温泉科学（J. Hot Spring Sci.），64，402-408（2015）

短報

Measuring Radon in High Humidity Air Using Anhydrous
Calcium Sulfate and an Activated Charcoal Detector
Yasunori Mori1）2）*, Yasutaka Omori3）, Hideki Yoshimura1）, Akira Maeda1）,
Masahiro Yamamoto1）, Takamichi Nishinaka1）, Tetsuo Ishikawa3）,
Akiko Wakabayashi4） and Yumi Yasuoka4）(Received Nov. 17, 2014, Accepted Feb. 27, 2015)

硫酸カルシウムと活性炭型検出器を用いた高湿度環境下における空気中ラドンの測定森康則1）2）*，大森康孝3），吉村英基1），前田 明1），山本昌宏1），
西中隆道1），石川徹夫3），若林亜希子4），安岡由美4）要旨

温泉法および鉱泉分析法指針では，地下水中ラドン濃度が 74 Bq/kg 以上で温泉，111 Bq/kg 以上で療養泉に該当すると規定されている．ラドン（222Rn）は希ガスの天然放射性核種であり，容易に空気中に揮散することから，放射能泉の利用による被ばく線量評価のためには，簡便で正確な空気中ラドン濃度の測定手法の確立が求められている．これまでに筆者らは，活性炭型検出器と液体シンチレーションカウンタを用いた空気中ラドンの簡易測定法を報告してきたが，同検出器はラドン捕集部に活性炭を使用しているため，放射能泉利用施設の浴室内など，特に高湿度環境における測定の際，環境中の水蒸気が活性炭の多孔質部に吸着し，感度低下を引き起こすことが分析上の課題とされてきた．
本研究では，硫酸カルシウムを主成分とする乾燥カラムを活性炭型検出器に装着することにより，除湿処理を伴う空気中ラドンの分析の検討を試みた．相対湿度 30%，60%，80% の各ラドン標準場で，空気中ラドン濃度約 1,000 Bq/m3 の曝露を行った検出器を液体シンチレーションカウンタによる測定に供した．その結果，本測定条件下での変換係数が 2.42±0.14（湿度1)

Mie Prefecture Health and Environment Research Institute, 3684-11 Sakura-cho, Yokkaichi-shi, Mie-ken
512-1211, Japan. 1）三重県保健環境研究所 〒512-1211 三重県四日市市桜町 3684-11．*Corresponding
author：E-mail moriy04@pref.mie.jp, Tel. 059-329-2917, Fax 059-329-3004
2)
Graduate School of Bioresources, Mie University, 1577 Kurimamachiya-cho, Tsu-shi, Mie-ken 514-8507,
Japan. 2）三重大学大学院生物資源学研究科 〒514-8507 三重県津市栗真町屋町 1577．
3)
Fukushima Medical University, 1 Hikarigaoka, Fukushima-shi, Fukushima-ken 960-1295, Japan. 3）福島県立医科大学 〒263-8555 福島県福島市光が丘 1．
National Institute of Radiological Sciences (former institute), 4-9-1 Anagawa, Inage-ku, Chiba-shi, Chiba-ken
263-8555, Japan. 独立行政法人放射線医学総合研究所（旧所属） 〒263-8555 千葉県千葉市稲毛区穴川 4-9-1．
4)
Kobe Pharmaceutical University, 4-9-1 Motoyamakita-machi, Higashinada-ku, Kobe-shi, Hyogo-ken
658-8558, Japan. 4）神戸薬科大学 〒658-8558 兵庫県神戸市東灘区本山北町 4-19-1．

402

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硫酸カルシウムと活性炭型検出器を用いた高湿度環境下における空気中ラドンの測定

80% 温度 20℃）と算出された．乾燥カラム装着による若干の感度低下は認められるものの，測定値の定量性は担保されていることから，本手法が高湿度かつラドン高濃度環境下における簡易的・汎用的な空気中ラドンの測定手法としての有効性を示唆しているものと考えられる．
キーワード：ラドン，高湿度環境，液体シンチレーションカウンタ，活性炭型検出器，放射能泉

