Assessment of radiation shielding materials for protection of space crews using CR-39 plastic nuclear track detector

J. M. DeWitt1, E. R. Benton1, Y. Uchihori2, N. Yasuda2,E. V. Benton3, and A. L. Frank3

1Dept. of Physics, Oklahoma State University, Stillwater, OK 74078 USA

2National Institute of Radiological Sciences, Chiba, Japan3Dept. of Physics, University of San Francisco,

San Francisco, CA 94117 USA

1. Provide data to validate existing transport models

2. Test new multi-functional materials

3. Systematically develop a method…

a. Using baseline materials (Al, Cu, PE, etc.)

b. Using appropriate ions and energies

c. Using ground-based testing and modeling components

d. Use the method to produce a weighted Figure of Merit

Motivation

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Limitation

•We can’t expose test materials to the whole of the space radiation environment

•We can expose test materials on the ground with beams of fixed Z and E

•This provides information for particles of similar Z and E, but we are limited by what the accelerators can give us

Solution

•Develop a method to combine results from accelerator exposures to a limited—but representative—set of beams

Figure of Merit (1)

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Figure of Merit (2)

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Solution (cont.)

Results should be weighted so as to reflect the relative abundances in the GCR spectrum

ReflectsLET

Figure of Merit (3)

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1 GeV protons and 1 GeV/n heavy ions is well-representative; add lower-E protons (e.g. 150 MeV) to simulate SPEs

Solution (cont.)

Figure of Merit (3)

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Solution (cont.)

• Since CR-39 is not sensitive to protons > 12 MeV, use Al2O3:C Optically Stimulated Luminescence Detectors (OSLDs) to measure this dose contribution

• The goal (and challenge) is to generate a single Figure of Merit that characterizes a given test material’s shielding efficacy relative to a series of baseline materials (PE and Al in particular)

The Space Radiation Environment

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Detector Exposures

• Simulate the SRE using 1H, 4He, 12C, 16O, 20Ne, 28Si, 56Fe, etc.

• 1 GeV/n for heavy ions; lower energies for protons and alphas

• BNL NSRL (AGS Booster), HIMAC, etc.

• Shielding targets: baseline (Al, Cu, PE, etc.) and multi-functional (carbon composite, Kevlar composite, etc.)

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Target-Detector Configuration

Mono-Energetic

ParticleBeam

(5000/cm2)

Front CR-39Detector Back CR-39

Detector

Shielding Target(e.g. Al, Cu, PE, etc.)

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e.g. 28Si, 56Fe

Detector Processing and Read-Out

• Chemically etch…

“low ” (6.25 N NaOH at 50° C)

“slow” (7 days)

• Bulk etch is determined using the Henke-Benton method

• Optical read-out is done semi-automatically using the Samaica system (Heinrich et. al)

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LETH2O (keV/m)

40 80 120 160 200 240 280 320

Nor

mal

ized

Diff

eren

tial F

luen

ce (c

m-2

ion-1

)

10-4

10-3

10-2

No target30 g/cm2 Cu

Differential LET Fluence Spectra in CR-39 PNTD 956 MeV/n 56Fe at BNL NSRL

No target and behind 30 g/cm2 copper

1.) Primary ionization peak shifts to higher LET

3.) Passage through the absorber leads to range straggling and broadens the peak

2.) Nuclear interaction

s produce projectile

fragments

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Differential LET Fluence Spectra in CR-39 PNTD 956 MeV/n 56Fe at BNL NSRL

0–30 g/cm2 aluminum

LETH2O (keV/m)

40 80 120 160 200 240 280 320 360 400 440 480 520

Nor

mal

ized

Diff

eren

tial F

luen

ce (c

m-2

ion-1

)

10-4

10-3

10-2

10-1

No target5 g/cm2 Al10 g/cm2 Al15 g/cm2 Al20 g/cm2 Al30 g/cm2 Al

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LETH2O (keV/m)

10 15 20 25 30 35 40 45 50 55 60

Nor

mal

ized

Diff

eren

tial F

luen

ce (c

m-2

ion-1

)

10-4

10-3

10-2

10-1

Differential LET Fluence Spectra in CR-39 PNTD 975 MeV/n 28Si at BNL NSRL

10 g/cm2 polyethylene

Z = 14

13121110

9

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Depth (g/cm2)

0 5 10 15 20 25 30 35

Nor

mal

ized

Dos

e (

Gy/

ion)

0.000

0.005

0.010

0.015

0.020

0.025

0.030AlCuPE

Depth (g/cm2)

0 5 10 15 20 25 30 35N

orm

aliz

ed D

ose

Equi

vale

nt (

Sv/i

on)

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Dose-Depth Profiles (1)

Range of 975 MeV/n 28Si in aluminum: 56.3 g/cm2

copper: 64.8 g/cm2

polyethylene: 47.8 g/cm2

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Depth (g/cm2)

0 5 10 15 20 25 30 35

Nor

mal

ized

Dos

e (

Gy/

ion)

0.04

0.05

0.06

0.07

0.08

0.09

0.10AlCuPE

Depth (g/cm2)

0 5 10 15 20 25 30 35N

orm

aliz

ed D

ose

Equi

vale

nt (

Sv/i

on)

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Dose-Depth Profiles (2)

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Range of 956 MeV/n 56Fe in aluminum: 31.5 g/cm2

copper: 36.2 g/cm2

polyethylene: 26.6 g/cm2

Depth (g/cm2)

0 5 10 15 20 25 30 35

% F

ragm

ente

d

0

10

20

30

40

50

60

70

80

90

100AlCuPE

Percent of Primaries Fragmented: 956 MeV/n 56Fe

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Conclusions and Future Work

• A systematic way of assessing space radiation shielding performance

• Do this by varying beam Z and E, along with target composition and depth

• Develop using baseline materials; test using multi-functional materials

• Compliment these tests using a computer model (e.g. FLUKA)

• Major Emphasis: Use the developed method to produce a weighted Figure of Merit for a given material

• Pragmatic approach; can say little about physics involved

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