The study of the induced radioactivity of samples intentionally placed aboard the Long Duration Exposure Facility (LDEF) and samples obtained from the LDEF structure is reviewed. The eight laboratories involved in the gamma-ray counting are listed and the scientists and the associated counting facilities are described. Presently, most of the gamma-ray counting has been completed and the spectra are being analyzed and corrected for efficiency and self absorption. The acquired spectra are being collected at Eastern Kentucky University for future reference. The results of these analyses are being compiled and reviewed for possible inconsistencies as well as for comparison with model calculations. These model calculations are being revised to include the changes in trapped-proton flux caused by the onset of the period of maximum solar activity and the rapidly decreasing spacecraft orbit. Tentative plans are given for the storage of the approximately 1000 gamma-ray spectra acquired in this study and the related experimental data

Fishman, G. J.

Harmon, B. A.

Laird, C. E.

Parnell, T. A.

English

NASA Technical Reports Server

N93-29631
COLLECTION, ANALYSIS, AND ARCtIIVAL
OF LDEF ACTWATION DATA*
C, E, I_aird
Department of Physics and Astronomy
Eastern Kentucky University
Richmond, KY 40475
B.A. Harmon, G. J. Fishman, and T. A. Parnell
NASA Marshall Space Flight Center
Huntsville, AL 35812
SUMblARY
The study of the induced radioactivity of samples intentionally placed
aboard the Long Duration Exposure Facility (LDEF) m_d sarnples obtained from
the LDEF structure is reviewed. The eight laboratories involved in the galnma-
ray counting are listed and the scientists and the associated counting facilities
are described. Presently, most of the gamma-ray counting has been completed
and the spectra are being analyzed and corrected for efficiency m_d self
absorption. The acquired spectra are being collected at Eastern Kentucky
University for future reference. The results of these analyses are being
compiled and reviewed for possible inconsistencies as well as for comparison
with model calculations. These model calculations are being revised to include
the changes in trapped-proton flux caused by the onset of the period of
maximum solar activity mad the rapidly decreasing spacecraft orbit. Tentative
plans are given for the storage of the approximately 1000 gamma-ray spectra
acquired in this study and the related experimental data.
PRECEDING PAGE BLANK NOT FILMED
* Work supported by NASA coqtracts It-08071D ,'rod ti-13029D.
111
https://ntrs.nasa.gov/search.jsp?R=19930020442 2020-03-17T04:29:26+00:00Z
INTRODUCTION
Samples intentionally placed aboard the Long Duration Exposure Facility
(LDEF) and samples obtained from the LDEF structure have been studied at
NASA Marshall Space Flight Center and seven national laboratories to
determine the specific radioactivity produced in orbit. The gamma-ray spectra
from these studies have provided information concerning the type and
quantity of radioactive nuclei produced by various activating particles. The
gamma-ray spectra, the resulting activation, and the experimental
arrangements are being collected at Marshall Space Flight Center and Eastern
Kentucky University for review, further analysis, and future archival. An
overview of this process and the type of information that will be available for
future reference will be given here. This information includes the samples
studied, the location of the samples on LDEF, the amount and type of covering
material, the types of detector systems, the format of the gamma-ray spectra
and the corrections for geometry, self-absorption, detector efficiency and
background needed to obtain accurate specific activities (actiyation per
kilogram) of material. Tentative plans are given as to the archival of t=he data
for such future reference and how other scientific investigators or spacecraft
designers can access the data.
DATA COLLECTION
. _. _ _
Approximately 400 samples I have been obtained from LDEF and most, if not
all, have been studied for radioactivityat one or more of seven counting
facilities. The LDEF activation samples include 20 elementally pure rectangular
slabs of original 2" x 2" x 1/8" dimensions although some were cut to smaller
dimensions for mounting. The slabs are made from the elements V, Ni, Co, In,
and Ta. The first three elements (V,Ni,Co) represent materials having very
well-known cross sections for proton-induced reactions up to 200 MeV2 and
fairly well known cross sections up to 1 GeV or higher. 3 The last two samples
have well known large thermal neutron cross sections 4 and have.recently been
