Active Measurements of Indoor

Concentrations of Radon and Thoron Gas

using Charcoal Canisters

K. N. YU,* Z. J. GUAN, E. C. M. YOUNG and M. J. STOKES

Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue,Kowloon Tong, Kowloon, Hong Kong

(Received for publication 31 January 1997)

Standardized charcoal canisters with diameters of 4 inch, of a type recommended by the USEPA forpassive measurement of radon gas concentrations, have been modi®ed for active air sampling. Simul-taneous measurement of the concentrations of radon (Rn) and thoron (Tn) in air can be obtained bycounting the areas under g-ray peaks. Sample results obtained in a laboratory and in a residence withdi�erent conditions are also given. The largest uncertainties are about 2% and 3% for Rn and Tn re-spectively. The minimum detection limits are around 2.0 Bq/m3 for both gases. If only Rn concen-tration is required, the measurement time can be signi®cantly shortened. # 1998 Elsevier Science Ltd.All rights reserved

Introduction

Many methods exist for measuring concentrationsof radon (222Rn, herein referred to as Rn) and

thoron (220Rn, hereafter referred to as Tn) in air. Ingeneral, sensitivities of practical grab sampling

methods to detection for Rn are low and for Tn iseven lower. Integrated passive sampling based upon

use of an activated charcoal canister has becomeavailable in recent years (Cohen and Cohen, 1983;

George, 1984; Cohen and Nason, 1986; Jenkins,1991). Because of its simplicity, one such radoncounting system has found favor with the United

States Environmental Protection Agency (USEPA).The system, which employs a 4 inch activated char-

coal canister and a NaI gamma spectrometer(Environmental Protection Agency, 1986), has

found widespread applications in Rn detection (Yuet al., 1992, 1993).

In recent years, increasing attention has beenpaid to the thoron problem (Bigu, 1986;

Steinhausler et al., 1994). However, to date theUSEPA charcoal canister has seldom been used in

detection of environmental levels of Tn. Rn and Tnhave similar physical and chemical properties and,

as for Rn, adsorption of Tn can be used as a basisfor measurement of Tn gas. Bigu (1986) has per-formed active sampling of Tn gas by use of acti-

vated charcoal canisters, followed by measurement

using the 212Pb g peak with a high purity germa-nium (HPGe) detector. These measurement

involved Tn concentration several orders of magni-tude higher than those of environmental levels.Typically it is found that environmental Tn levelsare an order of magnitude lower than those of Rn.

In addition, since the half life of Tn is less than oneminute, measurement sensitivity for Tn needs to beespecially high. This has remained a di�cult pro-

blem.Current interest focuses on the possibility of

modifying this system for active sampling, also

allowing simultaneous measurements of concen-trations of Rn and Tn. Solomon and Gan (1989)have demonstrated that a viable modi®ed system

can be obtained using a charcoal coil. However,several di�culties have been encountered in tryingto use the modi®ed arrangement as an environmen-tal monitoring device. In practice, the coil tube is

long and di�cult to manufacture; the adsorbed Rnis di�cult to desorb; and the power needed for theair pump is large. The current proposal represents

an alternative system which avoids such problems.

Experimental Design

Activated charcoal canister and the sampling system

A collection of small holes, with diameters of4 mm, have been drilled in the bottom of a testcharcoal canister. A layer of temperature resistant

Appl. Radiat. Isot. Vol. 49, No. 12, pp. 1691±1694, 1998# 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0969-8043/98 $19.00+0.00PII: S0969-8043(98)00033-5

*To whom all correspondence should be addressed.

1691

permeable ®lter has then been placed inside, cover-

ing the entire bottom of the canister to preventcharcoal from being pumped out. The weight of the

charcoal has been increased from 70 g as used byUSEPA to 80 g in order to increase the sensitivity.Prior to sampling, the charcoal canister is opened

and heated for activation, cooled and then coveredby the original metal lid and by an extra plastic lid

covering the bottom.The sampling system consists of a ®lter, one or

more ¯ow meters, a desiccator, the activated char-coal canister and a pump. Figure 1 is a schematicdiagram of such a system. The pump is a high

volume air sampler (Model 08-601, NuclearAssociates) with an internal air ¯ow meter. The air

inlet has a diameter of 4 inch and is therefore highlysuitable for use with the 4 inch charcoal canisters.

For the actual air sampling, both the metal top lidand the plastic bottom lid are removed. The canis-ter is then tightly screwed into the plastic holder

with plastic O-rings. One or other of the setupsshown in Fig. 1 is adopted, depending on the size

of the ®lter paper used. The ®lter, used for trapping

Rn and Tn progeny in order to allow measure-ments, will not be discussed in the present paper.The setup shown in Fig. 1(a) is for use with a smallsized ®lter. The normal air ¯ow rate is 7±10 l/min.

At such small ¯ow rates, the air pump can becomevery hot. To avoid overheating of the air pump,extra air is drawn through. The setup shown in

Fig. 1(b) is used in conjunction with a large sized®lter. The air ¯ow rate can achieve levels of up toseveral hundreds of l/min, and no extra air is

required to provide for heat dissipation. As a result,the internal ¯ow meter can be used. After airsampling, the charcoal canister is immediately cov-

ered by the lids, which are then sealed against thecanister using adhesive tape, to prevent air leakage.

High purity germanium (HPGe) g-spectrometrysystem

After adsorption in the activated charcoal canis-ter, the Rn and Tn decay to their short-lived pro-geny. When the progeny reach equilibrium with

Fig. 2. The calibration system for the active sampling system for measuring concentrations of radonand thoron gas employing an activated charcoal canister.

