Results are given from sets of fission foil detectors (FFD's) (Ta-181, Bi-209, Th-232, U-238) which were included in the A0015 experiment to measure combined proton/neutron fluences. Use has been made of recent FFD high energy proton calibrations for improved accuracy of response. Comparisons of track density measurements have been made with the predictions of environmental modeling based on simple 1-D (slab) geometry. At 1 g/cm(exp 2) (trailing edge) the calculations were approximately 25 percent lower than measurements; at 13 g/cm(exp 2) (Earthside) calculations were more than a factor of 2 lower. A future 3-D modeling of the experiment is needed for a more meaningful comparison. Approximate mission proton doses and neutron dose equivalents were found. At Earthside (13 g/cm(exp 2) the dose was 171 rad and dose equivalent was 82 rem. At the trailing edge (1 g/cm(exp 2) dose was 315 rad and dose equivalent was 33 rem. The proton doses are less than expected from TLD doses by 16 percent and 37 percent, respectively. These differences can be explained by uncertainties in the proton and neutron spectra and in the method used to separate proton and neutron contributions to the measurements

Armstrong, T. W.

Benton, E. V.

Colborn, B. L.

Frank, A. L.

English

NASA Technical Reports Server

N95- 23811
FISSION FOIL MEASUREMENTS OF NEUTRON AND PROTON
FLUENCES IN THE A0015 EXPERIMENT"
A.L. Frank and E.V. Benton
Eril Research, Inc.
E O. Box 150788
San Rafael, CA 94915-0788, U.S.A.
T. W. Armstrong and B.L. Colborn
Science Applications International Corporation
Route 2, Prospect, TN 38477, U.S.A.
SUMMARY
Results are given from sets of fission foil detectors (FFDs) (lSXTa, 2°9Bi, 232Th, 23su) which were
included in the A0015 experiment to measure combined proton/neutron fluences. Use has been made of
recent FFD high energy proton calibrations for improved accuracy of response. Comparisons of track
density measurements have been made with the predictions of environmental modeling based on simple
1-D (slab) geometry. At 1 g/cm 2 (trailing edge) the calculations were ,-,25% lower than measurements; at
13 g/cm 2 (Earthside) calculations were more than a factor of 2 lower. A future 3-D modeling of the
experiment is needed for a more meaningful comparison. Approximate mission proton doses and neutron
dose equivalents were found. At Earthside (13 g/cm 2) the close was 171 rad and dose equivalent was
82 rein. At the trailing edge (1 g/cm 2) dose was 315 rad and dose equivalent was 33 rein. The proton
doses are less than expected from TLD doses by 16% and 37%, respectively. These differences can be
explained by uncertainties in the proton and neutron spectra and in the method used to separate proton
and neutron contributions to the measurements.
INTRODUCTION
The A0015 radiation experiment consisted of sets of passive integrating detectors which were
contained in three different sealed canisters on the LDEF satellite. Canisters #1 and #2 (at the Earth end
and near the trailing edge of LDEF, respectively) were filled with the detectors while Canister #3 (near
the trailing edge) was only partially filled. Both #1 and #2 carried a selection of FFDs. The FFDs consist
of heavy metal foils in contact with muscovite mica films. The foil types included lalTa, 2°9Bi, 232Th and
238U.
•Work partially supported by NASA Contract No. NAS8-38610, Marshall Space Flight Center,
Huntsville, AL 35812
149
https://ntrs.nasa.gov/search.jsp?R=19950017391 2020-06-16T08:32:13+00:00Z
101
I I J ! I I ' , I ' ' I '
10°
10-1
104 I i , 1
10o lC 10 2 10 _
I I
10s
Figure 1: Fission cross sections for neutrons and protons incident on heavy metal foils.
Heavy metal nuclei have significant cross sections for fission when irradiated with neutrons and
protons. Each isotope is characterized by threshold energies for the fission reactions and pax:ticular
