Journal of Radioanalytical and Nuclear Chemistry
Title page
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Names of the authors: Raphael Haudebourg, Pascal Fichet
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Title: A non-destructive and on-site Digital Autoradiography-based tool to identify
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contaminating radionuclide in nuclear wastes and facilities to be dismantled
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Affiliation(s)
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DEN/DANS/DPC/SEARS/LASE, F-91191 Gif-sur-Yvette Cedex, France
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E-mail address of the corresponding author: raphael.haudebourg@cea.fr
and
address(es)
of
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1
the
author(s):
CEA
Saclay,
Journal of Radioanalytical and Nuclear Chemistry
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A non-destructive and on-site Digital Autoradiography-
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based tool to identify contaminating radionuclide in
11
nuclear wastes and facilities to be dismantled
12
Raphael Haudebourg1, Pascal Fichet1
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1
CEA Saclay, DEN/DANS/DPC/SEARS/LASE, F-91191 Gif-sur-Yvette Cedex, France
Abstract
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As part of the recent developments of Digital Autoradiography-based methods in the
16
context of nuclear dismantling (alpha and soft-beta on-site detection, mapping, wastes
17
and sample characterization), two novel approaches are proposed to enable a preliminary
18
identification of the contaminating radionuclide. An energy-storage radio luminescent
19
phosphor screen stacking method is described, and can be completed with a comparison
20
method between two different types of screens, for the case of non-through radiations
21
(alpha, H-3 and C-14 beta, Fe-55 X-rays). Tests were carried out on fifteen common
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radionuclides as well as on real samples, and a fast stack-scan method was developed, to
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provide industry-ready operational tools.
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25
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Keywords
Dismantling, digital autoradiography, phosphor screen, radio luminescent, alpha, beta
Introduction
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The characterization of the radiological status of a nuclear installation to be dismantled is
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one of the most crucial steps in the decommissioning process. Representative and
29
accurate measurements have to be performed on the structure of the facility (floor, walls,
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Journal of Radioanalytical and Nuclear Chemistry
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ceiling, tanks, chemical fume hoods, furniture, piping, wiring…) and on various kinds of
31
wastes originated from it (scrap, rubble, dust). Such measurements are undertaken in off-
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site dedicated nuclear laboratories, which implies expensive and tedious radioactive
33
material transport and raises the question of sampling (type and spatial resolution).
34
Moreover, discriminating analyses like spectroscopies, liquid scintillation counting…, are
35
generally preceded by sample preparation steps (solution treatments and radiochemical
36
separations) and sometimes followed by decontamination steps. Overall analysis duration
37
is therefore time- and manpower consuming, and nuclear waste-producing, which in turn
38
necessitates a compromise to limit their number while at the same time ensuring
39
sampling representativeness.
40
In this context, it is easy to understand why preliminary in-situ and non-destructive
41
measurements, even only semi-quantitative, are of initial interest in the decommissioning
42
process. Each piece of information on the location, the nature, the homogeneity and the
43
activity of contamination is a precious clue for sampling policy definition and laboratory
44
protocol optimization. To this purpose, a wide range of operational efficient tools have
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been developed, with specifications and corresponding limitations. X-ray and gamma
46
detection is performed with probes and cameras [1], which are not sensitive to other
47
radiations. They are therefore accompanied by complementary beta probes (e.g. SB beta
48
probes series by CANBERRA), which are not very sensitive to very-low energy beta
49
radiations (H-3, C-14, Ni-63…), and by alpha probes or alpha camera [2] (the alpha
50
camera being ineffective for facility investigation because of the usual inability to totally
51
remove light pollution).
52
To address this issue, the repurposing of Digital Autoradiography (DA) from
53
biomolecular research to nuclear dismantling was proposed by the Laboratory of
54
Analyses and Operators’ Support (LASE) at CEA Saclay: in recent publications [3-5], we
55
showed how this technique could be of the most useful help in the dismantling process,
56
either for facility radiological mapping or for sample and waste characterization. DA is
57
non-destructive, sensitive to all types of radiations (including alphas and H-3-emitted
58
betas) and to both labile and fixed contamination (in comparison to wipe tests), and is
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carried out in-situ, requiring neither operators’ presence nor power supply during
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radioactivity imaging. It consists of the exposure of a reusable two-dimensional screen to
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the area to investigate. Particles emitted from a contaminated surface induce partial
62
ionization of the radiosensitive layer of the screen and electron-hole pair trapping in
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metastable sites. A latent energy, proportional to particle flux and exposure time, is thus
64
stored in the screen. After exposure, the energy is released in the form of near-ultraviolet
65
photons (390 nm) by laser stimulation in a dedicated small-sized scanner. A high-
66
resolution image is finally obtained, showing ionized areas [6]. Screens can then be reset
67
to their initial state by exposing them to an intense white light for a few minutes, and are
68
reusable thousands of times. Thus the technique produces very little waste.
