Rep. Sci.Engrg.
Saga Univ.
40-2 (2011)

Reports of Science and Engineering,
Saga University, Vol. 40, No.2, 20111

Measurement of 134,137Cs Concentrations in the Air of Fukushima-City
By
Kouichi TOYOSHIMA, Takasi ENDO and Yutaka HIRAYOSHI
Abstract: Concentrations of radioactive Cs isotopes in Fukushima-City were measured by collecting theair dust on the air filter for automobile on Oct. 21 and 22, 2011. Gamma-rays from the filter paper werecounted by a NaI scintillation counter and a Ge detector. Dividing the activities by the volume of air passed
through the filter, the concentrations of 134Cs and 137Cs were estimated to be 6.13 10-4 Bq/m3 and 8.29
10-4 Bq/m3 respectively, both with 31% statistical error.
Key words: Fukushima Daiichi Nuclear Accident, radioactive cesium, gamma-ray spectrum, atmosphericconcentration

1. Introduction
Severe accident in Fukushima Daiichi Nuclear PowerPlant after the big earthquake and tsunami on March 11th
this year caused a heavy radioactive pollution over a vast
area of Japan’s soil, including the capital city ofFukushima Prefecture. Radiation level inFukushima-City is still high, lingering round 1μSv/h.
Measurements of radiation levels everywhere andconcentrations of radioactive nuclides in everything are
essential to protect people from the radiation andradioactivity. Dose rates due to gamma-rays from
activities on the ground are widely measured andmapped, as well as the contaminations of the earth.
However the data on the pollution of the air in
Fukushima Prefecture and near-by areas are rather sparse.
The only report we found for Fukushima City is from theUniversity of Fukushima(1). Another systematic
monitoring of nuclides in the air is performed by theCTBT radionuclide observatory station in Takasaki,
which is located more than 200km from Fukushima
Daiichi, and the data are published regularly on theirwebsite(2). According to these observations, even today,
more than seven months after the accident, considerable
atmospheric concentrations of Cs isotopes arecontinuously observed.
Since one of the author K. Toyoshima had an
opportunity to visit Fukushima City last October, he tried
to collect dust in the air to measure its radioactivity
concentration. For this purpose, a simple air dust
collector was devised by combining an air filter element
for a car and a small fan. This kind of filter has a rather
Received November 25, 2011
Department of Physics@Graduate School of Science and Engineering, Saga University

large mesh, but they are easily obtained. In addition agreat number of this “collectors” are always working on
the roads. So this kind of filter could be a standard
device for widely monitoring the radioactivity in the air.
2. Collection of the air dust
The air filter unit used is “SU-102” of Nitto Kogyo Co.
Ltd, a cylindrical one for a small car. The outer diameteris 120mm and the length is 130mm. Small fan, Model“AD0812HS-A70GS” of ADDA CORP was attached toone end and the other end was closed by a wooden board.
Both battery and AC adapter were prepared to drive thefan continuously.
Performance detail of this filter is not known, butaccording to a literature(3) this kind of filters “capture
more than 98% of 7 to 9μm diameter particles”. The
power of the fan is 65.59m3/h or 38.6 CFM.
This device was run continuously throughout the stayof Toyoshima in Fukushima City, starting at 15:56 on
Oct. 21, 2011, and terminating at 10:16 next day, carried
always by him. Total running time is 18 hours and 30
minutes, which means the air of volume 1.2134 103 m3
in total passed through this filter. Places where thisdevice traveled were: JR Fukushima station, JR
Kanayagawa station, Fukushima University, and finallythe central area of Fukushima City within 1km from JRstation - lyceum, pub, and hotel.
3. Activity measurement of the air filter
The filter paper of the unit was then stripped off andthe gamma-rays from it were observed by a NaI
scintillation counter and a Ge detector. It was wound2Kouichi TOYOSHIMA, Takasi ENDO and Yutaka HIRAYOSHI

around the side faces of these detectors, counts
which are both cylindrically shaped. The 4000
Gamma-ray spectrum from air filter exposed in
NaI counter is Bicron 2M2/2PR, whose
Fukushima City. Oct. 21-22 2011
crystal size is 2"φ 2". To reduce thegamma-ray background, sample roomand the detector itself were enclosed by 3000
11 lead bricks. The Ge detector is theair filter element
equipment of the Radioisotope Institutefor Basic and Clinical Medicine of134
backgroundCs
Kurume University and we were granted 2000
137
to use it.
Cs
134
Gamma-ray spectrum taken by theCs
NaI counter is shown in fig.1, with the40
Kbackground data normalized to the same 1000
live-time(activetimeof
themultichannel pulse–height analyzer) as
the measurement of the sample.
0
Background spectrum was taken by
100
150
200
250
300
350
putting new filter unit (but different lotchannelno.
to the used one). Gamma-ray peaks from134,137
Cs are fitted by a software
Fig. 1. Gamma-ray spectra taken by NaI counter.
FitzPeaks (Demo and NaI version) (4).
Efficiency calibration was done bycounts
Gamma-ray spectrum by Ge detector
putting a calibrated 137Cs source of25
LMRI (Laboratoire de Métrologie des222Rn
Rayonnements Ionisants) at several
510keV
points aroundthe NaI crystal.
20
Considering the geometry of the sample
wound around the NaI, the position of137Cs
134Cs
15134Cs
11mm from the cylindrical face and 20.4
661.7keV
mm backward (to the photomultiplier’s
604.7keV
795.8keVside ) from the front face was chosen as10the typical point, and the efficiency there
was obtained by averaging the nearest 2points actually measured.
5Measurement and analysis by the Gedetector were performed automatically0by the attached computer and software.
900
1000
1100
1200
1300
1400
1500
1600
1700
134
137
Three peaks fromCs andCs arechannel no.
clearly visible, as shown in fig.2. Since
the efficiency of this detector wasFig. 2. Gamma-ray spectra taken by the Ge detector.
available only for the front surface of thedetector but the sample geometry is
get the concentrations of 134Cs and 137Cs as 6.13 10-4complex, absolute activities cannot be known. However,Bq/m3 and 8.29 10-4 Bq/m3 respectively. They are
since the relative efficiencies across gamma-ray energies
about ten times greater than the record of Takasaki
are meaningful, activities of each peaks were evaluatedby the efficiencies at the front surface. Results of bothobservatory, which reads 0.73 and 0.98 respectively indetectors are summarized in table 1.
the unit of 10-4 Bq/m3, for the intake of Oct. 21 to 22.
This difference may reflect the difference in distance4. Estimation of atmospheric concentration andfrom the Fukushima Daiichi between these twocomparison to other observations
observing positions. Considering the roughness of mesh
If we divide the activity values in table 1 by the airof our filter, these ratios may be greater.
volume of 1,213.4 m3 which passed through the filter, weMeasurement of 134,137Cs Concentrations in the Air of Fukushima-City

