James R. Gaier and Stephen Berkebile

Glenn Research Center, Cleveland, Ohio

Edward A. Sechkar and Scott R. Panko

ASRC Aerospace Corporation, Cleveland, Ohio

Preliminary Design of a Galactic Cosmic 

Ray Shielding Materials Testbed for 

the International Space Station

NASA/TM—2012-216055

December 2012
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James R. Gaier and Stephen Berkebile

Glenn Research Center, Cleveland, Ohio

Edward A. Sechkar and Scott R. Panko

ASRC Aerospace Corporation, Cleveland, Ohio

Preliminary Design of a Galactic Cosmic 

Ray Shielding Materials Testbed for 

the International Space Station

NASA/TM—2012-216055

December 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The authors gratefully acknowledge Quang-Viet Nguyen (NASA HQ) who fi rst suggested this study and was a constant source 

of encouragement. K.K. de Groh and D.A. Jaworske (NASA GRC) acted as our interfaces to the MISSE-X project. J.H. Adams 

(U. Alabama, Huntsville), B.A. Banks (Alphaport), A.F. Barghouty (NASA MSFC), J. Wrbanek (NASA GRC) and S. Wrbanek 

(NASA GRC) provided us with valuable feedback and constructive criticism. Funding for S. Berkebile was provided by the NASA 

Advanced Exploration Systems through the NASA Postdoctoral Program, administered by Oak Ridge Associated Universities.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-216055 1

Preliminary Design of a Galactic Cosmic Ray Shielding Materials 

Testbed for the International Space Station

James R. Gaier and Stephen Berkebile*

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Edward A. Sechkar† and Scott R. Panko‡

ASRC Aerospace Corporation

Cleveland, Ohio 44135

Abstract

The preliminary design of a testbed to evaluate the effectiveness of galactic cosmic ray (GCR) 

shielding materials, the MISSE Radiation Shielding Testbed (MRSMAT) is presented. The intent is to 

mount the testbed on the Materials International Space Station Experiment-X (MISSE-X) which is to be 

mounted on the International Space Station (ISS) in 2016. A key feature is the ability to simultaneously 

test nine samples, including standards, which are 5.25 cm thick. This thickness will enable most samples 

to have an areal density greater than 5 g/cm2

. It features a novel and compact GCR telescope which will 

be able to distinguish which cosmic rays have penetrated which shielding material, and will be able to 

evaluate the dose transmitted through the shield. The testbed could play a pivotal role in the development 

and qualification of new cosmic ray shielding technologies.

Introduction

It is widely recognized that galactic cosmic rays (GCRs) pose an exposure risk to astronauts on long 

term missions in deep space. It has even been suggested that this may be an insurmountable hurdle to long 

duration human space missions (Ref. 1). This recognition is reflected in the NASA Office of the Chief 

Technologist (OCT) Roadmap in TA06—Human Health, Life Support and Habitation Systems, 2.5 

Radiation: “The radiation area is focused on developing knowledge and technologies...(among other) 

...to minimize exposures through the use of material shielding systems.“ “The major technical challenge 

for future human exploration is determining the best way to protect humans from the high-charge and 

high-energy galactic cosmic radiation (GCR) permeating interplanetary space.“ “...shielding GCR is 

much more difficult than shielding terrestrial radiation...” (Ref. 2). 

Developing ground tests to evaluate the effectiveness of GCR shielding materials is difficult. There 

are no ground-based facilities available which can subject shields to the full spectrum of GCRs. The 

current evaluation method is to measure the candidate shielding material response to mono-energetic ions 

of a single type in a facility such as the NASA Space Radiation Laboratory (NSRL) at Brookhaven 

National Laboratory (Ref. 3) and use that to verify a model of the shielding material. This model is used 

to explore the putative effectiveness of the shielding in a typical cosmic ray background. Although there 

have been several instruments flown on spacecraft that measure the cosmic ray flux (Refs. 4, 5, and 6)

there are no in-space facilities to verify the effectiveness of cosmic ray shielding materials.

