JPR SYMPOSIUM Current status and future control of cesium contaminationin plants and algae in Fukushima

Measurements of gamma (c)-emitting radionuclides with a high-purity germanium detector: the methods and reliability of ourenvironmental assessments on the Fukushima 1 Nuclear PowerPlant accident

Tetsuro Mimura • Mari Mimura • Chiyo Komiyama •

Masaaki Miyamoto • Akira Kitamura

Received: 9 July 2013 / Accepted: 17 August 2013 / Published online: 15 December 2013

� The Botanical Society of Japan and Springer Japan 2013

Abstract The severe accident of Fukushima 1 Nuclear

Power Plant due to the Tohoku Region Pacific Coast

Earthquake in 11 March 2011 caused wide contamination

and pollution by radionuclides in Fukushima and sur-

rounding prefectures. In the current JPR symposium, a

group of plant scientists attempted to examine the impact

of the radioactive contamination on wild and cultivated

plants. Measurements of gamma (c) radiation from radio-

nuclides in ‘‘Fukushima samples’’, which we called and

collected from natural and agricultural areas in Fukushima

prefecture were mostly done with a high-purity Ge detector

in the Graduate School of Maritime Sciences, Kobe Uni-

versity. In this technical note, we describe the methods of

sample preparation and measurements of radioactivity of

the samples and discuss the reliability of our data in regards

to the International Atomic Energy Agency (IAEA) Inter-

laboratory comparisons and proficiency test (IAEA profi-

ciency test).

Keywords A high-purity Ge semiconductor detector �137Cs � 134Cs � Fukushima 1 Nuclear Power Plant � IAEA

proficiency test � Radionuclides

Abbreviations

F1NPP Fukushma 1 Nucelar Power Plant

IAEA International Atomic Energy Agency

HPGe High-purity germanium

MCA Multi-channel analyzer

cps Counts per second

gps Gamma-ray counts per second

Introduction

Measurements of gamma-emitting radionuclides, such as137Cs, 134Cs, and 131I, are essential for evaluating the extent

of contamination by fallouts from Fukushima 1 Nuclear

Power Plant (F1NPP) in the Fukushima prefecture. Various

data are presented in the following web pages (see Refer-

ences; Environmental Radioactivity and Radiation in

Japan; Fukushima Radiation Monitoring, Water, Soil and

Entrainment; IAEA, Fukushima Nuclear Accident). Radi-

ation from these nuclides can be detected using various

instruments including Geiger-Mueller counter, scintillation

counter, and semiconductor detectors. Using a high-purity

Ge (HPGe) semiconductor detector, it is possible to mea-

sure the energy spectrum of gamma radiation and distin-

guish different radionuclides in the samples with high

resolution [Environmental Radioactivity and Radiation in

Japan and in its web site (http://www.kankyo-hoshano.go.

jp/en/index.html), from which we can download technical

manuals written in Japanese for HPGe semiconductor

‘Fukushima Daiichi Nuclear Power Station’ is cited as ‘Fukushima 1

Nuclear Power Plant’ in the present manuscript.

T. Mimura (&) � M. Mimura

Department of Biology, Graduate School of Science, Kobe

University, Rokkodai, Nada, Kobe 657-8501, Japan

e-mail: mimura@kobe-u.ac.jp

C. Komiyama � A. Kitamura

Department of Marine Engineering, Graduate School of

Maritime Sciences, Kobe University, Fukaeminami,

Higashinada, Kobe 658-0022, Japan

M. Miyamoto

Radioisotope Division, Center for Supports to Research and

Educational Activities, Kobe University, Rokkodai, Nada,

Kobe 657-8501, Japan

123

J Plant Res (2014) 127:91–97

DOI 10.1007/s10265-013-0594-y

detector standardized by the Ministry of Education, Cul-

ture, Sports, Science and Technology, Japan].

Samples of the following works in this symposium

(Kawai et al. 2014; Kobayashi et al. 2014; Mimura et al.

2014; Ohmori et al. 2014; Sekimoto et al. 2014; Terashima

et al. 2014), were collected from Fukushima, Motomiya,

and Iwaki city area (we call these places Fukushima area)

from May, 2011, and the radioactivity of ‘‘Fukushima

samples’’ were mostly measured with an HPGe detector at

the Graduate School of Maritime Sciences, Kobe Univer-

sity. In this report, we describe technical details of the

measurements with the HPGe detector at Kobe University

and discuss the validity of our methods and reliability of

our data in regards to the IAEA proficiency test.