Abstract
According to the Japanese Hot Springs Law and the Guidelines for Analysis of Kosen(fluid-type hot springs), spring water containing more than 74 Bq/kg of radon is regarded asa hot spring and one with a radon level more than 111 Bq/kg is regarded as a medicalspring, also called a radioactive spring. Radon (222Rn) is a radioactive element derived fromthe earth and a chemically inert rare gas. Therefore, to evaluate the exposure dose due toradioactive spring usage, it is important to establish an easy and accurate method tomeasure radon concentration in air. We have previously reported on a relatively convenient
handling and highly versatile measurement method of radon concentration in air that uses
both an activated charcoal detector and a liquid scintillator. The radon trapping point of thedetector is made of activated charcoal. When activated charcoal detector is set up inenvironment with high humidity such as the bathing rooms of radioactive spring facilities,
the porous component of the activated charcoal adsorbs water vapor in the air, resulting inchange of detector sensitivity and issues with this measurement method.
In this study, we installed an acrylic desiccant column containing anhydrous calcium
sulfate on the activated charcoal detector and performed dehumidified examinations ofradon concentration in air. In a radon chamber, the radon concentration in air was set toabout 1,000 Bq/m3 and the relative humidity was set to 30% (low), 60% (normal), and 80%(high). Immediately after the exposure, we added a toluene solution of DPO(2,5-diphenyloxazole)＋POPOP (1,4-bis(5-phenyl-2-oxazolyl)benzene) as a liquid scintillator, andused it to measure radon concentration with a liquid scintillation counter. The conversion
coefficient under these measurement conditions was 2.42±0.14 (relative humidity : 60% ; airtemperature : 20℃). Although detector sensitivity was slightly lowered by installing theacrylic desiccant column, the examinations reveal that this measurement method is valid forquantitative analysis. Therefore, our measurement method allows for convenient handling,high versatility, and efficient measurement of radon concentration in air under high radon
concentration and humidity.
Key words : radon, high humidity environment, liquid scintillation counter, activated charcoal
detector, radioactive spring

1.

Introduction

The ledger record of Mie Prefecture lists about 250 hot springs, including about 10
radioactive springs. Radon ( 222Rn) is a main feature of radioactive springs. Rn-222 is aradioactive element formed by decay of 226Ra in uranium series in the earth’s crust and it is achemically inert rare gas. Based on the Japanese Hot Springs Law (Government of Japan, 1948)
and the Guidelines for Analysis of Kosen (fluid-type hot springs) (revised) (Ministry of theEnvironment, Nature Conservation Bureau of Japan, 2014), a spring is considered a “hot spring”
if 1 kg of the spring water contains at least 74 Bq (20×10－10 Ci, 5.5 Mache) of radon. Similarly, if
1 kg of spring water contains at least 111 Bq (30×10－10 Ci, 8.25 Mache) of radon, the spring is
considered a “medical hot spring” with “radioactive hot spring” properties.
403

森 康則，大森康孝，吉村英基，前田 明，山本昌宏，西中隆道，石川徹夫，若林亜希子，安岡由美

温泉科学Radon rapidly diffuses into air, so it is necessary to measure radon in the air at radioactive
spring facilities to estimate the exposure dose (Mori et al., 2013a ; 2014). We have previously
reported on a relatively convenient handling and highly versatile measurement method of radon
concentration in air that uses an activated charcoal detector and a liquid scintillator (Mori et al.,2013b). The radon trapping point of the detector is made of activated charcoal. When samples
are collected in high humidity environments, such as the bathing rooms of radioactive spring
facilities, the porous component of the activated charcoal adsorbed water vapor in the air,decreases the detector sensitivity and causes problems with this measurement method (Iimoto etal., 2005).
The present study focuses on this issue and investigates a suitable analytical method formeasuring the concentration of radon in high humidity air, with detector comprising a desiccantmade of anhydrous calcium sulfate, an activated charcoal detector, and a liquid scintillation
counter.