69
In the earliest developments of the technique in the dismantling application, the purpose
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was firstly to locate possible contamination spots, and to characterize them in terms of
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homogeneity and relative intensity. Contaminating radionuclide identification was then
72
undertaken at the laboratory, after sampling, transport, sample preparations and analyses
73
in the different apparatuses. Thanks to a library of calibrations, the relative intensities of
74
spots observed in autoradiographs could be linked to activities on the whole investigated
75
area in a semi-quantitative way. However, the ability to achieve the identification of the
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contaminating radionuclide (or the main contaminating one) directly through the first DA
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investigation would be a desirable advance in the performances of the method.
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Two main approaches to recognize the nature and the energy of an ionizing particle
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directly through DA could be found in the literature. The first one, proposed by Takebe et
80
al., consisted of comparing the optical signals obtained at two different LASER
81
wavelengths during the scanning step [7-9]. They observed that the energy loss dE/dx of
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the incident ionizing electron (and, in turn, its initial energy) was correlated to the ratio of
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the signals obtained with a stimulation at 600 nm and at 500 nm. The main limitation of
84
their approach in our context was that they used 100 keV band-wide electron beams
85
provided by transmission microscopes, providing a number of particles equivalent to an
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exposure of 24 hours to a source of 150 MBq/cm². This is hardly comparable to the
87
orders of magnitude met in dismantling applications. The second one, proposed by
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Zeissler et al., was based on pixel intensity spectra calculations and plots [10-12]:
89
intensity frequency distribution around one event was a fair indicator of particle type, but
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this approach is relevant in the case of particle by particle counting, which is obviously
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usually not the case in contaminated spots.
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The latest work, presented in this paper, aimed at developing a new autoradiographic
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operational method based on the stacking of several screens, in order to deduce particle
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type, energy, and, thus, identity. The decrease of the signal through the successive
95
screens was expected to provide such information. In the case of non-penetrating
96
particles, a comparison between the signals obtained on two different types of screens
97
was assumed to enable recognition.
98
99
Experimental
Emission energies called upon in the paper stemmed from LNHB’s (Laboratoire National
100
Henri Becquerel, France, www.nucleide.org) recommended data.
101
Two different DA systems were used to provide the results presented herein: Cyclone
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Plus by Perkin Elmer and Typhoon FLA-7000 by GE Healthcare. The Cyclone Plus
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system was involved in all experiments except those described in the subsection “Design
104
of a stack to be scanned in one run”; the Typhoon system was involved in the subsection
105
“Design of a stack to be scanned in one run” only. The system is anyway specified for
106
each experiment.
107
When scanning the screen, pixel size was always set to its highest value (169 µm for the
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Cyclone Plus system, 200 µm for the Typhoon system), because highest sensitivity and
109
shortest scan duration were preferred to sharper resolution (as it is usually the case in the
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dismantling context).
111
Screens were type TR by Perkin Elmer, and of a size of 12.5 x 25 cm² (see fig. 1). TR
112
stands for “tritium”, meaning that such screens do not have the usual protection overcoat,
113
in order to be sensitive to 3-H beta radiations (6 keV). In subsection “Identification of
114
non-penetrating radiations”, MS screens by Perkin Elmer (size of 12.5 x 25 cm²) were
115
used. MS stands for “multi-sensitive”, and MS screens radiosensitive layer is coated with
116
a thin protection layer. n screens were stacked in a pile for exposure (see fig. 2); in most
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experiments, n was 7 or 10, but depending on practical constraints, a few experiments
118
were carried out with n=3. In section 5 “Design of a stack to be scanned in one run”, TR
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screens by GE Healthcare (size 20 x 40 cm²) were diced down with a steel punch to make
120
small circular elementary screens (diameter of 5 cm).
121
122
Fig. 1 Picture of a TR screen (12.5 x 25 cm²) by Perkin Elmer for system Cyclone Plus
123
124
Fig. 2 Screen stacking pattern; screens were actually in contact to each other and sources,
125
which were standard or samples placed at the top of the stack
126
Radioactive sealed sources were purchased at CERCA LEA (France) and at American
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Radiolabeled Chemicals (USA). Characteristics are listed in table 1.
