Initial results from LDEF include radiation detector measurements from four experiments, P0006, P0004, M0004, and A0015. The detectors were located on both the leading and trailing edges of the orbiter and also on the Earthside end. This allowed the directional dependence of the incoming radiation to be measured. Total absorbed doses from thermoluminescent detectors (TLDs) verified the predicted spatial east-west dose ratio dependence of a factor approx. 2.5, due to trapped proton anisotropy in the South Atlantic Anomaly. On the trailing edge of the orbiter a range of doses from 6.64 to 2.91 Gy were measured under Al equivalent shielding of 0.42 to 1.11 g/sq cm. A second set of detectors near this location yielded doses of 6.48 to 2.66 Gy under Al equivalent shielding of 0.48 to 15.4 g/sq cm. On the leading edge, doses of 2.58 to 2.10 Gy were found under Al equivalent shielding of 1.37 to 2.90 g/sq cm. Initial charged particle LET (linear energy transfer) spectra, fluxes, doses and dose equivalents, for LET in H2O greater than or = 8 keV/micron, were measured with plastic nuclear track detectors (PNTDs) located in two experiments. Also preliminary data on low energy neutrons were obtained from detectors containing (6)LiF foils

Benton, E. R.

Benton, E. V.

Csige, I.

Frank, A. L.

Parnell, T. A.

Watts, J. W., Jr.

