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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. HPA PADC dosemeter & EuCPD. EuCPD. - PowerPoint PPT Presentation
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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