arXiv:0901.1359v1 [physics.med-ph] 10 Jan 2009
Measurement Limits to134Cs Concentration in
J.K. Ahn,a,cJ.S. Kim,b,∗H.M. Lee,bT.H. Kim,cJ.N. Park,c
Y.S. Kang,aH.S. Lee,aS.J. Kim,aJ.Y. Park,aS.Y. Ryu,a
H.-Ch. Kim,d,∗W.G. Kang,eS.K. Kimf
aDepartment of Physics, Pusan National University, Pusan 609-735, Korea
bDepartment of Geology, Pusan National University, Pusan 609-735, Korea
cNuclear Physics and Radiation Technology Institute (NuRI), Pusan National
University, Pusan 609-735, Korea
dDepartment of Physics, Inha University, Inchon 402-751, Korea
eDepartment of Physics, Sejong University, Seoul 143-747, Korea
fDepartment of Physics, Seoul National University, Seoul 151-742, Korea
We investigate the caesium concentrations in soils in mountain areas near Gori
nuclear power plant in Korea, focusing on the measurement limits to the134Cs. In
order to lower the minimum detectable amount (MDA) of activity for the134Cs, we
have used the ammonium molybdophosphase (AMP) precipitation method to get
rid of the40K existing in natural radioactivity, which reduces the MDA of activity
about ten times smaller than those without the AMP precipitation method. The
MDA results for the134Cs were found to be in the range between 0.015 and 0.044
Bq/kg-dry weight. In order to diminish the background, we also have measured
a part of the soil samples in Yangyang, a small town in the east coast of Korea.
However, it turns out that in order to detect the134Cs in the samples the MDA
should be reduced to the level of mBq/kg-dry weight.
method, minimum detectable amount (MDA)
134Cs,137Cs, ammonium molybdophosphate (AMP) precipitation
∗Corresponding authors: email@example.com (J.S. Kim), firstname.lastname@example.org (H.-
Preprint submitted to Elsevier13 January 2009
It is of great importance to study the anthropogenic caesium radioisotopes
137Cs and134Cs, since their production and emission rates are much higher
than other radioisotopes from nuclear fissions at nuclear power plants (NPP)
and they have rather low movility in soils. Thus,137Cs are widely used as
tracer radionuclides for monitoring the NPP-related environmental radioac-
tivity. The137Cs nucleus (t1/2= 30.07y) is a long-lived beta emitter decay-
ing a 95% branching ratio to the metastable state (t1/2= 2.55m) of137Ba∗
at 661.7keV. While the134Cs nucleus is also a beta emitter, its decay time
(t1/2= 2.065y) is much shorter and emits many gamma-rays correlated with
the beta electron. Due to these different life-times, it is possible to date the
anthropogenic radioactivity with the ratio of134Cs to137Cs concentrations in
soil measured. However, since the134Cs has much shorter half life-time com-
pared to the137Cs, the dating range depends solely on a minimum detectable
amount (MDA) of activity for the short-lived134Cs radioisotope.
Moreover, it is well known that the137Cs contamination arises mainly from
three different sources: Atmospheric nuclear weapon tests (NWT) in the pe-
riod of the late 1950s and early 1960s, in particular, in the nothern hemisphere
(UNSCEAR, 2000), the Chernobyl accident taken place in May 1986, and dis-
charges from nuclear power plants (NPPs). On the contrary, the134Cs does
not come from the NWT, since it is induced only by the neutrons impinged
on the fission radionuclide133Cs, which rarely occurs in the course of a nu-
clear explosion due to a lack of the reaction time. However, the134Cs can
be produced in reactors at NPPs. Thus, knowing the ratio of134Cs to137Cs
concentrations in soil may determine even their origin. Note that the fallout
ratio of the Chernobyl explosion is known to be about134Cs/137Cs ≃ 50% in
1986 (Thomas and Martin, 1986).