English

NASA Technical Reports Server

RADIATION EXPOSURE OF LDEF: INITIAL RESULTS
E. V. Benton, A. L. Frank, E. R. Benton and I. Csige
Physics Department, University of San Francisco
2130 Fulton St. San Francisco, CE 94117-I080
T. A. Parnell and J. W. Watts, Jr.
ES-62, NASA-Marshall Space Flight Center
George C. Marshall Space Flight Center, AL 35812
ABSTRACT
Initial results from LDEF include radiation detector measurements from four
experiments, POOO6, PO004, MO004 and AO015. The detectors were located on both
the leading and trailing edges of the orbiter and also at the Earthside end.
This allowed the directional dependence of the incoming radiation to be measured.
Total absorbed doses from thermoluminescent detectors (TLDs) verified the predicted
spatial east-west dose ratio dependence of a factor_2.S, due to trapped proton
anisotropy in the South Atlantic Anomaly (SAA). On the trailing edge of the orbiter
a range of doses from 6.64 to 2.91Gy were measured under A1 equivalent shielding
of 0.42 to i.ii g/cm 2. A second set of detectors near this location yielded doses
of 6.48 to 2.66 Gy under A1 equivalent shielding of 0.48 to 15.4 g/cm =. On the
leading edge doses of 2.58 to 2.10 Gy were found under A1 equivalent shielding of
1.37 to 2.90 g/cm 2. Initial charged particle LET (linear energy transfer) spectra,
fluxes, doses and dose equivalents, for LET in H20>18 keV/_m, have been measured
with plastic nuclear track detectors (PNTDs) located in two experiments. Also
preliminary data on low energy neutrons were obtained from detectors containing
6LiF foils.
INTRODUCTION
The LDEF orbiter carried four experiments which contained passive
integrating detectors from the University of San Francisco. The POOO6 LET
Spectra Measurements experiment consisted of a single canister in Tray F-2 in
which TLDs, PNTDs and neutron detectors were included. The PO004 Seeds in Space
experiment consisted of seven canisters in Tray F-2 in which packets containing
TLDs, PNTDs and neutron detectors were distributed within the seed component. The
MOOO4 Radiation Effects in Electronics experiment included two detector canisters
in Tray F-8 containing TLDs and PNTDs. The AOOI5 Biostack experiment consisted
of two USF (Tray C-2 and G-I-2) and one partial USF (Tray F-2) canisters
containing TLDs, PNTDs and neutron detectors.
The purpose of the detectors was to define the radiation environment as a
function of shielding depth at the experimental sites on the orbiter. As seen
from the Tray locations PO006, POO04 and two AO015 canisters were near the
trailing edge. The third AOOl5 canister was at the earth end while MOOO4 was
at the leading edge.
Work partially supported by NASA Grant No. NAG8-168 (NASA-Marshall Space Flight
Center, Huntsville)
325
https://ntrs.nasa.gov/search.jsp?R=19920014059 2020-03-17T11:48:06+00:00Z
EXPERIMENTAL PROCEDURES
The passive radiation detectors were arrayed in stacks within the LDEF
flight canisters. All the canisters (except the one partial A0015 unit) were
sealed with O-rings to prevent venting to space. The placement of the different
detector types was made to enable measurements to be made as a function of
shielding depth.
TLD Procedures
Single batches of TLD - 700 were divided into flight, calibration and
background portions. The calibration TLDs were irradiated with a standard
137Cs source at two-month intervals over the 5.7 year orbital period in order
to approximate both the LDEF absorbed doses and any signal fading which might
occur. A high-dose TLD response supralinearity study was also conducted with
137Cs standard doses up to 100 Gy.
After the mission TLDs were read out with a model 4000 Harshaw reader.
Calibration and backgrounds were read out along with the flight detectors. The
measured signals, averaged over a series of mission TLDs, were then converted
to absorbed doses (137Cs gamma ray equivalent). The minimum vertical shielding
between each series of TLDs and space was measured and the shielding materials
converted into the equivalent mass thickness of AI.
PNTD Procedures
The five types of PNTDs included on the LDEF experiments were pure CR-39,
CR-39 with DOP plasticizer, Tuffak and Sheffield polycarbonates and Melinex
polyester. Some of the CR-39 has been processed and read out to yield particle
flux, dose rate and dose equivalent rate for LEToo'H20>j8 keV/_m.
The CR-39 was processed in 6.25N NaOH solution at 50"C for 36 or 48 hr.
After processing pairs of CR-39 layers from the flight stacks were reassembled
in their original configurations. The pairs were scanned at the inner, adjacent
surfaces under an optical microscope. Coincident track pairs were located in
the adjacent surfaces (#2 and #3), then surfaces #I and #4 were examined to
determine whether the particle was of long range (it penetrated both CR-39
layers and resulted in four aligned tracks) or short range (it penetrated only
the adjacent surfaces or the adjacent surfaces and one outer surface, resulting
in two or three aligned tracks). The long range particles were classified as
galactic cosmic rays (GCRs) and also included projectile fragments. The short
range (SR) tracks are mainly stopping primary protons and secondary particles
