Journal of Nuclear and Radiochemical Sciences, Vol. 20, pp. 25-27, 2020

Notes
Pretreatment conditions for detecting 134Cs -Eight years after the Fukushima Daiichi nuclear
accidentKatsumi Shozugawaa,*, Mayumi HoribaGraduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo, 153-8902, Japan
Komaba Organization for Educational Excellence, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo, 1538902, Japanb

Received January 5, 2020; Accepted May 12, 2020; Published online June 3, 2020
134Cs is an activation product with a short half-life (2.06 years) that was released during the Fukushima Daiichi
nuclear disaster. It is an important indicator of contamination from the disaster because it has been more than 30years since the last event when 134Cs was released. The radioactivity ratio of 137Cs to 134Cs was approximately 1 at
the time of the accident, and thus, it is possible to quantitatively evaluate the sources of 137Cs from the radioactivity of 134Cs in environmental samples. In particular, because the radioactivity of 134Cs in foodstuffs in Japan islow due to its short half-life, pretreatment (concentration) is essential for quantification. For analyses using a 2-L
Marinelli vessel and a conventional HPGe semiconductor detector (relative efficiency 25%), the detection limit
for 134Cs was approximately 0.05 Bq/kg without pretreatment. For the 137Cs contamination source segregation in
Japan in March 2020, the radioactivity in samples must be at least 0.83 Bq/kg of 137Cs. This study provided
guideline requirements for sample volume and pretreatment.

1. Introduction

2. Methods

Since the Fukushima Daiichi nuclear disaster, significant
amounts of radioactive nuclides have been released to theenvironment, mainly in the North Pacific Ocean and easternJapan.1 Japan has devoted substantial financial and technicalresources to measuring radioactive nuclides in foodstuffs, 2,3
and regulated radioactive nuclides include radioactive cesium(134Cs and 137Cs). Since April 2012, the regulated radioactivity
level for total radioactive cesium has been set to 100 Bq/kg for
general foods and 570 Bq/kg-dry for dried shiitake mushrooms (Lentinula edodes) under the Japanese domestic law.4-6
As of 2019, very few foods in markets exceeded the regulatorylevel. The detection of 134Cs (T1/2 = 2.06 y) is crucial for quantitative evaluation of the contamination sources (FukushimaDaiichi nuclear disaster or pre-Fukushima, such as earlier
nuclear bomb incidents and tests or the Chernobyl disaster).7134
Cs is a gamma-emitted neutron activation product from stable 133Cs and representative of nuclides released to the environment by the Fukushima Daiichi nuclear disaster.
Many crops in Japan are harvested annually, and there are
two major implications for high-accuracy gamma ray analysisof annual crops. First, at supermarkets, there can be many
crops from all over Japan without any environmental sampling. Second, the ability to detect small amounts of 134Cs in a
crop can more clearly identify the areas affected by theFukushima nuclear accident in Japan. In 2019, 8 years after
the Fukushima Daiichi nuclear disaster, the radioactivity of134
Cs had decreased to less than 1/16 of the radioactivity at thetime of the accident, and thus, high-accuracy gamma ray measurement is needed with increased measurement time.
In this study, we present specific measurement and pretreatment methods for improving the detection limits of 134Cs using
a conventional Ge semiconductor detector.

A conventional HPGe semiconductor detector (GC25187500 SL-2002CSL) manufactured by Mirion Technologies,
Canberra, was used to analyze gamma rays with a relative
efficiency of 25% (actual catalog spec 27.7%). The absolutedetection efficiencies at 0.6 and 1.4 MeV using a 2L Marinelli
vessel were 0.81% and 0.4%, respectively. To determine 137Cs,
the 662 keV photopeak was used, whereas the peaks at 604
and 795 keV were used for calculating 134 Cs; the detection
limit was set to 3 sigma.8
Studied food samples were purchased from markets andtreated following the official methods in Japan.9*Corresponding author: cshozu@mail.ecc.u-tokyo.ac.jp