In order to determine the ultra-low level caesium concentrations quantitatively
in soil, it is essential to understand the natural background originated mainly
from uranium-, thorium-, actinum-series nuclides, and the potassium40K. The
γ-ray peaks of the134Cs, in particular, those at 604.7keV and 795.6keV are
overlapped with a great deal of γ-ray peaks from natural radionuclides. More-
over, strong peaks such as a 1460keV line of the40K at high energies produce
the Compton-continuum background that covers the wide range of the gamma
energies for caesium decays. Thus, we need to get rid of the natural background
as much as possible. One of the best ways known to eliminate the40K is to
use the ammonium molybdophosphase (AMP) precipitation method. AMP
is an inorganic compound that selects the caesium exclusively and has high
adsorption capacity (Suss and Pfrepper, 1981). This method has been widely
applied to sea-water samples in order to enrich the Cs concentration. However,
this radiochemical extraction for a soil sample should be emphasized by its
effective background suppression, thereby significantly lowering the minimum
In the present work, we want to investigate the minimum detection limit to
the measurement of the134Cs in surface soils in mountain areas near the Gori
NPP in Korea, aiming at determining the ratio of the134Cs to137Cs. In Korea,
eighteen nuclear reactors are being operated at four different NPP sites. One
of the NPP sites is located in Gori, a small town in the south-east coast of
Korea, where four pressurized-water reactors are in operation. The first Gori
reactor has been operated first time in Korea since 1978. The environmental
radioactivity in the vicinity of the Gori NPP has been monitored last years.
The samples were taken from soils, rain water, sea water, surface water, milk,
seaweed, egg, etc. Recently, high concentrations of the137Cs were first reported
in the soil samples from the mountain areas near the Gori NPP in the range of
50−460 Bq/kg-dry, while the137Cs concentrations in surface soils of Korea are
known to be ranged in 7.86−70.1 Bq/kg-dry with a mean value of 33.2±16.1
Bq/kg-dry (Kim et al., 1998).
Taking into account the fact that the Chernobyl accident took place more than
thirty years ago, the134Cs concentration was reduced by three orders of mag-
nitude, while a half of the original137Cs concentration is survived (Pourcelot
et al., 2003). If the ratio of134Cs to137Cs is higher than the average ratio
to be mapped out for the distant area from the Gori power plant, the origin
should be investigated thoroughly. In this regard, it is of great significance to
conduct the methodological study for the measurement of the ultra-low level
concentration of134Cs in its own right.
The present work is organized as follows: In Section 2, we briefly explain the
process of the experiment. In Section 3, we discuss the detection limits to the
134Cs concentration. In the final Section, we summarize and draw conclusions
of this work.
2.1Sampling and Preparation
Soil samples were collected from 96 sites at top areas of four different moun-
tains near the Gori NPP: Mt Daleum (35◦18′N, 129◦12′E), Mt Ilgwang (35◦16′N,
129◦12′E), Mt Samgak (35◦21′N, 129◦13′E), and Mt Daewoon (35◦24′N,
129◦13′E). All these mountains are located within 7-12 km distance from the
Gori NPP. Uncultivated soil was sampled from the 5−10cm layer. Each sam-
ple was taken in close proximity to give approximately 0.5 to 1.5 kg. Since we
are mostly interested in measuring both134Cs and137Cs, we have selected the
samples with the highest137Cs concentration: 461 ± 9 Bq/kg-dry. From that
sample, we took two samples for the measurement and prepared them with
different methods: Sample A was air-dried at 65oC for 10 hours and sieved
with 2mm mesh size. It was then filled in a 450ml Marinelli beaker. On the
other hand, sample B was heated at 450oC for 10 hours. One liter of 37% HCl
solvent was added to sample B, and the solution was stirred with heat for
4 hours. Soil contents in the sample solution were sieved through two layers
of the normal filter(5C, 125mmφ) and the glass-fiber filter(GF/C, 110mmφ)
in the Buhner filtering system. Unfiltered sediment was washed out with hot
For precipitation with ammonium-molybdophosphate(AMP), the sample was
stirred with the 80g AMP solution at PH2 for 10 hours. The sample was then
sieved with a membrane filter of 0.45µm pore size, and after cleansing the
sample bottle with 1% HCl solution, the cleansing solution was put to the
filter to sieve remaining contents. After the AMP precipitation, the sample
was dried with infrared light, and put into a standard 55ml Marinelli beaker
for γ-ray measurement.