deriving from target nuclei within the PNTDs. Because of their short
registration ranges protons are classified as SR particles.
The detected track parameters were measured at the #2 surface to determine
particle LET. The semimajor and semiminor axes of the elliptical track surface
openings were measured with an electronic micrometer. With the PNTD bulk etch
and the LET calibration curve for the detectors, the track measurements were
converted to LET spectra.
326
Low Energy Neutron Detectors (LENDs)
The LENDs were composed of 6LiF radiation foils and CR-39 PNTDs. Alpha
particles from the 6Li(n, _)T reaction were emitted from the foils and detected
in the CR-39. The LENDs were exposed in pairs with one detector covered by Gd
foils. This allowed the separation of the neutrons into thermal (< 0.2 eV)
and resonance (0.2eV - IMeV) energy regions.
The CR-39 PNTDs were processed in 6.25N NaOH solution at 70" C for 1.25
hours. The alpha particle track densities on the PNTDs were counted manually
at 430 x under an optical microscope. The backs of the detectors were also
counted to provide the backgrounds due to other charged particle sources
present in space. The track densities were converted to neutron fluences and
dose equivalents by previously established calibrations. The dose equivalent
conversion factors (ref. I) incorporated QF values of 2 for thermal neutrons
and 6.4 for resonance neutrons.
MEASUREMENTS
TLD Results
The TLD measurements from experiments PO006, PO004, MOO04 and AO015 are
given in Tables I, II, III and IV respectively. The trailing edge TLDs (PO006,
PO004) are seen to measure higher dose rates than the leading edge (MOO04) and
earthside (AO015) TLDs, although the shieldings are somewhat different for the
maximum dose rate.
PNTD Results
Measurements of LET spectra from the leading and trailing edges of the
orbiter are shown in Figures 1 and 2, respectively. Integral particle flux is
plotted against LET=_'H20 for the Total, GCR and SR particles. Integral flux,
dose rate and dose equivalent rate from these measurements are given in Tables
V and VI, respectively. In comparing the spectra it is seen that the MOO04
(leading edge) curve is much steeper than that of PO006. MOO04 has a higher
total integrated flux but a smaller flux in the LET region above _ 12 keV/_ m.
The greater importance of high LET particles in contributing dose and dose
equivalent can be seen by comparing Tables V and VI, where the PO006 spectrum
leads to higher dose rates and considerably higher dose equivalent rates. The
differences in the spectra in the two experiments are due both to the position
of the experiment on the orbiter and to the considerably different shielding
depths. Note that the GCR spectra are truncated, and also perhaps under-measured,
due to the difficulty in discriminating between GCR and SR particle tracks in
the very high track densities found on the LDEF PNTDs.
LEND Results
Measurements of the low energy neutrons from the trailing edge of the
orbiter are given in Tables VII and VIII. The PO004 (Table VIII) fluences and
dose equivalents are larger than those in PO006 (Table VII). The PO004 detectors
327
were surrounded by a greater mass of hydrogenous material (seeds) which
contributed to the moderation of high energy neutrons. The two PO004
measurements also have significant variations. The LENDs were located in two
different canisters with that in #3 having higher neutron levels than that in #6.
From Table II it is seen that the TLDs in mid #6 yielded higher doses than those
in mid #3. There was probably a shielding difference from the side for the
two canisters.
CONCLUSIONS
Radiation measurements have been made at different positions and shielding
depths on the LDEF orbiter. Total absorbed doses measured with TLDs ranged
from 6.64 to 2.66 Gy, for shielding of 0.42 and 15.4 g/cm 2, at the trailing
edge to 2.58 to 2.10 Gy, for shielding of 1.37 and 2.90 g/cm 2, at the leading
edge. This difference reflects the East-West anisotropy of trapped protons
at the South Atlantic Anomaly. For heavy particle measurements with PNTDs
(LET_H20>_8 keV/_m) absorbed doses of 19 and 31 mGy were found at the
leading and trailing edges. The shielding at the two positions was 2.74 and
8.88 g/cm _, respectively. The corresponding dose equivalents were 124 and 328
mSv. Neutron detectors at the trailing edge measured from 0.12 to 0.82 mSv
for thermal neutrons and from 7.0 to 14.2 mSv for resonance neutrons. The
shielding varied from 16.8 to 6.1 g/cm 2 for the extremes. Further information
on charged particle measurements being performed with the LDEF detectors is
given by Csige et al (ref. 2).
The preliminary measurements have revealed some of the differences in
radiation levels over the surface of the LDEF orbiter and with shielding
depth. Future measurements will allow the development of a more comprehensive
picture of the quantities and of directional radiation variations.
328
REFERENCES
I. NCRP Report No. 38, NCRP, Washington D.C., 1971.
2. Csige, I; Benton, E. V.; Frank, A. L.; Frigo, L. A.; Benton, E. R.; Parnell,
T. A. and Watts, J. W. Jr.: Charged Particle LET-Spectra Measurements
Aboard LDEF, First LDEF Post-Retrieval Symposium, NASA CP,3|34,1992.
