V. FUKUSHIMA SURVEY& RELATED RESEARCH

Ⅴ．Fukushima Survey & Related Research

RIKEN Accel. Prog. Rep. 45 (2012)

Preface to Special Section

Our Mission as Nuclear Physicists

On March 15th, 2011 at 10:00 am, high radiationlevel shown on the monitor prevented a radiation
worker to leave the controlled area of the Nishina RI
Beam Factory (RIBF), followed by many others who
encountered the same problem. We soon realized thata high level of radioactivity did not originate fromanything indoors but already existed outdoors (theradioactivity inspection is usually performed only at
an exit). Ironically, the fallout from the Fukushima
Dai-ichi Nuclear Power Plant made the controlled areaof our accelerator facility the least contaminated placein the Wako campus of RIKEN.
For older generation, this situation wasn't new.
Many years ago, atmospheric nuclear-bomb tests thattook place overseas caused fallout to drift over toJapan that led to the similar radioactive situation
observed in the Japanese accelerator facilities. Thistime, however, the fallout was due to the nuclear
power plant disaster.
We gathered in the radiation proof control room of
RIBF, and watched the monitoring posts to assess thesituation. Then, we decided to upload the logged data
to RIKEN’s public website, and began taking several
other measurements to determine the radioactive
nuclear species of the fallout. While desperately
hoping that the situation would not worsen, weconvened special meetings every week to exchange
latest knowledge and information.
The Nishina Center also functioned as the base to
send radiation surveyors to Fukushima’s affected area
to measure the dosage of bodies, clothes, furniture,cars and any items that the refugees had with them.
At the beginning, it was tough since no public
transportation was available. Even in such diresituation however, it was very remarkable that many
nuclear physicists from all over Japan joined to
conduct this survey. Later as a follow-up, we joined
the large-area soil survey initiated by the Research- 237 -

Center of Nuclear Physics of Osaka University and theCenter of Nuclear Study of the University of Tokyo.

This special section of the Accelerator Progress
Report records the works we were involved in 2011 in
the aftermath of the Fukushima nuclear disaster. The
Nishina Center is the largest Japanese research
institute for nuclear physics, and our RIBF was built
to study the nuclear synthesis at the beginning of theuniverse. Nuclear fuel is a gift from the nuclear
synthesis, and atomic energy is the most importantsocial application born from nuclear physics. As
nuclear physicists, many of us feel that we could not
escape from original sin for creating the world that led
to the Fukushima nuclear accident. We have therefore
done whatever we could as radiation workers withexpertise in nuclear physics to deal with the aftermathof the disaster.
Yet what we were able to do and achieve were very
limited in the face of nuclear crisis of such magnitude,
we hope from the bottom of our hearts a swift
recovery from the tragedy and that this kind of tragedy
will never happen again.
Hideto En'yo
Director,
RIKEN Nishina Center for Accelerator-Based ScienceⅤ．Fukushima Survey & Related Research

RIKEN Accel. Prog. Rep. 45 (2012)

Diffusion of radioactive materials from Fukushima Daiichi Nuclear Power
Plant detected by gamma-ray measurements on expressways†
H. Matsumura*1, K. Saito*1, J. Ishioka*2 and Y. Uwamino
The accident at the Fukushima Daiichi Nuclear Power
Plant caused by the enormous earthquake and subsequenttsunami on March 11, 2011, scattered a large amount ofradionuclides in the Tohoku and Kanto areas. On therequest of the Ministry of Education, Culture, Sports,
Science and Technology, we visited Fukushima city on
March 14, and we were asked by the Fukushima prefectural
government to measure the dose rates along expressways in
Fukushima prefecture.
By using an NaI(Tl) scintillation survey meter and a
LaBr3 -ray spectrometer, whose energy resolution issignificantly superior to that of NaI(Tl), we measured -rays
along the roads, shown as thick lines in Fig. 1, on March
15−17 and April 8.
As an example, the measured data on the Tohoku
expressway are shown in Fig. 2. As indicated by the open
squares, when we started measurements in Fukushima city
at 1:33 p.m. on March 15, the dose rate was less than 0.1
Sv/h, which was the dose rate of the natural background. A
radioactive plume that contained 133Xe, 132Te, 132I, 131I, 134Cs,
and 136Cs was first observed at the Koriyama-Higashi
interchange at 3:25 p.m. We measured dose rates in Iwaki
city, and found them to be approximately 2 to 3 Sv/h.
However, when we returned to Fukushima city at 8 p.m.,

Fig. 1. Route map of the gamma-ray measurements.
†

Condensed from the article in Transactions of the Atomic Energy
Society of Japan 10, 152 (2011), in Japanese.
*1
High Energy Accelerator Research Organization*2
Safety Division, RIKEN- 238 -

we found the city to already be polluted by as much as
approximately 10 Sv/h.
On March 16, the dose distributions in south Fukushima
and the Aizu area were measured. The dose rate at
Shirakawa interchange, which is the southern end of
Fukushima, was 4.6 Sv/h at 2 p.m.; however, it was lower
than the 1 Sv/h dose rate measured in southern Aizu. In
Aizuwakamatsu city, the dose rate was approximately 1−2
Sv/h.
The dose distribution till the Ibaraki prefecture wasmeasured on March 17. The dose rate in Fukushima
prefecture was similar to that measured on March 15 and 16.
The dose rate remained high outside Fukushima prefecture
and was higher than 1 Sv/h up to the Yaita-Kita parking
area in Tochigi prefecture.
The dose rate distribution was measured again on April 8
from Ibaraki prefecture to Miyagi prefecture, and it was
found that the relative distribution had not changed. The
attenuation of the absolute value was explained as the decayof radionuclides that were detected by the -ray
spectroscopy. This implied that for the measured area, mostof the radioactive material was deposited with the rain on
March 15, that it did not move from the initial location, andthat there was effectively no additional deposition ofradioactive material after March 15.
Although the dose rate distributions of 132Te, 132I, 134Cs,
136
Cs, and 137Cs were similar, the distribution of 131I was
different from those of the others. The dominant nuclides interms of the dose rate were 132Te and 132I for the March 15
to 17 measurements and 134Cs and 137Cs for the April 8
measurements.

Fig. 2. Comparison of distributions of the dose ratemeasured along Tohoku expressway, from the complete
measurement series.

Ⅴ．Fukushima Survey & Related Research
RIKEN
Accel. Prog. Rep. 45 (2012)

RIKEN Accel. Prog. Rep. 45 (2012)

Radiation screening of evacuated people in Fukushima
N. Aoi, H. En’yo, T. Fujinawa, H. Hasebe, K. Ikegami, T. Kambara, Y. Kanai, M. Kase, I. Nakagawa,
H. Onishi, H. Otsu, H. Sakai, A. Taketani, H. Ueno, M. Wada, Yu. Watanabe
Immediately after the accidents of the Fukushima
Daiichi nuclear power plant in March 2011, the experimental nuclear physics society organized a collaborative eﬀort to aid evacuated people in Fukushima.
The joint action was initiated by RNC together with
RCNP, Osaka University, and was supported by thecrisis management team of RIKEN and the MEXT.
In the early stages, RNC served as a logistics base forthe detachments from west Japan. Since public transportation to Fukushima was disrupted and gas supply
was limited, we could only access Fukushima using automobiles with special permission from the police. The
eﬀort commenced on 21st March and was conducted
continuously till May end and intermittently till earlyAugust. In total, RNC contributed 103 man-day to theeﬀort, which was about 28% of the total contribution
from the nuclear physics society team.
The team worked together with many other teams
from diﬀerent organizations such as universities andlocal autonomous bodies under the medical team of
Fukushima prefectural government, which was supported by the Radiation Emergency Medical Assistance Team (REMAT) from National Institute forRadiological Science.
All teams would assembleat the temporary Fukushima prefectural oﬃce inFukushima city and were assigned the next day’stasks. The screening center for the evacuated people
were scattered throughout the Fukushima prefecture,and the participants from RNC visited the centers
in Koriyama-city, Kawamata-town, Yamakiya-chiku in
Kawamata, Nakoso in Iwaki-city, and Tamura-city forthe early emergency screening. Since late May, we havealso contributed to the screening of residents within 20
km of the power plant who temporarily visited theirhome town. The base stations for the temporary visits were Bajikoen in Minamisouma-city, Hirono-town,
and Hurumichi in Tamura-city.
In the end of April, the nuclear physics society teamFig. 1. Screening of local citizens at Kawamata publichealth center on 24th March.

