determination of the influence of primary protons on the assessed neutron doses in the iss

Centre for Radiation, Chemical and Environmental Hazards

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

R J Tanner, L G Hager and J S EakinsINTS24, Bologna, September 2008

ID 236, Neutron Dosimetry

© HPA

HPA PADC dosemeter & 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

EuCPD

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Representative neutron energy distributions

10-3

10-2

10-1

100

101

102

103

104

105

106

107

108

109

1010

1011

Neutron energy, E (eV)

0

0.05

0.10

0.15

0.20

No

rmal

ized

/l

n(E)

Ersmark calculated ISS Columbus Module

Sato calculated NASA STS shuttle

Goldhagen measured ER-2 56 g cm-2

Wilson calculated NASA STS-36

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Methods – need to get rid of the charged particles

Simple method to get neutron dose

ISSR

BNH

But, charged particles are recorded by other elements of the dosemeter – these can be detected via entry and exit tracks, NCP

ISSR

BNNH CP

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Residual range vs Emax in PADC

100 101 102 103 10410

1

102

103

104

105

Emax (MeV amu-1

)

CS

DA

ra

ng

e ( m

)

t

d

p

3He

4He

6Li7Li

9Be

12C

20Ne

Thickness of dosemeter = 500 m Can produce tracks on both faces

Cannot produce tracks on both faces

ISSR

BNNNH LIHI

If E > Emax thenLET < LETcrit

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Calculated protons inside ISS Columbus module

SAA Belt

GCR

(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 > 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

Ions

• 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

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HIMAC 4He: 105.6 MeV into PADC

2 4 6 8 10 12 14 160

50

100

150

200

250

300

350

Mea

sure

d t

rack

s, N

ne

t (cm

-2)

Distance into PADC stack, x (mm)

TRIM calculation

Measurement

• 116.8 mm PMMA

• Trim does not include air

• Straggling modelled well

2.5 m air577.14 MeV +

116.8 mm PMMA

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Rcrit

D

D = dosemeter thickness

Rcrit = etchable length of track (i.e. LET > LETcrit)

= fluence

Quoted FluenceEtchable 4He tracks

4He: Etchable range in PADC

Stack

DN

Rcrit

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HIMAC 12C: 1493 MeV into PADC

0 10 20 30 40 50

Distance into PADC stack, X (mm)

0

500

1000

1500

Net

tra

cks

cm-2

Region of mixedtrack diameters

4582 MeV 12C + 185.2 mm PMMA

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HIMAC 56Fe: 9.727 GeV into PADC

0 5 10 15 20 25 30 35

Distance into PADC stack, X (mm)

0

1000

2000

3000

4000

Net

tra

cks

(cm

-2)

23.3 GeV 56Fe + 47.6 mm PMMA

Quoted = 5000 cm-2

Measured ~ 3900 cm-2

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LET threshold in PADC

IonEtchable track

length, LEnergy, E

(MeV)

LET∞ PADC

(keV m-1)

LET200 PADC

(keV m-1)

4He 130 µm 12.75 58 33

12C 2.9 mm 403.6 77 44

56Fe > 3 m - - > threshold

20Ne - 6020 44 25

• 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, θ vs Ep

40 60 100 200 300 400 500 600 800

Proton energy, Ep (keV)

0

10

20

30

40

50

60

70

An

gle

of

inci

den

ce,

(°)

Minimum protontreeing energy

Maxiumum proton treeing energy

Analysis needs refinement. c 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 , the energy range which produces etchable tracks

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Revised estimate of etchable proton tracks

p TOTAL p Etchable

Origin (cm-2 d-1) (cm-2 d-1) Ratio

SAA belt 1.24 x 105 1.79 1.44 x 10-5

GCR 5.50 x 104 0.17 3.09 x 10-6

TOTAL 1.96

1.96 cm-2 d-1 ~ 3.5 d-1 (read area = 1.767 cm2)

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Neutron dose estimateMATROSHKA 2A

Total tracks

HI(17%)

Non-HI(83%)

Protons (14%)

Neutrons(69%)

9349 1589 7760 1285 6475

Neutron tracks

Hp(10)(mSv)

EISO

(mSv)Hp(10) rate(mSv d-1)

EISO rate(mSv d-1)

6475 70.4 53.1 0.19 0.14

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 Z>2• direct protons & -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: -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