April 2013

NASA/TP–2013-217983

Radiation Shielding Optimization on Mars

Tony C. Slaba, Christopher J. Mertens, and Steve R. Blattnig

Langley Research Center, Hampton, Virginia
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April 2013

NASA/TP–2013-217983

Radiation Shielding Optimization on Mars

Tony C. Slaba, Christopher J. Mertens, and Steve R. Blattnig

Langley Research Center, Hampton, Virginia
Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076-1320

443-757-5802
Y

Contents

Abstract .......................................................................................................................................................... 1

1. Introduction ................................................................................................................................................ 1

2. Computational Models ............................................................................................................................... 2

3. Results ........................................................................................................................................................ 4

4. Summary and Conclusions ......................................................................................................................... 6

5. References .................................................................................................................................................. 7
YL

Figures 

1. Representative geometry (not to scale) used for the surface calculations. .............................................. 2

2. Cumulative atmospheric thickness for several inclination angles is plotted as a function of density 

(g/cm3

) in the left pane. Total atmospheric thickness (g/cm2) is plotted as a function of inclination 

angle in the right pane. ............................................................................................................................ 3

3. Dose equivalent (left pane) and effective dose (right pane) as a function of shield thickness on the 

Martian surface. ....................................................................................................................................... 5

4. Comparison of surface exposure to 1⁄2 free space exposure as a function of shield thickness. ................ 6
vLL

Tables 

1. Comparison of present calculations to results generated with the Geant4-based application, MEREM 

[McKenna-Lawlor et al. 2012b]. Dose equivalent values are in mSv/year and values in parentheses are 

the relative percent difference between present calculations and MEREM results. ................................ 4
1

Abstract 

Future space missions to Mars will require radiation shielding to be optimized for deep 

space transit and an extended stay on the surface. In deep space, increased shielding 

levels and material optimization will reduce the exposure from most solar particle events 

(SPE) but are less effective at shielding against galactic cosmic rays (GCR). On the 

surface, the shielding provided by the Martian atmosphere greatly reduces the exposure 

from most SPE, and long-term GCR exposure is a primary concern. Previous work has 

shown that in deep space, additional shielding of common materials such as aluminum or 

polyethylene does not significantly reduce the GCR exposure. In this work, it is shown 

that on the Martian surface, almost any amount of aluminum shielding increases 

exposure levels for humans. The increased exposure levels are attributed to neutron 

production in the shield and Martian regolith as well as the electromagnetic cascade 

induced in the Martian atmosphere. This result is significant for optimization of vehicle 

and shield designs intended for the surface of Mars. 

1. Introduction 

Future missions to Mars will include a relatively long duration transit time in deep space as well as 

an extended stay on the surface. Possible mission durations are likely to range from 500-1000 days with 

surface stays ranging from 30-500 days [NASA 2009]. The transit will include continuous exposure from 

Galactic Cosmic Rays (GCR) as well as the possibility for one or more Solar Particle Events (SPE). 

Vehicle design and material optimization in deep space radiation environments has been studied 

extensively [Cucinotta et al. 2012, Wilson et al. 1997, 2004], and it has been found that exposure from SPE 

can be effectively mitigated with appropriate shielding materials and thicknesses. However, exposure from 

GCR is only moderately reduced by shield optimization, and exposure levels can actually be slightly 

amplified within a vehicle due to neutron build-up if shielding thicknesses exceed 50 g/cm2

 for certain 

materials such as aluminum. 

 The radiation environment on the Martian surface is characteristically different than in deep space. 

SPE are greatly attenuated as they pass through the atmosphere, and the exposure level on the surface is 

reduced by approximately an order of magnitude. The exposure from GCR is somewhat reduced in passing 

through the atmosphere; however, interactions between incoming ions and the atmospheric constituents 

result in pion production and an electromagnetic cascade that contributes to exposure on the surface. There 

are also nuclear collisions between GCR ions and the Martian soil creating an ambient field of neutrons that 

contribute to the total exposure. Other secondary reaction products are also present on the surface, but their 

contribution to total exposure and biological risk is small [Dartnell et al. 2007a]. 

 For radiation shield optimization on the Martian surface, exposure levels need to be provided for a 

range of shielding thicknesses and materials and should include albedo neutrons as well as the pionelectromagnetic (π/EM) cascade induced in the atmosphere. Exposure levels and mitigation strategies for 

the Martian surface have been examined in the past [Cucinotta et al. 2010, Saganti et al. 2004, Zeitlin et al. 