When the Materials International Space Station Experiment-X (MISSE-X) proposal was formulated, 

a conceptual proposal to fly a testbed to measure the effectiveness of GCR shielding materials was 

submitted as a core experiment to populate its initial configuration. This proposal was selected to be 

 *

NASA Postdoctoral Program Fellow

†

Presently at ZIN Technologies, Cleveland, Ohio 44130

‡

Presently at Vantage Partners LLC, Cleveland, Ohio 44135
NASA/TM—2012-216055 2

included in the MISSE-X proposal. After the MISSE-X proposal was accepted by OCT, a more detailed 

version was submitted to OCT for a second approval. Subsequent cost overruns in the MISSE-X program 

caused the MRSMAT to be dropped from the initial MISSE-X configuration. The purpose of this report is 

to document the progress that was made in the design of MRSMAT in the hopes that the experiment will 

be continued at a later date.

The MRSMAT Flight Unit had a 22 month development cycle, concurrent with the MISSE-X 

development cycle. The design plan presented herein is for the Engineering Model that was to test out the 

major systems for the Flight Unit. Since the purpose of the Engineering Model was to mimic the Flight 

Unit with sufficient fidelity to reduce technical risk, it was to be built with the same geometry, detector, 

and electronics as the Flight Unit. But the Engineering Model was to be built to operate in air, so the 

thermal control and the materials and connectors were not space rated. It is anticipated that the Flight Unit 

design will evolve as lessons are learned from the operation of the Engineering Model, but the idea is that 

the flight unit’s salient features will be captured in the Engineering Model.

GCR Environment at ISS

GCRs emanate isotropically from space, but since ISS is in a low orbit (320 to 400 km), the Earth 

blocks out a nadir-facing cone with a half-angle of about 40° (Ref. 7). Shielding from the Earth’s 

magnetic field and atmosphere also attenuates the flux. Table I compares the ion flux expected at ISS 

orbit with that of deep space at 1 AU as predicted by the On-Line Tool for the Assessment of Radiation in 

Space (OLTARIS) radiation modeling program (Ref. 8). The GCR environment at ISS is about 15 

percent that of deep space, with the percentage of heavier ions somewhat higher than that of lighter. Thus,

shielding materials would have to be exposed 6 to 7 times longer on the ISS to acquire GCR exposure

equivalent to deep space.

TABLE I.—COMPARISON OF FLUX OF GCR PARTICLES IN DEEP SPACE AT 1 AU AND AT ISS

Ion Flux, ion/cm2-s H He Li - Ne Na – Fe

Deep space 3.72 0.50 0.026 0.0056

ISS 2010 0.53 0.10 0.0058 0.0013

The bulk of GCRs have energies ranging from 100 to 1000 MeV/nucleon. Although most GCRs are 

protons and alpha particles, the high energy heavy nuclei do so much biological damage per particle that 

they are of as much concern. These heavy ions are in fact more troublesome because they more efficiently 

transfer their energy to cellular material, and can cause cascades of ionizing secondary particles (Ref. 9).

General Parameters of MRSMAT

Since GCRs are so penetrating, any shielding will have to be relatively thick to stop an appreciable 

fraction of them. A requirement of the experiment is that it be able to discriminate between GCR shield 

effectiveness of 1 mGr/day. This will require samples with a thickness of about to 5 g/cm2 to distinguish 

among shielding candidates. Practical considerations have led to the sample size of a 5.25 cm cube.

Table II lists the areal density for 5.25 cm of some candidate materials. The greater thickness also enables 

the testing of more complex GCR shielding structures, including laminar composites.

TABLE II.—AREAL DENSITY OF 5.25 cm OF CANDIDATE MRSMAT MATERIALS

Material Aluminum Polyethylene Composite Br-Composite

Areal density, g/cm2 14.2 4.8 8.9 9.0

Side-by-side testing of candidate materials is considered important for this to be a fair comparison 

among materials. A 3×3 matrix of samples sharing a common detector is proposed. The middle sample of 

the array would be 6061 Al as a reference and one of the samples will be polyethylene, widely considered to 
NASA/TM—2012-216055 3

be the leading radiation shielding material. The remaining samples will be selected from candidates solicited 

from numerous sources. Since this experiment was proposed to the OCT program, novel materials and 

architectures would receive priority. In order to avoid environmental degradation of the shielding materials, 

the sample array would not be exposed directly to the atomic oxygen and ultraviolet of the low Earth orbit 

(LEO) environment, but would be covered by a 0.32 cm layer of 6061 Al. This is also consistent with the 

environments that radiation shields would probably be exposed to on application.