We have measured the137Cs concentration in the soil samples by directly
measuring the 661.7keV, 604.7keV, and 795.8keV gamma rays with an HPGe
detector. The HPGe γ-ray detector has a 30% relative efficiency at 1330keV
of60Co, and the schematic setup of the detector and the shielding system is
shown in Fig. 1.
Fig. 1. Schematic view of the measurement setup
The HPGe detector was coupled to standard NIM electronics and a PC with
Gamma-VisionTMspectroscopy software. The system was shielded with a uni-
form lead castle of 10cm in thickness. Certified reference materials (CRM) of
mixed gamma-ray sources from KRISS (Korea Research Institute of Standards
and Science) were employed for the efficiency calibration of the system. The
CRM provides with 13 gamma-ray peaks from 10 radioactive elements:241Am
(59.5keV),109Cd (88.0keV),57Co (122.1keV, 136.5keV),139Ce (165.86keV),
51Cr (320.08keV),113Sn (391.70keV),85Sr (514.00keV),137Cs (661.66keV),88Y
(898.04keV, 1836.05keV), and60Co (1173.23keV, 1332.49keV).
Fig. 2. Efficiency curves of the HPGe detector for the 450ml dry-soil sample (square)
and the 45ml AMP sample (circle), respectively.
Figure 2 represents efficiency curves of the HPGe detector, fitted with the
following empirical logarithmic polynomials:
where Eγdenotes the gamma-ray energy. The anare the fitting parameters of
the polynomial. The efficiency for the 450ml sample is lower than that for the
45ml AMP sample in the low-energy region, because of a larger self-absorption
of gamma-rays in the sample volume.
Figure 5 depicts the gamma-ray spectrum of the AMP sample overlaid with
that of the dry-soil sample. The chemical separation shows a significant im-
provement in a signal-to-background ratio. The background is reduced dramat-
ically by almost a order of magnitude due to the AMP method. The upward
arrows in Fig. 5 designate the positions of gamma-ray energies from134Cs and
Fig. 3. Measured gamma-ray energy spectrum of the AMP precipitation sample
overlaid with the spectrum for direct measurement. Upward arrows indicate gam-
ma-ray energies from134Cs and137Cs.
We are now in a position to discuss the MDA of activity of samples. The
criterion for limits of detectability is usually given by the MDA of activity.
The most widely used definition of the MDA was first defined by Curie (Curie,
1968) as follows:
MDA =2.71 + 4.65√Nb
ε · Pγ· t
where Nbdenotes the number of background events measured during the time
t. The ε represents the detection efficiency as defined in Eq.(1), and Pγstands
for a branching fraction for134Cs emitting the respective gamma-ray. The
minimum detectable activity depends on the square root of the number of
background events which mainly come from three different reasons: The sam-
ple itself, Compton continuum, and natural radioactivity. Note that it actually
depends on the inverse square root of the time, since the background events
also increase as a function of time.
In Fig. 4, we draw the gamma-ray spectrum of the dry-soil sample measured at
a normal laboratory without the AMP precipitation taken into account. The
peak at 609 keV corresponds to that of the gamma-ray from214Bi. Because of
the background, it is very difficult to identify the spectrum of the sample, in
particular, in the higer region of the gamma-ray energy.
Fig. 4. Gamma-ray energy spectrum of the sample measured at a normal laboratory.
Fig. 5. Gamma-ray energy spectrum of the AMP-precipitated sample measured at
a underground laboratory. The 795 keV gamma-ray from134Cs cannot be resolved
from the strong 794.7 keV gamma-ray peak from228Ac.
In order to reduce the background of the sample, we have brought the sample
to an underground laboratory located in Yangyang, a small town in the east
coast of Korea. The underground laboratory is specialized to search for dark
matter for which is it essential to develop low-background measurement. It uti-
lizes the space in a tunnel of Yangyang Pumped Storage Power Plant located
deep under a mountain. Figure 5 shows the gamma-ray spectrum of the AMP-
precipitated sample measured at the underground laboratory in Yangyang. As
compared with Fig. 4, it is shown that the background is drastically reduced.