studied for incident protons up to 200 MeV3 and for neutr0ns at" 200 MeVS.
The samples of opportunity, or "unintentional" samples, include aluminum
clamp plates and trays, titanium clips, lead ballast, and the stainless-steel
trunnions.* The trunnions have been cut into 1/2-2 inch sections and sections
D, g and L have been layered by cutting at different radial distances from the
center. (See Figure 1 in ref. 6). Except for the outer layer(layer number 1),
*Ilarmon, B. A.: Space Science Laboratory, NASA/MSFC, private
communication.
112
these layers were carefully flattened to give an approximate rectangular slab
geometry. The aluminum pieces are of various shapes mid sizes. In addition,
bolts were taken froln the structure and have been studied.
These samples, mostly having rectangula_r slab geometries, have been
studied for radioactivity at MSFC and six other counting facilities. These
facilities are listed in Table 1 ,along with the collaborating scientists. These
facilities could be categorized as being shielded, low-background, m_d ultra-low
background. A shielded facility is one where shielding is used primarily to
prevent contamination from other smnples being counted in a multiple sample
count facility. The TVA/Muscle Shoals faci!ity might be considered such a
facility. Low-background facilities would have several inches of shielding
consisting of layers of lead, stainless-steel (or copper), and aluminum. The
facility at the Space Science Laboratory at MSFC could be categorized amongst
these. The ultra-low background facilities often have a low background facility
placed in an underground location. The facilities used by Bill Winn at SRL 6 and
by A1 Smith at LBL7 are ultra-low-background facilities.
At SRL the facility is located in a clean room S0 feet underground with the
equivalent of 104 feet of water shielding. (See Figure 1, ref. 6). At LBL, or
rather at their Oroville Dam facility, the facility is located inside the dam under
600 feet of bedrock. Such locations with low-background arrangements make
excellent facilities for very low-activity samples.
The detectors used in these facilities consist of low resolution large volume
NaI detectors mad HPGe and Ge(Li) high resolution gamma-ray detectors which
in some cases possess active shielding in addition to the passive shielding
already described. The NaI(TI) detectors include the 47v detector at JSC as well
as the one at PNWL already mentioned by Jim Reeves in this conference. The
germanium detectors a_re etficiency rated in relation to a 3" diameter, 3" long
NaI(TI) detector at a gamma-ray energy of 1332 keV. Those used in these
studies have efficiencies r,-'mging up to 90%. YVith shielding and electronics
these ,',,re definitely state-of-the-art systems.
Analysis
Most radioactivity studies are done with very small, moderately active
samples placed 10-25 cm from the detector. Such a sampl e is considered to be
a point source and the determination of the aCtivity is greatly simplified.
However, the LDEF samples are quite large and must be counted in close
proximity to the detectors to accurately determine gamma-ray yields. In order
113
to properly determine the activity, the efficiency of the detector for such an
extended source must be determined and the correction for the self attenuation
of the source must be made.
Each laboratory is responsible for determining the efficiency of their
detectors and correcting for the self attenuation. The unique experimental
arrangements of each iaboratory prevent outside determination of these
factors. However, to facilitate such corrections 2" x 2" mixed gamma-ray
sources have been made by Charles Frederick of TVA and MSFC has prepared a
stack of 2" x 2" stainless steel absorbers. These have been distributed to the
counting laboratories to establish common reference data.
Figure 1 shows the efficiency of the HPGe detector at SSL/MSFC along with
the fit found using an appropriate energy-dependent function:
lnE = a/ET+ b + c (lnET) + d(lnET) 2 + e(lnET) 3.
Figure 3 of reference 6 shows similar effic!encies for the Los Alamos detectors
at several different distances made with one of the 2" x 2" sources. These
curves are typical of those obtained for ttPGe detectors:
Since the absolute efficiency of these detectors decreases with distance
between source and detector and with the increase of material between them,
the absolute gamma-ray activities required a correction to the measured
gamma-ray rate, Various laboratories have developed their own correction for
..=
efficiency and self attenuation. Bill Winn_ has done a careful study of his
systems and has developed an excellent model for these corrections. A similar