As GCRs impinge on the detector from all directions, it is important to be able to determine which 

particles came through which sample, and avoid counting those particles that passed through a more 

complex shielding environment, such as multiple samples or part of the ISS. This would be accomplished 

by utilizing two sets of scintillator arrays, one above the other. The incidence angle of the particles can be 

determined from the x-y coordinates of the two detectors, and only those particles penetrating a particular 

sample would be counted.

The scintillator arrays would be made up of 15.75 cm long fibers of polystyrene of 3.5 mm square 

cross section. Each of the two arrays would have a set of fibers oriented in the x direction directly above a 

set oriented in the y direction. If the particle events are rare (as they will be for GCRs) there would be 

only one event within the coincident time window, so the x, y position can be determined without 

ambiguity. Thus, only 4n scintillators and photon detection units are required rather than the 2n2 if an 

array with a separate scintillator at each x, y position were used. Between these arrays, a thick Tl-doped 

CsI crystal scintillator (CsI(Tl)) will be used to estimate the linear energy transfer of an incoming particle 

and distinguish whether the incoming particle was a proton, alpha particle, or heavier ion using pulse 

shape discrimination similar to the method described by Guinet (Ref. 10). 

A ray-tracing analysis has been done to optimize the number and size of the pixel elements (= fiber 

cross-section), and the spacing of the two detectors. This analysis was constrained by the maximum total 

thickness of a MISSE-X experiment, and the desire to have the scintillator crystals large enough to 

capture a countable fraction of the GCR particles that penetrate the shielding samples. Additionally, the 

scintillating fibers must be thick enough to produce a measureable light pulse when a minimally ionizing 

particle passes through the detector. Figure 1 shows part of the analysis that lead to the selection of 

15 square pixels, 3.5 mm wide, with a detector thickness of 3.1 cm and a total sample/detector thickness 

of 8.35 cm.

Figure 1.—Acceptance angle as a function of detector thickness for different numbers of fibers/pixels (left), and the 

dimensions of the optimum design corresponding to the circled point on left (right).
NASA/TM—2012-216055 4

Figure 2.—Preliminary design of the MRSMAT Flight Unit.

The geometry of the samples, the constraints of the Modular Experiment Container (MEC) of 

MISSE-X, and the detector and electronic requirements feed into the preliminary design of the MRSMAT 

Flight Unit (Figure 2). Details of the mechanical coupling with the MEC, the electrical interface with the 

Data Acquisition Unit (DAU), and the thermal control structures are unknown at this time, and so not 

included in Figure 2.

The detector recovery time required to ensure a low probability (P) of multiple events being counted 

as a single event can be calculated using Poisson statistics (Eq. (1)).

r!

e P

r

r

−μ μ = (1)

where r is the number of events, and μ is the product of the probability density over time and the time.

Such an analysis for a 15.75 cm square plate, with the probability density defined using the values in 

Table I, yields that with a recovery time of 100 μs only 1 in 9300 measurements will have the results of 

two particles in it, and one in 1.9 million will have the results of three particles in it. Thus, if there are 

multiple signals in a measurement, 99.3 percent of the time it will be due to a cascade of secondary 

particles from a single incident particle. So if the recovery time is 100 μs or shorter, cascades of 

secondary particles will be distinguishable from multiple particles. Because of other considerations 

detailed below, our sampling times would actually be on the order of 10–2

μs.

GCRs are not the only high energy particles that would impinge on the MRSMAT surface. The solar 

wind particles have a flux that is 1012 times as high as the GCR flux, but the particle energies are 10–5 to 

10–6 that of GCRs. It is important that the solar wind particles be blocked so they do not saturate the 

detector. The Bragg curve indicates that ionizing particles lose their energy and stop within a well-defined 

length of matter (Figure 3(a)). Figure 3(b) shows that range as a function of energy for H, He, C, O, and 

Fe ions in Al and polystyrene. Note that the 0.32 cm cap of aluminum over the samples will stop H+ with 

energies up to about 10 MeV. This is well above the energy of solar wind protons. This is not to say there 

are no secondary particles generated, but no significant number of primary solar wind protons would 

make it even to the upper surface of the candidate shielding materials. The difference in the projected 

range between 0.32 cm of Al and 5.25 cm of Al is the amount of radiation predicted to be stopped by the 

Al shield sample.

