J. Jpn. Soc. Soil Phys.
土壌の物理性No. 139, p.15 ∼ 20 (2018)

Handmade Becquerel meterusing commercial scintillation survey meter
Shuichiro YOSHIDA1 , Sho SHIOZAWA1 , Takuya ZAITSU2 , Hiroaki YAMANO3
and Kazuhiro NISHIDA1

Abstract: After devastating nuclear disasters, the Becquerel meter should be very important for monitoring thecontamination of our surrounding environment and judging the risk of exposure. However, the commercial Becquerel meters are too expensive for the citizens to purchase. Even the availability for the administrative agency
is not ensured in emergency. Considering these risks, aself-produced Becquerel meter using a commercially available scintillation survey meter is proposed. The sample formeasurement should be set in a cylindrical bottle having
a diameter of 3 cm and the height of 5 cm. Although theshielding of the system with lead is not perfectly given,
the subtraction of the background dose enables the measurement of the radiocesium concentration of the contaminated soil samples with sufficient accuracy. The theoretical error analysis revealed the relative error depends on the
total counts and the contribution of the background radiation. The counts required for the measurement with 10% of error is 4500 if the contribution of the backgroundradiation is 80 % of the total counts, while the required
counts reduced to 600 if the contribution of backgroundis 50 %. The correction methods to apply this system tothe sample with variable thickness in the bottle is also presented. Because the relationship between the counts rate
and the radiocesium concentration depends on the proportion of 134 Cs to 137 Cs, the calibrations are generally required at each measurement. However, no significant difference was revealed for the calibration lines measured atJan. 2013 and Aug. 2013 during which the proportion of
134 Cs highly decreased. Thus, any frequent calibrations
are not necessary for the practical use of the system. Whenthe aforementioned conditions are satisfied, the proposed
Becquerel meter meets the practical demand for knowing
the radioactivity of soil samples.
Key Words : radiocesium concentration, radioactive contamination, simple measurement, soil contamination, contamination of agricultural products
1 Graduate School of Agricultural and Life Sciences, The University ofTokyo, 1-1-1, Yayoi, Bunkyo, Tokyo, 113-8657, JAPAN. Corresponding
author: Shuichiro Yoshida, 東京大学大学院農学生命科学研究科.
2 Ministry of Agriculture, Forestry and Fisheries, 1-2-1 Kasumigaseki,
Chiyoda-ku, Tokyo 100-8950, Japan.
3 Kajima Corp. 3-1, Motoakasaka 1-chome, Minato-ku, Tokyo 107-8388,
Japan.
2017 年 12 月 22 日受稿 2018 年 5 月 23 日受理

1. Introduction
On unforeseen contamination by radionuclides in a nation scale such as an accident of nuclear power station, notonly researchers or governmental officers who should prepare apparatus to monitor the situation, but citizens also eager to know the real situation. At the first step, they wouldpurchase dose meter or survey meter, to monitor the surrounding environment. But it is impossible to measure theconcentration of radionuclide in samples of soil or foods
without having an expensive professional instrument. For
the official purposes, the methods to measure the concentration of radionuclides contained in foods or soil is defined in the guidelines published by the government. For
example, Ministry of Health Labor and Welfare (2012) established a guideline on the analytical method of radionuclides in foods in which the analysts must use a germanium
semiconductor spectrometer (Ge spectrometer) having relative efficiency larger than 15 % and surrounded by leadshield with a thickness of 10 to 15 cm. NaI (TI) spectrometer which is much cheaper than Ge spectrometer havebeen also used for screening or voluntary analysis of foods.
Although the lower limit of the measurable concentrationby the system is as dilute as 1 Bq kg−1 , it is yet too expensive and bulky for citizens to purchase. The availability is also questioned after devastating accidents. In fact,
the shortage of the instruments was reported just after theaccident of Fukushima Daiichi Nuclear Power Station in
2011, leading researchers on the front-line to seek alternative methods to measure radioactivity of soil samples using
commercially available scintillation survey meter (Nemoto
et al., 2013). They proposed to measure the radiation fromthe surface of the 1 kg of soil sample packed and filled
in a 2 L beaker. The background radiation is subtracted
from the total counts to determine the net radiation fromthe sample. Although the principle to measure the radioactivity of samples is common for all methods, the mass ofthe sample, the shielding capacity, and the uniformity of16

土壌の物理性第 139 号(2018)

Fig. 1 Schematic of the proposed apparatus.