energy-dependent cross sections. In the FFDs, fission fragments produced by the reactions are emitted
from the foils and create latent particle tracks in the adjacent mica films. When the films are processed,
surface tracks are formed which can be optically counted. The track densities are indications of the
fluences and spectra of neutrons and/or protons.
FFDs have previously been used for spaceflight measurements[4, 2, 3, 6, 5]. In cases where the proton
contribution to track densities can be subtracted out, the FFDs can be used as high energy (> 1 MeV)
neutron dosimeters. These detectors have been calibrated with neutrons of energies up to -,_15 MeV and
found to have efficiencies e = 1.16 x 10 -s tracks/neutron barn[8]. More recently, calibrations have
been performed with high energy protons. At high energies, either proton or neutron calibrations are
sufficient since the cross section data, plotted in Figure 117, 10, 9], show that the proton and neutron
fission cross sections are approximately equal.
Detector efficiencies, plotted in Figure 2, have been found for the four FFD types by combining low
150
Io" , , 'I ' ' Jl ' ' 'I ' ' Jl ' ' '
10-7
1
I
I j
I I
I I
I I'
.I
i !
J I
! I
I !
I
II
I I
I
II
I I
i !
/
,I
10
Pb
,/ / ,I , , / ,I l ,
102 10 3
205_i
ii I i
10 k
NUCLe_ _ (Y,eV)
Figure 2: Sensitivities of fission foil detectors to neutrons and protons.
I
I0_
energy neutron and high energy proton calibrations with the published fission cross sections to cover the
energy region from 1 MeV to 100 GeV. Armstrong and Colborn[1] have shown that the energy range of
interest in space applications extends to 100 GeV for proton/neutron spectra.
The FFD efficiencies _(£), together with calculated proton or neutron energy spectra, N(E), are used
to generate predicted track densities. These values are then compared with measured track densities, for
an evaluation of the calculated spectra and determination of LDEF fluences and doses.
EXPERIMENT
The aluminum canisters containing the A0015 detectors had acrylic liners with inner dimensions of
9.7 cm diameter by 8.6 cm depth. The detectors were 7 cm × 7 cm in dimension (with corners clipped)
151
Table 1: Shieldingof LDEF A0015FissionFoil Detectors.
Canister Plate Min.VerticalShielding
(g/cm2) A1 Equivalent
Min.-Max. Horizontal
Shielding (g/cm _)
AI Equivalent
#1 _°gBi 12.7 2.75- 10.0
lSlTa 13.1 2.70 - 22.6
232Th 13.4 1.87 - 8.92
23su 13.6 1.87- 13.4
#2 2°°Bi 1.65 2.75- 10.0
lSlTa 1.42 2.70- 22.6
232Th 1.20 1.87 - 8.92
23su 0.97 1.87- 13.4
Canister #1 was located on the Earthside end of LDEF and Canister #2 near the trailing edge (Tray C2).
and were stacked through the depth of the canisters. The FFDs were contained in cutouts in acrylic
plates. The shielding of the FFDs is given in Table 1. The aluminum equivalent values are given but most
of the volume of the canisters was filled with plastics (polycarbonate, acrylic and CR-39). This has a
significant effect on the scattering of neutrons in the vicinities of the FFDs. FFDs are at one end of the
detector arrays and, from the shielding in Table 1, it is seen that the FFDs were oriented toward space in
Canister #2 (trailing edge) but away from space in Canister #1 (Earthside). There were 6-10 cm 2 of each
of the four foil types in each canister.
After the return of LDEF, the FFDs were removed from the canisters and disassembled. The mica
films were processed in 50% HF solution at 21°C for 1.25 hr in order to delineate the fission fragment
tracks for counting. The mica was given a pre-flight processing for 3 hr to enlarge the fossil tracks. The
films were then counted under an optical microscope at 200 x.
MEASUREMENTS
The average track densities from the mica films are given in Table 2. The standard deviations given
are due to counting statistics. In addition, there were differences of up to 16% from the mean in track
densities across the detector layers in Canister #1 and up to 37% in Canister #2 which seem to have been