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submitted a written proposal to the Fukushima prefectural government, outlining their suggestions on how
best to conduct the screening activity.
Many in the RNC staﬀ were also very supportive of
our eﬀorts, in particular, the members of the administration oﬃce of RNC led by Mr. S. Asakawa. Someparticipants from RNC reported their experiences during the support work in Ref. 1, and in the following.
T. Fujinawa: I participated in the screening overthree separated periods: late March to early April,May, and late July to early August. In late March, I
had to drive a car from Wako to Fukushima to transport colleagues from Osaka and Kyushu. The highway
was damaged by the earthquake, which made it difﬁcult to drive a car. Access to Fukushima improved
drastically once the Tohoku Shinkansen restarted operation on 29th April.
I participated in the emergency screening of evacuees
at Big-palette in Koriyama but none of them exceeded
the oﬃcial reference level of 13 kcpm; however, I advised the residents to wash their hands and shoes, andgargle frequently, because many of them showed morethan our standard level. In the early stages, the foodsupply at the hotel was limited and the temporary ofﬁce would run short of copy paper. We brought manyboxes of paper, batteries for survey meters, disposable
body warmers, as well as our own food. The situation there changed with every visit I made. In August,only a few volunteers from our team remained, butmuch work had yet be done; I was even asked to staylonger. All of us hoped that the lives of those in thestricken area would soon return to normalcy.
H. Otsu: On March 28, I visited Kawamata public health center, where our team was conducting an
emergency survey as well as thyroid gland screening
for children. The radiation dose rate outside the building was 3 µSv/h, and 131 I had been found to be themain component. The radiation dose rate inside thebuilding at 9:30, the beginning of the screening was 0.1µSv/h and gradually increased as people entered andexited. About 500 people underwent surface screening
and 230 children underwent thyroid gland screening.
Almost the same number of people visited on March
29. Kawamata is located between Fukushima Dai’ichi
nuclear power plant and Fukushima city. Hence, a variety of people, not only residents from towns near the
power plant, such as Namie, but also prefecture oﬃcials, self-defense personnel, police, and radiation inspectors, passed through this screening center.
Surface survey were performed on the upper body,lower body, stomach, back, arms, legs; and soles of1

Ⅴ．Fukushima Survey & Related Research

RIKEN Accel. Prog. Rep. 45 (2012)

shoes using a GM survey meter. If the count rate wasmore than 100 kcpm, we asked the self-defense personnel to wash the entire body, while if the count rate wasmore than 13 kcpm, we cleaned the contaminated partwith a wet tissue or ﬂowing water.
Even with high radiation doses outside, the surfaceof the cloths were not so contaminated. The countrates were 200∼500 cpm, almost equivalent to theroom background in that area. High contamination
was sometimes found on the soles of shoes. In one
instance, the backs of a lady’s shoes showed a countrate exceeding the reference level. We accompanied her
while she washed them under ﬂowing water, but thiswas in vain, because even after washing, the counting
rate was not signiﬁcantly reduced. We advised her to
let us dispose of them. The self-defense oﬃce gave hera pair of sandals, which the Crocs company provided.
Subsequently, we noticed that the adhesion of contaminants to polyvinyl acetate was much lower than thatfor other materials such as polyvinyl chloride.
In an exception, high count rate of 4 kcpm was detected on the upper body of a ﬁre ﬁghter in Namie
town. He continued to work and provide water andtreat the bodily wastes of people who had been evacuated from towns near Namie. He had to handle water
hoses with his hands, and his upper jacket was contaminated. We advised him to take oﬀ the jacket outside
the entrance of his house and not wear it indoors to
reduce the contamination in residential spaces.
As mentioned above, one of the key points we focused on was the soles of shoes. We had the choiceof recommending that they be disposed of. However,a girl of the same age as my daughter told me before
being surveyed that her shoes were a birthday present
from her grandmother. I held my breath while surveying her from top to her shoes. The count rate of her
shoe soles was about 1 kcpm, much lower than the ﬁrst
reference level. I could just advise her mother to keepthem outside the entrance and to wash hands every
time when she came back to home.
During that time, I read a local newspaper every
morning in order to gauge residents’ mood as well. The
anger of the people of Fukushima was evident there.
However, although many would express their anger,
there were also many who kept calm and tried to keep
themselves informed about the radiation situation as
much as possible. I admire their behavior.
M. Wada: An important task in the ﬁrst weeks after
the explosion was to investigate the radioactive iodine
concentration in the thyroid gland, in particular forchildren. Together with other teams, we surveyed several hundred people and found that no one exceeded
the oﬃcial limit. The survey was performed in a relatively low background environment in the center with a
NaI(Tl) scintillation survey meter. However, the background level was much higher than ordinary and the2- 240 -

statistical treatment in the survey meter was not sufﬁcient to avoid criticism afterwards. We could havetaken more precautions during the measurements. For
example, using a simple metal tube to cover the sidesof the detector could have reduced the background rate
signiﬁcantly and the use of a simple scaler, which cancount the number of pulses, could have aﬀorded theresults with much higher statistical accuracy.
In the later stages, we encountered few cases where
the contamination of the body surface exceeded the reference levels, which coincided with increased demands
to survey food, drinking water, soil in rice ﬁelds. However, screening of such materials was not an oﬃcial
mission and there were no suitable detectors for such
purposes at the screening center. Occasionally, we received some samples and performed measurements at
RIKEN with HPGe spectrometers. We found thatthe drinking water in the wells, including a well in
Nagadoro-chiku of Iitate-mura which was known as ahighly contaminated area, were clean from radioactive
iodine or cesium at all. The soil in the rice ﬁled washighly contaminated, but the water in the rice ﬁelds
was also as clean as drinkable level. However, we did
not have an oﬃcial channel through which we could
convey such information. We could only inform prefectural oﬃcers on the next visit. When the surfacesof vegetables were highly contaminated, we could detect using a GM survey meter, however, the sensitivity
was much lower than the reference level of 500 Bq/kg.
If we could have taught the residents a simple ashingmethod2) , they could have prepared the concentrated
samples at home, which could have been easily tested
at the screening center. Unfortunately, we were notaware of this simple and sensitive method at that time.
Overall, we did contribute some extent; however, weserved more as radiation workers. As researchers, we
could have worked towards ﬁnding better solutions to
the problems and contributed in a more organized way.

Fig. 2. Screening of thyroid of local children at Yamakiya
community center on 24th March.

References
1) H. Hasebe, J. Particle Accelerator Society of Japan, 8,
No. 3, (2011) 132-136.
2) M. Wada et al., Acc. Prog. Rep. 45 (2012).

Ⅴ．Fukushima Survey & Related Research

RIKEN Accel. Prog. Rep. 45 (2012)

Gamma-ray dose resulting from the accident at Fukushima Daiichi Nuclear
Power Plant, as measured by the radiation monitoring posts in Wako campus
H. Mukai, S. Hashiguchi, H. Sakamoto, A. Akashio, R. Higurashi-Hirunuma and Y. Uwamino
134

Cs, 38%; and 137Cs, 15%. From the half-lives of these
nuclides, the accumulated dose during a year from March
15, 2011, was estimated to be about 0.5 mSv. The internal
dose due to the inhalation of the airborne radionuclides,
which was measured by Haba et al.2), was also calculated,
and the total annual dose due to the accident was estimatedto be 0.55 mSv.

A large heavy-ion accelerator complex, RIBF, and theRadioisotope Center are used in the RIKEN Wako campus,
and the dose rates at the site boundary are continuously
measured by six radiation monitoring posts, as shown in Fig.
1.
In March 2011, a magnitude-9.0 earthquake and asubsequent severe tsunami occurred, and huge amounts ofradionuclides were released from the Fukushima Daiichi
Nuclear Power Plant. Plumes containing radionuclides fromthe power plant passed over the Wako campus several times
in March, and some amounts of radionuclides were
deposited.

Fig. 2. Hourly averaged dose rate measured by RIC Westpost, along with the rainfall amount.

Fig. 1. Locations of radiation monitoring posts.
As shown in Fig. 2, a strong spike of dose rate wasobserved at 10 a.m. on March 15. This spike was mostly
due to the -rays from 133Xe gas1), and the dose rate quickly
decreased again because the radionuclide deposit on the
ground was small. However, a second big plume came on
March 21, and the rain on that day caused the deposition of132
Te, 131I, 134Cs, and 137Cs.1) This deposit has been keeping
the environmental dose rate high, as shown in Fig. 3.
In Fig. 2, small spikes occurring simultaneously with therainfall are seen. These spikes are not due to the accident
but due to the deposit of natural descendant radionuclides ofuranium and thorium. Spikes are also seen in Fig. 3. The
longer-term fluctuations in Fig. 3 are thought to be due tochanges in the conditions around the monitors, that is,heavy rainfall, pileup of fallen leaves, etc.
The dose rate due to the accident was estimated to be 0.1
μSv/h on March 30. The contributions due to individual
radionuclides were estimated on the basis of theradionuclide concentration in the soil: 132Te, 22%; 131I, 25%;

Fig. 3. Daily averaged dose rates measured at six posts.
References
1) H. Otsu et al., elsewhere in this issue.
2) H. Haba et al., elsewhere in this issue.

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Ⅴ．Fukushima Survey & Related Research

RIKEN Accel. Prog. Rep. 45 (2012)

Measurement of radioactivity concentrations of airborne radionuclides in
Wako after the Fukushima Dai-ichi nuclear power plant accident
H. Haba, J. Kanaya, H. Mukai, T. Kambara, and M. Kase

6

0

200

400

695.8
Te
99
739.5 Mo
110m
95g
763.9
Ag, 765.8 Nb
132g
772.6 I
134g
795.6
Cs
134g
802.0
Cs
136g
Cs
818.5

I
132g

954.6

131

I

884.7

110m

Ag

I
137
661.7 Cs
132g
667.7
I
637.0

131

600

722.9

Bi
214

132g

609.3

630.2
I
110m
657.8
Ag

132g

129g

487.4

0

10

522.7

Te

Pb
212

238.6

1

10

I

2

10

129m

129g

Cs
131
364.4 I
136g

340.5

Te
I
284.3

132

131

228.2

Tc
99m

Cs

140.5

3

10

136g

4

10

176.6

Counts per 0.5 keV

5

10

Te
459.6
134g
Cs
475.4
510.8 208Tl
140537.3 Ba134g
563.2, 569.3
Cs
208
583.2 Tl
134g
Cs
604.7

10

800

1000

Energy (keV)

Fig. 1. The γ-ray spectrum for the air dust sample
collected in the period Mar. 29 9:40–Mar. 30 11:15(JST). The filter sample was measured for 35866 s,
1353 s after the end of the sampling.

