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

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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
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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

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Representative neutron energy distributions
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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
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Residual range vs Emax in PADC
If E gt Emax then LET lt LETcrit
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Calculated protons inside ISS Columbus module
GCR
SAA Belt
(T Ersmark PhD thesis, June 2006, Royal Institute
of Technology, Stockholm)
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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 gt 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

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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
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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

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4He Etchable range in PADC
Etchable 4He tracks
Quoted Fluence
D
Stack
Rcrit
D dosemeter thickness Rcrit etchable length
of track (i.e. LET gt LETcrit) F fluence
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HIMAC 12C 1493 MeV into PADC
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HIMAC 56Fe 9.727 GeV into PADC
Quoted F 5000 cm-2Measured F 3900 cm-2
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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

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Angle of incidence, ? vs Ep
Analysis needs refinement. qc is probably in the
35o 50o range

• 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

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Revised estimate of etchable proton tracks
1.96 cm-2 d-1 3.5 d-1 (read area 1.767 cm2)
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Neutron dose estimateMATROSHKA 2A
i.e. 31 lower than the uncorrected doses
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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 Zgt2
• direct protons a-particles
• So far the assessment has considered subtraction
  of
• Z gt 2 ions by measurement after secondary
  chemical etch 17
• Direct protons by calculation 14
• Still to be considered
• a-particles

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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

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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