due mainly to shielding differences through the sides of the canisters. The larger gradients in track
densities in Canister #2 would be expected near the trailing edge of LDEF and under smaller shielding.
It is of interest to note that the track density ratios between Canisters #1 and #2 change very little for
the four foil types. Since the foils have different threshold energies one can conclude that the ratios of
high energy to low energy nucleons are equal to within a few percent for the two FFD positions.
152
Table2: AverageTrackDensitiesfrom theLDEF A0015FissionFoil Detectors.
Measured Calculated
TrackDensity(cm2) TrackDensity (cm 2)
Canister #1
_SlTa 131 4- 4 51.8
2°9Bi 2340 + 47 103
232Th 23880 4- 240 9870
238U 34490 4- 500" 18800
Canister #2
lSlTa 148 + 5 49.0
_°gBi 2825 + 5 2030
232Th 27030 4- 315 20300
238U 39490 + 500 34600
"Corrected for spontaneous fission background and surface oxidation of foils.
Canister #1 - Earthside
Canister #2 - Trailing Edge
The calculated track densities are based on proton and neutron spectra derived from primary/secondary
particle propagation in a simple slab (one-dimensional) shield.
CALCULATIONS
Numerical integrations were carried out, as discussed above, to calculate theoretical track densities
for comparison with measurements. These results are approximate since the simple slab geometry
models developed by Armstrong and Colbom[1] were used to propagate the incident particles through
shielding. Secondary particles are included in the calculations.
The equation for numerical integration is
E,-_.z E,,_,_
De= __, %(E)Np(E)+ _ e.(E)N.(E) (1)
Err, ln Emi.
where % and _,_ are the detector efficiencies for protons and neutrons, respectively (Figure 2), and Np and
N, are the calculated proton and neutron spectra in the vicinity of the FFDs. An example of the proton
and neutron spectra for a slab thickness of 10 g/cm 2 AI is given in Figure 3.
The calculated track densities for each of the four FFD types as functions of slab thickness are plotted
in Figure 4 with the measurements. Values corresponding to the vertical shielding of the FFDs in Table 1
are given in Table 3 along with the measurements. There is better agreement between calculation and
measurement for Canister #2, where the shielding is small. For thicker shielding the slab calculations fall
well under the canister measurements because of the large difference in shielding from the sides.
153
5x10 8
10 8
10 7
10 6
105
I0h
10 3
102
101
Neutron
Proton
\
\
\
\
\
\
\
10 0 101 102 103 10W 105
NUCLEON ENERGY (MeV)
Figure 3: Differential spectra of neutrons and protons accumulated during the total LDEF mission under
10 g/cm 2 A1. from data by Armstrong and Colborn (1990). Summations and extrapolations by the authors.
154
105
' I ' I ' I ' I ' I ' I ' I ' I ' I ' I
o103
I- zo=Bi,- -
102
101 , I i I i I , I _ Z I I l I i I j I
0 20 40 60 80 100
Shielding (g/cm z )
Figure 4: Calculated and measured fission fragment track densities from the combined p,f and n,f reactions
in the A0015 fission foil detectors on LDEF. The calculations are based on a simple 1-D (slab) geometry
(Armstrong and Colborn, 1990). The measured values lie at approximately 1 and 13 g/cm 2.
The calculations for Canister #2 are low by 67, 28, 25 and 12% for 18tTa, 2°9Bi, 232Th and 238U,
respectively. The higher the energy threshold, the greater the deviation. This suggests that the calculated
proton and neutron energy spectra may be deficient at higher energies.
The proton absorbed doses and the neutron dose equivalents have been approximated for the FFD
measurements by scaling the calculated proton and neutron track densities to the total measured values.
The proton dose, in rad, is given by
E2 dE
O = 1.602 x 10-s _--_+N(E) dx (2)
Et
where E1 and E2 are 10 MeV and 100 GeV, the energy end points in this study. N(E) is the differential