2004, Kim et al. 1998, Simonsen and Nealy 1991], and most of these studies utilized the radiation transport 

code, HZETRN (High charge (Z) and Energy TRaNsport), within the straight-ahead approximation 

[Wilson et al. 1991, 2006]. In this radiation transport model, all particles are assumed to propagate along a 

common axis, and the contribution from backscattered neutrons is not included. The contribution from 

π/EM interactions is also not included. Clowdsley et al. [2000] did include a model for backward moving 

neutrons into HZETRN and estimated the neutron energy spectrum on the Martian surface; however, no 

detailed shielding studies were included. Monte Carlo analyses have also been performed [McKennaLawlor et al. 2012a, 2012b; Borggrafe et al. 2009; Dartnell et al. 2007a, 2007b; Morthekai et al. 2007], but 

results have only been provided for no shielding or very specific shielding configurations. 

 Recent extensions to HZETRN have included an improved bi-directional neutron transport model 

that is fully coupled to light ion (Z < 2) target fragment production [Slaba et al. 2010a, 2011]. The model 

has been compared to three dimensional Monte Carlo codes for both slab geometries and lunar surface 

calculations and found to agree with the Monte Carlo codes to the extent they agree with each other. The 

π/EM transport model originally developed by Blattnig et al. [2004] was also recently integrated into 
2

HZETRN [Norman et al. 2012, 2013] and compared to Monte Carlo codes [Slaba et al. 2013]. While 

improvements to the model are necessary, the initial comparisons clearly showed the importance of π/EM 

interactions in GCR transport calculations, and reasonable agreement was found between the codes at large 

shielding thicknesses. For transport through the Martian atmosphere, vertical atmospheric shielding ranges 

from 16-22 g/cm2

 [Simonsen and Nealy 1991], but off-axis rays closer to the horizon easily exceed 100 

g/cm2

. 

 In this work, dose equivalent and effective dose results are presented as a function of aluminum 

and polyethylene spherical shielding thickness on the Martian surface. The contributions from backscattered neutrons and the π/EM cascade are explicitly separated. It is shown that surface exposures 

increase with almost any amount of aluminum shielding. These findings are significant for vehicle design 

and material optimization on the Martian surface. This paper is organized as follows. In section 2, the 

computational tools used to compute exposure on the Martian surface are described. Results and discussion 

are given in section 3. Summary and conclusions are given in section 4. 

2. Computational Models 

Martian surface exposure 

 In Figure 1, a representative geometry that was used for the surface calculations is shown. In order 

to calculate dose equivalent or effective dose at the target point, the particle fluxes are needed. The fluxes 

are computed by using a ray-by-ray transport methodology with HZETRN including the π/EM and bidirectional neutron transport (HZETRN- π/EM) [Wilson et al. 1991; Slaba et al. 2010a, 2010b; Norman et 

al. 2013]. In this approach, a large number of rays are extended in all directions from the target point 

through the shield material and through either the atmosphere or regolith, depending on the ray-direction. 

The atmospheric thickness and density profile along any slant angle is determined from a vertical density 

profile through trigonometric relationships [Simonsen et al. 1990]. The vertical density profile provides 

atmospheric density as a function of elevation above the surface. The profile used in this study will be 

discussed later in this section. The regolith thickness traversed by any ray extending down into the surface 

is set to 300 g/cm2

. This approximates Mars as a large hemisphere under the spherical shield. Sensitivity 

studies were performed to ensure that 300 g/cm2

 is sufficiently thick to ensure equilibrium in the backscattered neutron field. The atmospheric composition from De Angelis et al. [2004] and regolith 

composition from McKenna-Lawlor et al. [2012a] are used in the present work. The atmospheric 

composition is mainly CO2 with a trace amount of argon. The regolith composition is a global average 

containing 51.2% SiO2, 9.3% Fe2O3, and 7.4% H2O with the remaining 32.1% comprised mainly of 

elements between oxygen and calcium. It should be noted that the small amount of water included in the 

composition corresponds to a hydrogen mass fraction of < 1%. The majority of the regolith composition is 

given by 47% oxygen and 24% silicon. 

 

Figure 1. Representative geometry used for the surface calculations. 