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Table 1 Details of all sealed surface sources used in the study
Radionuclide
Th-232
U-233
Emissions per disintegration (mean energy)
1 α (3995 keV)
1 α (4817 keV)
Pu-239
1 α (5148 keV)
Cm-244
1 α (5795 keV)
6
Source #
1
2
3
4
5
6
7
Type
A
A
A
A
A
B□
A
Activity (Bq)
584
330
320
310
2930
434
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Journal of Radioanalytical and Nuclear Chemistry
Am-241
1 α (5542 keV), 0.38 X (17 keV),
0.38 γ (58 keV)
H-3
1 β (6 keV)
C-14
1 β (49 keV)
Pm-147
Tl-204
1 β (62 keV)
0.97 β (244 keV)
Cl-36
0.98 β (316 keV)
Sr-90 / Y-90
1 β (562 keV)
Cs-137
1 β (188 keV), 0.85 γ (662 keV)
Cs-134
0.75 eA (7 keV), 0.35 ec (85 keV), 0.28 β
(279 keV), 0.87 X (35 keV), 1.5 γ (704 keV)
1 β (157 keV), 2.23 γ (697 keV)
Co-60
1 β (96 keV), 2 γ (1253 keV)
Na-22
0.90 β+ (216 keV), 2.81 γ (1359 keV)
Fe-55
0.29 X (6 keV)
Eu-152
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
A
A
A
A
A
C
D
D
B○
B○
B□
B□
A
A
A
A
A
A
A
B□
B□
A
A
A
B□
B□
B□
E
A
A
B○
B□
B□
C
C
C
A
A
A
B□
B□
C
A
C
C
265
3008
248
320
3290
453500
22 to 80000
0.43 to 49000
3282
3318
5899
3380
3844
41
5678
99
3330
3007
3472
6332
5831
83
3250
2542
3553
4247
5580
≈ 0.2
3100
2610
4428
3232
5800
315000
330000
25000
17
3100
1420
123
4262
167000
724
82000
1055
Journal of Radioanalytical and Nuclear Chemistry
Co-57
Ba-133
Mn-54
Y-88
0.59 X (6 keV), 1.06 γ (114 keV)
1.36 X (29 keV), 1.35 γ (266 keV)
0.26 X (5 keV), 1 γ (835 keV)
0.63 X (14 keV), 1.94 γ (1372 keV)
53
54
55
56
57
58
59
C
C
C
C
C
C
C
5042
1996
5045
3622
17000
7426
35000
Emissions refer to the non-negligible radiations emitted by the radionuclide at disintegration. α=alpha,
β=beta, γ=gamma, X=X-ray, eA=Auger electron, eC=conversion electron.
Types refer to commercial catalogs:
- “A” refers to disk-shaped sources of a few cm²: see CERCA LEA catalog, “sources ponctuelles et
étendues”, « sources beta »
- “B” refers to homogeneous disk-shaped sources (○) or rectangle-shaped sources (□) of 15 to 150
cm² with very low spatial variations of the activity: see CERCA LEA catalog, “sources ponctuelles et
étendues”, « Sources de référence pour la radioprotection»
- “C” refers to isolated X-ray and gamma sources of a few mm² (possible alphas and low energy electrons
are stopped by an attenuating layer): see CERCA LEA catalog, “sources ponctuelles et étendues”,
« sources gamma »
- “D” refers to rectangle-shaped standards of 35 mm²: see ARC catalog, autoradiography standards, H-3
or C-14 standards on glass slides
- “E” refers to home-made precipitates on filters (1 µL of radioactive solution dropped on a 1 cm diameter
filter, which was then coated with Mylar)
129
130
To illustrate the application of the method to practical issues, two examples were chosen
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to show how DA was able to provide useful information concerning the nature of the
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contamination of real samples: steel blocks (as shown on fig. 3), and concrete drilled
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cores (as shown on fig. 4). These samples were intended to be fully analyzed through
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conventional (including destructive) techniques at the LASE.
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136
Fig. 3 Contaminated steel sample to be analyzed
137
138
Fig. 4 Contaminated half drilled core arrangement for exposure of a phosphor screen
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(TR, Perkin Elmer).
140
All exposures were carried out in opaque black boxes, at the laboratory, in a gamma
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background similar to the natural one. Between the end of the exposure and the insertion
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of the screen in the scanner, screens were carefully protected from light, to prevent signal
143
loss.
