A systematic study of the induced radioactivity of the Long Duration Exposure Facility (LDEF) is being carried out in order to gather information about the low earth orbit radiation environment and its effects on materials. The large mass of the LDEF spacecraft, its stabilized configuration, and long mission duration have presented an opportunity to determine space radiation-induced radioactivities with a precision not possible before. Data presented include preliminary activities for steel and aluminum structural samples, and activation subexperiment foils. Effects seen in the data show a clear indication of the trapped proton anisotropy in the South Atlantic Anomaly and suggest contributions from different sources of external radiation fluxes

Fishman, G. J.

Harmon, B. A.

Laird, C. E.

Parnell, T. A.

English

NASA Technical Reports Server

N92-2 3()0 _ 
INDUCED RADIOACTIVITY IN LDEF COMPONENTS
B. A. Harmon
NASA Marshall Space Flight Center
Huntsville, AL 35812
Phone: 205/544-4924, Fax:205/544-7754
G. J. Fishman and T. A. Parnell
NASA Marshall Space Flight Center
Huntsville, AL 35812
Phone:205/544-7693, Fax:205/544-7754
C. E. Laird
Department of Physics
Eastern Kentucky University
Richmond, KY 40475
Phone:606/622-1526, Fax:606/622-1020
SUMMARY
A systematic study of the induced radioactivity of the
Long Duration Exposure Facility (LDEF) is being carried out
in order to gather information about the low earth orbit
radiation environment and its effects on materials. The
large mass of the LDEF spacecraft, its stabilized
configuration, and long mission duration have presented an
opportunity to determine space radiation-induced
radioactivities with a precision not possible before. Data
presented include preliminary activities for steel and
aluminum structural samples, and activation subexperiment
foils. Effects seen in the data sh_w a clear indication of
the trapped proton anisotropy in the South Atlantic Anomaly
and suggest contributions from different sources of external
radiation fluxes.
301
https://ntrs.nasa.gov/search.jsp?R=19920014057 2020-03-17T11:48:10+00:00Z
INTRODUCTION
A systematic study of induced radioactivity in materials
carried aboard the Long Duration Exposure Facility has
provided a significant and very useful database for space
radiation-related applications. This study was broad-based
to include as many different materials as could be obtained
from the LDEF structure and experiments. In essence, the
entire spacecraft was used as a passive detector to sample
the radiation environment in low earth orbit.
The uniqueness of the LDEF spacecraft for radiation
studies not only stems from its extended flight time
(mission duration 5.8 years), but also its large mass and
passively stabilized geometry. The stabilized configuration
was designed to control exposure of various experiments to
the space environment, and in particular, allowed
directional aspects of the induced radioactivity to be
studied on the leading (eastern), trailing, north and south
sides of the spacecraft, as well as the earth and spaceward
directions.
The induced radioactivity is produced by several sources
of particle fluxes: galactic protons, trapped Van Allen
protons (encountered in the South Atlantic Anomaly and
accounting for the bulk of the activity), atmospheric and
secondary neutrons, and to a small extent heavier ions. All
of these particles induce radioactivity by colliding with a
stable nuclide in the spacecraft material, and occasionally
forming a radioactive nuclide. If its half-life is long
enough, it can be detected in the laboratory following
retrieval. The sources of external radiation flux, the
nuclear reactions with the spacecraft material, and the
spacecraft geometry can be combined, in principle, into a
model to predict the experimentally measured activities.
Such a comparison of calculation and experiment can be very
useful for future long duration missions in low earth orbit
(LEO), such as Space Station Freedom and the Earth Observing
System, where accurate radiation dose predictions are
required.
SAMPLE PROCESSING AND DISTRIBUTION
Following retrieval of the LDEF in January of 1990,
samples for measurement of induced radioactivity were
obtained from the Kennedy Space Center over a period of a
302
few months. Some samples were obtained from the structural
components of the spacecraft; others were taken from
experiment trays under agreements with the experiment
investigators. Approximately 400-500 samples were processed
at the Marshall Space Flight Center and distributed to eight
nationally recognized laboratories for analysis: Lawrence
Berkeley Laboratory, Los Alamos National Laboratory, Johnson
Space Center, Lawrence Livermore National Laboratory,
Savannah River-Westinghouse, the Tennessee Valley Authority-
Western Area Radiological Laboratory, Battelle-Northwest
Laboratory, and the Marshall Space Flight Center.
High-purity germanium detectors are used to detect gamma
rays from decaying radionuclides, and must be well-shielded
from gamma rays produced by secondary emission from cosmic
rays and naturally radioactive materials in the laboratory.
Typical measured results are in the range of 0.i-i00
picoCuries per kilogram of material in the LDEF spacecraft.