the geometry are the highly substantial factors to establish
useful methods. The method proposed by Nemoto et al.
(2013) does not require any apparatus except the scintillation survey meter. However, the large mass of soil sample
is necessary to assure the consistent accuracy.
In-situ measurement of vertical profile of radiocesium
concentration in soil inexpensively was also studied.
Suzuki et. al. (2013) arranged photodiodes along a rod
to detect the vertical radiation profile from which the concentration profile is analyzed. Due to the low sensitivity ofthe photodiode sensors, long counting time is required for
accurate measurements. But the required time can be reduced by replacing the sensors to more sensitive ones, withadditional cost.
The present report aims to provide an information about
making a cheap but reliable Becquerel meter from a commercially available gamma ray survey meter and rolled
lead sheet. The accuracy of the apparatus is thoroughly
discussed to present the applicability of the method.

2. Design of the apparatus
Measurement of radiocesium concentration of contaminated soil samples by arranging a commercial scintillation probe (Model 5530) and survey meter (Model 5000CYPHER) was tested. The gamma sensitivity of the scintillation probe is approximately 30,000 cpm µ Sv−1 h−1 for
137 Cs. according to the manufacturer. The survey meter
including the probe approximately costs 4,000 US dollars.
The GM (Geiger-Muller) tube survey meters are cheaper
than scintillation survey meters, though the measuring time
would have been longer due to the lower sensitivity. Fig.

1 shows the design of the instrument. The instrument consists of a shielded sample holder and shielded scintillation
probe.
A cylindrical shielding cap (a) and shielding frame (b)were made of lead sheets which is soft enough to shape.
The cost for the 6.5 kg of rolled lead sheet used for theshielding cap and frame was about 200 US dollars. We
wrapped multiple thin lead sheets having thickness of 3
mm around a cylindrical mold having a diameter of 35mm, each of which was glued with adhesive agent (Super
Glue). The resultant thickness of the shielding wall was 20mm, which attenuates direct gamma beam to 7 %. Because
the lead wall alone is not enough to shield the backgrounddose completely, subtraction of the background radiation isnecessary. The procedure will be discussed later. The topof the cylindrical cap (a) was filled with a top lead cap (h)having thickness of 20 mm. The probe (c) must be set at thepredefined position in the cylindrical shielding to be fixedby the screws when used. Thus, the probe can be takenout from the frame easily to be used as a survey meter tomonitor the dose rate in environment. The NaI scintillator(d) having a diameter of 25.4 mm and a length of 25.4 mm(1 inch) is installed at the top of the probe, where gamma
beam from the sample and the background is converted tolight and then the light is converted to electronic pulse signal by the photomultiplier. The probe is connected to thesurvey meter (g) (analyzer) which displays count of photon
in a predefined period. Because the system we used cannot detect the energy spectrum, the response depends onthe energy. The major energies of the gamma beam fromradiocesium are 661 keV (137 Cs), 605 keV (134 Cs) and 796
keV (134 Cs) (e.g. Malonda and Carles, 2012). Due to the

研究ノート：Handmade Becquerel meter using commercial scintillation survey meter

17

Fig. 2 Blank counts per 10 seconds; (a) Fluctuation during 600 seconds; (b) Correlation incounts per 10 seconds between a couple of the apparatus put next to each other.

dependency of the sensitivity on energy, the relationshipbetween the counts and the dose can change when the proportion of 134 Cs and 137 Cs changes due to time. This drawback will be discussed later. A couple of the measurement
sets: probes, caps, frames and the controller, were constructed to compare the individual specificity.

tors influencing the accuracy, duration of the measurement,
the background dose rate, and the thickness of the sample
in the bottle were examined. All the measurements were
performed in an unshielded laboratory in the University ofTokyo located in Bunkyo-ku, Tokyo. Only the Set A wasused for the measurement except for the comparison between the probes.

3. Methods of calibration
and accuracy test

4. Results and discussion

The contaminated soil samples were taken from a profile
of a paddy field located in Iitate Village, Fukushima, Japanand the bottom of a reservoir in Motomiya City, Fukushima
Japan. The samples were subject to drying in an oven at
105 ◦ C. The highly contaminated sediment of the agricultural reservoir was mixed with non-contaminated volcanic
ash soil to make a specimen with variety of predefined radiocesium concentration, which was 100 %, 80 %, 60 %,40 %, 20 % and 10 % of the original sediment by weight.
The volcanic ash soil was taken from the experimental fieldof The University of Tokyo located in Nishi-Tokyo city
before the accident of Fukushima Daiichi Nuclear Power
Station. The radiocesium concentration of these samples
were measured by the Ge semiconductor detector run byRadioisotope (RI) Center of the Graduate School of Agricultural and Life Sciences, The University of Tokyo. The
samples were also subject to the counting of photons by
the proposed system. The contribution of the sample tothe detected count of photons were evaluated by subtracting the background from the total. The background canbe determined by measuring the radiation without setting
the samples, to be called blank. Among the expected fac-