Sample preparation

Plants, algae, and soils collected in Fukushima area were

dried at 105 �C for 2 days in an oven after recording the

fresh weights. The dried samples were then mechanically

crushed into powder with Waring blender (J-spec Blender

7011JBB, Warning Commercial, Torrington, USA), and

the powdered samples were packed into a U8 container (U-

8, 48 mm diameter 9 58 mm height, Mano Kagaku Youki

Co. Ltd., Osaka, Japan). Sample weight was determined by

differential weighing of the container before and after

packing. If the powdered sample was insufficient to fill in

the whole container, the sample was covered with a poly-

acrylic plate and the remaining space filled with styrene

foam beads. The amount of each sample was separately

determined by measuring its height (or thickness) at three

different positions in the U8 container using a vernier

caliper and averaged (Environmental Radioactivity and

Radiation in Japan; Yoshihara et al. 2013).

Measurement of gamma radiation with an HPGe

detector

Gamma rays emitted from the samples were detected with

a general-purpose HPGe detector GC3019 (Canberra

Industries Inc., Connecticut, USA) at a nominal detection

efficiency of 30 % and a resolution of 1.9 keV, as evalu-

ated from the full width at half maximum (FWHM) of a

1.332-MeV peak of 60Co. The U8 sample container was

mounted on the top of the 76-mm-diameter endcap of the

liquid–nitrogen–cooled Ge crystal. The detector head with

a preamplifier mounted on a 30-litre dewar was surrounded

by a 5 cm-thick lead wall on all sides except the bottom.

Accordingly, this system inherently counts a part of natural

background radiation, which was subtracted each time

from measuring empty samples.

Each gamma-ray incident on the Ge crystal produces an

electric pulse in the electronic circuit at the above-men-

tioned detection efficiency. Since the pulse height is pro-

portional to the energy of gamma rays, pulse height

analysis is necessary in order to identify the gamma-ray-

emitting radionuclide. At the beginning of our measure-

ments of Fukushima samples, we employed a multi-chan-

nel pulse height analyzer MCA 2100C (Laboratory

Equipment Corp.), which required manual data handling.

Figure 1 shows the gamma-ray spectrum of a soil sampled

from Iwaki city in late March and measured on April 5,

2011. The results clearly showed that several radionuclides

are involved in the present radiation problems: 134Cs

Fig. 1 An example of gamma-

ray spectra. The sample soil was

collected in Iwaki city on late

March and measured on April 5,

2011. The blue (upper) line is a

spectrum of the sample

(2011.4.5) and the pink (lower)

line is a background spectrum

(BG)

92 J Plant Res (2014) 127:91–97

123

[Ec = 0.6047 MeV (97.6 %) and 0.7958 MeV (85.4 %),

half-life s = 2.06 y] and 137Cs (Ec = 0.6616 MeV,

s = 30.1 y) were detected. In addition, radionuclides with

much shorter half-lives, such as 131I (s = 8.02 d) and132Te-132I (s = 3.20 d), were detected in this early-stage

measurement. In measurements made in autumn 2011,

however, these short half-life radionuclides were not

detected in the gamma-ray spectrum. 40K and 208Tl are

typical naturally occurring radionuclides and were detected

at approximate counting rates of 0.2 and 0.02 cps,

respectively. These counts constituted the main back-

ground in the spectra for the plant, algal and soil samples.

In the second round of sampling, all HPGe detector data

for Fukushima samples were taken by a more sophisticated

MCA, MP2-1U (Canberra Industries Inc., Connecticut,

USA), and analyzed using a commercially available soft-

ware (Gamma Explorer, Canberra Japan KK), which made

automatic identification of the isotope species and their

specific radioactivities, as well as calibrations for effective

solid angle, self-absorption effect and sum-coincidence

effect (Knoll 2000).

Calibration of the HPGe-MCA system for energy was

done using the peaks of natural 40K and 208Tl emitting

1.4608- and 2.6146-MeV gamma rays, respectively. Cali-

bration of the system for detection efficiency was performed

using a set of standard U8 samples containing nine different

radionuclides dispersed in alumina powder with five different

amounts of the content giving five different levels of each

radioactivity (MX033U8PP_0818 through MX033U8

PP_0822). The set was supplied by RIKEN for calibration of

radioactivity measurements of Fukushima environmental

samples organized by the Japanese Society of Radiation

Safety Management. The isotopes were 109Cd

(Ec = 0.0880 MeV; s = 462.6 d), 57Co (Ec = 0.1221 MeV,

0.1360 MeV; s = 271.7 d), 139Ce (Ec = 0.1659 MeV;

s = 137.6 d), 51Cr (Ec = 0.3201 MeV; s = 27.7 d), 85Sr

(Ec = 0.5140 MeV; s = 64.84 d), 137Cs (Ec = 0.6617

MeV; s = 30.1 y), 54Mn (Ec = 0.8349 MeV; s = 312.1 d),88Y (Ec = 0.8980 MeV, 1.8361 MeV; s = 106.7 d), and60Co (Ec = 1.1732 MeV, 1.3325 MeV; s = 5.27 y).