1x103

Radioactivity concentration (Bq m -3)

On Mar. 11, 2011, an earthquake of magnitude 9.0
occurred near the east coast of Honshu, Japan, and was
followed by a large tsunami. The disasters caused damage
to the Fukushima Dai-ichi nuclear power plant (FDNPP),
resulting in the release of radionuclides into theenvironment. On Mar. 15, at a monitoring post of the
RIKEN Wako Institute, located about 220 km to thesouthwest of FDNPP, we observed a rapid increase of dose
rate from the usual 0.03 μSv h–1 at 4:00 (JST) to 1.2 μSv h–1
at 10:00. Hence, we initiated an urgent measurement of theradioactivity concentrations of airborne radionuclides.
Air dust was collected using a commercially available air
dust sampler (M&F Enterprise SP-30) installed at the
RIKEN Wako Institute (35°46'32'' N, 139°37'04'' E). The
sampling flow rate was 30 L min–1. A cellulose glass-fiber
filter (ADVANTEC HE-40T) was used. No activatedcarbon filter was used; hence, our measurements were notsensitive to gaseous radioiodine. Dust was collected for 30
min for the first two samples in the period Mar. 15
11:15–11:45 and Mar. 16 13:15–13:45. After the thirdsample (Mar. 16 18:32–Mar. 17 9:00), dust wascontinuously collected for about one year, except for shortinterruptions due to the filter change. The filter samples
were subjected to γ-ray spectrometry using a Ge detector.
The detector efficiency was calibrated to an accuracy of
2%–10% using a multiple γ-ray standard source with thesame size as the sample. A calibrated 134gCs source was also
used to correct for the coincidence summing for 134gCs.
In this work, radionuclides of 140La, 140Ba, 137Cs, 136gCs,
134g
Cs, 133gI, 132gI, 132Te, 131I, 131mTe, 131gTe, 129mTe, 129gTe,
110m
Ag, 99mTc, 99Mo, and 95gNb were identified. Figure 1
shows a typical γ-ray spectrum of the sample collected in
the period Mar. 29 9:40–Mar. 30 11:15. The peaks ofvolatile 131I, 137Cs, and 134gCs are very intense since they
have a high release probability. In Fig. 2, the time

131I

Mar. 15 11:15 - Mar. 15 11:45
Mar. 20 10:05 - Mar. 21 10:00
Mar. 22 10:05 - Mar. 23 10:00

1x10

2

1x10

0

1x10

-2

1x10

-4

137Cs

1x101

134gCsMar. 29 9:40
- Mar. 31 10:00

1x10-1

1x10-3

0

5

10

15

20

25

35

40

1x10-5

1x10-7
0

50

100

150

200

250

300

350

Time since Mar. 15, 2011, 0:00 JST (d)Fig. 2. Time variation of the activity concentrations of
131 137
I, Cs, and 134gCs, as observed at the RIKEN Wako
Institute. Inset shows the time variation for 0–40 days.
variations of the activity concentrations of 131I, 137Cs, andCs are shown at the end of sampling as reference time.
In the first sample, the maximum activity concentrations of
131 137
I, Cs, and 134gCs were 35 ± 1, 8.8 ± 0.2, and 8.5 ± 0.2
Bq m–3, respectively. Although the activity concentrations
decrease with time, one can see two prominent peaks
corresponding to Mar. 20 10:05–Mar. 21 10:00 and Mar. 29
9:40–Mar. 31 10:00 as indicated by arrows in Fig. 2. It isinteresting to note that a distinct peak corresponding to Mar.
22 10:05–Mar. 23 10:00 is observed only for 131I, indicatingthat the behavior of radioiodine is different from thebehaviors of 137Cs and 134gCs. These time variations are very
similar to those measured at Japan Chemical Analysis
Center, Chiba, located about 220 km to the south-southeastof FDNPP.1) Amano et al.1) measured the ratios of gaseousiodine to total iodine to be 0.52–0.71, using the cellulose
glass-fiber filter in combination with an activated charcoal
cartridge. Thus, the total activity concentrations of 131I
measured at the RIKEN Wako Institute are estimated to be
2.1–3.4 times those presented in Fig. 2. The activity
concentrations of 137Cs and 134gCs are almost equal for all
samples. The 134gCs/137Cs ratio is calculated to be 1.0 ± 0.1
for Mar. 15, and this value is consistent with otherobservations related to the FDNPP accident.1)
We also determined the activity concentrations for otherradionuclides such as 140Ba, 136gCs, 133gI, 132Te, 129mTe,
110m
Ag, 99Mo, and 95gNb.2) Our data, together with themeasurements from other monitoring stations, will help
provide further information on the characteristics of the fuel,release mechanism, and the transport of the radionuclides.
134g

References

1) H. Amano et al.: J. Environ. Radioact. (in press).
2) H. Haba et al.: submitted to Geochemical Journal.

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30

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RIKEN Accel. Prog. Rep. 45 (2012)

Environmental radioactivity at Wako
from Fukushima Daiichi Nuclear Power Plant

H. Otsu, N. Aoi, *2 H. Baba, R.J.Chen, H. En’yo, T. Fujinawa, S. Go, *1 H. Hasebe, T. Hashimoto, *1,*2 H. Watanabe,
T. Ichihara, E. Ideguchi, *1 T. Isobe, J. Lee, K. Ozeki, K. Kisamori, *1 S. Kubono, *1 M. Kurokawa, G. Lorusso, K. Miki, *2
S. Michimasa, *1 H. Miya, *1 T. Motobayashi, S. Ota, *1 P. Doornenbal, M. Sako, H. Sakurai, S. Sakaguchi, D. Steppenbeck,
M. Takaki, *1 M. Takechi, *3 S. Takeuchi, A. Taketani, Y. Togano, *3 H. Tokieda,*1 Y. Wakabayashi, *1 K. Yoshinaga,
T. Kawahara, *1 H. Imao, Y. Ito, and N. Sakamoto (fGamma collaboration)*
Energy spectra obtained on March 24 and April 1 areshown in Fig. 1. Many of the observed peaks correspond to
gamma-ray radiations, which would originate from the
Fukushima Daiichi Nuclear Power Plant. Among them,132
Te, 131I, 132I, 134Cs, and 137Cs isotopes are observed asprominent peaks. Gain shift and offset deviation in thedetector or electronics caused by fluctuations inenvironmental parameters such as temperature or humidity
were corrected by referring to each gamma ray peak.
The time dependence of the radiation is shown in Fig. 2.
Before March 21 (6th day on the horizontal axis in the
figure), the main contributions were from 131I, 132Te, and its
daughter 132I. March 21 and 22 were rainy. During thesetwo days, the radiation from these four isotopes increased
very significantly. However, after the 22nd, we did notobserve such increases associated with the rain.

After the severe accident at the Fukushima Daiichi Nuclear
Power Plant, we started monitoring the environmental
radioactivity at Wako campus in order to detect a possible
increase in the radioactive elements dispersed from theplant.
We set a germanium detector with volume of 146 cm3outside the RIBF building, next to the entrance facing theopen space with a lawn. Gamma rays could be measured
directly in the environment for 23 days, from March 15 toApril 8. Initially, the data were collected by a multi-channel
analyzer at 15-min intervals. On March 19, we installed an
automatic data acquisition system that continuously
processed the signals and generated histograms from thedata every 15 minutes. Data accumulation continued with
24h a day except during the intervals when liquid nitrogenwas supplied for cooling the germanium crystal andpreamplifier.

Count / 15 min.
3/24 13:00

Cs

I
132

134

Cs
I
132

137

Cs

132

Te

134

131

I

4/1 13:11

Eγ[keV]*1 CNS, The University of Tokyo*2 Present affiliation: RCNP, Osaka University*3 Present affiliation: GSI- 243 -

Fig. 1. Gamma-ray energy spectra
obtained using the Ge detector on
March 24 (cyan) and April 1 (red).
Top frame shows energy spectra withlogarithmic vertical axis, whereas
bottom frame shows the same spectrawith linear axis. Inset of the bottom
frame shows spectra from 600 to
800 keV region.

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RIKEN Accel. Prog. Rep. 45 (2012)

Count/sFig. 2.
Time dependence of observed radiation.
Horizontal axis corresponds to days after
March 15 at 0:00 A.M. and vertical axis
indicates the counts per second.
March 21 and 22 were rainy. A drastic
difference of trends occurred before andafter these days. Before March 21 (6thday on the horizontal axis in the figure) the
main contributions were from 131I, 132Te,
and its daughter 132I. During rain, theradiation from the isotopes indicatedincreased very significantly.
However, we did not observe suchincreases associated with rain after the
22nd. After the rains, iodine isotopes
decreased with their decay half lives or on
the same order. Cesium isotopes did notdecrease in this period. In fact, they
seemed to increase slightly.

