 4(a)).For 20 d mission, at 20 g cm −2 layer with 50 g cm −2 Aluminum the effective dose equivalent values (mSv/day) for polyethylene, Kevlar, Polycarbonate, Kapton, Epoxy, Dacron, Vectran are 0.93, 1.08, 0.995, 1.056, 1.007, 1.031, 1.042 respectively.For Natural polymers with same setup the effective dose equivalent values (mSv/day) for natural rubber, cotton, jute and silk are 0.95, 1.004, 1.036 and 1.004 respectively.
For GCR spectra effective dose equivalent reduction for polyethylene, Kevlar, polycarbonate, Kapton, Epoxy, Dacron and Vectran are 12%, 4%, 6.10%, 0.37%, 5%, 2.73%, 1.69% (compared to 50 g cm −2 Aluminum without the 20 g cm −2 of the chosen material figure 4(b)) where for natural polymers the dose reduction are 10.24%, 5.28%, 2.26% and 5.28% for natural rubber, cotton, jute and silk respectively.The flux of proton in GCR spectra is very high compared to the high Z particles like alpha and Iron and others, that's why we see considerable variation in figures 3(b), and (c) with 4(b).Also figure 3(b), (c) shows for absorbed dose for monoenergetic particles and figure 4(b) deals with effective dose equivalent with GCR spectra within 1 astronomical unit.But the nature of the results are pretty similar.

Conclusion
In this study, we investigated the capability of natural polymers to shield against space radiation, and compared their performance to materials that are already used for this purpose.We employed the Monte Carlo simulation with MULASSIS/GEANT4 for calculating absorbed dose and OLTARIS for getting the results of effective dose equivalent (mSv/per day) with a Male Adult VoXel (MAX) phantom.The results are shown in figures 2-4.The Monte Carlo study with mono-energetic (1 GeV n −1 ) proton, alpha and iron shows that with the increase of the layer of chosen materials attached with 50 g cm −2 Aluminum the absorbed dose decreases and maximum reduction takes place at 20 g cm −2 .The layer attached with 50 g cm −2 Aluminum performs better for high Z (Z>1) particles (Alpha, iron ion).The effectiveness of the natural polymers are comparable with the conventional space radiation shielding materials.For conventional space radiation shielding materials, with a 20 g cm −2 additional layer with 50 g cm −2 Aluminum, the average dose reduction for proton, alpha, and iron 1 GeV n −1 are around 2%, 57%, and 59%, respectively, when compared with not using a 20 g cm −2 additional layer with 50 g cm −2 Aluminum layer.As it pertains to natural polymers, with an additional layer of 20 g cm −2 with 50 g cm −2 aluminum, the average absorbed dose reduction for protons, alpha particles and iron particles of 1 GeV n −1 is approximately 2%, 59% and 61% respectively as opposed to not using the additional layer of 20 g cm −2 with 50 g cm −2 aluminum.
The results of OLTARIS with GCR spectra are aligned with the results of Monte Carlo simulations performed.Also here, the natural polymers are found to be similar in shielding capability to conventional space radiation shielding materials.For GCR spectra (within 1 astronomical unit, Event: 2010 solar minimum), the effective dose equivalent values (mSv/day) for a 20-day mission with a 20 g cm −2 layer and 50 g cm −2 aluminum shielding were measured for Polyethylene, Kevlar, Polycarbonate, Kapton, Epoxy, Dacron, and Vectran and the results were 0.93, 1.08, 0.995, 1.056, 1.007, 1.031, and 1.042, respectively.The effective dose equivalent values (mSv/day) for natural polymers (natural rubber, cotton, jute, and silk) under the same conditions were 0.95, 1.004, 1.036, and 1.004, respectively.The effective dose equivalent reduction for conventional space radiation shielding polymers i. e. Polyethylene, Kevlar, Polycarbonate, Kapton, Epoxy, Dacron, and Vectran was 12%, 4%, 6.10%, 0.37%, 5%, 2.73%, and 1.69%, respectively (This was compared to 50 g cm −2 aluminum without the 20 g cm −2 of the chosen material (figure 4(b)).For natural polymers, the rates were 10.24%, 5.28%, 2.26%, and 5.28% for natural rubber, cotton, jute, and silk, respectively.The dose reduction capability of natural rubber is comparable to polyethylene which is regarded as one of the best materials for space radiation shielding.
While natural polymers show promise as space radiation shielding materials, there are still several challenges that need to be addressed before they can be widely used in this application.Natural polymers have not been extensively tested for their radiation shielding properties, so their effectiveness in this application is not well understood.They should be tested in space environment like the ISS.Natural polymers are susceptible to degradation.With time natural polymers will degrade and the process can be accelerated by exposure to radiation.This could lead to the breakdown of the polymer and a decrease in its shielding effectiveness.Cosmic rays will interact with the atoms in natural polymers, causing them to break apart.This could lead to the formation of free radicals, which are unstable molecules that can damage other molecules in the polymer.It is possible that this may lead to the radiation shield setup breaking down.Natural polymers can degrade when exposed to high temperatures, which could occur in the harsh environment of space.This could lead to the destruction of the polymer or make it less effective.These reasons limit their utility for long-duration space missions.Missions to Mars or the Moon, where the mission duration is not relatively longer we may be able to utilize natural polymers.However, missions that extend beyond one year would be more challenging.Doping natural polymers with other materials, such as light metals or carbon nanotubes, may improve their stability and make them more viable for long-duration space missions.Further studies in this arena are required to unravel the true potential of natural polymers in space radiation shielding.