Some radionuclide activities, as in the steel and aluminum
structural components, were strong enough to allow mapping
of their directional and depth-dependent characteristics.
Figure 1 shows the positions of various materials on the
LDEF spacecraft that were analyzed for induced
radioactivity. The steel trunnions (alloy 17-4PH) from the
end support frame (Earth end) of the LDEF were the first
components acquired for analysis. Other structural
materials obtained later included aluminum experiment tray
clamp plates and trunnion clamp assemblies, titanium
structural clips, and lead ballast plates. An activation
subexperiment consisting of metal foils (ref. I) was also
included in several experiment trays. These included sets
of five different metals (cobalt, vanadium, tantalum, indium
and nickel). These were chosen because of their simple
isotopic makeup and significant long-lived radionuclide
production. In addition some samples were obtained by
agreement from other experimenters, such as magnesium,
copper, germanium, niobium, silver, and teflon. Many of
these samples, however, were not of sufficient mass to yield
good signal-to-background ratios for accurate measurement.
PRELIMINARY RESULTS
In this section, preliminary results obtained in the
analysis of LDEF induced radioactivity are presented.
Absolute activities have been corrected for decay since
retrieval and for detection geometry, and are estimated to
be good to 20%.
303
A detailed representation of the west trunnion (adjacent
to experiment tray row 3 in figure I) with respect to the
external environment and the pattern in which samples were
prepared from it is shown in figure 2. Samples were cut in
1.3, 1.9 and 2.5 cm- (0.5, .75 and 1 in) thick cylindrical
sections 8.3 cm (3.25 in) in diameter for bulk activity
measurements, and thinner 5 cm (2 in) square layers of
varying thickness to study depth and directional
dependences.
Examples of spectra showing peaks from gamma-decaying
radionuclides produced in the end section (section A in
figure 2) in each of the two steel trunnions are shown in
figure 3. These two samples were activated primarily by the
trapped proton flux from (a) the east (onto the leading side
of the spacecraft) and (b) the west (onto the trailing side
of the spacecraft), respectively. These spectra indicate
production of 56Co, 58Co, 54Mn and 46Sc with half-lives ranging
from 71 to 312 days. An enhancement of the intensity of 835
keV 54Mn peak by a factor of -2 in the west-facing sample
relative to the east-facing sample can clearly be seen.
This effect is caused by the interaction of the trapped
protons in the South Atlantic Anomaly with upper atmospheric
gases. In the SAA the flux encountered by the leading
(east) side of the spacecraft is attenuated relative to the
trailing (west) side flux because the east side flux is
traveling about a magnetic field line below the spacecraft,
and thus penetrates deeper into the atmosphere. This effect
has been quantified recently by Watts, et al. (ref. 2) and
is being incorporated into the radiation models being
developed currently. The 478 keV line observed on the
leading side of the spacecraft (top figure) was determined
to be caused by a deposition of atmospheric ZBe on the
surface of the spacecraft, and was not produced by
spallation within the spacecraft material (ref. 3) (See also
J. C. Gregory, et al., these conference proceedings).
Figure 4 illustrates the effect of the anisotropic SAA
flux, where the west/east ratio of the 54Mn activities as a
function of distance is plotted along the axis of the
trunnion. The difference in activity from one side to the
other decreases with depth due to attenuation of the proton
flux, even though the anisotropy of the external flux is
known to increase with energy. The majority of the SAA
protons which activate the steel are in the range of 20-120
MeV, and are stopped in the first 2 cm (0.8 or 15 g/cm 2) of
material. The bulk activity for 54Mn in the trunnion
interior does not drop to zero, however, but reaches approx-
imately 80 picoCuries/kg (see C. E. Moss and R. C. Reedy;
3O4
W. G. Winn, these conference proceedings), which may be
caused by secondary activation by neutrons as well as high
energy background fluxes of cosmic ray protons in the energy
range of several GeV. These contributions are currently
being included in a simplified three-dimensional spacecraft
mass model (See B. L. Colborn and T. W. Armstrong, these
conference proceedings).
A large number of aluminum experiment tray clamps (alloy
6061-T6) were obtained from the LDEF following de-
integration of the spacecraft. The clamps were
approximately 5 cm (2 in) by 12.7 cm (5 in) and 0.47 cm
thick (.185 in or 1.3 g/cm2). A total of 50 clamps have
been counted at the TVA Western Area Radiological Laboratory
to investigate the variation of activating flux with
direction. Clamps were obtained from the spacecraft on each
row to allow measurement of the change in flux every 15
degrees. In figure 5, the 22Na activity based on the 1275
keV line is shown as a function of angle from the leading
direction of the spacecraft (east). A comparative one-
dimensional calculation is also shown (see T. W. Armstrong