The GCRs which make it through the shielding sample must interact with the scintillator material to 

generate the light to be detected. Discussions with the scintillator manufacturer Saint-Gobain revealed 

that, in doped polystyrene fibers, about 7 percent of the light generated in a 3.5 mm diameter scintillator 

would reach a detector that fully covers one end of the scintillator. As is illustrated in Figure 4, the 

minimum ionizing particle is a H+ with a kinetic energy of about 2 GeV which will produce around 300 

detectable photons. All other particles at all energies produce more photons. A 1.2 cm thick CsI(Tl)
NASA/TM—2012-216055 5

(a) 

(b)

Figure 3.—The Bragg curve (a) shows that essentially all of a particle’s kinetic energy 

is given up within a well defined range as it travels through a material. The Bragg 

range for several ions through 0.32 cm (~1/8 in.) and 5.25 cm (~2 in.) of Al (b). 
NASA/TM—2012-216055 6

(a)

(b)

Figure 4.—The Bragg range for several ions passing through a 3.5 mm (~0.133 in.) 

polystyrene (PS) scintillator (a) and a 1.2 cm CsI crystal (c). Light output in PS for 

several ions as a function of energy assuming that 7 percent of the photons 

generated are captured by the detector (b) and the total light output for several ions 

in a CsI(Tl) crystal of 1.2 cm thickness (d).
NASA/TM—2012-216055 7

(c)

(d)

Figure 4.—Concluded.
NASA/TM—2012-216055 8

crystal produces a much greater number of photons than the polystyrene fibers. The number of photons 

produced in polystyrene and CsI(Tl) is drawn in solid lines for those ions which have a range greater than 

the thickness of the scintillator. Lower energy ions would not be counted, as they would not penetrate all 

levels of the detector. The detector should be sensitive to H+ with energies higher than 65 MeV, and Fen+

with energies higher than 16 GeV (280 MeV/nucleon).

The pulse intensity in the scintillating materials due to a penetrating particle is dependent on the 

stopping power within the material, modified by quenching effects. The quenching effects increase with 

greater energy transfer, but they are generally well known and can be described by modeling. In the 

CsI(Tl) crystal, the quenching effects are much less than the polystyrene fibers. The CsI(Tl) crystal also 

has the advantage that the decay of the pulse is a function of the stopping power, so that particles of 

different mass (and different stopping power) can be distinguished. By determining the particle type and 

stopping power of that particle, the linear energy transfer can be estimated for each event.

The photons produced by the scintillators would be collected using silicon photomultipliers (SiPMs), 

which are compact arrays of miniature avalanche photodiodes operated in Geiger mode. They are 

compact, are insensitive to magnetic fields, require less than 70 V to operate and are available with 

3×3 mm square active areas (among other sizes). Models are available with quantum efficiencies of 20 to 

60 percent within the emission wavelength range of the scintillating materials and gains of around 105

. 

They can operate between 0 and 40 °C but are sensitive to temperature changes. The temperature must be 

monitored so that the bias voltage can be adjusted to maintain constant gain (∼50 mV/°C).

The electronics to read the SiPMs are summarized in the schematic shown in Figure 5. They are 

grouped into three types of modules: 1) four fiber scintillator detection modules, 2) one CsI(Tl)

scintillator detection module, and 3) one mainboard. The function of each is described below.

One SiPM would be attached to each of the 45 scintillating fibers in each of the four fiber scintillation 

detector modules, two in the x direction and two in the y, for a total of 180 fibers. When a SiPMs detects 

a scintillation event, it outputs a current pulse which is converted to an analog voltage pulse. When a 

pulse exceeds a threshold value (to minimize noise) a digital signal is sent from a discriminator to the 

FPGA. The temperature of the SiPMs is monitored so that the gain on the bias voltage can be controlled, 

as described above.

Figure 5.—Schematic of electronics system for scintillation light detection and signal processing.
NASA/TM—2012-216055 9

A few SiPMs (to ensure adequate signal strength) would be attached to the CsI(Tl) scintillator. A 

series of measurements by a voltage integrator will be triggered by a valid signal from the four layers of 

fibers. The voltage integrator would measure two or more 1 to 2 μs intervals during the scintillator pulse 

duration. The integrated value is read by a fast analog to digital converter (ADC).