 
3

 Radiation transport is performed separately on each ray that enters the top of the atmosphere, 

passes through the vehicle, and propagates into the surface. These rays will be referred to as "downward" 

pointing rays. The exposure along all other rays (i.e. rays entering from the regolith beneath the vehicle and 

extending into the atmosphere) is assumed to be zero, thereby accounting for the 2π blockage provided by 

the surface. The total flux at the target point is obtained by integrating the fluxes computed for each ray. 

 In order to compute effective dose, a human phantom model is placed in the sphere, and 1208 

target points are distributed throughout the body in radiosensitive tissues [Slaba et al. 2010c]. Transport is 

performed along downward rays for each human target point separately. In this work, 968 rays were used 

for all calculations. It should be noted that whether or not the human is included, the ray-by-ray transport 

approach imposes no restriction on the number or order of materials along each ray. This is in contrast to 

interpolation-based methods using HZETRN that have been generally restricted to at most 3 or 4 layers and 

require strict ordering of materials. In this work, dose equivalent is calculated using the ICRP 60 quality 

factor [ICRP 1990], and for effective dose, the FAX (Female Adult voXel) human phantom [Kramer et al. 

2004] is used with the ICRP 103 tissue weights [ICRP 2007]. Unless specified otherwise, the free space 

GCR environment is described by the Badhwar-O'Neill 2010 (BON2010) model [O'Neill 2010] using a 

solar modulation parameter of 475 MV to represent solar minimum conditions. 

Atmospheric density profile 

 The vertical density profile used in this work to derive slant angle atmospheric thicknesses and 

density profiles is computed with the Mars Climate Database (MCD) model [Millour et al. 2008]. The 

MCD model is a meteorological database derived from numerical simulations and validated against 

available measurements. As inputs into the MCD model, the approximate landing time and location of the 

Curiosity rover were used. Curiosity landed on August 6, 2012 corresponding to a Martian solar longitude 

of Ls = 150.6°, and the coordinates of the landing site in Gale Crater were set to -4.6° N, 137.4° E. In the 

right pane of Figure 2, total atmospheric thickness (g/cm2

) is plotted as a function of inclination angle. For 

inclination angles greater than ~45°, the cumulative atmospheric thickness ranges from 20-30 g/cm2. 

However, it is clear that lower inclination rays have much larger thicknesses that can exceed 100 g/cm2

. 

 In the left pane of Figure 2, the cumulative atmospheric thickness is plotted as a function of 

atmospheric density. The density changes by over 4 orders of magnitude between the top of the atmosphere 

and the surface. Previous versions of HZETRN do not include decay processes, and only the total thickness 

along each ray was required for transport procedures. However, the charged pion and muon transport and 

EM cascade depends on the density of the medium. Consequently, the variation of density with 

atmospheric thickness along each ray is included in the transport procedure so that decay lengths are 

properly computed. 

Figure 2. Cumulative atmospheric thickness for several inclination angles is plotted as a function of density 

(g/cm3

) in the left pane. Total atmospheric thickness (g/cm2) is plotted as a function of inclination angle in 

the right pane. 
4

3. Results 

Comparison to Monte Carlo 

 In this section, the computational models described in section 2 are compared to published Monte 

Carlo results. McKenna-Lawlor et al. [2012b] used the Geant4 application, MEREM (Mars Energetic 

Radiation Environment Model), to compute the unshielded dose equivalent rate during solar minimum and 

solar maximum conditions on the Martian surface near the Viking 1 landing site. The atmospheric profile 

was retrieved from measured data but was not provided in the paper. The regolith composition was 

provided in a related paper [McKenna-Lawlor et al. 2012a]. The GCR environment was computed with the 

ISO (International Organization for Standardization) model based on the work of Nymmik et al. [1996]. 

December 2006 was used for the solar minimum period, and April 2002 was used for the solar maximum 

period. 

 In the current work, the atmospheric profile was computed with the MCD model described in 

section 2 using the Viking 1 landing site coordinates (Ls = 97°, 22.48° N, -49.97° E). The regolith 

composition provided by [McKenna-Lawlor et al. 2012a] was used directly. A plot of the GCR particle 

spectra was not provided in the paper; therefore, in the present work, results are generated with three 

different GCR models and compared. The BON2010 model, the model of Matthia et al. [2013], and the 

ISO model evaluated through the SPENVIS (SPace ENVironment Information System) [SPENVIS] 

website have been used. The BON2010 has been commonly used in space radiation analysis, and the 

Matthia model is derived from the ISO model. 

