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

© HPA

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

© HPA

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

© HPA

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

© HPA

Calculated protons inside ISS Columbus module

SAA Belt

GCR

(T Ersmark PhD thesis, June 2006, Royal Institute of Technology, Stockholm)

© HPA

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

© HPA

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

© HPA

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

© HPA

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

© HPA

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

© HPA

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

© HPA

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

© HPA

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

© HPA

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)

© HPA

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

© HPA

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

© HPA

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

© HPA

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