Table 1 Gamma-rays from the filter paper observed
by NaI and Ge detectors.
─────────────────────
Nuclide energy peak error efficiency activity(keV) area(%)(Bq)──────────────────────
NaI detector. live time :167,465s
134
Cs604.7
2,576 796 2.43
0.744
137
Cs661.65 2,747 838 1.917
1.006
Ge detector. live time : 7,200s
Cs 604.7
32.1 13.6 3.27
0.140*
134
Cs 795.85
39.6 9.2 2.570.251*
137
Cs 661.65
57.9 14.2 3.01
0.315*
─────────────────────* Relative values are only meaningful because of thelack of absolute efficiency (see text).
134

Fukushima University measured radioactive materials
in the atmosphere of their own place from May 18 to
July 15 with their high-volume air sampler(1). Theyobserved a highest value of 3.29 10-5 Bq/l, namely 3.29
10-2 Bq/m3 for total Cs on May 23. A member of thisobservation group, Professor A. Watanabe gave us theirrecent data(5) observed on Sept. 2, 6 and 10, which show
sharp decreases from 6.89 to 1.78 for 134Cs, from 8.01 to
2.11 for 137Cs, all in the unit of 10-4 Bq/m3 with error barsof 11 to 12%. Our result are comparable to the values
observed on Sept. 2 but higher than the last data byfactors of 3 to 4. Considering the difference inobservation conditions etc., we should say our result isroughly consistent to the observations of CTBT station inTakasaki as well as the observations of Fukushima
University.
Inhalation dose is calculated by multiplying theamount of intake and conversion coefficients. The former
is the product of activity concentration, breathing rate
and time. If we use the largest value (type ‘S’) of theICRP conversion coefficients for adults(6), committed
effective doses due to 1-day inhalation are 2.72 10-10
Sv and 7.18 10-10 Sv for 134Cs and 137Cs respectively. If
the conversion coefficients of ECRR(7) are employed,
value for 137Cs rises to 1.2 10-9 Sv. Breathing rate of
22.21m3/day is used throughout the above calculation.
5. Discussion
Even for full 1-year breathing of this contaminated air,
above doses are very small compared to the dose due togamma-rays from the radioactive materials deposited on
the earth’s surface or on buildings. However, recentdecontamination works may cause a risk of increasing
the air contamination. Worst of all, the accident has not
terminated yet and no one knows what happens next.
Therefore a continuous and systematic monitoring of the3radioactivity in the atmosphere will be necessary.
The inhalation dose may be significant in the earlydays after 3.11 all over Fukushima Prefecture. Air dustsof this period are kept on the enormous number of air
filters carried by cars which were then on the roads.
Among them there will be a number of filters whichstarted or stopped to work during the days of denseradioactive plume. If one can compare the activities offilters which began or stopped aspiration at differenttimes, we could possibly obtain time-differential valuesof radioactivity concentration. Collection of these filters
is desirable with their records of time-profile of use.
We express thanks to Mr. R. Doi at Radioisotope
Institute for Basic and Clinical Medicine, Kurume
University, for granting to use the Ge detector. We also
thank Prof. A. Watanabe at Fukushima University forsending their recent data on atmospheric concentration ofradioactive Cs measured at his campus.

References(1) “Results of the observation of radioactive materials in theatmosphere with a high-volume air sampler at Fukushima
University” (in Japanese), Press release by Fukushima
University, Aug. 3, 2011.
(2) Published regularly on the website of Center for thePromotion of Disarmament and Non-Proliferation.
(3) Y. Nakashima and S. Muranaka (ed.), “Gasoline Engine forAutomobiles” (in Japanese), Sankaido, Tokyo, 1999, ISBN
4381101332.
(4) Jim Fitzgerald, JF Computing Services, 17 Chapel Road,STANFORD IN THE VALE, Oxfordshire, SN7 8LE.
(5) Private communication with Prof. Watanabe via Email on
Nov. 10, 2011.
(6) ICRP Publication 72 (1992), Table A.2, p.62.
(7) Chris Busby (Ed.), “2010 Recommendations of theEuropean Committee on Radiation Risk”, 2010.