Full text of DOI:10.1093/oxfordjournals.rpd.a006645

Radiation Protection Dosimetry
Vol. 97, No. 2, pp. 105–108 (2001)
Nuclear Technology Publishing
EXPERIENCES IN THE DETERMINATION OF ACTIVITY IN
THORIUM CONTAMINATED MATERIALS IN WORKING
AREAS, USING GAMMA SPECTROMETER MEASUREMENTS,
FROM THE VIEWPOINT OF PRACTICAL RADIATION
PROTECTION
R. Kreh and J. Stainer
Siempelkamp Nuklear- und Umwelttechnik GmbH & Co
D-47803 Krefeld, Siempelkampstra␤e 45, Germany
Abstract — In industrial processes, classification and clearance of thorium contaminated material is often a time problem. Short-
term decisions have to be made about acceptance limits, industrial safety as the workers are in contact with the contaminated
material, and the environmental aspect regarding the disposal of the wastes. The use of gamma ray spectrometry measurements
is an easy method to estimate the level of activity in materials. Under the precondition of equilibrium of activity within the
thorium decay chain, which can be assumed only with knowledge of the materials’ history, adequate results can be produced in
a reasonable time. The advantages of gamma ray spectrometry are the low investment and operational costs, simple sample
preparation, and a system that is relatively quick and easy to operate.
INTRODUCTION
with gamma ray emission is the gamma ray spec-
trometer with a semiconductor detector, in particular
When carrying out maintenance, dismantling of
industrial plants, and the use and disposal of production
materials (e.g. welding rods, and electric filaments etc.),
it has become necessary to measure radiation in individ-
ual working areas caused by thorium and its decay pro-
ducts. According to EURATOM radiation protection
(2)
high purity germanium (HPGe) detectors . In compari-
son with a scintillation detector, up to 100 times higher
energy resolution can be achieved. The principle is to
generate a voltage signal that is proportional to the
energy produced by gamma-quant in the semiconductor.
Fundamentally a semiconductor detector functions simi-
larly to an ionisation chamber. The average energy
necessary to create an electron–hole pair is only
(
1)
recommendations
, measurements within working
areas are necessary if the dose due to natural radioactive
−
1
materials exceeds 1 mSv.a . The external dose
exposure from thorium and its products can be classed
as being trivial, but measurements of air in working
areas (inhalation), and in effluents are of great impor-
tance. By measuring the activity of incoming materials,
(2,3)
3
eV . Detailed description of the physics, the cali-
bration and the measuring methods of semiconductor
(4,5)
radiation detectors can be found in the literature
.
and from the knowledge of their use and processing, an EQUIPMENT
estimate of the dose exposure in the individual working
The Siempelkamp Nuklear- und Umwelttechnik lab-
oratory has two HPGe detectors (n-type and p-type)
used for gamma ray spectrometer measurements. The n-
type detector with a relative efficiency of 73% is mainly
used for the detection of low activity, in gamma ray
areas can be made. The accuracy of these measurements
depends strongly upon a representative sample, its prep-
aration, and the available analysis time. Costs and the
number of samples to be measured have a practical
influence on the analysing time. With the help of gamma
ray spectrometry measurements of air filters and
material samples, (dependant upon geometry and
nuclides of the thorium decay chain) a minimal detect-
210
emitters of low energy (i.e. Pb). This detector has a
carbon window for gamma ray energy measurements
down to approx. 5 keV. Figure 1 shows the gamma
spectrometric equipment, the measuring chamber with
the detector and the attached power supply with ampli-
fier and AD converter. The software, Gammavision by
EG&G Ortec is used for spectrum analysis. Depending
upon the state of the sample (density,geometry etc.) and
the measuring time, a minimal detectable activity for
nuclides of the thorium decay progeny of between
able activity can be achieved of approx.
3 ϫ
−
3
−3
−3
−1
1
0 Bq.m for filters, 2 ϫ 10 Bq.g for incoming
−
2
−2
materials, and 10 Bq.cm for surface contamination
wipe tests. Analysis time is between 1800 and 54,000 s.
MEASURING METHOD
2
−3
−1
5
ϫ 10− and 5 ϫ 10 Bq.g can be achieved. Table 1
The current method used to identify radionuclides
compares minimal detectable activity, in Bq, for wipe
tests, air filters and material samples in beakers, for vari-
ous measuring times. For clearance of contaminated
material leaving a controlled area, or to classify con-
Contact author E-mail: rainer.krehȰsiempelkamp.com
1
05