15

131I 364.5 keV

10

5132I 667.7 keV

134Cs 604.7 keV
137

0
0

5

10

Cs 661.7 keV

15

20

Days after 3/15 0:00

These tendencies indicate that most of the radioactive
elements in the air were fell down to the ground by the rainaround the 21st. Subsequently the cesium 134 and 137
isotopes exhibited a small but continuous increase during
the measurement because of the further deposition of theseisotopes.
In order to evaluate the amount of radioactive elements
accumulated on the ground, we performed a Monte Carlosimulation with the GEANT41) toolkit by taking into
account the area (ΔS) corresponding to the scoping angular
acceptance, detection efficiency (ε(Eγ)), and absorption
effects (f(Eγ,r)) by air and other materials. We obtained
the coupled parameters fεΔS as:�dS.
f �∆S = �(Eγ ) f (Eγ , �r)cos(θ)S-4
The obtained values were 6.2×10 and 5.0×10-4 [m2] at
364.5 keV and 660 keV, respectively. By taking thebranching ratios into account, we evaluated the surface
density of radioisotopes on the grass in the Wako Campus
as 26 kBq/m2 for 131I and 2.8~3.0 kBq/m2 for each of the- 244 -

other three isotopes on March 25. In these analyses, we
assumed for simplicity that all radioactive elements
remained on the ground surface and did not penetrate or
sank into the soil. Further analyses are needed forquantitative estimation, especially for the consideration of
depth distribution inside the soil. Systematic errors are
estimated to be about 20%, mainly due to ambiguity in thesimplified model employed for the simulation.
In summary, we measured the environmental
radioactivity in the Wako area in the period following the
Fukushima Daiichi Nuclear Power Plant accident andevaluated the results.
References
1) http://geant4.cern.ch/Ⅴ．Fukushima Survey & Related Research

RIKEN Accel. Prog. Rep. 45 (2012)

Radioactive Xe arriving in and passing through Wako area
H. Otsu, fGamma collaboration1
Counts/s

131I 364.5 keV

Pb(Natural)
214

212

Pb(Natural)

214

131
I 284 keV
Pb(Natural)

132Te 228 keV

133

Xe 81.0 keVArea Monitor [μSv/h] 1.5
150
Immediately after the severe accident that occurred at the
Fukushima Daiichi Nuclear Power Plant, we started thespatial radiation dose measurement in Wako campus1). In
100
1
this article, we report on the evidence of gaseous xenon
133Xe 81.0 keV
isotopes arriving in and passing over the Wako area. The
major components were 133Xe, 133mXe, and 135Xe, as50
0.5
identified by our Ge detector measurements. Experimental
131I 364.5 keVdetails can be found in Ref. 1 of this volume.
00
From March 15 to 16, abrupt changes in the
134Cs 604.7 keV
2)
0:00
12:00
0:00
12:00
0:00
0
0.5
1
1.5
2
environmental dose were observed at three instances by
3/16
3/15
area monitors at the RIKEN Nishina center as burst-like
Fig. 2. Time dependence of Xe 81.0 keV peaks (left verticalphenomena. The equipment was not ready for the first andaxis) with dose read out by area monitor in µSv/h unit (rightvertical axis). The 81 keV peak is not very prominent duringsecond abrupt changes. In the early morning on the 16th,
daytime on the 15th.
the third abrupt change occurred, although the scale of the
burst was smaller than those of the previous two events.
At that occasion, we successfully observed gamma-ray
we identified 133Xe, 133mXe, and 135Xe, respectively. Other
possibilities for the origin of the 81 keV peak, for example,
peaks originating from the burst.
80 keV from 131I, are rejected because no lines
Figure 1 shows gamma-ray energy spectra recorded
corresponding to their branches were observed. The
between 5:09 and 8:50. Representative spectra are shown inbump-like structure below the 81 keV peak is interpreted as
red, with a maximum at 6:15, and in cyan, as passing overcorresponding to the Compton scattering component.
the period at 8:39. The spectrum at 13:50 on the 15th is also
Figure 2 shows the time dependence of the 81 keVshown by solid green line for comparison. Magnitudes ofpeak in counts per second and the radiation dose measured
the three peaks at 81, 233, and 250 keV had alreadyby the area monitor in µSv/h unit for comparison. The 133Xe
increased at 5:09. They rapidly reduced at 8:50 while thoseyield varied on the morning of the 16th in a manner similarof other peaks stayed almost in constant during these 4
hours. From gamma-ray energies of 81, 233, and 250 keV, 	  	  	
 to the area monitor dose, whereas the peak yields for 131I
and 134Cs remained almost constant.
Counts/15 min.
These comparisons indicate that rapid increases in the
133m
Xe 233 keVdose were caused by gaseous Xe activities. Another135
Xe 250 keV
observation is the more rapid decrease of the yield than anyof the lifetime of the Xe isotopes: 5.25 d for 133Xe, 2.19 dfor 133mXe, and 9.14 h for 135Xe. This suggests that gaseous
Xe passed through the area as a cloud without causing alarge amount of activity accumulation on the ground.
The efficiency of the germanium detector with the same
geometry (56.8 mmφ × 61.7 mm) was calibrated using
several checking sources with a facially distribution.
Detection efficiency at 81 keV was calculated with theextrapolation as 0.11. From these properties og the detector,
we estimate the gamma-radiation flux density as 2.5×105[m-2s-1] at the observed burst maximum (70 Counts/s).
Eγ[keV]
Fig. 1. Gamma-ray energy spectra for the early morning ofIn summary, we observed a rapid increase andthe 16th of March with green indicating a reference, which
decrease in radiation on March 16 at the Wako campus. We
corresponds to the spectrum at 13:50 on March 15. Horizontal
conclude from our gamma-ray measurements that theyaxis indicates gamma-ray energy. Vertical axis shows detection
originate from radioactive Xe gases. The Xe isotopes werecounts within 15 minutes. In the inset, spectra are enlarged forconsidered to have passed over the Wako area.
the range between 220 and 260 keV. Here, data are limited for

6:15 (Red), 8:39 (Cyan), and reference (Green). Peaks related
to 133Xe, 133mXe, and 135Xe were observed during the burst.

References

1)

1

Gamma collaboration members are listed in the previous reportin this volume.

2)- 245 -

H. Otsu, fGamma collaboration, this volume.
Y. Uwamino, this volume, private communication.

Ⅴ．Fukushima Survey & Related Research

RIKEN Accel. Prog. Rep. 45 (2012)

Measurement of 137 Cs and 90 Sr in a Surface Soil Sample
Obtained from Namie, Fukushima
Y. Kobayashi, Y. Saito1, M. Iwasaki, and R. Hayano2
An enormous quantity of the radionuclides has beendischarged owing to the hydrogen explosion of the nuclear
reactor in the Fukushima Daiichi Nuclear Power Plant inMarch 2011. We performed γ-ray spectroscopy for
detecting 137Cs and the liquid scintillation technique fordetecting 90Sr with the rapid chemical separation of the soil
obtained from Namie, Fukushima Pref., to evaluate the
distribution and deposition of the radioactive fallout fromthe accident1). A soil sample with 2-cm thickness wascollected from the surface and from an area of 200 cm2 in
Namie on April 29, 2011. The sampling point, at thelatitude 37º 35’ 34.77” North and the longitude 140º 45’
12.85” East, is located approximately 30 km northwest fromthe site of the explosion. The spatial dose rate was 25~30µSv/h at the sampling time. The total wet weight of thesample was 19.8 g.
The complete sample was placed in a plastic bottle for the
γ-ray spectrometry that was performed using a Ge detector.
The γ-ray spectrum of the soil sample obtained from Namie
is shown in Fig. 1. Most γ-peaks could be assigned tolong-lived radioactive 137Cs (T1/2 = 30.1 y) and 134Cs (T1/2 =2.06 y). The concentration of 137Cs was estimated from thepeak area of a typical 661.9-keV γ-ray. The activity of 137Cs
was estimated to be 63,400 Bq/kg (wet soil) by acalculation performed using the counting rate of 20.4 cps,
the counting efficiency of the Ge detector, and thebranching ratio of the 661.9 keV γ-ray.
90

Sr can achieve a secular equilibrium with thehigh-energy β-emitter 90Y in a few weeks. 90Sr
concentration was determined by means of the lowbackground liquid scintillation technique with rapid
chemical separation through oxalate coprecipitation2). The
soil was heat-treated at 450 ºC for 24 h and dissolved inFig. 1. γ-ray spectrum of the soil sample obtained from
Namie, Fukushima.  and  indicate the 661.9-keV γ
peak of 137Cs and the γ-rays of 134Cs, respectively. The
measuring time was 1800 s.