and B. L. Colborn, these conference proceedings.) based on
the proton anisotropy model (ref. 2) normalized to AP8 omni-
directional flux, and measured cross sections for protons on
aluminum, which is within 30% agreement with the measured
activation. The peak of the 22Na activity in the trailing
side plates is clearly apparent.
The activation subexperiment foils of vanadium, cobalt,
indium, tantalum and nickel were analyzed as they were
obtained from LDEF experiments M0001, M0002, P0006, and
AOII4. The results for each of the four nickel samples
counted at Marshall Space Flight Center are shown in figure
6 along with predictions of a one-dimensional calculation
with the AP8 flux for proton activation of the
5 cm (2 in) square by 0.32 cm (.125 in) thick
nickel foils. The upper and lower limits of the
calculation represent the range of proton-induced activation
caused by (a) normally incident flux on the nickel surface,
(b) a uniform exposure in all directions, and (c), same as
(a) and (b) but shielded by 1 cm (0.4 in) of aluminum.
These calculations represent the range of shielding/flux
conditions to which the nickel samples were exposed around
the spacecraft. By modeling the local geometry of these
samples, it may be possible to separate shielding effects
from contributions due to different activation sources. For
example, the small amount of 6°Co observed in these samples
can be produced alternately by an (n,p) reaction on 6°Ni
(abundance 26.1%) above 5 MeV, or proton reactions on the
other stable isotopes of nickel (61Ni, 62Ni and _Ni, total
abundance 5.6%). More refined calculations may be able to
distinquish these two contributions. Further analyses of
305
the nickel samples and the indium cobalt, tantalum, and
vanadium samples are in progress.
ANALYSIS PLAN/CONCLUSION
Most of the low level counting has now been completed
and the effort has shifted to collection and analysis of
data from the counting laboratories. Much of the analysis
and archiving of these data will be performed at Eastern
Kentucky University to produce a large database of the
measured induced radioactivities. The current scope of this
effort is to be able to provide specific activities for
different materials whenever the detection geometry is
reasonably convenient for normalization of the gamma ray
counting measurements. In other cases, where only relative
measurements were possible, information about depth and
directional dependences can still be extracted.
Measurements and analyses of the induced radioactivity
in the Long Duration Exposure Facility will continue through
1991. Detailed plans can be found in the Long Duration
Exposure Induced Radioactivity Analysis Plan (ref. 4). A
program of calculations in order to extract as much
information as possible about various sources of
radioactivity is now underway (See T. W. Armstrong and B. L.
Colborn, these conference proceedings.). It is hoped that
these studies will yield a complete and accurate picture of
the low earth orbit radiation environment.
3O6
REFERENCES
,
•
•
•
Clark L. G.; Kinard, W. H.; Carter, D. J.; and Jones,
J.L.: The Long Duration Exposure Facility Mission 1
Experiments. NASA SP-473, 1984, pp. 1-189.
Watts, J. W.; Parnell, T. A.; and Heckman, H. H.:
Approximate Angular Distribution and Spectral for
Geomagnetically Trapped Protons in Low-Earth Orbit.
AIP Conference Proceedings 186, 1989, pp. 75-85.
Fishman, G. J.; Harmon, B. A.; Parnell, T. A.; Gregory,
J. C.; Peters, P.; Haskins, P. S.; McKisson, J. E.;
Ely, D. W.; Weisenberger, A. G.; Piercy, R. B.;
Phillips, G. W.; King, S. W.; August, R. A.; Ritter,
J. C.; and Cutchin, J. H.: Observation of 7Be on the
Surface of the LDEF SPacecraft, Nature 349, 1991,
pp. 678-680.
Fishman, G. J.; Parnell, T. A.; and Harmon, B. A.: Long
Duration Exposure Facility (LDEF) Induced Radioactivity
Analysis Plan, NASA TM-I03545, 1991, pp. X-32.
307
LDEF Induced Activity Analysis
Structural Clips
Trunnion
(Steel)
1"_"_ Structural Clips
(Ti)
"-'-Trunnion
(Steel)
F
(East) X
Figure 1
Space
West
LDEF
Main Structure
East
South
Earth
/ Flight Vector
(East)
West (Trailing) Trunnion Orientation
Figure 2
308
LDEF Acquired Activity
700
Steel Trunnion
! ! | I 1
6O0
500
•,- 400
t-
O
o 300
20O
IO0
0
4O0
7O0
600
5OO
*- 400
C
0
O 3O0
2OO
100
0
4OO
(MSFC)
a°
Leading Side
835 keV
54 Mn
478 keV
7Be 511 keV
e+
500 600 700
E(keV)
8OO 900
I ! I
b,
Trailing Side
I
835 keV
54 Mn
511 keV
e+
811 keV
58C0
500 600 700 800
E(keV)
I
(MSFC)
847 keV
_6Co
889 keY
46S0
9OO
Figure 3
1000
1000
309
1.5
O
1.0
0.5
0
MN-S,4. WES,T/EASTACT,y,_',RATIO..iN.STEEL.,TRUNN'ONS,. .
-1-
i i i i I , J i , I i , _ , |
5 10 15
DISTANCEFROM END (in)
I I
Figure 4
22Na Activation of LDEF Aluminum Tray Clamps
o Measured (TVA)
-- Approx. Calculation Using MSFC Anisotropy Model
(1-D LDEF Geometry Model, 0.64 g/cm2 Depth, 450 kin.)
#S
i !
,t North West South E t
0 I , , , , , , , , I , , , , , , , , ' , , i , , I
0 45 90 135 180 225 270 315 360
Angle from LDEF Leading Edge (East) (deg)
Figure 5
|
20
310
30
G)
--, 18
o
0
.o_ 12
13..
6
0
Ni Samples-LDEF
I I
- r-] Predicted
- I Measured
I I I I 1 I
I I I I I I
60C0 58C0 57C0 56C0 54Mn
Figure 6
311



Induced radioactivity in LDEF components

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