 Results are given in Table 1. The current computational model is within 20% of the detailed 

Monte Carlo calculation. Differences can be attributed to GCR and transport model differences and to a 

lesser extent, differences in the atmospheric density profile. 

Table 1. Comparison of present calculations to results generated with the Geant4-based application, 

MEREM [McKenna-Lawlor et al. 2012b]. Dose equivalent values are in mSv/year and values in 

parentheses are the relative percent difference between present calculations and MEREM results. 

 MEREM Present 

ISO 

Present 

Matthia 

Present 

BON2010 

Solar minimum 273.8 252.5 (-8%) 245.8 (-10%) 224.5 (-18%) 

Solar maximum 156.4 183.0 (17%) 165.9 (6%) 155.3 (-1%) 

Shielding Analysis 

 In Figure 3, dose equivalent (left pane) and effective dose (right pane) are plotted as a function of 

aluminum and polyethylene shield thickness. The total dose equivalent and effective dose values exhibit 

similar, but not identical, trends for aluminum and polyethylene. In particular, the dose equivalent values 

monotonically increase with aluminum thickness by 63% from 0 g/cm2

 to 100 g/cm2. The effective dose 

values also initially increase with aluminum thickness but reach a maximum value around 60 g/cm2

. The 

total variation in effective dose values as a function of aluminum thickness is only 3%. For polyethylene, 

dose equivalent values are reduced by more than a factor of 2, and effective dose values are reduced by as 

much as 35%. For both materials, the variation in effective dose versus depth is smaller than the variation 

in dose equivalent versus depth. This is a result of secondary particle attenuation within tissue, particularly 

for neutrons. The effect is less pronounced in polyethylene because it moderates neutron production more 

effectively than aluminum. 

 There is a clear difference in the albedo neutron contribution to dose equivalent between 

aluminum and polyethylene. The decrease in albedo neutron contribution with polyethylene depth is a 

result of elastic collisions with hydrogen nuclei. The increase in albedo neutron contribution with 

aluminum depth is a result of inelastic collisions between neutrons and ions with target nuclei. The 

difference between the two materials is greatly reduced if effective dose values are considered. This is a 

result of elastic collisions with hydrogen nuclei occurring within tissue. The contribution from π/EM 
5

interactions appears to be similar for both materials, with consistently smaller values observed in 

polyethylene. The reduction in polyethylene is attributed to reduced neutron production and neutron 

attenuation associated with elastic collisions as already noted. The difference between the two materials is 

at most 31% for dose equivalent and 13% for effective dose. It is also important to notice that for effective 

dose, the π/EM contribution is actually greater than the albedo neutron contribution for both materials 

across all depths. 

Figure 3. Dose equivalent (left pane) and effective dose (right pane) as a function of shield thickness on the 

Martian surface. 

 In Figure 4, the total dose equivalent and effective dose values computed on the surface are 

compared to half of the free space results. The free space results are computed using the ray-by-ray 

methodology described in section 2 without the presence of the atmosphere or Martian regolith. The factor 

of 1⁄2 is applied to the end result to approximately account for planetary blockage. The increase in free space 

and surface dose equivalent values beyond 45 g/cm2

 of aluminum does not continue indefinitely. The 

curves reach a maximum value just beyond 100 g/cm2

, and decline monotonically with increasing shield 

thickness. The exact location of the peak value within the Martian subsurface has been examined with 

Monte Carlo simulations for dose [Dartnell et al. 2007b, Morthekai et al. 2007], but not dose equivalent or 

effective dose. Nonetheless, in order to obtain any meaningful reduction in exposure, one would need to 

use shielding thicknesses much larger than where the peak value occurs, corresponding to several hundred 

g/cm2

 of shielding mass. 

 The comparison in Figure 4 is given to clarify that shielding optimization strategies will be 

different for deep space transit and surface operations. In free space, the exposure is reduced by about 45-

65% for dose equivalent and 25-35% for effective dose over the first 20 g/cm2

 of shielding. In the case of 

aluminum, a clear minimum occurs in the dose equivalent-depth curve and a less pronounced minimum 

occurs in the effective dose-depth curve at about 45 g/cm2

. The variation of effective dose near the 

minimum value is within the transport code uncertainty as determined by comparisons with Monte Carlo 

simulations [Slaba et al. 2013]. It is therefore possible that the effective dose minimum is an artifact of 

transport code error, and the curve should actually continue to gradually decrease with depth. From a 

design standpoint, whether it is actually a minimum is less important than the fact there is no significant 

benefit from shielding after 45 g/cm2

 if the composition is similar to aluminum. This suggests a shielding 

strategy that would take advantage of the sudden drop due to thin amounts of shielding (< 20 g/cm2) by 

distributing mass isotropically around the vehicle, as one might expect given the non-radiation related 

design constraints. After this initial stage of optimization, additional parasitic shielding mass could be 

added in specific regions to mitigate directional exposures. 
6

 

Figure 4. Comparison of surface exposure to 1⁄2 free space exposure as a function of shield thickness. 