In this design the mainboard contains a radiation-tolerant field programmable gate array (FPGA) with 

270 digital I/O connections. It monitors all digital discriminators and ADC outputs listed above. It would

also monitor the temperatures of the SiPMs and adjust five separate voltage regulators which adjust the 

SiPM reverse bias voltage on each of the four fiber scintillator strips and the CsI scintillator. Additionally 

the FPGA determines which events are valid, formats the data, stores the data temporarily, and 

communicates with the DAU.

When the x, y coordinates of excited scintillators indicate a particle has penetrated a single shielding 

sample, the CsI(Tl) scintillation signal is counted and processed by the FPGA. Each event contains the

x, y coordinates, the pulse intensity of the CsI(Tl) scintillator as a function of time, and a timestamp. The 

luminescent decay of the CsI(Tl) is well below the 100 μs recovery time limit established above.

Accepted events are transferred as a serial string to the DAU which formats the data such that it can be 

transmitted by the ISS to a ground control station daily. The MISSE-X project would provide data 

decoding, and would send the data to the PI on a regular basis. The PI would analyze the data and provide 

both raw data and the analysis to the Co-Is on a regular basis.

Once the data trends describing the relative performance of the GCR shields are clear, nominally 

1 year, the MRSMAT would be discarded. Evaluation of additional samples would be carried out by 

subsequent MRSMATs.

MRSMAT Engineering Model Testing

The principal testing to be carried out on the Engineering Model is the operation of the detector 

system. In lieu of testing on GCRs, modeling and testing would be carried out at high energy particle 

beam facilities yet to be determined. Detector response would be determined for proton beams varying in 

energy from 50 to 500 MeV directed from multiple angles in a yet to be identified particle accelerator.

The response to α particles and heavier ions in the 50 to 500 MeV/nucleon energy range would also be 

measured, perhaps at the NASA Space Radiation Laboratory (NSRL).

After beam testing, the electronics of the Engineering Model would be tested for vacuum 

compatibility. After vacuum testing, the Engineering Model would be vibration tested at GRC up to loads 

at least 50 percent higher than the greatest loads expected during launch.

Conclusions

The preliminary design of a testbed to evaluate the effectiveness of GCR shielding materials is 

presented. The testbed is intended to fly on the ISS as part of the MISSE-X platform. A key feature is the 

ability to simultaneously test nine samples, including standards, which are 5.25 cm thick, which will 

enable most samples to have an areal density greater than 5 g/cm2

. A novel and compact GCR telescope 

would be used to distinguish which cosmic rays have penetrated which shielding material sample, and 

would be able to evaluate the dose transmitted through the shield. The testbed could be key in the 

development and qualification of new cosmic ray shielding technologies.

References

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2. http://www.nasa.gov/pdf/500436main_TA06-ID_rev6_NRC_wTASR.pdf

3. http://www.bnl.gov/medical/nasa/nsrl_description.asp
NASA/TM—2012-216055 10

4. M. Aguilar et al., “Cosmic-Ray Positron Fraction Measurement from 1 to 30 GeV with AMS-1,”

Physics Letters B, Vol. 646(4) (2007) pp. 145-154.

5. R. Bogue, “Mars Curiosity: Sensors on the Red Planet,” Sensor Review Vol. 32(3) (2012) pp. 187–

193.

6. E.C. Stone et al., “The Cosmic Ray Isotope Spectrometer for the Advanced Composition Explorer,” 

Space Science Reviews Vol. 86 (1998) pp. 285-356.

7. D.F. Smart and M.A. Shea, “A Review of Geomagnetic Cutoff Rigidities for Earth-Orbiting 

Spacecraft,” Advances in Space Research 36 (2005) pp. 2012–2020.

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NASA/TP-216722 (2010).

9. M. Durante and F.A. Cucinotta, “Physical Basis of Radiation Protection in Space Travel,” Reviews of 

Modern Physics, Vol. 83 (2011) pp. 1245–1281.

10. D. Guinet, et al., “Using the Combination of CsI(Tl) and Photodiode for Identification and Energy 

Measurement of Light Particles,” Nuclear Instruments and Methods in Physics A278 (1989) 614-6.