model made for the MSFC detector has been incorporated into the inverse-
square, self-absorption program EFFATN. This program was originally :
developed to correct spectra obtained from intentional samples activated with
200 MeV neutrons, s Figure 2(ref. 5) gives the activity of the 122 keV gamma
rays from STCo taken through increasing thickness of stainless steel. The solid
line is the predicted activity including inverse-square and self-absorption
attenuation of this gamma ray. The plot was made using the average of the
corrected activity through each absorber thickness. The "poor" fit at zero
thickness may be due to incomplete correction for gamma-ray summing into
the continuum. Such studies (results) indicate that these corrections can be
accurately determined.
Table C-l.c of reference 7 from SRL shows an example of the exchange of
samples. Consistent with the sharing of calibration sources and the exchange of
samples is the goal of assuring that the results from the various counting
laboratories are consistent. To date the data from the laboratories have been !
i
114 _-[
very consistent. A few comparisons currently being studied are shown in
figures 3-6. Alan Harmon will present additional results in the next paper.
To date, a large set of resultant specific activities have been reported.
Primarily, these have been from SRL, LLNL and LANL. They have been
compiled to a spreadsheet and are currently being reviewed. They are being
correlated with the position on LDEF so that the effects of the surrounding
material may be determined. The shielding provided by the covering material,
the moderation of thermal neutrons and the production of secondary particles
will be studied in relation to sample specific activity.
The specific activity of each LDEF sample depends on the flux of activating
particles, on the half-life of the decaying nuclei, and on the production cross
section for the particular nuclei. The sample activities, therefore, will be
significantly different from those originally estimated. The most significant
reason for this is the rapid lowering of the LDEF orbit in the iast two years and
the transition from a period of minimum solar activity to one of maximum solar
activity. Figure 7 shows the trapped proton flux at a proton energy of 50 MeV
for various times in the LDEF-1 mission.a Figure 8 shows the projected
activation rate for S4Mn for the same time period. Obviously, short-lived
radioisotopes will significantly decay in the last few months in the orbit. A
careful prediction of the activation taking into account these reduced fluxes
must be made for activation comparisons.
Archival
As the counting of the samples is nearing completion, we are planning the
archival of the specific activities and the spectra obtained at the counting
laboratories. To be stored with the spectra is a directory containing the specific
details of the activation study. These include sample material, shape,
dimensions, mass, and location on the spacecraft as well as detector efficiencies,
energy calibrations, and self-attenuation corrections.
Perhaps as many as 1000 spectra will be available for future review. The
spectra in the original binary format will be stored on a convenient magnetic
media in a specified computer center. A catalog will specify the data
acquisition system(ADCAM, ND66, Canberra 100, etc.) and will give the data
format. Programs to change from one format to another will be available as
well as general procedures to change to other formats. Figure 9 shows a
sample spectrum from SRL which has been changed from the original ADCAM
115
format to that for a Tennelec/Nucleus PCA-II system. Automatically analyzed
peaks are indicated in the figures.
In addition, the spectra will be trm_slated to a text format. Table 2 is a
tentative sample of such a file from the SRL spectra. Included in the file is a
channel number indicator in column 1 and a header giving the name of the
original file and other pertinent details from the header of the spectrum file.
Other data will be available in the overall directory.
Hopefully, if we can obtain the spectra and analysis from the counting
laboratories in this calendar year(1992), then the archived spectra will be
available during calendar year1993 for outs!d e users. - Th_en, _scientists and
engineers needing information concerning: activation of spacecraft material in
low-earth orbit will have a source of data that can greatly aid:them in their •
individual projects.
: ' = 7
£7
h
116
REFERENCES
1. Harmon, B. A., Fishman, G. J., Parnell, T. A., and Laird, C. E. : Induced
Radioactivity in LDEF Components, p. 301-311, part 1, First Post-Retrieval