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144
Background contribution was measured and subtracted from the raw pixel intensity signal
145
to obtain the net signal (see a typical autoradiographic image on fig. 5). In the particular
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example of drilled core analysis, the irrelevant contribution of natural K-40 emissions
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(508 keV mean-energy betas and 1461 keV gammas) was evaluated thanks to the
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exposure of the screens to a non-contaminated core of the same origin and dimensions,
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and subtracted from the raw signal as well.
150
151
Fig. 5 An example of autoradiographic image resulting from the exposure of a 12.5 x 25
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cm² TR screen to sources 23 (Cl-36, left) and 29 (Sr-90, right) for 18 h 34 min; red
153
rectangle corresponds to background measurement. System Cyclone Plus
154
The net signal measured on the screen nth screen of the stack was called Sn (n=1 referred
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to the screen in contact with the sample), as illustrated in fig. 6; the sequence Sn+1/Sn was
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plotted for each experiment, in order to directly compare fading- exposure-time- and
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activity-independent sequences between sources and radionuclides. Fading refers to the
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spontaneous self-erasing of the signal due to thermal agitation-induced release of the
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electrons from the metastable sites. This phenomenon occurs during exposure and during
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the delay between end of exposure and scanning. The relative signal loss depends on
161
screen temperature and on time. The sequence Sn+1/Sn (as displayed in fig. 7) was
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preferred to the sequence Sn/S1 because it stressed more efficiently differences between
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sequences.
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Journal of Radioanalytical and Nuclear Chemistry
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165
Fig. 6 Sn sequence for source 36 (Cs-137) using an exposure of 4 h 06 min of a stack of
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10 TR screens; system Cyclone Plus
167
168
Fig. 7 Cs-137 mean Sn+1/Sn sequence for sources 36 to 41. Standard deviations for each
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ratio are displayed as well. Deviations higher than 5 % could be explained either by
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strong differences in sealed source preparation (resulting in different matrix and sealing
171
layer auto-attenuation effects), or by differences in the delay between end of exposure
172
and scanning (experiments involved exposure places located at different distances from
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Journal of Radioanalytical and Nuclear Chemistry
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the scanning place). Standard deviations ranged between 3 % and 18 % (in S n+1/Sn units).
174
Screens type was TR and the system Cyclone Plus
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Results and discussion
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Repeatability & intermediate precision
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In order to assess the feasibility of radiation identification, preliminary and basic
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precision tests were carried out, as summarized in table 2. Whether it was for
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repeatability or for screen to screen precision, relative standard deviations ranged around
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5 % of mean value. Regarding source to source intermediate precision with 11 different
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sources of Sr-90/Y-90, relative standard deviations were 12 % of mean value for S2/S1
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and 5 % for S3/S2. Displayed in Sn+1/Sn units, i.e. in %, these variations around the mean
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value could be expressed as S2/S1 = 35 ± 4 % and S3/S2 = 58 ± 3 %. For Cs-137 (sources
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36 to 41), which is a common radionuclide in the context of dismantling, very similar
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standard deviations were found: S2/S1 = 18 ± 4 % and S3/S2 = 37 ± 3 %. In the higher
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ranks of the sequence (i.e. S4/S3 to S10/S9), a poorer precision was observed, as shown on
187
fig. 7: standard deviations ranked between 6 % and 18 % (in absolute value of Sn+1/Sn
188
units). Identification protocol was therefore set in the following manner: the values of
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S2/S1 and S3/S2 for the sample to investigate are first compared to the values of S2/S1 and
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S3/S2 for different standards of the possible radionuclides. Thus, high precision and
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strong differences between standards (as shown in next section) are expected to provide
192
identification of the contaminating radionuclide. In case of remaining doubt, the rest of
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the sequence can be used as additional marks, but with higher care, due to the lower
194
precision.
195
Table 2 Summary of preliminary precision tests. Perkin Elmer Cyclone Plus scanner and
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TR screens
Test
Repeatability
Screen to screen
intermediate precision
Description
Standard deviations
10 exposures (5 min) of 1 screen to source 3 5 %
10 exposures (5 min) of 10 screens to source 5 %
3
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Journal of Radioanalytical and Nuclear Chemistry
Source to source 11 exposures of 10 screens
intermediate precision to sources 29 to 35 (Sr-90)
6 exposures of 10 screens
to sources 36 to 41 (Cs-137)
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4 % for S2/S1
3 % for S3/S2
4 % for S2/S1
3 % for S3/S2
Identification of radionuclides by screen stacking method
198
The sequences Sn+1/Sn for all investigated radionuclides are displayed on fig. 8, excepted
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for Th-232, U-233, Pu-239, Cm-244 (alpha emitters without significant X-ray emissions),
200
H-3 (6 keV-beta emitter) and Fe-55 (6 keV-X-ray emitter), for which no signal could be
201
detected on screen 2 (non-through particles).
