Space radiation dosimetry and the fluorescent nuclear track detector

1. Space radiation dosimetry and the fluorescent nuclear track detector Nakahiro Yasuda National Institute of Radiological Sciences

2. Contents • Space Radiation Monitoring (Passive dosimeter) - Requirements to be measured (ICRP 1991) - Technique for personal dosimetry for astronauts - Recent experiments and topics • Fluorescent nuclear track detector

3. Radiation of Space • Sources: • GCR (Protons~87%, He~11%, HZE~1%) with large scale of energy range • Solar particles (Dominated by protons) with the energy of ~ 100 MeV (300@ max) • Trapped protons (Dominated by protons) with the energy of below 250 MeV • Characteristics: • Mixed radiation field • Fluctuations (time and space) • Track traversal frequency for biological cell nucleus (~100mm2) • Proton / every few days • He ion / every month • Fe ion / every 100 years • = one Fe ion is hitting the surface of body for every second Space craft walls Secondary

4. Elemental Abundance of the Galactic Cosmic Rays

5. Galactic Cosmic Ray Energy Spectra

6. Radiation of Space • Sources: • GCR (Protons~87%, He~11%, HZE~1%) with large scale of energy range • Solar particles (Dominated by protons) with the energy of ~ 100 MeV (300@max) • Trapped protons (Dominated by protons) with the energy of below 250 MeV • Characteristics: • Mixed radiation field • Fluctuations (time and space) • Track traversal frequency for biological cell nucleus (~100mm2) • Proton / every few days • He ion / every month • Fe ion / every 100 years • = one Fe ion is hitting the surface of body for every second Space craft walls Secondary

7. Quantifying Space Radiation Exposure Dose is the amount of energy deposited per unit mass: D = E/m; 1 Gy = 1 Joule/kg • F is the Fluence, the number of incident particles per unit area, usually in particles/cm2, • LET (dE/dx) is the amount of energy deposited per unit distance by the particle as it traverses matter often in unit of keV/mm (unit used in radiation protection),

8. Conventional method for assessing radiation risk Evaluation of the risk of cancer mortality has been to estimate the dose equivalent at points in the various organ or tissue of interest within the individual. Assumption: Same dose equivalent for each radiation type results in the same risk Quality Factor (Q) - Universal function of particle LET (keV/mm) - Defined under the assumption that the same radiological effectiveness is obtained for different particle with the same LET at the point of interest

9. Dosimetric values and Quality factor Fe dE/dx ~ LET (keV/mm) conventional assumption r = 1 g/cm3 (water) Dose Equivalent is expressed in Sieverts 1 Sv = Q(LET)  1 Gy.

10. Contributions to dose and dose equivalent Fe Proton He

11. Requirements for radiation monitoring for astronauts • - Large dynamic range (0.1~1,000 keV/mm) • Real time (area monitor) and personal dosimetry CPDS (Si stack) DB-8 (Si) R-16 (IC) Shuttle TEPC

12. Passive dosimeters • Photogenic (nuclear) emulsion • * No charge resolution to heavy ions (up to Fe) • * Sensitive to MIP • Thermoluminescence Detectors (TLD) - Optically Stimulated Luminescence Detectors (OSLD) • * Measures total absorbed Dose (Gy) • * No LET information, so can’t be used by itself to determine • Dose Equivalent (Sv) • CR-39 plastic nuclear track detector • * High charge resolution, but no sensitivity to lower • LET particles (below 5 keV/mm)

13. Operation of TLD and OSLD

14. Passive dosimeters

15. Combine method with CR-39 an TLD or OSLD • Combine method using TLD and CR-39 • TLD for low LET particles (0.1 – 5 or 10 keV/mm) • CR-39 for High LET particles (~5 or 10 keV/mm – 1,000 keV/mm) Dtotal = DTLD– k D>5keV/mm + D<5 keV/mm = DTLD + (1-k) DCR-39 H<5keV/mm = DTLD– k D>5 keV/mm = DTLD– k DCR-39 Htotal = H>5keV/mm + H<5keV/mm = DTLD– k DCR-39 + HCR-39 CR-39 TLD T. Doke et al., Radiat Meas.24(1995)74.

16. BRADOS phase-2 experiment in the ISS (Russian Service Module ) Phase-2 - Spacial distributions of dose (rate) at 5 locations - Intercomparison for dosimeters of NIRS and IBMP - Exposure duration: 268.5 days

17. Locations of BRADOS boxes and exposed durations

18. Shielding functions in the Service Module model

19. Results

21. Typical Radiation Exposures

22. Target fragment Sample of target fragmentation event in nuclear emulsion P148 290 MeV/u Carbon Nuclear emulsion (H, C, N, O, Br, Ag) 50mm

23. Fluorescent nuclear track detector • Ideal detector for space radiation measurement as personal dosimeter • Large dynamic range (0.1 – 1,000 keV/mm) • No fading • No chemical treatment • Able to readout on board • (mobile, no electricity)

24. Characteristics of luminescence detectors ～500mGy

25. Idea saturate Fact : Signal will be saturated when the exposed dose becomes high Can be explained by overlapping tracks Individual tracks? High Low

26. Luminescence detector response to heavy ion

27. Material developed by Landauer Inc. Al2O3:C, Mg single crystal Trapping center ～ 104-105/mm3 Stable ～600℃ No fading

28. Optics Laser：635nm Emission：750nm Objective 60x, 0.85NA

29. Heavy ion track in Crystal

30. Heavy ion track in Crystal 400 MeV/n Ne 400 MeV/n Kr

31. 3D

32. Linearity of signal

33. Conclusions • Introduction of space radiation measurement • Lack of information for short range recoils • Introduction of Fluorescent nuclear track detector