 The surface results show that any reasonable amount of aluminum does not have a noticeable 

impact on effective dose, and consequently, adding or rearranging shielding mass on the surface will not 

appreciably reduce exposures. An in-situ shielding strategy will also be of little help unless several hundred 

g/cm2

 of regolith is utilized. Such a strategy would probably require large scale excavation making it an 

unlikely candidate. Instead, the shielding strategy would rely primarily on material optimization. Options, 

such as replacing aluminum structures with high hydrogen content carbon composites, could be pursued. 

4. Summary and Conclusions 

 In this work, a computational model was described that allows exposures to be computed on the 

surface of Mars. The model utilizes the HZETRN-π/EM transport code in a ray-by-ray approach that 

includes the atmospheric cascade and secondary albedo neutrons. The model was found to be within 20% 

of a published Monte Carlo result; more comparisons are needed for further verification. Dose equivalent 

and effective dose values were computed as a function of aluminum and polyethylene shield thickness 

during solar minimum for surface and free space conditions. It was found that any reasonable amount of 

aluminum thickness increases dose equivalent and effective dose values on the surface. In the case of dose 

equivalent on the surface, the increase was quite dramatic (63%); in the case of effective dose on the 

surface, exposures varied by less than 3%. Both exposure quantities decrease monotonically with 

increasing polyethylene thickness. The marked difference between the materials is attributed mainly to the 

hydrogen content in the polyethylene. 

 The results shown in this work may be significant for vehicle optimization studies related to future 

Mars missions. In particular, the optimization strategy implemented for deep space transit will be able to 

take advantage of a reduction in exposure over the first 20-40 g/cm2

 of shielding. Though the shielding 

effectiveness of polyethylene is better than aluminum, both materials provide some exposure reduction 

over thicknesses typically found in transportation vehicles. A noticeable minimum in the exposure-depth 

curve near 45 g/cm2

 also helps constrain the optimization phase space. As discussed in section 3, whether 

the minimum is an artifact of transport code error or not, the response curve beyond 45 g/cm2

 is not likely 

to decline rapidly enough to justify the mass incurred with such large shielding thicknesses. Conversely, 

optimization strategies on the surface will not be able to take advantage of any exposure reduction from 

aluminum shielding, and in-situ shielding strategies will require prohibitively large shielding thicknesses 

(greater than several hundred g/cm2

) to provide any meaningful reduction. Material optimization strategies 

utilizing hydrogenated carbon composites could be pursued instead. 
7

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Research, Volume 33, pp. 2204-2210 (2004). 

 
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Technical Publication

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Radiation Shielding Optimization on Mars 

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Slaba, Tony C.; Mertens, Christopher J.; Blattnig, Steve R.

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NASA Langley Research Center

Hampton, VA 23681-2199

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National Aeronautics and Space Administration

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NASA

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14. ABSTRACT

Future space missions to Mars will require radiation shielding to be optimized for deep space transit and an extended stay on 

the surface. In deep space, increased shielding levels and material optimization will reduce the exposure from most solar 

particle events (SPE) but are less effective at shielding against galactic cosmic rays (GCR). On the surface, the shielding 

provided by the Martian atmosphere greatly reduces the exposure from most SPE, and long-term GCR exposure is a primary 

concern. Previous work has shown that in deep space, additional shielding of common materials such as aluminum or 

polyethylene does not significantly reduce the GCR exposure. In this work, it is shown that on the Martian surface, almost any 

amount of aluminum shielding increases exposure levels for humans. The increased exposure levels are attributed to neutron 

production in the shield and Martian regolith as well as the electromagnetic cascade induced in the Martian atmosphere. This 

result is significant for optimization of vehicle and shield designs intended for the surface of Mars. 

15. SUBJECT TERMS

Deep space; HZETRN; Mars radiation; Risk; Space radiation

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