In Fig. 6, we present the full energy-range gamma-ray energy spectrum of
the AMP-precipitated sample measured at the underground laboratory. As
mentioned before, the AMP precipitation method has greatly reduced the
MDA by almost one order of magnitude. It filters selectively out Cs isotopes
from the sample. Moreover, the Compton continuum background from40K is
also shown to be noticeably reduced. However, the 795 keV peak from228Ac
Fig. 6. Full energy-range gamma-ray energy spectrum of the AMP precipitation sam-
ple measured at a underground laboratory. It turns out that our natural background
level is almost four orders of magnitude smaller than the level of Cs radioisotope
is interfered with the 795.6keV from134Cs. Therefore, we found it difficult to
resolve the peak from134Cs even with very low background. The 604.7 keV
peak from134Cs can be well resolved from underlying background. The 609.3
keV peak from214Bi appears in the vicinity of the 604.7 keV peak, but a good
energy resolution of the HPGe detector helps the two peaks resolved clearly.
It therefore should be noted that the 604.7 peak should be searched for in
the134Cs survey. It is then necessary to suppress background contributions
underneath the 604.7 keV peak. The region near the peak is between the
661.7 keV137Cs peak and its Compton edge. The yield in the region is thus
mainly due to multiple Compton scattering of the 661.7 keV137Cs gamma
ray in a HPGe crystal. A possible experimental approach is a gamma-ray
detection with a Compton suppressed system so that it may be possible to
suppress such multiple Compton scattering events and to lower the MDA
significantly. Figure 7 summarizes how to resolve peaks of134Cs and137Cs
from the background.
Table 1 lists the values of the MDA from the present work, with and without
the AMP precipitation method considered.
The values of the MDA for the134Cs concentration: The unit is given in Bq/kg-dry.
Locationwithout AMPwith AMP
Fig. 7. Simplified gamma-ray spectrum of the AMP soil sample.
4 Summary and Conclusion
In the present work, we investigated the caesium concentrations in soil sam-
ples taken from mountain areas in the vicinity of the Gori nuclear power plant,
emphasizing the minimum detection limit to the measurement of134Cs con-
centration. We carried out the sampling by taking 5 − 10 cm layer of surface
soil at the top areas of four different mountains within the 7−12 km distance
of the Gori nuclear power plant. We have used the ammonium molybdophos-
phase (AMP) precipitation method to get rid of the40K existing in natural
radioactivity, which reduces the minimum detectable activity (MDA) of activ-
ity about ten times smaller than those without the AMP precipitation method.
Even though we were able to reduce the MDA for the134Cs from 0.2 to 0.015
by one order of magnitude smaller, using the ammonium-molybdophosphate
precipitation method in order to lower the background mainly arising from
the natural radioacnuclide40K, it is not possible to determine the134Cs con-
While it is of great difficulty to track down the origin of the137Cs that are
found in higher concentrations in soil samples near the Gori nuclear power
plant, the results imply that the137Cs found in the samples are at least 20
years old. Taking into account the fact that the Chernobyl accident took place
22 years ago from now, one can assume that the present137Cs concentrations
may have come from the Chernobyl explosion. Since it is known that the fallout
ratio of the Chernobyl accident is about134Cs/137Cs ≃ 50% in 1986 (Thomas
and Martin, 1986), the present ratio should be dropped to be about 0.1%,
considering that the half life-times of the134Cs and137Cs are, respectively,
2.065y and 30.07y. In fact, the fallout due to the Chernobyl accident did
not affect Korea much, since the distribution of137Cs is similar to those of
other countries (C.S. Kim et al., 1998). Thus, we can infer from it that the
present concentration of the134Cs should be less than 0.01 Bq/kg-dry which
is definitely lower than the minimum detection limit reached in the present
In order to reduce the minimum detectable amount of134Cs activity further,
we need to get rid of the Compton continuum, using the Compton suppresion
method, which is now under investigation.
The present work is supported by Inha University Research Grant (INHA-
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