3. Results and discussion
3.1. Relationship between the detection limit and livetime. Generally, the detection limit decreased with increasing
measurement time. Table 1 presents the limit of quantification(LOQ) for 137Cs in a 2-L Marinelli vessel using the HPGe
detector with a relative efficiency of 15%.10 The regulatory
limit for milk in Japan since April 2012 has been 50 Bq/kg.
The LOQ for 137Cs was 0.8 Bq/L when measuring gamma rays
in a 2-L Marinelli vessel for one hour. Since the specific gravity of milk is approximately 1.03, a radioactivity of 0.8 Bq/L is
TABLE 1: The limits of quantification (LOQ) for different food samples at measurement times (live times) of 1 and10 hSample

SampleamountMilk
Green Vegetable
Seaweed/FishGrain/Meat/Egg

2L
1 kg
2 kg
2 kg

LOQ(live time)
Unit(1 hour) (10 hours)
0.8
0.3
Bq/L
1.60.5
Bq/kg-raw0.80.3
Bq/kg-raw
0.80.3
Bq/kg-raw© 2020 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/03/2020

26J. Nucl. Radiochem. Sci., Vol. 20, 2020

Shozugawaconsidered equivalent to 0.8 Bq/kg. Even one hour was toolong to determine the 137Cs activity concentration in milk if it
exceeded the regulatory level.
As described previously, although a measurement guideline
is available, quantitative evaluation of the measurement time
and the LOQ or detection limit remains sparse for gamma rayanalysis. Here a general solution is examined using resultsfrom actual measurements.
Figure 1 presents the radioactivity of 134Cs, 137Cs, and 40K
with respect to measurement time (live time) and detection
limits for dried shiitake mushrooms that were manufactured
and sold in Yugawara City, Kanagawa Prefecture, in January
2014. The dried mushroom sample had a density of 0.422 g/
cm3 and weight of 0.844 kg and was crushed and measured in
a 2-L Marinelli vessel without pretreatment.
The radioactivities of 134Cs, 137Cs, and 40K at a measurement
time of 67,500 s (live time) were 26.3 ± 0.2, 120 ± 0.6, and 578± 5 Bq/kg, respectively. The radioactivity of radioactive
cesium (sum of 134Cs and 137Cs) in the dried mushroom sample
was 147 ± 0.6 Bq/kg at the time of measurement. This value
was below the regulatory limit of 570 Bq/kg for dried shiitake
mushrooms and thus suitable for distribution in Japan.
40
K was first determined to be 774 ± 200 Bq/kg at a live
time of 75 s. Since the detection limit was 420 Bq/kg 75 s
after measurement started, it was determined in excess of 3σ.
However, the coefficient of variation (c.v.), which is an indicator of accuracy, fell below 10% after the measurement timeexceeded 1000 s.
Although the sum of the radioactivities of 134Cs and 137Cs
was lower than the 40K activity concentration, the measurement time was not required to determine accuracy. The c.v.
for the radioactivity of 134Cs and 137Cs were below 10% at 496
and 257 s, respectively. This was mainly because the detection
efficiency near 0.6 MeV was approximately two times higher
than that near 1.4 MeV.
The detection limit could be approximated as a function ofCs-134Cs-137
K-40