2.

Methods

We used a PICO-RAD radon detector (AccuStar Labs, USA) (hereafter referred to as “thedetector”) to collect radon in air. The detector adsorbs radon onto activated charcoal, providing
convenient handling and high versatility. The air measured in a radon chamber under eachhumidity level was dehumidified with an anhydrous calcium sulfate desiccant (Drierite, 8 mesh ;W.A. Hammond Drierite Company, Ltd., USA) to avoid
the decrease in detector sensitivity caused by watervapor adsorbed onto activated charcoal (Fig. 1). The
desiccant itself adsorbs almost no radon from the air(Tajika et al., 2014).
The radon chamber at the National Institute ofRadiological Sciences (NIRS) allows for the adjustmentof airborne radon concentrations and humidity settings.
We built three acrylic columns, with lengths of 73 mm(A), 85 mm (B), and 110 mm (C), to determine theoptimal desiccant column length.

We filled eachcolumn with calcium sulfate, and placed them in theradon chamber. We also prepared a detector with adesiccant column (No. 1), a detector with no desiccantcolumn in a resealable polyethylene bag with thickness
0.04 mm (No. 2), and a detector with no desiccantcolumn or polyethylene bag (No. 3).
The detectors equipped with desiccant columns
were exposed to the radon chamber at NIRS for 48 h.
The radon concentration in the air was set to about
1,000 Bq/m 3 , the temperature was 20℃, and therelative humidity was set to 30% (low), 60% (normal),
404

Fig. 1 Schematic drawing and crosssectional views of the PICO-RAD
activated charcoal radon detector(after Iimoto et al., 2005), equipping
an acrylic column of desiccant(anhydrous calcium sulfate).   Thelengths of the column were set as
A＝73 mm, B＝85 mm, and C＝110mm to determine the most appro­
priate desiccant column length.

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硫酸カルシウムと活性炭型検出器を用いた高湿度環境下における空気中ラドンの測定

Table 1 Analytical methods used in this study for measuring radon in air.  Themeasurement conditions follow our previous study (Mori et al., 2013b)

and 80% (high). A toluene solution of 2,5-diphenyloxazole (DPO) and 1,4-bis(5-phenyl-2-oxazolyl)
benzene (POPOP) was used as a liquid scintillator. The liquid scintillation counter (LSC ;ALOKA LSC LB-5, Hitachi Aloka Medical, Ltd., Japan) was also used. The measurementconditions and reagents (Table 1) follow our previous study (Mori et al., 2013b).
From the results of the LSC measurements, net counts per second (netcps) were converted
into radon concentration according to Eq. (1).
…(1)
netcps : net counts per second
t1 : starting time of exposure in the radon chamber
t2 : ending time of exposure in the radon chamber
t3 : adding time of the liquid scintillator
t4 : measurement time using the liquid scintillation counterA : conversion coefficient
Detailed specifications for Eq. (1) are given by Passo and Floeckher (1991), Shefsky (1998), andMori et al. (2013b). Radon concentrations in the chamber were measured with the AlphaGUARD(PQ2000Pro, Genitron Instruments, GmbH) at the NIRS, which was accurately calibrated in the405

森 康則，大森康孝，吉村英基，前田 明，山本昌宏，西中隆道，石川徹夫，若林亜希子，安岡由美

温泉科学

Physikalisch Technische Bundesanstalt, Germany (on March 2013), and was assumed to givetrue values. This study aims to determine the conversion coefficient A under the measurement
conditions using the desiccant column.

3.
3⊖1.

