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
Centre for Radiation, Chemical and Environmental Hazards
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
© HPA
Representative neutron
energy distributions
0.20
Ersmark calculated ISS Columbus Module
Sato calculated NASA STS shuttle
-2
Goldhagen measured ER-2 56 g cm
Wilson calculated NASA STS-36
Normalized  /ln( E)
0.15
0.10
0.05
0 -3
10
-2
10
10
-1
10
0
10
1
2
10
10
3
10
4
5
10
10
6
Neutron energy, E (eV)
10
7
8
10
9
10
10
10
10
11
© HPA
Methods – need to get rid of
the charged particles
Simple method to get neutron dose
N B
H
RISS
But, charged particles are recorded by other elements of
the dosemeter – these can be detected via entry and
exit tracks, NCP
N  NCP  B
H
RISS
© HPA
Residual range vs Emax
in PADC
N  NHI  NLI  B
H
RISS
If E > Emax
then
LET < LETcrit
© HPA
Calculated protons inside ISS
Columbus module
GCR
SAA Belt
(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
• 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
Ions
• Data allow etchable
range of an ion to be
estimated
• LETcrit can be inferred
© HPA
HIMAC 4He: 105.6 MeV into
PADC
577.14 MeV a +
2.5 m air
116.8 mm PMMA
• 116.8 mm
PMMA
• Trim does not
include air
• Straggling
modelled well
© HPA
4He:
Etchable range in PADC
Etchable 4He
tracks
Quoted Fluence
Stack
D
Rcrit
Rcrit 
DN

D = dosemeter thickness
Rcrit = etchable length of track (i.e. LET > LETcrit)
 = fluence
© HPA
HIMAC 12C: 1493 MeV into
PADC
4582 MeV
12
C + 185.2 mm PMMA
Net tracks cm
-2
1500
1000
Region of mixed
track diameters
500
0
0
10
20
30
Distance into PADC stack, X (mm)
40
50
© HPA
HIMAC 56Fe: 9.727 GeV into
PADC
4000
23.3 GeV
Fe + 47.6 mm PMMA
Quoted  = 5000 cm-2
Measured  ~ 3900 cm-2
3000
-2
Net tracks (cm )
56
2000
1000
0
0
5
10
15
20
25
Distance into PADC stack, X (mm)
30
35
© HPA
LET threshold in PADC
Ion
Etchable track
length, L
Energy, E
(MeV)
LET∞ PADC
(keV mm-1)
LET200 PADC
(keV mm-1)
4He
130 µm
12.75
58
33
12C
2.9 mm
403.6
77
44
56Fe
>3m
-
-
> 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
70
Maxiumum proton
treeing energy
60
Angle of incidence,  (°)
Minimum proton
treeing energy
50
Analysis needs refinement. c is
probably in the 35o – 50o range
40
• 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
30
20
10
0
40
60
100
200
Proton energy, Ep (keV)
300
400
500 600
800
© 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 estimate
MATROSHKA 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 & 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
© 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
© HPA