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

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

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 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 (~100  m 2 ) - 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

Elemental Abundance of the Galactic Cosmic Rays

Galactic Cosmic Ray Energy Spectra

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 - 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 (~100  m 2 ) - 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

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/cm 2, LET (dE/dx) is the amount of energy deposited per unit distance by the particle as it traverses matter often in unit of keV/  m (unit used in radiation protection),

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/  m) - Defined under the assumption that the same radiological effectiveness is obtained for different particle with the same LET at the point of interest

Dosimetric values and Quality factor dE/dx ~ LET (keV/  m) conventional assumption  = 1 g/cm 3 (water) Dose Equivalent is expressed in Sieverts 1 Sv = Q(LET)  1 Gy. Fe

Contributions to dose and dose equivalent Proton He Fe

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

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/  m)

Operation of TLD and OSLD

Passive dosimeters

Combine method with CR-39 an TLD or OSLD  Combine method using TLD and CR-39 D total = D TLD – k D >5keV/m + D <5 keV/m = D TLD + (1-k) D CR-39 H 5 keV/m = D TLD – k D CR-39 H total = H >5keV/m + H <5keV/m = D TLD – k D CR-39 + H CR-39 T. Doke et al., Radiat Meas.24(1995)74. CR-39 TLD - TLD for low LET particles (0.1 – 5 or 10 keV/m) - CR-39 for High LET particles (~5 or 10 keV/m – 1,000 keV/m)

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: days

Locations of BRADOS boxes and exposed durations Box # (Panel #) LocationExposure duration (days) A46 (P#443) Starboard side91.5 A41 (P#445) Starboard side268.5 A42 (P#240) Port side268.5 A43 (P#111) Floor, Starboard side A44 (P#445) Starboard side268.5 A45 (P#326) Ceiling near the R-16, port side 268.5

Shielding functions in the Service Module model

Results

Typical Radiation Exposures Limit: Annual Public 1 mSv Limit: Annual Radiation Worker 50 mSv Average yearly exposure to natural background 2.4 mSv Living 1 year in Japan * 2.3 mSv Living 1 year in Kerala, India 13 mSv STS-57 (473 km, 28.5  ) 19.1 mSv STS-60 (352 km, 57  ) 4 mSv 270 day mission on ISS (400 km,  ) ~50 mSv *Excluding exposure to Natural Background

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

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

Characteristics of luminescence detectors ～ 500mGy

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

Luminescence detector response to heavy ion

Material developed by Landauer Inc. Al 2 O 3 :C, Mg single crystal Trapping center ～ /  m 3 Stable ～ 600 ℃ No fading

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

Heavy ion track in Crystal

400 MeV/n Kr 400 MeV/n Ne

3D

Linearity of signal

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