Fig. 2. β-ray spectra of (a) a oxalate salt extracted after
chemical separation, (b) the blank test, and (c) the reference
which was a precipitation of Y(OH)3 obtained using a 90Sr/90Y
tracer with 160 Bq activity.

aqua regia. Then, the Y and Sr carrier solution was added toit. 90Y was extracted from bis(2-ethyl-hexyl) phosphoricacid (HDEHP) and toluene solution after the radioactive
equilibrium of 90Sr/90Y was reached. Yttrium was separated
as a hydroxide, and then, oxalate salt was precipitated withthe condition for which the pH was ~1.5. The chemicalyield was estimated to be 70%. The sample for the β-ray
measurement was prepared by adding a liquid scintillator(Perkin Elmer Ultima Gold AB). The measurement was
performed in a Packard low-background detector. Figure 2
shows the β-spectra of a soil sample prepared by rapid
chemical separation, a blank test, and a hydroxide asreference including 90Sr/90Y with 160 Bq activity. The end
point (Emax = 2.2 MeV for 90Y) could not be detected in the
spectrum of the soil sample, and the line shape in its casediffered visibly from that of the reference precipitation. It
was concluded that the activity of 90Sr in the soil sample
was below the detection limit. The detection limit of thepresent method was estimated to be 35 Bq/kg from thecounting efficiency and counting rate of the reference
sample. The 90Sr/137Cs ratio caluculated in this study wasconsistent with the analytical report published by MEXT on
May 31, 2011.
References

1) http://www.radiochem.org/kinkyu/index.html
2) http://www.kankyo-hosyano.go.jp/series/lib/No2.pdf
3) http://radioactivity.mext.go.jp/ja

College of Science and Engineering, Aoyama Gakuin Univ.
2
Department of Physics, University of Tokyo1- 246 -

Ⅴ．Fukushima Survey & Related Research
RIKEN
Accel. Prog. Rep. 45 (2012)

RIKEN Accel. Prog. Rep. 45 (2012)

Measurement of β-ray emitting isotopes from the Fukushima Daiichi
nuclear power plant accident
F. Sakuma, M. Tokuda∗1 , Y. Sada∗2 and T. Hashimoto∗3[Fukushima reactor accident, β-ray isotopes]
The Fukushima Daiichi nuclear power plant accident, caused by the earthquake that occurred on March
11, 2011, resulted in the release of large amounts ofradioactive materials into the atmosphere. Emissions
from γ-ray emitting isotopes such as 137 Cs and 131 I
were observed and reported immediately after the accident. However, the isotopes that emitted only β-rays,
such as 90 Sr, were not detected owing to the diﬃcultyin the measurement of β-rays. As it is known thatthe 90 Sr/137 Cs activity ratio was observed to be up
to 1.5 at the Chernobyl accident1) , we attempted tomeasure the β-ray emitters using plastic scintillatorsand photomultiplier tubes (PMTs) in March and April
2011. The experimental setup is shown in Fig. 1. We
wiped the roof of a car (∼1.5 m2 ) with a cleaning tissue (Nippon Paper Crecia, Kimwipe S-200) on March
23 at around 17:00: the car was parked in the RIKEN
Wako campus from before the earthquake to the timeof wiping. In order to measure the spectra of β-rays
with a few MeV of energy, a thin (t1 mm, PMT1)
and a thick (t50 mm, PMT2) plastic scintillator wereused. A sample was set on the thin scintillator, andan event trigger was generated by a coincidence signal of the PMT1 and the PMT2. The ADC spectrumof the β-rays was obtained using the thick scintillator.
In addition, two large plastic scintillators (PMT3 andPMT4) that covered the complete system were used ascosmic-ray veto counters.

tively. The endpoint energy of 90 Sr is 2.280 MeV, and90
β − 0.5459MeV
its half-life is 28.79 y, i.e., 90) → 39 Y
38 Sr → (28.79y90
β − 2.280MeV) → 40 Zr. In the ﬁgures, the spectrum→ (64.053h
shape can be seen to obviously change, i.e., the countrate in the range over 500 ch decreased with time. As
the half-life of 90 Sr is very long, this decreasing radioactivity indicates that we could not have observed
the β-rays emitted by 90 Sr. In this case, what isotope had been observed that emitted β-rays similar
to 90 Sr? Figure 3 shows the count rate in the ADC
range between 500 ch and 800 ch after subtracting thebackground, in which each data point is obtained on adiﬀerent day and the start time corresponds to April
1, 21:30. The ﬁtting result of a half-life is 3.20 ± 0.23
d, which is consistent with the half life of 132 Te, i.e.,− 2.137MeV
132
132
Te → (β3.20d) →I. In fact, a radiation monitor group in KEK reported the observation of 132 Te in
March 20112).
In conclusion, we could not observe β-rays emitted
by 90 Sr, although we observed a β-ray emitting isotope,132
Te, which was released from the Fukushima reactor
after the accident in March 2011.

Fig. 2. ADC spectra obtained from the sample. (see text)

Fig. 1. Schematic drawing of the setup.

The ADC spectra obtained from the sample areshown in Fig. 2; these spectra were obtained with thesame setup but on a diﬀerent days: April 1, 21:30 toApril 2, 4:36, and April 20, 16:22 to April 21, 16:26.
Each ADC spectrum was ﬁtted with the backgroundcosmic-ray spectrum and referenced 90 Sr spectrum in
the ADC range between 500 ch and 800 ch. The
cosmic-ray and 90 Sr spectra were obtained with anempty sample and 90 Sr source of ∼0.9 MBq, respec∗1∗2∗3

Fig. 3. Count rate in the ADC range between 500 ch and800 ch after subtracting the background.

References
1) Environmental Consequences of the Chernobyl Accident
and their Remediation (IAEA, 2006)2) http://www.kek.jp/quake/radmonitor/

Department of Physics, Tokyo Institute of Technology
Department of Physics, Kyoto University
Department of Physics, The University of Tokyo- 247 -

1

Ⅴ．Fukushima Survey & Related Research

RIKEN Accel. Prog. Rep. 45 (2012)
RIKEN
Accel. Prog. Rep. 45 (2012)

Pilot survey of feral cattle in the vicinity of Fukushima Daiichi
Nuclear power plant
M. Wada, H. Haba, S. Sato,∗1 S. Roh,∗1 and M. Toyomizu∗1
A pilot survey of radioactive contamination of feral
cattle in Minamisoma city was performed in late August 2011. After the accident of Fukushima Daiichi nuclear power plant, many cattle left their enclosures andsurvived by consuming plants and water in a ditch that
may have been radioactively contaminated. Therefore,
it is worth surveying the radioactive contamination in
the muscles, organs, blood, urine, and feces of cattle
before slaughter disposition.
Increasing internal radiation dose in human body
because of contaminated food became a sensitive problem especially after contaminated beef was found in themarket. The beaf cattle consumed contaminated straw
immediately before being butchered, and the meat contained more than 2 kBq/kg of radioactive cesium. The
transfer rate of cesium from the contaminated food tothe body and the metabolism rate have been previously
studied1,2) . If the speciﬁc radioactivities in cattle muscle could be evaluated from that in the blood or urine
when the cattle are alive, it might aid in assessing theuse of those cattle.
In the ﬁrst sampling, we evaluated 5 cows captured
at Long. 140.929E, Lat. 37.572N, about 19 km NNWfrom the nuclear power plant. Samples of blood, urine,
feces, and amniotic ﬂuid were taken in 20 cc vial bottles, and samples of muscles and other organs weretaken in 100 cc bottles at the location. The 5 cowsthat were evaluated inhabited a trench with abundantavailability of water. We observed that the urine colorof these cows was clearer than that of cows in a stock
farm. One cow was pregnant and we sampled both thefetus and amnionic ﬂuid.
The radioactivity levels of samples were measured
using a well shielded HPGe detector with a relative efﬁciency of 20%. The absolute eﬃciency for each sample������������
������
����
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������������
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����

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����

��
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�

�

Cattle ID�������
������
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Fig. 1. Speciﬁc activities of radioactive Cs in individual
cattle samples. #6 is the fetus of #5.
∗1

References
1) J.E. Johnson, et al.: J. of Nutrition, 94, 282 (1968).
2) G. Voigt et al.: Science of Total Environment, 85, 329(1989). S. Ambe et al.: Chem. Lett. 2001, 149.

Specific Activities [Bq/kg]

Specific Activities [Bq/kg]

��bottle was calibrated using a mixed standard source ina water matrix. The sum correction factors for the
γ-rays from 134 Cs were evaluated with a sample itself
but placed at 10 cm apart from the detector.
The speciﬁc activities of the sum of 134,137 Cs wereplotted for individual cattle (Fig. 1). Cs activity did
not show a uniform distribution even within the muscles, and the internal organs showed much lower radioactivity level than that in the muscles. The speciﬁc
activities were also plotted as a function of the blood
activity (Fig. 2). Positive correlations were found, forexample, ratios of the ﬁllet and tongue to the blood
were approximately 33 and 12, respectively. Since evaluation of the total amount of urine per day was diﬃcult
for such feral cattle, we could not use urine samples forquantitative evaluation of whole body activities. In aliver sample, some amounts of 110m Ag 148(5)Bq/kg)
and 129m Te 20(1) Bq/kg) were found. These radionuclides were also detected in the soil of the area inhabited by the cattle; further, we speculated that the cattle consumed contaminated soil when drinking the water in the ditch, which resulted in the heavy metals toconcentrate in the liver.
Although we planned to continue the survey, becauseof an organization issue, only the ﬁrst pilot survey was
conducted, and the number of samples was limited because of the allowed sampling time at the location.
However, we consider our limited data worth reporting, because our data may be used in combination withother surveys.

