2004.12.4 @ 京都大学 μ-PICを用いた位置有感生体等価比例計数管（PS-TEPC) の宇宙放射線線量等量計測器への応用 PS-TEPC : Position-Sensitive Tissue Equivalent Proportional Counter 早稲田大学・理工学総合研究センター ＆ 宇宙航空研究開発機構・総合技術研究本部 寺沢和洋、道家忠義 京都大学大学院・理学研究科 身内賢太朗、永吉勉 高エネルギー加速器研究機構・放射線科学センター 佐々木慎一、俵裕子 宇宙航空研究開発機構・総合技術研究本部 松本晴久 Introduction • Radiation hazard to the astronaut in space → Dose Equivalent : Dose on the ground ≪ Dose in space (~ 1 mSv / year) (~ 1 mSv / day) • Radiation exposure limit (dose equivalent) on the ground : 50 mSv / year , 100 mSv / 5 years [ICRP-60 recommendation, International Commission on Radiological Protection] in space : lifetime excess risk < 3 % ex. male, 25 years old: 400 mSv / year male, 35 years old: 900 mSv / year → 750 mSv @ 95 %C.L. (σ= 10 %) → 560 mSv (σ= 30 %) [ref. T. Abe et al., Mut. Res. 430 (1999) 177.] • Main contribution to dose in space : protons and heavy ions cf. no protons and no heavy ions on the ground Dose evaluation D k f (L )LdL H k Q(L ) f (L )LdL H Q D D : Absorbed dose H : Dose equivalent Q : Effective quality factor L∞ : LET in water (Stopping power of water) Q (L∞) : Quality factor as a function of LET f (L∞) : Differential LET distribution k : Conversion coefficient (constant) → Measurement of LET distribution is essential to evaluate dose equivalent. LET = E / R [keV/μm-water] E : energy, R : range LET [keV/μm] QF (ICRP-60) < 10 1 10 ~ 100 0.32 L – 2.2 100 < 300 / L1/2 Quality factor as a function of LET TEPC (Cylindrical) DOSTEL TEPC (Spherical) Kiel Univ., Germany NASA, America DOSimetric TELescope Si1 1.27 mm 6.35 mm 1 mm Si2 Thickness : 315μm 1) Mean σ(%) 0.67 51 0.67 35 Response functions and their standard deviations 1.2 17 1) “Path length = 1” means the thickness of a silicon detector (a) (b) DSSD1 DSSD2 t DSSD3 (a) For penetrating particles LET=Ed / (t/cos) (b) For stopping particles LET=Ed / R Particle identification by E・ E Range – Energy relation → determination of R LETwater[keV/μm] = LETSi ×1.193 / 2.33 R • LET of each particle can be evaluated. ・ Incident angle ・ Path length in the detector Size : 2 mm× 2 cm Thickness : 0.5 mm Spacing : 5 mm No. of strips : 16 Ed : Deposited Energy in the detector t : Thickness of the detector θ : Incident angle of each particle R : Range of a stopping particle in the detector 1.193 : Conversion factor for relativistic particles 2.33 : Density of Si RRMD: Real-time Radiation Monitoring Device DSSD: Double-sided Silicon Strip Detector Measurement of LET in real time by RRMD-III (waseda-jaxa) Geographical map of count rate (STS-84) • Peak around 0.2 keV/μm : relativistic protons • Clear shoulders and peaks : major abundant elements C (7.2 keV/ μm) O (12.8) Ne (20.0) Mg (28.8) Si (39.2) Fe (135.2) LET distribution for GCR particles (STS-84) • Differential flux @ SAA region : 10 – 100 times higher in LET ( 1 – 10 keV /μm) than that of GCR particles • Broad peak at around 0.5 keV/μm : trapped protons • LETmax of proton : 90 keV/μm → Low energy protons : dominant component @ SAA region LET distribution for trapped protons (STS-84) Comparison of dose data between RRMD-III, TEPC and DOSTEL onboard theSpace Shuttle (STS-84, -91) • Difference between RRMD-III and DOSTEL is reasonable, if surrounding materials of DOSTEL are taken into consideration. • Dose equivalent (0.4 – 600keV/μm) of TEPC is 66 % higher than that of RRMD-III • Quality factor of TEPC in SAA region is over 2. (cf. 1.2 – 1.3 for DOSTEL & RRMD-III) → Inter-comparison experiment (ICCHIBAN Project) → Absorbed dose : TEPC LET distirbution & Dose equivalent : RRMD-III (for space dosimetry to charged particles) Next step → Neutron dosimeter or both (neutron & charged particle) → PS-TEPC Requirements for active detectors for space dosimetry Requirements ＼ Detectors TEPC DOSTEL RRMD-III Precise measurement of LETs (Standard deviation of response function) LET ≒ y (lineal energy) ×1) ○1) ○ (51%, 35%) 17 % ○ ○ － Real-time measurement ○1) ○1) Event-by-event evaluation ○1) ○1) ○ ○ Tissue equivalent ○ × × Sensitivity to neutrons ○ × × Compact system ○ 4π acceptance ○ ○ × ○ ×2) Detection of m.i.p. △ ○ ○ LET range [keV/μm] 0.4 ~ 1200 0.1 ~ 120 0.1 ~ 700 1) Deconvolution process is necessary to obtain the real LET distribution. 51 % for Cylindrical. 35 % for Spherical. 2) RRMD-IV, which is a cubic type detector, has acceptance for 4π.(△ → ○) Requirements for active detectors for space dosimetry Requirements ＼ Detectors TEPC DOSTEL RRMD-III PS-TEPC Precise measurement of LETs (Standard deviation of response function) LET ≒ y (lineal energy) ×1) ○1) ○ ○ (51%, 35%) 17 % ○ ○ － － Real-time measurement ○1) ○1) Event-by-event evaluation ○1) ○1) ○ ○ ○ ○ Tissue equivalent ○ × × ○ Sensitivity to neutrons ○ × × ○ Compact system ○ 4π acceptance ○ ○ × ○ ×2) ○ ○ Detection of m.i.p. △ ○ ○ ○ LET range [keV/μm] 0.4 ~ 1200 0.1 ~ 120 0.1 ~ 700 0.1 ~ 1000 1) Deconvolution process is necessary to obtain the real LET distribution. 51 % for Cylindrical. 35 % for Spherical. 2) RRMD-IV, which is a cubic type detector, has acceptance for 4π.(△ → ○) Summary and Plan • Small-size PS-TEPC ・ 3-dimensional tracks ・ Effective volume : 20×20×20 mm3 ・ Position resolution : ~ 1 mm ・ LET range : 0.2 ~ 1000 keV/μm • Performance test with tissue equivalent gas (CH4 or C3H8) + CO2 and N2 • Irradiation test with tissue equivalent gas or Ar + C2H6 (10 cm μ-TPC) ・ μ, electron(γ, X), proton and α ・ Heavy ions such as C, Si and Fe (~ 500 MeV/n)
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