The variation of the background radiation was examinedusing a couple of the proposed apparatus which were thesame model of the survey meter but the sensibility to the
gamma ray fluence are different due to the shielding capability. Fig. 2 shows the temporal variation of the background detected by the probes (Set A and Set B) in 10 seconds. Observed numbers of photons in space at predefinedtime interval is widely known to follow Poisson distribution (e.g. Malonda and Carles, 2012) represented by P(λ ).
When the probability variable X follows the Poisson distribution and the mean value is λ , the variance of X is also
λ . The observed mean (Mean) and variance (σb 2 ) shownin Fig. 2a are consistent with this theoretical knowledge.
Because the mean count is proportional to the duration of
the measurement, the theory tells that the standard deviation increases proportionally with the root square of duration of measurement. This means that the longer we spent
for the measurement, the less the relative error becomes.
This principle is important in measuring dilute samples.
Because the count due to the sample alone (Ns ) is evaluated by subtraction of the count due to the background(Nb ) from the measured count (N):

土壌の物理性

18

第 139 号(2018)

dition because Nb /N cannot be reduced. For instance, thetotal counts required for the concentration measurementwith 10 % of error (σs /Ns = 0.1) is 4500 if the contribution of the background (Nb /N) is 80 %, while the required
counts reduced to 600 if the Nb /N is 50 %. In general, therelative contribution of Nb : Nb /N is not known in advance.
Thus, the rate of counts should be measured for the sample with the minimum radioactivity among the samples forsome minutes to roughly estimate Nb /N.

Fig. 3 Theoretical estimate of relative error in the countdue to sample Ns as a function of the proportion of the background radiation Nb to the total count N.

Ns = N − Nb(1)

the range of error in the count due to the sample can be
estimated by eq. (2) based on the principle of propagationof error:

σs2 = σ 2 + σb2(2)where σs 2 , σ 2 and σb 2 are variance of Ns , N and Nb , respectively. Since N = σ 2 and Nb = σb 2 ,σs2 = N + Nb

(3)

Then the relative error in Ns can be written as√σs
N + Nb=Ns
N − Nb(4)

The diagram of this relationship is provided as Fig. 3,
which is useful to estimate the minimum total count to obtain the count due to the sample with practical accuracy.
We should usually wait until the total count meets the con-

Although two probes were set on the same table, thecounts in 10 seconds measured by the both probes were
never synchronized to each other. Fig. 2b examines thecorrelation between the counts by both probes. It is obvious no correlation was found in the temporal trends ofthe counts in 10 seconds detected at the neighboring location. This basic characteristic show real-time subtractionof the background does not work to increase the accuracyof measurements.
Table 1 shows the variation in the averaged blank counts
per minute (CPM) due to the measurement dates. The measurement was kept for an hour and the counts per every
minute were averaged. The variation due to the dates canbe negligible compared to the fluctuation observed in every
minute. Therefore, frequent measurement of blank CPM is
not required unless the environmental radiation is known
to be considerably change. However, needless to write, themeasurements of samples and blanks must be done in thesame place because the background radiation highly depends on the place.
Fig. 4 shows the relationship between total radiocesium concentration of mixture of the contaminated sediment and andosol measured by Ge semiconductor detector
and Counts in 400 seconds by the proposed apparatus. The
temporal decay of the concentration is corrected based on
the date of the measurements. The relationship was so linear that the CPM obtained by the proposed apparatus canbe converted to concentration. However, we should notethat the relationship could change depending on height,
density and 134 Cs/137 Cs ratio of samples.
The thickness (height) of the sample perpendicular tothe scintillator affects the self-attenuation of the gamma

Table 1 Variation in averaged blank CPM due to the measurement dates.