Figure 1 .
Figure 1.Simulation setup for this study.Figures 1(a), (b) shows for MULASSIS/Geant4.The first cyan region (1) specifies the Aluminum layer.The next red region (2) designates the layer made of different chosen materials in this study with varying thicknesses.The remaining portion (3) (Grey) implies the soft tissue region.Figure 1(b) shows the setup without the 5-20 g cm −2 thick layer of the chosen materials.Figure 1(c) shows the setup in OLTARIS.The first grey hollow sphere indicates the Aluminum layer.The yellow sphere followed by the grey sphere specifies the layer of the selected materials and at the center of the setup, the light green portion indicates the Male adult voxel (MAX).

Figure 2 .
Figure 2. Absorbed dose variations for different thickness of the selected materials with 50 g cm −2 Aluminum for proton, alpha, and Fe ions with energies of 1 GeV n −1 .Figure 2(a) shows for proton.Figure 2(b) shows for alpha and figure 2(c) shows for iron ion.The solid lines are for conventional space radiation shielding materials and the dotted lines are for natural polymers.

Figure 2 (
Figure 2. Absorbed dose variations for different thickness of the selected materials with 50 g cm −2 Aluminum for proton, alpha, and Fe ions with energies of 1 GeV n −1 .Figure 2(a) shows for proton.Figure 2(b) shows for alpha and figure 2(c) shows for iron ion.The solid lines are for conventional space radiation shielding materials and the dotted lines are for natural polymers.

figure 1 (
figure 1(b), the initial hollow sphere in gray represents aluminum.Subsequently, the yellow sphere adjacent to the gray one represents the layer of the chosen materials, while at the center of the arrangement, the light green portion represents MAX.

Figure
Figure 3. Absorbed dose reduction scenario at 50 g cm −2 Aluminum with the chosen materials.Figure 3(a) shows for 1 GeV n −1 proton.Figure 3(b) shows for 1 GeV n −1 alpha.Figure 3(c) shows for 1 GeV n −1 iron.

Figure 4 .
Figure 4. Results for OLTARIS study using GCR spectra.Figure 4(a) shows the variation of effective dose equivalent (mSv/per day) with thickness.The solid lines are for conventional space radiation shielding materials and the dotted lines are for natural polymers.Figure 4(b) shows the effective dose equivalent values for 50 g cm −2 Aluminum with 20 g cm −2 of chosen materials in this study.

Figure 4 (
Figure 4. Results for OLTARIS study using GCR spectra.Figure 4(a) shows the variation of effective dose equivalent (mSv/per day) with thickness.The solid lines are for conventional space radiation shielding materials and the dotted lines are for natural polymers.Figure 4(b) shows the effective dose equivalent values for 50 g cm −2 Aluminum with 20 g cm −2 of chosen materials in this study.

Table 1 .
Description of the chosen materials in this study.