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Journal of Radioanalytical and Nuclear Chemistry
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Fig. 8 Sn+1/Sn sequences of the 15 radionuclides for which a significant signal could be
203
detected on screen 2. Non-penetrating radiations were alpha, H-3-emitted beta, and Fe-
204
55-emitted X-ray. Graphs are ranked according to the mean energy released per
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disintegration for each type of emission (O: beta emitters, ∆: beta + gamma emitters and
206
alpha + gamma emitter Am-241, X: X-ray or gamma emitters). Data correspond to the
207
average value of the different sources for each radionuclide. Screens type is TR and the
208
system Cyclone Plus was used.
209
For beta-emitting radionuclides, the last screen on which a signal could be detected was
210
as expected linked to the maximum energy of the emitted electrons: screen 1 for H-3 (20
211
keV), screen 2 for C-14 (150 keV), screen 3 for Pm-147 (220 keV), screen 6 for Cl-36
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(710 keV), screen 10 for Tl-204 (760 keV). For Sr-90/Y-90 (2280 keV), a significant
213
energy could still be stored and imaged on the last screen of an experiment involving 20
214
screens. Although transmission behavior could not be modeled simply, results trended to
215
prove that it should be very easy to deduce beta-emitting radionuclide from the sequence.
216
For photon-emitting radionuclides, a plateau (of a value roughly ranging between 80 %
217
and 100 %) could be observed from a given rank in the sequence. This plateau
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Journal of Radioanalytical and Nuclear Chemistry
218
corresponded to ionizations only caused by photons, while the range of other particles
219
like alpha, beta, and Auger electrons) was limited to the first screens only. For example,
220
the comparison between Cs-137, Co-60 and Am-241 (three very common radionuclides
221
in dismantling) is detailed: for Cs-137, emitted beta particles have a maximum energy of
222
514 and 1176 keV, and therefore easily reached the sixth screen, while screens 8 to 10
223
were only irradiated by 662 keV gamma photons; for Co-60, beta particles (317 keV
224
maximum) barely reach the third screen and the plateau started for smaller n than Cs-137,
225
but was located at a higher value due to the higher energy of the gamma (1250 keV) (the
226
higher the energy, the lower the interaction probability); for Am-241, alpha particles
227
released all their energy in the first screen only (hence the very low value of S 2/S1), and
228
the plateau was located at an even smaller value due to the lower energy of the photons
229
(around 40 keV).
230
The first conclusion was that, in this ideal case of one radionuclide involved and of
231
laboratory experiments with a limited number of sealed sources, radionuclide
232
identification could be measured, even taking account of method imprecision detailed in
233
section 1, except for: non-through radiation (alpha without photons, H-3 beta, C-14 beta
234
at very low doses, Fe-55 X-ray), pure gamma-emitters (Co-57, Ba-133, Mn-54 and Y-88,
235
whose signatures were very similar), and Cs-134, for which limited experimental
236
conditions (stack of 3 screens) led to a poorly-specific signature.
237
The next step consisted in developing a method to perform the discrimination between
238
non-through radiations, for which screen stacking method was irrelevant.
239
Identification of non-penetrating radiations by screen type method
240
Non-penetrating radiations that could not be identified through screen stacking method
241
could be discriminated through a “MS/TR” method, which simply consisted in
242
calculating the ratio of the signal measured with a MS type screen on the signal measured
243
with a TR type screen, in identical exposure conditions, as shown in table 3.
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Journal of Radioanalytical and Nuclear Chemistry
244
H-3-emitted beta particles lost most of their energy in the protective overcoat of a MS
245
screen, which resulted in a very weak response and in the lowest MS/TR ratio (0.001).
246
Alpha particles could easily be detected on the MS screen, but the response, for the same
247
reason, was lower than on the TR screen, and ratio values ranged around 0.5; moreover,
248
these values were as expected ranked according to the mean energy of the alpha
249
radiation. For all other beta emitters investigated, beta particles easily reached MS screen
250
radiosensitive layer. Because MS phosphor layer was more radiosensitive than TR
251
phosphor layer, MS/TR ratio value was then higher than 1 (with the higher energy, the
252
higher MS/TR ratio). This was also the case of beta + gamma emitters, because the
253
radiosensitive layer of screens is far more sensitive to beta particles than to photons. The
254
MS/TR ratio value for Fe-55 X-rays was found to be 2.0. Finally, MS/TR ratio value for
255
natural radioactivity due to K-40 presence in uncontaminated concrete was 7.4.