proton spectrum in cm -2 MeV -t and aE is the energy absorption of protons in tissue in MeV cm 2 g- 1.
The dose equivalent for neutrons, in rem, is given by
E2
DE = 10-3 _--_d(E)n(E) (3)
Et
where Ex and E2 are 1 MeV and 60 GeV, d(E) is the dose equivalent conversion factor in rem cm 2 and
n(E) is the differential neutron spectrum in cm 2 MeV -1. The d values were taken from NCRP (1971) up
to 500 MeV and extended to 60 GeV from that energy.
155
Table 3: High Energy Proton and Neutron Doses for the A0015 Fission Foil Detectors.
Canister Position Shielding Proton Dose Neutron Dose TLD
(g/cm 2) A1 (rad) (rem) Proton Dose
Equivalent (rad)
1 Earthside 13.2 171 82 200
2 Trailing Edge 1.3 315 33 500
The doses were determined by scaling calculated values of proton and neutron induced track densities to
total measured values.
The doses correspond to proton energies of 10 MeV to 100 GeV and neutron energies of 1 MeV to 60 GeV.
The doses are given in Table 3. The proton doses can be compared to measured TLD doses in the
same canisters. Extrapolating from the shielding dose distribution measured with TLDs, we would expect
about 225 rad in Canister #1 and 550 rad in Canister #2. About 90% of these doses would be due to
protons in the energy range of the FFDs. The TLD proton doses are therefore about a factor of 1.2 higher
in Canister #1 and 1.6 higher in Canister #2. Given the approximations involved in the slab calculations
of proton and neutron spectra and simple scaling of track densities to get measured proton doses, these
differences are within expectations.
DISCUSSION
A comparison has been made between fission foil detector measurements (track densities) induced by
proton/neutron fluences encountered on LDEF and predicted track densities based on calculated proton
and neutron spectra. The calculations employed a simple slab geometry[ 1]. The predicted track densities
on the detectors were less than measurements by --,25% at 1 g/cm 2 shielding (trailing edge) and more
than a factor of 2 at 13 g/cm 2 (Earthside). The differences are primarily due to the slab geometry
approximation of actual spaceflight conditions. Future calculations based on a geometrical model of the
A0015 experiment are needed for a more accurate test for the radiation modeling code.
The proton doses derived by combining measurements and calculations were 16% and 37% less than
TLD doses measured in the flight canisters, when extrapolated to equivalent shielding. The differences
can be explained by the approximations involved in the calculations.
In Table 4 the A0015 neutron dose equivalent rates are compared with measurements from other
spaceflights. The LDEF rates are seen to be higher than all other measurements, by factors of
approximately 6 to 40. The primary reason for the large differences is in the greater LDEF flight
altitude, with higher primary proton and secondary neutron fluxes. Shielding differences may also play a
significant role.
156
Table4: SpaceflightHigh Energy(> 1MeV) NeutronMeasuredComparisons.
Space Experi- Shielding Altitude Incli- DoseEquiv.
flight ment (km) nation Rate
(mrem/d)
LDEF A0015 1g/cm2 478 28.5° 16
A0015 13g/cm2 39
P0006 17g/cm2 33
STS-9(SL-1) VFI Pallet 241 57° 4.2
STS-51F(SL-2) VFI Pallet 322/304 49.5° 4.0
STS-3 Locker 280 40.3° 0.95
STS-4 Locker 297 28.5° 1.3
STS-5 Locker 284 28.5 ° 2.2
STS-6 Locker 293 25.5 ° 1.3
Cosmos 936 Inside Spacecraft 419/224 62.8 ° 6.8
Cosmos 1129 Inside Spacecraft 394/226 62.8 ° 6.8
Cosmos 2044 Outside Spacecraft 294/216 82.3 ° 3.3
Due to approximations made in separating neutron and proton contributions to the fission foil detector
measurements, the accuracy of neutron dose equivalents is estimated to be within a factor of 3.
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Fission foil measurements of neutron and proton fluences in the A0015 experiment

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