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Determination of the influence of primary protons on the assessed neutron doses in the ISS

Determination of the influence of primary protons on the assessed neutron doses in the ISS. ID 236, Neutron Dosimetry. R J Tanner, L G Hager and J S Eakins INTS24, Bologna, September 2008. HPA PADC dosemeter & EuCPD. EuCPD.

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Determination of the influence of primary protons on the assessed neutron doses in the ISS

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  1. Determination of the influence of primary protons on the assessed neutron doses in the ISS ID 236, Neutron Dosimetry R J Tanner, L G Hager and J S Eakins INTS24, Bologna, September 2008

  2. HPA PADC dosemeter & EuCPD EuCPD • Routine issue for neutron personal dosimetry – electrochemical etch rear face • Calibrated for neutrons ≤ 173 MeV • Electrochemical etch produces indistinguishable tracks for neutrons, direct protons, and heavy ions • Forms a component of European Crew Personal Dosimeter – to assess neutron dose equivalent only

  3. Representative neutron energy distributions

  4. Methods – need to get rid of the charged particles • Simple method to get neutron dose But, charged particles are recorded by other elements of the dosemeter – these can be detected via entry and exit tracks, NCP

  5. Residual range vs Emax in PADC If E > Emax then LET < LETcrit

  6. Calculated protons inside ISS Columbus module GCR SAA Belt (T Ersmark PhD thesis, June 2006, Royal Institute of Technology, Stockholm)

  7. The problem • Need to avoid overestimates of the dose to astronauts on the ISS because a component of the reported neutron dose is due to charged particles • A correction is required: the charged particles only exceed LETcrit near the end of their range • A > 2 particles can be excluded by detection of entry and exit tracks • Can estimate the number of tracks caused by protons if we know the LETcrit for protons • Shortage of available proton beams

  8. Detector stack arrangement for HIMAC irradiations • HIMAC irradiations provided data for 4He, 12C and 56Fe in May 2008 • Prior data for 20Ne • PMMA block in the beam to ensure ions stop in the PADC stack • Data allow etchable range of an ion to be estimated • LETcrit can be inferred Ions

  9. HIMAC 4He: 105.6 MeV into PADC 2.5 m air 577.14 MeV a + 116.8 mm PMMA • 116.8 mm PMMA • Trim does not include air • Straggling modelled well

  10. 4He: Etchable range in PADC Etchable4Hetracks Quoted Fluence Stack D Rcrit D = dosemeter thickness Rcrit = etchable length of track (i.e. LET > LETcrit) F = fluence

  11. HIMAC 12C: 1493 MeV into PADC

  12. HIMAC 56Fe: 9.727 GeV into PADC Quoted F = 5000 cm-2Measured F ~ 3900 cm-2

  13. LET threshold in PADC • Uncertainties yet to be determined • The LET for 56Fe will always be very high and will exceed the etching threshold, at any energy • Therefore the 56Fe measurement is a measure of the total fluence

  14. Angle of incidence, θ vs Ep • Envelope defines the energy range of etchable tracks • Assume dosemeter mounted on an ICRU tissue sphere inside ISS • Calculate tissue depth traversed to the detector surface for all incident angles • Calculate for each q, the energy range which produces etchable tracks Analysis needs refinement. qc is probably in the 35o – 50o range

  15. Revised estimate of etchable proton tracks 1.96 cm-2 d-1 ~ 3.5 d-1 (read area = 1.767 cm2)

  16. Neutron dose estimateMATROSHKA 2A i.e. 31% lower than the uncorrected doses

  17. Summary • The HPA PADC dosemeter can be used for the determination of the neutron dose in low earth orbit, but requires: • subtraction of high energy ions with Z>2 • direct protons & a-particles • So far the assessment has considered subtraction of: • Z > 2 ions by measurement after secondary chemical etch ~ 17% • Direct protons by calculation ~ 14% • Still to be considered: • a-particles

  18. Future Work • Determine experimentally the maximum proton energy threshold of detection: currently assumed 800 keV at the detector surface, equivalent to about 30 keV µm-1 • Determine experimentally the critical angle for protons as a function of energy • Simulate the results obtained using TRIM • Use a more realistic shape in the calculations than the ICRU sphere • Perform a full uncertainty analysis

  19. Acknowledgements • We would like to acknowledge the help of: • Tore Ersmark formerly of Royal Institute of Technology, Stockholm, for the calculated proton spectra • Staff at DLR, Cologne, Germany for assistance in arranging the measurement programme • Staff at HIMAC, Chiba, Japan for providing the irradiation facilities

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