256
Table 3 ratios of signal measured with a MS type screen on signal measured with a TR
257
type screen in identical exposure conditions.
Radionuclide
H-3
U-233
Pu-239
Am-241
Cm-244
C-14
Fe-55
Co-60
Cs-137
Tl-204
Cl-36
Sr-90/Y-90
K-40 (concrete)
Emission (mean energy)
beta (6 keV)
alpha (4817 keV)
alpha (5148 keV)
alpha (5542 keV)
alpha (5795 keV)
beta (49 keV)
X-ray (6 keV)
beta (96 keV) + gamma (1253 keV)
beta (188 keV) + gamma (662 keV)
beta (244 keV)
beta (316 keV)
beta (562 keV)
beta (510 keV) + gamma (1460 keV)
MS/TR
0.001
0.39
0.46
0.49
0.55
1-1.6
2.0
3.0
4.7
5.0
6.1
6.5
7.4
Radionuclides are ranked in the table according to MS/TR ratio values. Contrary to other
radionuclides, MS/TR ratio value for C-14 was much dispersed (0.98 for source 15, 1.44 for
source 16, 1.15 for source 17, and 1.64 for source 18, mean value for the 4 sources: 1.3); this
would be consistent with the strong influence of source matrix effects and beta particles auto
attenuation at these low energies.
258
Examples of application to real samples
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Journal of Radioanalytical and Nuclear Chemistry
259
The non-destructive methods described in previous sections were applied to various real
260
samples from dismantling sites and sent to the LASE (cores, pieces, rubble, crystalline
261
material, wastes etc.). Two analyses corresponding to the samples introduced on fig. 3
262
(steel blocks) and 4 (drilled cores) are presented.
263
Regarding steel blocks, preliminary autoradiographic measurements carried out on their
264
two main sides and on their edges proved that only one main side was contaminated and
265
not the other one, that contamination was roughly homogenous, and that contamination
266
depth was very small (less than one millimeter). Then, a screen stack was exposed to the
267
sample as well as to a few sources of radionuclides commonly encountered in
268
dismantling: various alpha emitters (from table 1), H-3, C-14, Cl-36, Sr-90, Cs-137, Co-
269
60, and Am-241. Experimental results are displayed in fig. 9, and strongly suggested a
270
contamination by Cs-137 mainly, which was later confirmed by accurate destructive
271
analyses. Accurate autoradiographic calibrations with sealed sources enabled an
272
evaluation of surface activity. All these results, i.e. preliminary assessments, qualitative
273
identifications, and activity quantitation, were helpful not only to design contamination
274
removal and analysis protocols, but also to support the results from other conventional
275
measurements results.
276
277
Fig. 9 Steel sample (as depicted on fig. 3) autoradiographic analysis through screen
278
stacking method (7 screens, TR type, Cyclone Plus system); screen stack was exposed to
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Journal of Radioanalytical and Nuclear Chemistry
279
standards (sealed sources) in the same conditions as it was to samples (i.e. same exposure
280
time of 18 hours 11 minutes, and sources placed in plastic bags, as the sample was)
281
Regarding drilled cores, preliminary autoradiographic measurements carried out on their
282
vertical surfaces (after cutting into two halves) showed a significant signal (fig. 10), and
283
provided a fair estimation of contamination depth. Installation history suggested that
284
contamination could be probably due to C-14. A first identification attempt was carried
285
out through the screen stacking method: no signal was detected on screens ranked higher
286
than second, and traces of signal (below quantitation limits) were detected on the second
287
screen, which was in agreement with installation history (contamination with C-14
288
mainly). The challenge was then to check whether contamination was only due to this
289
radionuclide, or if other ones - such as H-3, or pure alpha emitters etc. - were involved as
290
well. To this purpose, a MS/TR comparison experiment was set up. A value of 1.57 was
291
observed and that automatically excluded alpha emitters and H-3 as mainly
292
contaminating radionuclide. Even though the mean MS/TR ratio value calculated for the
293
four standards of C-14 was 1.3 (i.e. 17 % lower than the value calculated for the core),
294
this could be attributed to the imprecision of the method experienced for C-14; contrary
295
to other radionuclides and as mentioned in section 3, one of the standards had a MS/TR
296
ratio value of 1.64. Thanks to this non-destructive and preliminary approach,
297
contamination location (depth, hot points) and activity (in Bq/cm²) could be estimated on
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the vertical surface of the drilled half core.