Graduate School of Agricultural Science, Tohoku University- 248 -

��

��

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��

���

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Specific Activity in Blood [Bq/kg]

Fig. 2. Speciﬁc activities of radioactive Cs as functions of
the speciﬁc activity of blood.

1 Ⅴ．Fukushima Survey & Related Research

RIKEN Accel. Prog. Rep. 45 (2012)

Gamma ray spectroscopic analysis of underwater environmental
samples from ocean near Fukushima Daiichi Nuclear Power Plant
S. Moriya and H. Otsu

Radioactive materials have been released into the nearby
ocean from Fukushima Daiichi nuclear power plant after
severe accidents in March, 2011. Here, we discuss a seriesof analyses that we performed to access the incorporation ofsuch radioactivity into members of the coastal eco-system.
Oceanic biomasses and environmental samples werecollected at the Fishery port of Hisanohama and thesurrounding coastal regions (figure 1). Sample collection
was done through self contained underwater breathingapparatus (SCUBA) diving. The SCUBA sampling wasdone in collaboration with the Japanese Association ofUnderwater Science. We collected 10 litter of sea water andfiltrated it using a 0.22 µm Omuni pore filter (Millipore)
until the filter was clogged, after which we obtained
microbial biomass. The filtrated water was then evaporated
to obtain sea salts. We also collected biofilm materials fromthe surfaces of rocks and macro benthos (Ascidiacea,
Sponge, Sea urchin, Starfish and Shellfish) from seabed.

Fishery port of Hisanohama,
31km from the power plantArea 2
Area 4
Area 3

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Fig.1 : Sampling spots in Hisanohama.
Biofilm on the rock was collected using a wire brush with
distilled water. The samples of biofilm and macro benthos
were dried at 120  in a drying oven, then milled using afood mixer. The obtained samples were analyzed withgamma ray spectroscopy.
Results of the gamma ray spectroscopic analysis using a
Germanium detector are shown in table 1. According tothese results, the environmental water and primary
producers (oceanic microbes and biofilms) contain 134Cs
and 137Cs. However, the macro benthos on the sea floor
contain not only radioactive Cs but 110mAg in their body.
To estimate the source of 110mAg in such the organisms,
we performed additional sampling to obtain bottom mudusing a short piston core sampler operated by the SCUBA
divers. The core sampler is assembled with an end cap and a
PVC tube of 40 mm diameter and approximately 25 cm
length. The piston of the core sampler is made from craft
resin with the same PVC tube as that in the template (40mm diameter and approx. 5 cm length) and is inserted into

- 249 -

the PVC tube during the operation. Core samples from thedepth of 15-20 cm have been analyzed by sectioning depthof 5 cm each by using the Ge detector. The results (Table 1)show that the materials on surface of the seabed contain
110m
Ag and both of 134, 137Cs but bottom mud from deeper
position contain only radioactive Cs.
These results suggest that the once-precipitatedradioactive Ag is possibly re-suspended into the oceanic
water by the swell of the ocean and is fed by macro benthos.
The mechanism for 110mAg precipitation is still unclear.
Further analyses of the seawater contents by fine
fractionation and its relationship with land-originatedcompounds are needed to understand the entry of 110mAg
into the coastal eco-system.
Note that the gamma ray analyses are ongoing especiallyon the correction of the sum peaks from 134Cs and 110mAg
because of the relative geometry between a sample and theGe detector. These are planned to be corrected using the
GEANT4 code by considering the sample height
information.
In summary, we determined the radioactivity absorbed byoceanic plants and animals not only for Cs isotopes but also
for Ag isotopes released from the power plant accident.
These results will be utilized to understand the futureeco-system situation around these areas.

Ⅴ．Fukushima Survey & Related Research

RIKEN Accel. Prog. Rep. 45 (2012)
RIKEN
Accel. Prog. Rep. 45 (2012)

Analysis of Gamma-ray Emitters in Soils for Obtaining Maps of
Deposited Radioactive Substances around Fukushima Daiichi
Nuclear Power Plant
S. Shimoura∗1
In order to prepare maps of radioactive substances
deposited on the ground surface, soil samples fromthe 5-cm surface layer were collected for about 2,200
locations within approximately 100 km from the
Fukushima Daiichi Nuclear Power Plant (NPP) from
June 6 to 14 and from June 27 to July 8, 2011.
About 10,000 samples in total, corresponding to ﬁveor three diﬀerent points for each location, were analyzed by the Japan Chemical Analysis Center (JCAC)(40%) and by a collaboration (60%) among universities and institutes, including Center for Nuclear Study(CNS), RIKEN Nishina Center (RNC), Research Center for Nuclear Physics (RCNP), and other organizations. CNS acted as a hub of the collaboration,
where tasks such as receiving and checking about 6000
samples from Fukushima; sending them to 22 analysis groups depending on their capacities; and collecting, checking, and summarizing the results were performed. About 120 samples, collected from within 20
km of NPP, were analyzed by CNS and RNC.
The activities of Cs-134, Cs-137, and I-131 were analyzed by measuring γ-rays emitted by all the samples
using the germanium detectors of each analysis group.
Activities of other substances such as Te-129m, Ag110m were also analyzed for about 2000 samples. Eachanalysis group calibrated the absolute magnitudes of
the activities by using the IAEA-444 standard samples1) ﬁlled in the same U8 type containers (5 cmφ ×
5 cmH ) as the soil samples that were circulated among
the collaboration. Since Cs-134 and Cs-137 are involved in the IAEA-444 standard, their absolute activities were determined by the relative yields from the
IAEA-444 sample and each soil sample.
Measurement time for a sample was basically one
hour, whereas long runs with around 10 hours werealso performed for some samples to measure the radiation from I-131. However, large portion of the samples
have no signiﬁcant value for I-131 because of decay
out through more than 10 half lives and of Compton
backgrounds from the radiation of Cs-134 and Cs-137.
The same ∼300 samples were analyzed by multiple
groups, one of which is either CNS or JCAC, for the
purpose of a cross checking. Results from these diﬀerent groups were consistent within 30% for each sample
and within 15% for an average of ﬁve samples for eachlocation. The deviation is considered to be due to theinhomogeneity of the activities in soil samples, since it
was found that for some samples, counting rates de∗1pended on the direction of the sample relative to thedetector.
The maps based on the resultant activities (Cs-137,
Cs-134, I-131, Te-129m, and Ag-110m) are found fromthe Web pages of MEXT2) , JAEA3) , and RCNP.4)
As shown in ref.2) , the ratios of the activities of Cs134 to Cs-137 were independent of the locations about
0.9–1.0 . On the other hand, activity ratios of I-131
to Cs-137 of the soils in the southern coastal areas areabout 4 times larger than those in the northern areas.
The present results are expected to be used as a basis for assessing the radiation dose in the concernedareas, further investigations in the environmental research works, and other such purposes.
The measurements at RNC and CNS were performed by H. En’yo, Y. Watanabe, S. Yokkaichi,
H. Watanabe, M. Nishimura, T. Kishida, H. Otsu,
H. Ohnishi, T. Tada, K. Ozeki, K. Morita, D. Kaji,
H. Haba, J. Kanaya, E. Ikezawa, T. Maeyama,
M. Nabase, K. Ikegami, A. Yoshida, T. Hori, Y. Yamaguchi, S. Sonoda, Y. Ito, Y. Uwamino, H. Mukai,
S. Fujita, M. Fujinawa, H. Sakata, T. Kambara, K. Yamada, T. Otsuka, E. Ideguchi, S. Ota, D. Nakajima,
M.-H. Tanaka, R. Yoshino, Y. Sasamoto, S. Noji,
S. Go, M. Takaki, K. Kisamori, R. Yokoyama, T. Fujii,
Y. Kubota, R. Saito, and S. Shimoura.
The present work was in the course of the project
under the 2011 Strategic Funds for the Promotion of
Science and Technology, entitled “Establishment of theBase for Taking Measures for Environmental Impactof Radioactive Substances – Study on Distribution ofRadioactive Substances” of MEXT.
References
1) http://nucleus.iaea.org/rpst/Documents/
rs iaea 444.pdf
2) various reports can be found at
http://radioactivity.mext.go.jp/en/;
http://radioactivity.mext.go.jp/ja/ (in Japanese)3) http://ramap.jaea.go.jp/map/
4) http://www.rcnp.osaka-u.ac.jp/dojo/ (in Japanese)

Appendix:
The participants of the soil sampling from RNC andCNS are as follows: S. Fujita, K. Ikegami, Y. Ito,
T. Kageyama, S. Kubono, K. Morimoto, H. Mukai,
S. Nishimura, K. Ogawa, T. Sonoda, and M. Wada.