These calibration standards were used to determine the

geometry- and energy-dependent detection efficiency, as

shown in Fig. 2, and to find suitable polynomial fitting

functions of logarithmic energy. The efficiency is a

decreasing function of energy in a range above the maxi-

mum at about 0.14 MeV, which reflects the fact that the

probability of c-ray interaction with matter is a decreasing

function of c-ray energy below several MeV. The

Fig. 2 Calibration curves of detection efficiency for the HPGe

detector using a standard set of nine kinds of radionuclides dispersed

in alumina powder contained in U8 containers in different thickness:

0.53 cm, the most upper line with red circles; 1.03 cm, the second

line with blue triangles; 2.04 cm, the third line with green squares;

3.04 cm, the forth line with light blue triangles; and 5.08 cm, the

lowest line with purple triangles. Efficiency is expressed as counts per

second (cps) or gamma-ray counts per second (gps)

Table 1 Samples used for IAEA proficiency test

Sample

name

Material Volume Target

radionuclide

Radioactivity

(Bq kg-1)

01 Water 500 g 60Co, 133Ba,134Cs, 137Cs,152Eu, 241Am,3H

5–15

02 Water 500 g 60Co, 133Ba,134Cs, 137Cs,152Eu, 241Am,3H

3–8

03 Water 500 g 60Co, 133Ba,134Cs, 137Cs,152Eu, 241Am,3H

4–11

04 Soil (IAEA

377)

150 g 40K, 137Cs,234U, 235U,238U, 238Pu,239?240Pu

400–24,000

05 Grass (pure

nettle)a200 g 137Cs 2

06 Grass

(diluted

IAEA

372)

200 g 137Cs 3,000

07 Grass

(IAEA

372)

200 g 137Cs 10,000

08 Simulated

aerosol

filter

1 pc 57Co, 133Ba,134Cs, 137Cs,152Eu, 241Am

0.2–1.6

(Bq/pc)

09 Simulated

aerosol

filter

1 pc 57Co, 133Ba,134Cs, 137Cs,152Eu, 241Am

2–52 (Bq/pc)

10 Simulated

aerosol

filter

1 pc 57Co, 133Ba,134Cs, 137Cs,152Eu, 241Am

2–52 (Bq/pc)

a Urtica dioica

J Plant Res (2014) 127:91–97 93

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94 J Plant Res (2014) 127:91–97

123

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efficiency also decreases towards lower energy below the

maximum, since the shielding effect of material lying

between the sample and the HPGe detector also decreases

as a function of the energy. The efficiency of interest in the

present work lies between 0.02 and 0.06 depending on

energy and the thickness of the sample.

During the course of the sample measurement, accuracy

of the detector was checked by the peak energies of 134Cs

and 137Cs, and the amplifier gain was readjusted using a60Co checking source if required. Background radiation in

the measuring room was also measured frequently.

IAEA proficiency test

For evaluation of our measurements in Kobe University,

we participated in the IAEA Interlaboratory Comparisons

& Proficiency Test (IAEA proficiency test); an IAEA-TEL-

2011-8 test (for Fukushima radiation Monitoring, Water,

Soil and Entrainment) was conducted in 2011–2012, to

which 20 laboratories from universities, research institutes

or companies were participated. IAEA distributed ten dif-

ferent standard samples containing radionuclides in a spe-

cific radioactivity range from 2 Bq kg-1 to 10 kBq kg-1 in

different combinations of 3H, 60Co, 133Ba, 134Cs, 137Cs,152Eu, 241Am, 40K, 234U, 235U, 238U, 238Pu, 239?240Pu, or57Co (Table 1). The precision of our measurements of

these samples was evaluated by IAEA.

In this test, 38 radionuclides in ten different samples were

targeted by the instruction of IAEA. Among final scores for

our measurements, 23 were ‘‘A’’ (accepted or acceptable),

eight were ‘‘N’’ (not accepted or non-acceptable), one was

‘‘W’’ (with warning) and six were not reported (Table 2).

Among the eight radionuclides evaluated as ‘‘N’’, the

radioactivity of 60Co and 152Eu (Ec (MeV) = 0.122, 0.245,

0.964, 1.112, 1.408) in the Sample02 water and that of137Cs in the Sample03 water were at very low concentra-

tions, \10 Bq kg-1. Thus, the duration of our measure-

ment, 40,000 s, seemed not sufficient for precisely

measuring such low activity samples. To increase preci-

sion, longer measurement times or the usage of a larger

container such as marineri vessels would be necessary.