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Journal of Radioanalytical and Nuclear Chemistry
300
Fig. 10 Autoradiographic images of the half core (as depicted in fig. 4), obtained with an
301
exposure time of 24 hours of a TR screen (left) and of a MS screen (right); system
302
Cyclone Plus; color scales are identical on both images; measured MS/TR ratio (after
303
background and K-40 contributions withdrawal) was 1.57
304
Design of a stack to be scanned in one run
305
Scanning time for one screen at lowest spatial resolution was 2 minutes 50 seconds on the
306
Cyclone Plus system. In this first development of screen stacking method, taking into
307
account the time it took to name the acquisition, to put the screen inside the scanner, and
308
to remove it after scanning, overall scanning time was close to half an hour for one stack
309
of 10 screens. Therefore, a method was proposed to scan one whole stack in one run
310
(instead of as many runs as screens in the stack). It consisted in stacking hand-cut and
311
small-sized screens instead of the usual screens.
312
Perkin Elmer’s Cyclone Plus scanning system (a cylindrical rotary barrel on the surface
313
of which the screen was kept in place with two metallic clips) was hardly compatible
314
with multiple elementary small screens that were to be scanned in one run. On the
315
contrary, GE Healthcare’s Typhoon FLA 7000 scanning system (a simple translating
316
plate on which the screen is put) was much convenient to use. 21 5 cm-diameter circular
317
elementary screens could be made from one 20 x 40 cm² GE Healthcare TR screen, as
318
shown in fig. 11. Elementary screens were stacked in a pile for exposure, and then laid on
319
the plate for scanning. Overall scanning time (including acquisition naming, elementary
320
screens placement on the plate, plate insertion, scanning and plate removing) could be
321
reduced to less than 3 minutes. A few results for major radionuclides are shown on fig.
322
12.
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Journal of Radioanalytical and Nuclear Chemistry
323
324
Fig. 11 Picture of 8 elementary circular TR screens (diameter 5 cm) cut from a GE
325
Healthcare TR screen with a steel punch, in order to scan a whole stack in one run
326
Fig. 12 Sn+1/Sn sequences for 5 common radionuclides in the context of dismantling,
327
obtained with a stack of 10 elementary TR screens such as the ones shown one fig. 7, and
328
a one-run scan (2 minutes) with a GE Healthcare Typhoon FLA 7000
329
Results were comparable to those obtained with the previous method. The main
330
differences were observed on data points corresponding to screens receiving low dose
331
rates, i.e. screens irradiated by photons only (that is, screens 2 to 10 for both Am-241 and
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Journal of Radioanalytical and Nuclear Chemistry
332
Co-60, and screens 6 to 10 for Cs-137). The lower sensitivity of the Typhoon scanner
333
(compared to the Perkin Elmer one) produced a very weak and less precise signal from
334
those screens. A side comparison study (irrelevant in this paper) showed that screens
335
from the Typhoon system were practically identical to those from the Perkin Elmer
336
system, in terms of radio-sensitivity. Eventually, the crucial conclusion of this experiment
337
(specific signature for each radionuclide) was the same anyway.
338
Conclusions
339
In the application, once a contamination spot is observed through DA on a surface (floor,
340
sample, waste etc.), and if a doubt remained as to the presence of radionuclides
341
undetectable or indistinguishable through usual remote detection techniques (cameras,
342
probes), it is thus suggested to proceed to complementary autoradiography
343
measurements. A screen stack exposure, possibly completed with MS/TR comparison, is
344
recommended. Table 4 gathers the results of the study, so that at a glance it appears
345
possible to distinguish the radionuclide that causes the contamination of the material.
346
Table 4 Summary of values of interest for the identification of a radionuclide through
347
screen stacking and MS/TR comparison methods with a Perkin Elmer Cyclone Plus
348
system
Alpha
Alpha + Gamma
Beta
Beta + Gamma
Radionuclide
Th-232
U-233
Pu-239
Cm-244
Am-241
H-3
C-14
Pm-147
Tl-204
Cl-36
Sr-90/Y-90
Cs-137
Eu-152
Cs-134
S2/S1 (%)
0
0
0
0
0.17
0
0.09
2.1
21
27
37
18
35
24
21
S3/S2 (%)
0
0
0
0
61
0
0
20
27
27
63
37
65
17
MS/TR
0.39
0.46
0.55
0.49
0.001
1.4
5.0
6.1
6.5
4.7
Journal of Radioanalytical and Nuclear Chemistry
X-ray
Gamma
Co-60
Na-22
Fe-55
Co-57
Ba-133
Mn-54
Y-88
9.1
16
0
32
53
29
26
90
30
0
77
77
88
66
3.0
2.0
349
350
This paper proposed fully operational tools to identify the radionuclides from the
351
contaminated materials immediately on-site. It was shown that such methods were able to
352
provide crucial information in support of the off-site characterization process
353
(radiochemistry, destructive analyses, decontamination etc.).