Center for Nuclear Study (CNS), The University of Tokyo- 250 -

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RIKEN
Accel. Prog. Rep. 45 (2012)

RIKEN Accel. Prog. Rep. 45 (2012)

Simple screening method for radioactively contaminated food using
ashing method with Geiger-Müller counter
M. Wada and H. Haba
After the accident of the Fukushima Daiichi nuclear
power plant in the middle of March, 2011, radioactive
contamination of food has become a serious concern,
especially for those who live in the area where the soil
is highly contaminated. It is worth developing a simple
screening method, which can be used at home.
In general, β-rays can be detected with a muchhigher eﬃciency than γ-rays, and the shielding of βrays is considerably easier than that of γ-rays. However, β-rays can be shielded by the sample itself.
Only surface contamination can be detected by a thinwindow Gaiger-Müller (GM) counter. If the radioactivity could be concentrated into a thin sample, thetotal β radiation detection method would be a signiﬁcantly sensitive method for contamination surveys.
Most foods consist of water, carbohydrates, proteins,
lipids and ash. The concerned radionuclides, such as
Cs or Sr, belong to the ash that can be extracted fromthe sample by an ashing process. The content of ash
in food is on the order of 1%, which implies 100 times
concentration can be achieved.
First, we tested this method at home using a kitchen
grill for ﬁsh in order to ash the food samples without
contamination. This required approximately 20 min
under a medium ﬂame, and the temperature of thesamples was less than 500 ◦ C. The weight of ash wasless than 1 g. The ashes were ﬂattened and covered
with a 10 µm kitchen ﬁlm (polyvinylidene chloride).
The β-rays from the ash were detected using a 5 cmφaperture GM survey meter (TCS146B). The amount ofpotassium (K) in foods was noted from an ingredientstable and the speciﬁc activity of them could be calculated from the natural abundance of 40 K (0.0117%)and the half-life (1.28×109 y). The calculated counting rate ∆Ical from the table, with a branching ratio of
90% and a detection eﬃciency of 25%, was comparedwith the measured counting rate ∆Iexp after subtracting the background (Tab. 1). They agreed well within
the limits of a statistical error.
Next, samples that were radioactively contaminated

Table 1. Speciﬁc activities of 40 K in foods and the counting rates (Ical and Iexp ) obtained with a GM counter.
spicesSpinachSoy beanW. mellon
Onion
Milk

ash
%
1.7
1.4
0.2
0.4
0.7

K
%
0.69
0.49
0.12
0.15
0.11

S.A.
Bq/kg
219
155
38
48
35

m
g
59
33
100
114
76

∆Ical
cpm
174
70
51
73
36

∆Iexp
cpm
169(19)72(14)
67(11)
70(9)34(19)- 251 -

Table 2. Measurements of contaminated samples.
sampleBerry1
Berry2Urine1Urine2
Milk1
Milk2

Cs
Bq/kg
135
135
1727
549
145
268

K40
Bq/kg
76.7
76.7
532
377
48.8
47

mass
g
21.1
22.6
23.9
21.0
20.2
20.0

ash
g
0.53
2.1
4.0
0.9
0.2
<0.1

∆Ical
cpm
33
36
378
149
28
42

∆Iexp
cpm
38(4)
22(3)
217(5)
158(5)
24(3)44(3)

were tested at laboratory (Tab. 2). The speciﬁc activities of 134,137 Cs and 40 K were determined with a HPGe
detector before ashing. The expected β-ray count rate
Ical was evaluated with the cover area of this setup(75%); the eﬃciencies of β-rays (14.1%, 12.5% and22.5% for 137 Cs, 134 Cs and 40 K, respectively), whichwere evaluated by accounting for the self absorption
with a typical thickness (30 mg/cm2 ) and for the mean
β-energies and branching ratios. When the amount ofash was less than 1 g, the experimentally measured
β-ray counting rates ∆Iexp agreed with the calculated
values.
In the Milk1 measurement, the background count
NB was 350 in 10 min and the count from the sample
N was 592 in 10 min. The detection limit in this case
was Nm = 84 counts (8.4 cpm) with a decision criteriaof 3. It roughly corresponded to a total activity of
1.2 Bq in the sample. The maximum amount of thesample was limited by the amount of ash. Considering
the self shielding of β-rays, the amount of ash shouldbe less than 1 g. If a 50 g sample is used, the detection
limit, in terms of the total speciﬁc activity including
40
K, reaches 24 Bq/kg.
The contribution from 40 K is an important concern.
For most foods, the approximate amount of K can be
determined from an ingredients table, or, if an exact
value is needed, a commercial K ion meter, such as
C131K, can be used. We tested many samples with awidely varying K concentration and the values determined showed good agreement with the activity measurements recorded with a HPGe detector.
Although the total β radiation measurement method
has unavoidable drawbacks in the identiﬁcation of radionuclides and in the eﬃciency evaluation, it is stilla useful method for screening with a reference level of
100 Bq/kg. It should also be noted that this method
is sensitive to radionuclides that do not exhibit γ-ray
radiation, such as 90 Sr, even though Sr is not considered a major radionuclide in the presently discussed
accident.

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Ⅴ．Fukushima Survey & Related Research

RIKEN Accel. Prog. Rep. 45 (2012)
RIKEN
Accel. Prog. Rep. 45 (2012)

NaI scintillation detectors for quantitative measurement ofradioactive cesium in milk, blood, urine, and soil
M. WadaAfter the accident of the Fukushima Daiichi nuclear
power plant in mid-March in 2011, radioactive contamination of soil, food, and human’s body becamea serious concern in Japan. An HPGe semiconductor
detector is the best device to measure the speciﬁc activity of weakly contaminated samples, however, it isworth evaluating a possibility of using NaI(Tl) scintillation spectrometers for this purpose, as they areavailable within a reasonable price range and are relatively easy to operate. Several months after the accident, most short-lived nuclei disappeared, and major radionuclides that remained in the environment are
134,137
Cs. This implies that we can use low-resolution
spectrometers without identiﬁcation problems.
Statistically, the detection limit, in units of the countnumber, according to the Cooper’s method1) is√√(1)
Nm = (A2m + Am A2 + 8B)/2 ≈ Am 2B,
where Am is the decision criteria, and typically,
Am = 3 is used. B denotes background counts. The
minimum detectable speciﬁc activity is then,√(2)
Dm = Nm /(�f T m) ≈ Am 2B/(�f T m),

where � is the eﬃciency, f is the branching ratio, T
is the measurement time, and m is the weight of thesample. B and �, T , and m are strongly correlated,and hence, the actual dependence of Dm is√√
Dm ∝ Am w/(f �α mβ T ),(3)

where α and β are approximately 0.5, but are variable
due to the background and the sample condition, andw is the peak width. If we compare Dm for a typical
HPGe and a 3 in. NaI detector, wGe /wNaI ≈ 1/50 and�Ge /�√NaI ≈ 1/5. In such a naive evaluation, NaI shows
only 10 times less sensitivity as long as disturbing
γ-ray peaks are not present in the region of interest.
A typical spectrum of 134,137 Cs consists of threepeaks (Fig. 1): the one on the right at 800 keV is a
mixture of peaks at 795 keV (f = 85.44%) and 801 keV(8.73%) for 134 Cs; the one in the center is a 662 keV γray from 137 Cs; the one on the left at 600 keV is a mixture of peaks at 605 keV (97.56%), 569 keV(15.43%),
and 563 keV (8.38%) for 134 Cs. The speciﬁc activities (D) and the statistical errors (σD ) can be deduced
from the net counts of these peaks (Ni ) and the standard deviation (σN i ) using the mixed branching ratios(f600 = 121.73%, f662 = 85.1%, f800 = 94.17%), andthe detection eﬃciencies �i , including the sum corrections. For example,

Fig. 1. γ-rays from 134,137 Cs of 2.8(1), 3.4(1) Bq measured
with 3 in. well-type NaI spectrometer.

The net counts and their standard deviations canbe obtained by the least-square ﬁtting of the spectrumwith three Gaussian peaks and a linear background(Fig. 1). Alternatively, the classical Covell’s method,which determines the background line from the grosscounts in speciﬁed regions on both sides of the peak,can be used to evaluate the net counts. Although
this method cannot separate overlapped peaks, stablevalues of the sum counts NL and the standard deviation σN L can be obtained. The contribution from134
Cs can be subtracted with N800 using the ratio,
α = f600 �600 /(f800 �800 ). The speciﬁc activities of 137 Cs
and the statistical error are
D137 = (NL − αN800 )/(f662 �662 T m),√
2
2 2
σD137 = σN
L + α σ800 /(f662 �662 T m).

(5)
(6)

A 3 in. well-type NaI scintillation spectrometer with
an auto sample changer (Perkin Elmer) has been used
to measure milk, blood, and urine from cattle, andsoil samples. Although the volume of the sample islimited to 20 cm3 , the 4π solid angle provides a 15%
photo-peak eﬃciency for 662 keV γ-rays. The detection limit for each radioisotope reached 0.15 Bq (7.5
Bq/kg) with a low potassium abundance sample fora one-hour measurement. The limit depends on thelevel of the continuum background, which is mainly
caused by the Compton scattering of 1461 keV γ-rays
from 40 K. In less contaminated samples, particularly
for soil samples, peaks from environmental radioactivities such as 214 Bi and 214 Pb can cause misidentiﬁcation problems. The annihilation peak at 511 keV also
causes a problem in the background determination.
Well-shielded NaI spectrometers are indeed a useful
tool for conducting radioactive contamination surveys.
References
1) J.A. Cooper, Nucl. Instr. Meth, 82, 273 (1970).