Our mesurements for 57Co (Ec = 0.122 MeV) in the

Sample08 filter and 241Am (Ec = 0.061 MeV) in the

Sample08 through Sample10 filters were also evaluated as

‘‘N’’. These radionuclides emit very low energy c rays, and

the calibration efficiency of our system was not sufficient

for these nuclear species. As far as Fukushima samples are

concerned, we may neglect these low energy radionuclides,

since the problem with 134Cs and 137Cs emitting c rays with

energies higher than 0.6 MeV is more serious.

Another ‘‘N’’ was given to 137Cs in the Sample05 grass.

Since the specific activity of this raidonuclide was very low

(2 Bq kg-1), we measured this sample twice, for 40,000 and

130,000 s and obtain the radioactivity values of 3.97 ± 1.15

and 3.79 ± 1.13 Bq kg-1, respectively. A weighted average

of 3.83 ± 1.14 Bq kg-1 was calculated from a ratio of total

time which is 170,000 s. Although all these values were

similar and within a statistical deviation of our measure-

ments, they were still larger than the target value of

2.0 ± 0.4 Bq kg-1 provided from IAEA (in 250,000 s

measurement). Accordingly, we also measured the Sample05

in almost equal two portions, 05a and 05b. The Sample05a

and Sample05b recorded 1.18 ± 1.15 and 3.46 ±

1.41 Bq kg-1 in 571,000 and 641,000 s, respectively, and a

weighted average of 2.58 ± 1.39 Bq kg-1 (in 1,212,000 s)

was calculated as mentioned above. This value was more

close to the target value (2.0 ± 0.4 Bq kg-1 in 250,000 s)

compare to the first average (3.83 ± 1.14 Bq kg-1 in

170,000 s). Therefore, we consider that the relative position

of high-energy emitting radioactive samples in a U8 con-

tainer could affect our measurement. At present it remains

unclear why the difference existed between the target value

and our data for this sample. Finally, ‘‘W’’ was given to 137Cs

in the Sample04 soil. This sample recorded 2.57 ± 0.0032

and 2.56 ± 0.0023 kBq kg-1 in 140,000 and 260,000 s,

respectively, while the target value is 2.44 ± 0.03 kBq kg-1.

Because the amount of radioactivity in this sample was

higher by three orders of magnitude than that of the above-

mentioned Sample05 grass, we consider that the difference

from the target value for the Sample04 soil is not a matter of

statistical precision. Again, this could be caused by the

geometry problem of our measuring system. This is sup-

ported by the fact that our measurement of the lower energy

species 40K (0.389 ± 0.011 kBq kg-1) in the same sample

shows good agreement with the target value of

0.385 ± 0.020 kBq kg-1.

In conclusion, although our measurements do not pro-

duce reliable data as for some samples containing very low

levels of radioactivity (\10 Bq kg-1) or radionuclides

emitting very low c ray energies (such as 57Co or 241Am),

or for those consisting of uneven distribution of hotspot

material such as soils, our measurements with the HPGe

detector and analyses with Canberra Gamma Explorer

software in Kobe University are precise enough for prac-

tical evaluation of Fukushima samples with respect to

contamination, movement and persistence of radionuclides

in plant, water and food materials.

Acknowledgments The new multi-channel analyzer and data ana-

lysis software were purchased by the emergent financial support from

Kobe University. We are grateful to Prof. Yuichi Onda (University of

Tsukuba) for his kind support for using the set of standards for pre-

liminary calibration of the detector and joining the IAEA proficiency

test. We are also grateful to the laboratories participating in the ad-

hoc measurement group organized by Prof. Kazuyoshi Masumoto and

Michihiro Shibata, Japanese Society of Radiation Safety Manage-

ment, for providing the standard set of U8 samples for calibration. We

96 J Plant Res (2014) 127:91–97

123

thank Prof. Keiji Oda, Prof. Tomoya Yamauchi, Prof. Hideo Okamura

and Dr. Yuji Tooyama for their assistance with the use of the HPGe

detector, and also IAEA for evaluation of our measurements in the

proficiency test. We are grateful to Dr. Robert Reid, The University of

Adelaide, Australia, for his kind assistance with the English text. The

present technical work was partly supported by a grant from the

Mitsui & Co., Ltd. Environment Fund and Grant-in-Aid for Scientific

Research on Innovative Areas Grant Number 24110007.

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