354
However, two significant breakthroughs still remain to be made to fully take into account
355
the whole potential of this technique. The first one would be to enable radionuclides
356
identification in the case of a multi-element contamination. Indeed, the method in its
357
current state is efficient in the case of one contaminating radionuclide only (or one main
358
radionuclide, with other ones in negligible activities). The second breakthrough would be
359
to take into account matrix effects in the transmission sequence, because some materials
360
met in dismantling may often show a volume contamination like migration of radioactive
361
molecules, activation by penetrating radiations. In addition, materials may be dense
362
(concrete, steel…) with strong auto-attenuation power, implying dramatic changes in the
363
emerging energy spectrum, and in turn, in the transmission behavior through a stack of
364
screens. Tests with artificial multi-element sources in various matrices are planned for the
365
next developments, along with modeling studies resorting to conventional particles
366
transport codes such as Monte Carlo N-Particle.
367
Acknowledgements
368
The authors would like to thank Pr. J.-C. Bodineau at the INSTN for his precious
369
knowledge and sealed sources, R. Brennetot and C. Colin at LASE for giving us the
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Journal of Radioanalytical and Nuclear Chemistry
370
opportunity to prove our method relevant by analyzing their steel samples, and C. Gallou
371
for her relevant remarks concerning the manuscript.
372
References
373
1. Gal O, Izac C, Jean F, Lainé F, Lévêque C, Nguyen A (2001) CARTOGAM – a
374
portable gamma camera for remote localization of radioactive sources in nuclear
375
facilities. Nucl Instrum Methods Phys Res A 460:138-145
376
2. Mahé C, Chabal C (2013) Recent Improvement of Measurement Instrumentation to
377
Supervise Nuclear Operations and to Contribute Input Data to 3D Simulation Code.
378
Waste Management Conference, Phoenix, Arizona, USA
379
3. Fichet P, Bresson F, Leskinen A, Goutelard F, Ikonen J, Siitari-Kauppi M (2011)
380
Tritium analysis in building dismantling process using digital autoradiography. J
381
Radioannal Nucl Chem 291:869-875
382
4. Leskinen A, Fichet P, Siitari-Kauppi M, Goutelard F (2013) Digital autoradiography
383
(DA) in quantification of trace level beta emitters on concrete. J Radioannal Nucl Chem
384
298:153-161
385
5. Haudebourg R, Fichet P, Goutelard F (2015) Digital Autoradiography as a novel
386
complementary technique for the investigation of radioactive contamination in nuclear
387
facilities under dismantlement. ANIMMA Conference, Lisbon, Portugal
388
6. Takahashi K (2002) Progress in science and technology on photostimulable
389
BaFX:Eu2+ (X=Cl, Br, I) and imaging plates. J Lumin 100:307-315
390
7. Takebe M, Abe K (1994) A novel particle identification with an imaging plate. Nucl
391
Instrum Methods Phys Res A 345:606-608
392
8. Takebe M, Abe K, Souda M, Satoh Y, Kondo Y (1995) A particle energy
393
determination with an imaging plate. Nucl Instrum Methods Phys Res A 359:625-627
23
Journal of Radioanalytical and Nuclear Chemistry
394
9. Takebe M, Abe K, Souda M, Satoh Y, Kondo Y (1995) A wide range of electron
395
energy determination with an imaging plate. Nucl Instrum Methods Phys Res A 363:614-
396
615
397
10. Zeissler CJ, Wight SA, Lindstrom RM (1998) Detection and characterization of
398
radioactive particles. Appl Radiat Isot 49:1091-1097
399
11. Zeissler CJ, Lindstrom RM, McKinley JP (2001) Radioactive particle analysis by
400
digital autoradiography. J Radioannal Nucl Chem 248:407-412
401
12. Zeissler CJ, Lindstrom AP (2010) Spectral measurements of imaging plate
402
backgrounds, alpha-particles and beta-particles. Nucl Instrum Methods Phys Res A
403
624:92-100
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