Symposium(1991).
2. Michel, R., Peiffer, F., and Stuck, R.: Nucl. Phys. A441, 617(1985):
and references therein.
3. Laird, C. E." Study of Proton and Neutron Activation of Metal Samples in Low-
Earth Orbit, July, 1985, Final Technical Report, NASA Contract NAS8-35180.
4. ACTL82- a compilation of proton and neutron activation cross sections,
Brookhaven National Laboratory( 1982).
S. Laird, C. E.: Study of Activation of Metal Samples from LDEF-1 and Spacelab-
2, July, 1991, Final Technical Report, NASA Contract NAS8-36649.
6. Winn, W. G.: Gamma-Ray Spectroscopy of LDEF Samples, USDOE SRS Report
WSRC-RD-91-16 (February 19,1992).
7. Moss, C. E., and Reedy, R. C.:Measurements of Induced Radioactivity in some
LDEF Samples, p. 271-285, part 1, First Post-Retrieval Symposium(1991).
8. Watts, J. w., Jr.: Predictions of LDEF Fluxes and Dose Due to Geometrically
Trapped Protons and Electrons, Nucl. Tracks Radiat. Meas. 20, 85(1990).
117
Table 1. Counting Facilities
and Associated Scientists
Dr. Gerald Fishman
Dr. B. Alan Harmon
NASA/Marshall Space Flight Center(NASA/MSFC)
Dr. Ronald L. Brodzinski
Dr. James Reeves
Pacific Northwest Laboratory(PNWL)
Batelle Memorial Institute .... :
Dr. Alan R. Smith
Donna L. Hurley ....
Lawrence Berkeley Laboratory(LBL)
Dr. Calvin E. Moss
Dr. Robert C. Reedy
Los Alamos National Laboratory(LANL) . :
Dr. David C. Camp
Lawrence Livermore National Laboratory(LLNL)
Mr. Charles Frederick
Tennessee Valley Authority(TVA)
Western Area Radiological Laboratory
Dr. David J. Lindstrom
NASA/Johnson Space Center(JSC)
Dr. Bill Winn
Westinghouse Corporation
Savannah River Laboratory(SRL)
118
Table 2. Sample Text File for
Archived Gamma Ray Spectra
SPECTRUM FILE:I791NASA.PCA
SAMPLE COMPOSITION:ALUMINUM
START DATE:JAN 16, 1991
START TIME:f0:23:00 am
REAL/CLOCK TIME: 254?00
ELAPSED/LIVE TIME: 231216
seconds
seconds
CHANNEL COUNTS
1 0 0 0 0 0
9 0 0 0 0 0
17 0 0 0 0 0
25 133 141 136 132 136
33 145 132 125 121 117
41 104 I00 98 114 102
49 138 118 123 117 122
57 105 104 106 98 If5
65 103 99 101 119 87
73 105 107 102 102 90
81 114 94 I00 103 87
89 109 106 90 96 I01
97 88 93 84 95 92
105 115 88 92 123 112
113 112 88 91 103 99
121 107 93 109 95 105
129 195 149 82 90 I00
137 116 102 101 120 124
145 122 101 127 126 106
153 136 113 139 170 155
161 112 107 102 112 113
169 137 138 186 165 160
177 178 136 125 127 131
185 122 215 374 369 245
193 98 113 108 108 98
201 126 96 110 109 110
209 123 105 115 120 117
217 114 115 107 117 I19
225 133 124 130 142 121
233 II0 130 125 120 108
241 112 131 121 134 132
249 128 116 113 131 123
257 153 118 139 125 126
265 132 136 140 123 126
273 121 140 123 149 133
281 121 137 152 138 137
289 139 166 184 189 160
297 135 156 154 125 144
305 136 143 131 152 148
313 164 138 154 161 159
321 146 146 161 154 137
329 155 134 186 172 177
337 148 148 134 172 147
0
0
0
122
146
95
99
117
91
90
103
123
93
88
91
113
98
97
125
133
108
123
124
153
107
113
123
132
135
119
108
133
144
131
136
166
140
141
124
153
167
143
178
0
0
0
131
103
116
109
96
89
90
I01
102
87
106
112
164
88
101
164
I00
114
125
IIi
121
122
131
119
115
124
116
124
129
133
129
140
106
133
128
179
126
139
168
158
0
0
21
127
117
127
106
108
97
81
122
79
76
94
105
207
99
102
190
130
127
148
119
107
118
91
111
130
107
121
134
136
107
130
122
148
152
136
158
139
138
162
157
119
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75 1 O0 I 000
Gamma Ray Energy (keY)
Efficiency Curve for SSL/MSFC Detector
122 keV Gamma Ray
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1.5E4
1.4E4
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Figure 2.
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Thickness (or'n)
Yield of 122 keV Gamma Ray vs. EFFATt,J Prediction
120
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Figure 5. RHG Trunnion Layers(Earth Fnd) Counted
Face Up and Face Down.
15
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Figure 6. RHO
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Figure 7.
500 1000 1500 2000
Time in Orbit (Doys)
Proton Flux at 50 MeV as a Function of Time in Orbit
o
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Figure 8. ProducLion of Mn-54- os a Function of Time in Orbit
123
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Help File Calc _etup Option_ Mode Quit 2_9E
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I i
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Px,eset: 8
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Fiqure 9. Sample of SRL Soectra converted to
PCA-II Format.
|
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124


Collection, analysis, and archival of LDEF activation data

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