1. Assessment of radiation shielding materials for protection of space crews using CR-39 plastic nuclear track detector J. M. DeWitt 1, E. R. Benton 1, Y. Uchihori 2, N. Yasuda 2, E. V. Benton 3, and A. L. Frank 3 1 Dept. of Physics, Oklahoma State University, Stillwater, OK 74078 USA 2 National Institute of Radiological Sciences, Chiba, Japan 3 Dept. of Physics, University of San Francisco, San Francisco, CA 94117 USA

2. 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 21-5 September 200824th ICNTS 2008, Bologna, Italy

3. 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) 31-5 September 200824th ICNTS 2008, Bologna, Italy

4. Figure of Merit (2) 41-5 September 200824th ICNTS 2008, Bologna, Italy Solution (cont.) Results should be weighted so as to reflect the relative abundances in the GCR spectrum Reflects LET

5. Figure of Merit (3) 51-5 September 200824th ICNTS 2008, Bologna, Italy 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.)

6. Figure of Merit (3) 61-5 September 200824th ICNTS 2008, Bologna, Italy Solution (cont.) Since CR-39 is not sensitive to protons > 12 MeV, use Al 2 O 3 :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)

7. The Space Radiation Environment 71-5 September 200824th ICNTS 2008, Bologna, Italy

8. Detector Exposures Simulate the SRE using 1 H, 4 He, 12 C, 16 O, 20 Ne, 28 Si, 56 Fe, 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.) 81-5 September 200824th ICNTS 2008, Bologna, Italy

9. Target-Detector Configuration Mono- Energetic Particle Beam (5000/cm 2 ) Front CR-39 Detector Back CR-39 Detector Shielding Target (e.g. Al, Cu, PE, etc.) 91-5 September 200824th ICNTS 2008, Bologna, Italy e.g. 28 Si, 56 Fe

10. 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) 101-5 September 200824th ICNTS 2008, Bologna, Italy

11. Differential LET Fluence Spectra in CR-39 PNTD 956 MeV/n 56 Fe at BNL NSRL No target and behind 30 g/cm 2 copper 1.) Primary ionization peak shifts to higher LET 3.) Passage through the absorber leads to range straggling and broadens the peak 2.) Nuclear interactions produce projectile fragments 111-5 September 200824th ICNTS 2008, Bologna, Italy

12. Differential LET Fluence Spectra in CR-39 PNTD 956 MeV/n 56 Fe at BNL NSRL 0–30 g/cm 2 aluminum 121-5 September 200824th ICNTS 2008, Bologna, Italy

13. Differential LET Fluence Spectra in CR-39 PNTD 975 MeV/n 28 Si at BNL NSRL 10 g/cm 2 polyethylene Z = 14 13 12 11 10 9 131-5 September 200824th ICNTS 2008, Bologna, Italy

14. Dose-Depth Profiles (1) Range of 975 MeV/n 28 Si in aluminum: 56.3 g/cm 2 copper: 64.8 g/cm 2 polyethylene: 47.8 g/cm 2 141-5 September 200824th ICNTS 2008, Bologna, Italy

15. Dose-Depth Profiles (2) 151-5 September 200824th ICNTS 2008, Bologna, Italy Range of 956 MeV/n 56 Fe in aluminum: 31.5 g/cm 2 copper: 36.2 g/cm 2 polyethylene: 26.6 g/cm 2

16. Percent of Primaries Fragmented: 956 MeV/n 56 Fe 161-5 September 200824th ICNTS 2008, Bologna, Italy

17. 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 171-5 September 200824th ICNTS 2008, Bologna, Italy