Set A Meanσ
Set B Meanσ

2016/3/11

2016/3/14

2016/3/14

2016/3/15

2016/3/21

PM

AM

PM

AM

PM

490

491

490

488

492

21.7

23.3

25.5

24.3

21.7

311

309

312

310

307

18.0

17.1

18.1

17.0

17.6

研究ノート：Handmade Becquerel meter using commercial scintillation survey meter

19

beam. Fig. 5 shows the relative decrease in the CPM perunit weight of soil with an increase in the thickness of thesample in the bottle. Thickness of 33 mm was applied as
the reference. The 10 % of decrease in thickness caused
5 % increase in CPM for paddy soil; the 30 % decreasecaused 15 % of increase in CPM for the sediment. The dependency of CPM on the thickness had a slight difference
between the samples having different bulk density. The
dependency of CPM on the thickness for the denser sample (paddy soil) was less than that for the looser sample(sediment). If the thickness of the sample is adjusted tothe standard thickness, the difference in the soil density ispractically less important.
Fig. 4 Relationship between total radiocesium concentration measured by Ge semiconductor detector and counts per
400 seconds by the proposed apparatus (Set A). The sample
was the mixture of sediment in an agricultural reservoir andnon-contaminated upland soil. The temporal decay of theconcentration is corrected based on the date of the measurements.

Because the measurement of numerous samples usually
took so much time that the radiocesium concentration of
the samples were subject to the effect of natural decay.
Thus, the measured concentrations should be converted
into the ones at the reference date by considering the halflife period and existing ratio of 134 Cs and 137 Cs. However,since the survey meter we used cannot detect the energy
spectrum, the response depends on the component of theenergy of the gamma ray. Fig. 6 compares the differencein the relationship between the radiocesium concentration
and CPM due to the measurement date: 23 Jan. and 29
Aug. 2013, during which the proportion of 134 Cs to thetotal Cs should decreased from 0.34 to 0.28. Because thecovariance analysis reveals no significant difference in the
regression coefficients, the calibration is not frequently required unless the measurement spans many years.

5. Conclusion
Fig. 5 Dependency of CPM on thickness of sample inbottle. Normalized to the CPM for the sample with thickness of 3.3 cm. Set A is used for these measurements.

Self-produced Becquerel meter using commerciallyavailable scintillation detector is proposed. Sufficient accuracy can be ensured if the measurement is conducted foradequate time determined from the background radiationand the concentration of the sample. Shieling by lead sheet
helps reduce the effect of background radiation and shorten
the time required for the accurate measurement. Although
any types of detector can be principally used, the NaI scintillation probe should be the best regarding both the cost
and the sensitivity. Detectors with a function of counting
photons in any time interval is highly recommended to produce the system.

References
Fig. 6 Temporal change in relationship between CPM by
the proposed apparatus (Set A) and total radiocesium concentration. The samples were taken from a profile of a contaminated paddy field. The concentrations were correctedto the day when the CPM measurement was performed.

Nemoto, T., Sato, M., Ono, Y., Fujimura, K., Nohei, N. andSuzuki, Y. (2013): A simple method for the measurementof radiocesium in soil samples using a NaI(Tl) scintillation
counter. Research Report of Fukushima Agricultural Technol-

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第 139 号(2018)

ogy Center, Special Issue on countermeasures for radionu-

for Radioactive Substances in Food, Notice No. 0315 Article

clides, 15–18. (in Japanese)
Malonda, A.G. and Carles, A.G (2012): Radioactivity counting

Suzuki, S., Iwase, H., Noborio, K., Mizoguchi, M., Kobayashi, D.

statistics, in L’Annunziata, F., Michael (eds.) Handbook of Ra-

and Ito, T. (2013): Development of a device for measuring the

dioactivity Analysis 3rd Edition, pp.163–189, Elsevier Inc.

vertical distribution of radioactivityin soil using photodiode.

Ministry of Health, Labor and Welfare (2012): Testing Methods

要

4 of the Department of Food Safety, March 15, 2012.

Journal of Japanese Society of Soil Physics, 125: 29–33.

旨

原子力事故の後には，周辺環境の汚染モニタリングや被爆リスクの判断のためにベクレルメータが不可欠である．しかし，市販のベクレルメータは一般市民には高価であり，しかも非常時には行政機関でさえもすぐに入手することはできない．これらのリスクを考慮し，比較的入手が容易な市販のシンチレー
ションサーベイメータを用いてベクレルメータを自作し，その精度を検証した．作成したベクレルメー
タは，シンチレーションプローブの周囲に鉛板を巻き付けることで，簡易な遮蔽を行うとともに，ブランク測定によるバックグラウンドを差し引くことで，汚染試料の測定において実用上十分な精度を得ることができた．
キーワード：放射性セシウム濃度，放射能汚染，簡易計測，土壌汚染，農産物汚染