Results and Discussion

Difference in detector sensitivity by column length
Table 2 shows the values measured by the detector for the three column lengths. This

study focuses on differences in measured values for each column length and amount of desiccantin the column.
The result under measurement condition A for the shortest column was 1,027 Bq/m3 (areference value; radon concentration was calculated by conversion coefficient A＝1.55 (Mori etal., 2013b)) and the result under measurement condition C for the longest acrylic column was 790
Bq/m3. The longer column thus lowered sensitivity, by a factor of about 80%. The weight ofthe desiccant in measurement condition A was 4.031 g, and that under measurement condition C
was sevenfold higher, at 29.564 g. In the next examination we therefore focused on measurement
condition C, which had the longest column length, because despite the lower sensitivity, it
showed higher performance for dehumidification. We believe that the appropriate length of thedesiccant column depends on the humidity of the environment.
3⊖2.

Difference in detector sensitivity by dehumidiﬁcation method
We adjusted the humidity in the radon chamber, and exposed the detectors equipped witha desiccant column or polyethylene bag. Table 3 shows the conversion coefficients that werederived from the measurement results for each relative humidity.
The conversion coefficient (A) was from 2.36 to 2.48 under measurement condition No. 1, in
which the desiccant column was installed in the activated charcoal detector. The conversion
coefficient (A) was from 1.07 to 1.14 in control experiment No. 3, in which no desiccant column
was installed in the activated charcoal detector. Compared with experiment No. 3, the desiccantcolumn in No. 1 decreased the detector sensitivity somewhat. The coefficient of variation (CV)was 7.12 at 30% relative humidity, 2.46 at 60% relative humidity, and 5.78 at 80% relativehumidity. The small variation in these data suggests that this measurement method is valid forquantitative analysis.
Table 2406

Differences in sensitivity by acrylic column length and amount of desiccant

第 64 巻（2015）

硫酸カルシウムと活性炭型検出器を用いた高湿度環境下における空気中ラドンの測定
Table 3

Conversion coefficients (A) under each condition

In contrast, the conversion coefficient (A) was very large (from 25.3 to 30.8) undermeasurement condition No. 2, because the polyethylene bag enclosed water vapor but little
radon in the air. This is presumably because most radon cannot penetrate the polyethylene bag,and thus cannot reach the activated charcoal.
The air temperature in the radon chamber was set to about 20℃, which was assumed to beroom temperature. However, specific environments, such as saunas in hot spring facilities canbe at much higher temperatures. Previous studies indicate a temperature-dependent change ofairborne radon measurements with an activated charcoal detector (Shefsky et al., 1993). We
intend to investigate the temperature-dependent change in airborne radon measurements withan activated charcoal detector continuously. We will also validate a more practical measurement
method.

4. Conclusion
The results of this study can be summarized as follows.
・We measured the sensitivity decrease of an activated charcoal detector caused by acrylic
columns of various lengths containing various amounts of anhydrous calcium desiccant.
Although installing the desiccant column lowered the detector sensitivity somewhat, ourexamination reveals that this measurement method is valid for quantitative analysis.
・Assuming reference values from the accurately calibrated AlphaGUARD at NIRS to be true
values, a conversion coefficient of 2.42 was found under the measurement conditions, using anactivated charcoal detector with an 11.5 mm acrylic desiccant column under 80% relativehumidity.
・The conversion coefficient was very large when the detectors were put in a polyethylene bag.
This is probably because most radon in air cannot penetrate the bag, and thus the radoncannot reach the activated charcoal.
・In future work, we will investigate the temperature-dependent change of airborne radonmeasurements with the activated charcoal detector continuously. We will also validate themore practical measurement method.
Acknowledgments
The authors gratefully acknowledge a research grant from Japan Health & ResearchInstitute. In addition, the authors gratefully acknowledge anonymous reviewers, whose valuable407

森 康則，大森康孝，吉村英基，前田 明，山本昌宏，西中隆道，石川徹夫，若林亜希子，安岡由美

温泉科学

and helpful comments improved our manuscript.
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（A part of this study was presented at the 67th Annual Meeting of the Japanese Society of Hot
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