the live time under ideal conditions, such that the gamma ray
analysis ignored Compton scattering, summing peaks from thephotopeaks of higher energy than targeted peaks. For foodstuffs, this approximation was applied because naturalnuclides, except 40K, were often absent.
The detection limit for 137Cs was related to the live time by
an exponent based on values for the 11 data points in Fig. 1.
This relationship yielded a detection limit of 0.054 Bq/kg formeasurements from 14 consecutive days.
Measurements of the radioactivity of cesium in approximately 259 food samples7 using 2L Marinelli vessels and theHPGe detector revealed that the detection limit for 134Cs or137
Cs was 0.05 Bq/kg for a reasonable live time (up to 2 weeks)regardless of the radioactivity or density. Thus, increasing themeasurement time was an inefficient method for lowering thedetection limit.
Conversely, sample concentration before counting gamma
rays was very effective in lowering the detection limit.
Removing water from samples by heating was a suitable
method for concentrating radiocesium. According to our previous study,11 the 133Cs loss rate from heating was approximately 0.4%. Therefore, if the sample was prepared,
radiocesium could be determined more efficiently by concentrating samples by heating for water removal.
3.2. Pretreatment conditions for the detection of 134Cs
nine years after the Fukushima Daiichi nuclear disaster.
On the 8th anniversary of the Fukushima Daiichi nuclear
disaster (March 11, 2020), the activity ratio of 134Cs to 137Cs
was expected to be approximately 0.06.8 Therefore, if a conventional Ge semiconductor detector (134Cs detection limit 0.05Bq/kg) was used, the theoretical required radioactivity of 137Cs
would be at least 0.833 Bq/kg. This theoretical radioactivity
assumed that all detectable 137 Cs or iginated f rom theFukushima Daiichi nuclear disaster.
In Japan, pre-Fukushima events, such as nuclear bombs in

Detection limit of Cs-134
Detection limit of Cs-137
Detection limit of K-40

Log_Radioactivity (y, Bq/kg)

1000

100

1040K
y = 7137.5x-0.729
R² = 0.9904

134Cs

y = 366.59x-0.636
R² = 0.9895

1137Csy = 327.96x-0.637
R² = 0.980.150500
5000
Log_Live time (x, sec)

Figure 1. Radioactivity and detection limit (3 sigma) of 134Cs, 137Cs, and 40K relative to live time (s).

50000

Pretreatment conditions for detecting 134Cs -Eight years after the Fukushima

1945, atmospheric nuclear weapons testing in other countries(1950s and 1960s), and the Chernobyl disaster in 1986 introduced 137Cs to the environment. Therefore, 137Cs detected inmost samples exceeded the theoretical radioactivity calculated
from the 134 Cs/137Cs ratio based on the Fukushima Daiichi
nuclear disaster.
Pretreatment methods depended on the sample type. For
large fish living and caught in the North Pacific Ocean,
approximately 50% of the 137Cs content was attributed to theFukushima Daiichi nuclear disaster.7 Salmon caught inHokkaido, Japan had 137Cs yields of approximately 0.1 Bq/kgraw level. 7
Therefore, 137Cs source discrimination for fish samples
required quantification of at least 1.6 Bq/kg 137Cs after concentration. That is, concentration to 16 times by weight wasrequired prior to gamma ray counting. This is not a realistic
pretreatment method.
Conventional HPGe with a detection efficiency of 25% wasused in this study; however, other approaches, such as using aGe semiconductor detector with a higher detection efficiency,
a higher peak over Compton ratio, or a low-back environment,
could lower the detection limit.
Although the application range was limited to Japan, to efficiently detect 134Cs for investigating 137Cs sources, pre-measuring 137Cs radioactivity or extracting it from a database12 andcalculating the concentration ratio from the 134Cs/137Cs ratio
backwards from the radioactivity could be effective.
4. Conclusions
Several years after the Fukushima Daiichi nuclear disaster,
the quantification of 134Cs over time has become difficult. The
absolute detection efficiency of 134 Cs for the conventional
HPGe using a 2L Marinelli vessel was approximately 0.1%.
Under these conditions, the achievable detection limit wasapproximately 0.05 Bq/kg, even if samples were measured for
2 weeks. Concentrating the sample by removing water was
more effective for decreasing detection limits than extending
the measurement time. As of March 2019, to quantitatively
distinguish the sources of 137Cs from the Fukushima Daiichi
nuclear disaster and previous events like the Chernobyl accident, a sample with 137Cs of approximately 0.1 Bq/kg-fresh

J. Nucl. Radiochem. Sci., Vol. 20, 2020

27

would require at least 16 times 137Cs concentration on a weightratio basis. Since such an enrichment of radioactivity levels is
not feasible, a sample with higher radioactivity should be chosen, or a high-performance germanium semiconductor detector should be used.
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