D134 = N800 /(f800 �800 T m), σD134= σ800 /(f800 �800 T m). (4)- 252 -

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Ⅴ．Fukushima Survey & Related Research
RIKEN
Accel. Prog. Rep. 45 (2012)

RIKEN Accel. Prog. Rep. 45 (2012)

Development of Pulse Height Analyzer for Education about
Radiation
A. Taketani, T. Takahashi,∗1 I.Y. Kawamo,∗1 H. Murakami, M. Wada, M. Sekimoto,∗2 K. Ninomiya,∗1 Y.
Ashikaga,∗3 K. Imai,∗4 H. Okura,∗5 S. Kubono,∗6 K. Nakagawa,∗7 T. Fukui,∗8 Y. Maruo,∗9 and Y. Morimoto∗10
The ﬁrst prototype radiation detector for educational purposes was developed with the peripheral interface controller (PIC). After the Fukushima atomic
reactor accidents, people inadequate knowledge abouradiations and radio activities bocame worried about
their health and reacted improperly. We as physicists,
can contribute to addressing their concern by helping
them understand radiation and training them to takesuitable actions. We are a group consisting of teachers,
a curator, and physicists, and we have been discussing
on how to develop a good education system for this
puropose and have identiﬁed important items ”experience of radiation measurement”, ”understand of radiation detection principle,” and ”assembling of radiation
detector.” The feature requirements for a radiation
detector kit are(1) Easy to assemble without a soldering iron.
(2) Inexpensive, with easily procurable parts.
(3) Ablity to detect environmental radiation.
(4) Tunable detection threshold.
The complete design of the ﬁrst trial detector is described in Kawamo’s article1) . The detector consits of
two main items, a sensor component and a pulse height
analyzer (PHA). Radiations are injected into the sensor component and converted readable voltage signals.
The sensor component consists of a sensor, a preampliﬁer, a shaping ampliﬁer and a peak hold circuit.1)
Then, the PHA converts the voltage signal into digitalvalues and displays their energy spectrum. The PHA
consists of a comparator, an ADC, a pulse generator,
and a display. The function diagram is shown in Fig.
1. When the signal from the peak hold circuit of thesensor component is fed into the comparator and ADC,
only the signals exceeding the threshold are converted
into a digital pulse height value by the ADC. Then, thedigital values are transmitted to the PC for displaying the energy spectrum. The PHA was made from
PIC18F45502) by Microchip; it is an 8-bit CPU withan ADC, comparators, digital inputs, digital outputs(DO), and a USB interface within a single chip. The

chip has ﬂash ROM for programing, which can be written into by a dedicated writer or a USB interface. The
program for the PIC is written in C language3) . The
board with the PIC and other necessary parts costs
1100 yen. The typical AD conversion time required
after signal rises in the peak hold circuit is 10µs. After AD conversion, the digitized pulse height values are
stored in the PIC memory and transferred periodicallyto the PC through the USB interface. This chip canmeasure up to 100 KCPS radiation with adequate PIC
memory. However, this value is extreamly high radiation to handle in a classroom. The data acquisition
program on the PC was developed by using Microsoft
Visual C#.4) The program is event driven. When the
PIC transmits the pulse height data, the PC receives
them and updates the histogram display. Transmission
requires about the 5 ms inclusive of the USB interrupt
process. Fig. 2 shows the obtained histogram of 137 Cs
the checking source.
The prototype of the PHA for the radiation detector
was developed for educational purposes. It satisﬁed thefunctional requiements. We will produce 200 radiation
detector assembly kits for supplying to classrooms.

Fig. 1. Functional block diagram of pulse height analyzer.

Fig. 2. Histogram of the pulse peight for 137 Cs. Squares
and crosses represent backgrounds and signals respectively.

∗1

Rikkyo University, Japan
KEK, Japan∗3 Tottori University of Environmental Studies∗4 Japan Atomic Energy Agency∗5 Osaka Science Museum∗6 University of Tokyo∗7 Kobe University∗8 Hakuryo Junior High/Senior High School∗9 Hyogo Prefectural Kamigori High School∗10 Hyogo Prefectural Tobban High School∗2

References
1) Y. Kawamo in this APR.
2) PIC18F2455/2550/4455/4550 Data sheet.
3) MPLAB C18 C Compiler User’s Guide.
4) www.microsoft.com/visualstudio- 253 -

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Ⅴ．Fukushima Survey & Related Research

RIKEN Accel. Prog. Rep. 45 (2012)
RIKEN
Accel. Prog. Rep. 45 (2012)

Alternative method to evaluate internal dose using simple γ-ray
spectrometer
M. WadaInternal dose has been a critical topic of concern
after the accident at the Fukushima Daiichi nuclear
power plant. A signiﬁcant amount of radionuclides released from the power plant has been spread over awide area in Japan. After the such incident the humanbody may take radioactive substances directly from theplume in the initial days and indirectly from contaminated foods afterward. In order to evaluate the internal dose, the concentration of radioactive substancesin the whole body should be measured. A so-called
Whole Body Counter (WBC) has been exclusively used
for such a purpose. However, popularly used WBCs
have not been designed for low-level contamination andcannot be aimed for use in a high background environment. Furthermore, the number of WBCs for public
use is limited compared to the demand. Hence, an alternative way to evaluate the concentration of radioactive substance in the human body is worth considering.
The main radioactive substances of concern are
137
Cs and 134 Cs. From a survey conducted on animals,
it is known that radioactive Cs is mainly distributed in
the muscles and not in the internal organs or bones1) .
It would be more eﬀective to measure the “muscle-rich”
parts of a human body with a well-shielded γ-ray detector. A good candidate for such a part is the leg.
A “U”-form shielding can be placed on top of the leg
and a detector with a cylindrical shielding block can beplaced at the bottom. Although the palm would not bemuscle-rich, we evaluated the measurement of speciﬁc
activity in the palm using standard γ-ray spectrometers used for food screening, without any modiﬁcation
to the shielding.
A phantom sample was made using a globe ﬁlledwith 173 g contaminated rice with speciﬁc activitiesof 62.2(1.9) Bq/kg and 50.5(2.0) Bq/kg for 137 Cs and134
Cs, respectively. The phantom was placed on top of

Table 1. γ-ray measurement of phantom palm (173 g)(top) and human hand (bottom) for 10 min measurements using a HPGe spectrometer.
γ-ray
keV
662
604
795
1460
662
604
795
1460

eﬃciency%
1.40(5)1.28(2)
1.05(2)0.63(9)1.40(5)1.28(2)
1.05(2)0.63(9)count (σN )73.7(9.0)
68.3(9.0)
51.0(7.1)8(2.8)
3.8(2.8)
1.5(3.5)
1.8(2.5)19(4)ActivityBq
10.3(1.3)
9.1(1.2)
9.45(1.3)
16.5(7.0)
0.53(0.39)
0.20(0.47)
0.33(0.47)
43.9(9.9)

Dm
Bq
1.87
2.18
1.55
26
1.68
2.05
2.28
19.0

the standard HPGe spectrometer to mimic a palm isgrasping the head of the detector. At ﬁrst, the detection eﬃciency, including the sum correction, was calibrated with a long measurement time. Then, the samephantom and an actual hand were measured with a realistic measurement time of 10 min. The net counts
and their statistical error σN , the deduced activities,
and the detection limits Dm in case of the decision criteria of 3 were listed in Table 1. The detection limits
reached ≈2 Bq, which correspond to speciﬁc activitiesof ≈11 Bq/kg.
A similar test was also performed with a 3” φ×3”
NaI(Tl) scintillation spectrometer used for food screening (OKEN FNF-401). The eﬃciency, including thesum correction, for the mixed peaks of 795 keV and801 keV γ-rays (branching ratio is 0.942) from 134 Cs
was 3.1%, and the detection limit was 6.0 Bq for thesame 172 g sample with a 10 min measurement. The
result corresponds to a speciﬁc activity of 36 Bq/kg.
These two test measurements showed promising results. With a slight modiﬁcation of the shielding block,
γ-ray spectrometers for food samples can be used toevaluate the concentration of radioactive substances
in a human palm.
Another problem in the WBC involves determining
the detection eﬃciencies. Usually, phantom humanbodies are used for the eﬃciency calibrations. However, the geometry of the human body is quite diﬀerent for each individual, and the diﬀerent distributionsof radioactive substances within the body can cause
large ambiguities in the eﬃciency evaluation. We propose here a simple way to evaluate the concentration ofradioactive substances in the whole body as a ratio tothat of potassium. Although the actual distributionsof potassium and cesium are not equivalent2) , we canapproximate that the two alkali elements are equallydistributed to some extent. Under such an approximation, the ratio of the speciﬁc activities can be
A137CsN661 �1460 b1460
N661=≈ 0.056,
A40K
N1460 �661 b661
N1460

(1)

where Nx are the net counts, �x are the eﬃciencies, andbx are the branching ratios. It should be noted that
1) N1460 is usually too small for a short-time measurement with a small HPGe spectrometer; 2) a sum peakof 134 Cs at 1401 keV can be overlapped with the 40 K
peak in NaI measurements; and 3) the internal absorptions of diﬀerent γ-rays can be slightly diﬀerent.
References
1) M. Wada et al., In this report.
2) N. Yamagata, J. Radiation Research, 3, 9 (1962).

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1