329
TABLE I
TLD Plate
No.
PO006- LDEF ABSORBED DOSE MEASUREMENTS WITH TLD-700
Tissue Absorbed Dose Rate A1 Equivalent
Dose (Gy) (mGy/d) Shielding (glcm z)
1 6.48 -_ 0.24 3.07 - 0.II 0.48
2 3.92 _ 0.21 1.85 ± 0.I0 _.I0
3 3.16 ± 0.15 1.49 -" 0.07 8.34
4 2.76 ± 0.13 1.31 -+ 0.06 12.2
5 2.66 -* 0.12 1.26 -* 0.06 15.4
The doses were approximately uniform over Plates 1 and 2 and were non-
uniform over Plates 3, 4 and 5 (due to lesser shielding through the
sides than through the top of the detector assembly for the deeper TLD
plates) The minimum shielding to the side (for only the detector
assembly) of the individual TLDs was 1.96 to 6.66 g/cm 2 A1 equivalent.
All shielding was converted to A1 equivalent on the basis of the rela-
tive ranges of I00 MeV protons in the materials.
330
Detector
No.
TABLE II
PO004: LDEF ABSORBED DOSE MEASUREMENTS WITH TLD-700
Canister Tissue Absorbed Dose Rate Al Equivalent
No. Dose (Gy) (mGy/d) Shielding (_/cm z)
1 6 6.64 ± 0.29 3.14 ± 0.14 0.42
2 6 2.91 ± 0.07 1.38 ±0.03 II.I
3 6 3.88 ± 0.22 1.83 ± O.lO _5
4 4 3.12 ± 0.08 1.48 ± 0.04 6.11
5 2 3.05 ± 0.08 1.44 ± 0.04 6.10
6 5 3.09 ± 0.08 1.46 ± 0.04 6.10
7 7 2.93 ± 0.I0 1.39 ± 0.05 6.10
8 3 3.15 ± 0.08 1.49 ± 0.05 6.10
GCI 3.250.2x10 -3 1.3xl 0-3*
GC2 3.250.2xi0 "3 1.3xI0 -a*
* For a total detector assembly time of 2418 days. The flight detectors
are averaged over the LDEF orbital duration of 2115 days.
The minimum shielding to the side Ifor only the detector assembly) of
the individual TLDs was _12.4 g/cm Al equivalent. All shielding was
converted to Al equivalent on the basis of the relative ranges of lO0
MeV protons in the materials. The proton range in the seed was assumed
to be equal (in units of g/cm 2) to that of polycarbonate plastic.
331
TABLE III
MOO04: LDEF ABSORBED DOSE MEASUREMENTS WITH TLD-700
Detector TLD Plate Tissue Absorbed Dose Rate
No. No. Dose (Gy) (mG_/d)
A1 Equivalent
Shieldin 9 (_/cm 2)
1 2.10 ± 0.13 0.99 ± 0.06 2.90
2 2.37 ± 0.I0 1.12 ± 0.05 1.37
1 2.19 ± 0.12 1.04 ± 0.06 2.90
2 2.58 ± 0.09 1.22 ± 0.04 1.37
3(GC) 1 2.9-+0.2x10 -3` 1.3+-0.1x10 -3*
2 3.2+_0.2xi0"3 1.4±0.1xI0 _3.
4(GC) 1 2.9+_0.2xi0"-: 1.3+_0.1xi0-3"
2 2.9+_0.2x10-3 1.3±0.1x10 -3*
* For a total detector assembly time of 2271 days. The flight detectors
are averaged over the LDEF orbital duration of 2115 days.
All shielding materials were converted to A1 equivalent on the basis of
the relative ranges of lO0 MeV protons in the materials.
332
Canister
NO.
TABLE IV
AO015: LDEF ABSORBED DOSE MEASUREMENTS WITH TLD-700
TLD Plate Tissue Absorbed Dose Rate A1 Equivalent
No. Dose (Gy) (mGy/d) Shieldin_ (_/cm 2 )
1 3.93 ± 0.08 1.86 ± 0.04 1.66
2 2.74 ± 0.23 1.30 ± 0.II 6.23
3 2.41 +-0.18 1.14 ± 0.09 lO.O
1 4.49 ± 0.II 2.12 ± 0.05 3.85
2 3.29 ± 0.22 1.56 ± 0.I0 7.83
3 3.04 ± 0.32 1.44 -+0.15 ll.7
3 1 3.47 ± 0.22 1.64 ± 0.I0
Canister #I was to Earthside.
Canister #2 was at the trailing edge.
Canister #3 was vented to space and at the trailing edge.
Minimum shielding to the side (for only the detector assembly) of the
individual TLDs was 2.52 to 5.31 g/cm 2 A1 equivalent.
All shielding materials were converted to A1 equivalent on the basis
of the relative ranges of I00 MeV protons in the materials.
333
TABLEV. PNTDMEASUREMENTSFROMMOO04(7-IC-I, 2)
Flux Dose rate Doseequiv, rate
(cm-2.s-1"sr "I) (uGy d-I) (uSv d-I)
TOTAL 2.847xi0-4 9.09 58.8
GCR 6.322xI0-_ 0.47 5.0
SR 2.784xi0 "4 8.62 53.8
LET_-H20->8 keV/_m
Minimumshielding was 2.74 g/cm2 A1 equivalent
TABLE VI. PNTD MEASUREMENTSON PO006 (7-119, 120)
Flux Dose rate Dose equiv, rate
(cm-2-s-1.sr -I) (_Gy d-I) (_Sv d-I)
TOTAL 2.526xi0-4 14.8 155
GCR 7.980xI0 -G 1.15 1 7.0
SR 2.446xi0 -4 13.6 138
LET_.H20 > 8 keV/um
Minimum shielding was 8.88 g/cm 2 A1 equivalent
334
Table VII. Thermal and Resonance
Neutron Measurements for PO006
Neutron Energy Fluence Dose Equivalent
Ranse (cm -2) (mSv)
_0.2eV
0.2 eV - 1MeV
1.22 ± 0.24 x i07
1.43 ± 0.72 x i08
0.124 + 0.024
Dose Equivalent rate
( _Svd -' )
0.059 ± 0.011
7.0 + 3.5 3.3 ± 1.6
Minimum shielding above the detector was 16.8 g/cm = A1 equivalent. To the side
it was 3.3 g/cm 2 plus shielding external to the canister.
Table VIII. Thermal and Resonance
Neutron Measurements for PO004
Neutron Dose Dose
Canister Energy F1 uence Equi valen t Eq ui valen t
# Ranse (cm--5) (mSv) Rate ( _ Svd--')
3 ._< 0.2 eV 8.1 + 1.6 x 107 0.82 ± 0.16 0.38 + 0.07
0.2 eV - 1 MeV 2.9 + 1.4 x 108 14.2 +- 7.1 6.7 -+ 3.4
6 0.2 eV 4.0 + 0.8 x i0' 0.41 ± 0.08 0.19 + 0.04
0.2 eV - 1 MeV 1.9 ± 0.9 x 108 9.2 ± 4.6 4.3 + 2.2
Minimum shielding above the detector was 6.1 g/cm 2 AI equivalent. To the side
it was approximately 12.4 g/cm _ plus shielding external to the canister.
335
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31 -JLL-1991
Fig. 1 Integral LET flux spectra from the MO004 experiment on the leading
edge of the orbiter. The minimum shielding was 2.74 g/cm= AI equivalent.
336
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Fig. 2 Integral LET flux spectra from the PO006 experiment on the trailing
edge of the orbiter. The minimum shielding was 8.88 g/cm 2 AI equivalent.
337



Radiation exposure of LDEF: Initial results

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