R. KREH and J. STAINER
taminated materials for transport, a minimum detectable increased due to a high background. The counting
−
3
activity of at least 6 ϫ 10 Bq for wipe tests must be uncertainty will also increase in the individual energy
reached, which means a measuring time, using the lines.
above equipment, of approx. 90 min. An analysed wipe
test can be used to calibrate a contamination monitor
for direct measurements of large surfaces. If the sample
SAMPLE PREPARATION
contains not only nuclides of the thorium decay chain,
The accuracy of the measurements depends a great
but also nuclides of the uranium decay chain, which is deal upon how the sample has been prepared, and for
usual with NORM (naturally occurring radioactive an estimate of activity in a large quantity of material,
material), then the minimum detectable activity will be upon a representative sample. The gamma ray spec-
trometer equipment is calibrated for various measure-
ments with set sample shapes and sizes. Unfortunately
contaminated material is not always in a shape or size
that can easily be measured. Figure 2 shows three
examples of thorium contaminated material. The left
and middle pictures show two examples of material that
can be poured into a beaker, giving a reasonably homo-
geneous and representative sample.
The right picture shows material used in the pro-
duction of phosphate fertiliser, which is contaminated
with a scale containing thorium and radium. For the
measurement of activity this surface scale must be
removed, milled and then filled into a calibrated
geometry. To determine the total activity, it is necessary
to estimate the surface area of contamination and thick-
ness of scale. Often the analysis results have to be cor-
rected because they differ in density or geometry from
Table 1. Theoretical detection limits for several geometries
calculated by using Reference 6.
Nuclide
Filter/wipe test
Beaker 100 ml
800 s
Bq)
1
(
5
4,000 s
Bq)
1800 s
(Bq)
(
228
228
224
212
212
208
2.0 ϫ 10−1
6.1 ϫ 10−1
Ac
Th
Ra
Pb
Bi
1.1
1.5 ϫ 10
3.7
2.8
6.7 ϫ 10
6.8 ϫ 10
6.2 ϫ 10
7.5
1.9
1.9 ϫ 10
−1
−2
−1
−1
3.8 ϫ 10
3.4
−1
1.9
3.2 ϫ 10
−1
−1
−1
Tl
1.1 ϫ 10
5.8 ϫ 10
Figure 1. Gamma spectrometry equipment; 73% n-type HPGe detector with carbon window for low energy ␥ ray measurement;
power supply.
Figure 2. Examples of thorium-contaminated materials: filaments, welding rods, contaminated scrap.
1
06

ACTIVITY IN THORIUM CONTAMINATED MATERIALS IN WORKING AREAS
the standard calibration. Insufficient sample material in SUMMARY
a beaker must be geometry corrected, using a correction
factor related to the filling height of the material. If Mar-
inelli beakers are used for measurements they surround
the detector, and the sample material must reach up to
the filling mark. If not, a wedge shaped segment can be
placed into the Marinelli beaker that reduces the volume
of the sample material but not the filling height. This
does not effect the efficiency, because there is no sym-
metrical change in the measurement arrangement. The
density can be corrected before the measurement
through dissolving the sample in water, or adding a
specifically lighter material that is inactive (e.g. poly-
styrene particles), but the mixture must be homo-
geneous. Because of the costs and especially the inten-
sive time needed for sample preparation, which is, in
the industry, not always possible, the measurements can
be corrected after analysing the sample. The software
Measurements of thorium in working areas using a
gamma ray spectrometer are accurate enough as long as
the thorium decay family is in equilibrium, if not, then
232
determination of Th is not directly possible. A predic-
tion of the external dose for the workers can be made
by measuring the specific activity and the nuclide distri-
bution of the incoming material. Also it is possible to
observe and maintain inhalation limits in working areas,
3
−1
with a high volume aerosol collecter (min. 50 m .h )
and measuring times for air filters as shown in Table 1.
Table 2. Density correction factors for
rods.
tt ung ss ten welding
E
ne
r
gy
(
ke
V
)
D
en
si
ty
c
or
r
e
ct
io
n
f
a
ct
o
r
*
‘Gammatool’ by Amersham Buchler (AEA Technology)
74
.8
22 4.51
makes this possible, and takes into consideration the
self-absorption inside the sample. Using Gammatool,
correction factors for mixtures consisting of different
materials can be calculated. These factors are a ratio
between the peak efficiency of the calibration source
and the efficiency of the mixture at a given density.
Table 2 shows an example for density correction fac-
tors, for a sample containing tungsten welding rods.
Table 3 shows the density correction factors for the
8
7
.
.
.
.
.
.
.
.
6
1
2
0
5
2
0
2
22 1.40
22 .61
11 .84
11 .2
11 .12
11 .11
11 .10
11 .10
2
38
00
83
95
60
11
67
2
28
212
212
energy lines of Ac, Pb and Bi of a Thorium Net-
cc a ll ib rr aa ti oo nn s oo ur cc e (␳)tun gg sten
** ␩ // ␩␩ .
(
7)
work sample
.
(
7)
Table 3. Correction factors for liquid sample of 2nd intercomparison
.
Bq.g− before
1
Density
correction factor
Bq.g−1 after
correction
Average*
weighted by p
Nuclide
Energy
p
␥
correction
␥
2
2
28
12
Ac
Pb
911.2
0.2900
0.1700
0.1200
0.0052
0.4340
0.1750
0.1180
0.0617
0.0324
0.0675
0.0109
0.0102
10.12
10.98
11.64
17.93
13.48
14.67
14.58
13.37
11.58
12.45
8.49
1.085
1.083
1.131
1.736
1.152
1.417
1.438
1.345
1.137
1.091
1.091
2.787
9.33
10.14
10.29
10.33
11.70
10.35
10.14
9.94
9
3
67
38.3
5
7.7
9.77
238.6
7
7
8
7.1
4.8
7.2
3
00.1
727.3
85.5
9.9
10.18
11.38
7.79
11.00
10.73
2
12
Bi
7
3
26.58
9.54
*
average = ⌺ a
i
P
␥,i/⌺i p␥,i; a = specific activity of nuclide i.
i
i
REFERENCES
1
2
3
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Verlag der Wissenschaften (1992).
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1
07

R. KREH and J. STAINER
5
6
7
. Debertin, K. and Helmer, R. G. Gamma- and X-ray Spectrometry with Semiconductor Detectors (Amsterdam: North-
Holland) (1988).
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nd
. Thorium Network on the Analysis of Thorium and its Isotopes in Workplace Materials. Report on the 2 Intercomparison
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1
08