Fig. 1. Schematic diagrams for active sampling systems for measuring concentrations of radon andthoron gas employing an activated charcoal canister. (a) for a small sized ®lter; (b) for a large sized

®lter.

K. N. Yu et al.1692

their parent gases, the gas concentrations can bededuced from the gamma intensities emitted by theprogeny. The half life of Rn is 3.85 d, and the pro-

geny 214Pb and 214Bi will achieve secular equili-brium within a period of 3 h. Since the half life ofTn is only 55.6 s, the progeny 212Pb (T1/2=10.6 h)can achieve secular equilibrium within only 6 min.

However, it takes 2.5 h for 212Bi (T1/2=1.01 h) toreach transient equilibrium with 212Pb. In order toallow simultaneous measurements for Rn and Tn

concentrations, and considering the time which isrequired to bring canisters back from site to thelaboratory, measurements must start as soon as

possible to three hours after sampling. The exactelapsed time between completion of sampling andthe start of measurements should be recorded to

allow accurate radioactive decay correction.For our measurements, an EG&G ORTEC g-

spectrometry system consisting of a 4 inch cylindri-cal HPGe detector with a relative e�ciency of 90%,

housed in a 10 cm-thick lead shield has beenemployed. The available 4096 channels are made tocover the energy interval 0 to 700 keV. In the

gamma spectra which are obtained, it is observedthat the peaks at 295 and 352 keV for 214Pb,609 keV for 214Bi, 239 keV for 212Pb and 583 keV

for 208Tl are clearly separated from each other. Thepeaks at 352 and 239 keV are chosen as the charac-teristic peaks for Rn and Tn, and the areas underthe peaks are used for calculation of Rn and Tn

concentrations. The peak at 239 keV for 212Pb isthe most di�cult to resolve. In our measurements,this peak has a width of 30 channels, which is su�-

cient to be separated from the nearest peak at241 keV for 214Pb. The measurement time is mainlydetermined by the precision required for the Tn

measurements (due to its relatively low concen-trations). Our measurement time has been chosento be 120 min which is the same as the background

measurement time we have adopted.

Calibration of the System

For active sampling using activated charcoal can-isters and measurement of net areas under thecharacteristic g-ray peaks of the Rn and Tn pro-

geny, calibration of the whole system can be based

on that for passive sampling (Environmental

Protection Agency, 1986). The concentration of Rn

and Tn are calculated as

C � NETCPM=�V ��AE��DE��DF� �1�

where C (Bq/m3) is the concentration of Rn or Tn,

NETCPM (cpm) is the net count per minute (i.e.,

after subtraction of background) under the particu-

lar characteristic g-ray peaks, V (m3) is the volume

of the sampled air which is equal to uT, u being the

air ¯ow rate and T the sampling time, AE (%) is

the adsorption e�ciency of activated charcoal for

Rn or Tn, DE (cpm/Bq) is the detection e�ciency

which can be obtained through measurements on

known activities of Rn and Tn, and DF is the cor-

rection factor for radioactive decay which takes

into account non-equilibrium between the progeny

and the parent gas. We combine V, AE and DE

into a calibration factor CF (cpm/Bq/m3) which can

be obtained through the method shown in Fig. 2

employing the small ®lter sampling method shown

in Fig. 1(a). A barium radium carbonate liquid

source has been used for determining the CF(Rn)

while for CF(Tn) a 228Ra-equilibrium thorium

liquid source has been used. If their activities are re-

spectively ARnS and ATn

S , and if Rn and Tn are com-

pletely expelled by aged nitrogen gas into the

charcoal canister, the released Rn and Tn activities

per unit time are

ARn � lRnASRn and ATn � lTnAS

Tn �Bq=min� �2�

where lRn=1.266�10ÿ4 minÿ1 and lTn=0.7629 minÿ1 are the decay constants for Rn and

Tn respectively. The calibrated gas concentrations

entering the charcoal canister for a ¯ow rate u (m3/

min) are

CRn* � ARn=u and CTn* � ATn=u �Bq=m3�: �3�

Measurements of the canisters start at 180 min

after sampling. If the net count rate under the

Table 1. Sample results on indoor radon and thoron concentrations

Sample no. Background Peak area NETCPM Conc. (Bq/m3)

352 keV 239 keV 352 keV 239 keV 352 keV 239 keV Rn Tn

1 147 162 4553 295 36.7 1.11 140 6.102 136 186 4371 372 35.3 1.55 134 8.523 160 194 1598 327 12.0 1.11 45.6 6.104 144 174 3106 296 24.7 1.02 93.9 5.605 146 174 3063 281 24.3 0.892 92.4 4.906 143 172 3019 385 24.0 1.78 91.3 9.787 141 164 2678 343 21.1 1.49 80.2 8.198 190 232 3685 541 29.1 2.58 111 14.2

*1: laboratory with 24-hour air conditioning; 2: same as sample 1 but taken on a di�erent day; 3: laboratory without air conditioning; 4:laboratory with negative ion generator; 5: laboratory with negative ion generator and fan; 6: laboratory with positive ion generator;7: laboratory with positive ion generator and fan; 8: closed reading room in a high-rise residence.

Measurements of indoor radon and thoron gas concentrations 1693

characteristic g-ray peak at 352 keV is NETCPMRn,and that under the peak at 239 keV is NETCPMTn,

the calibration factors CF are expressed as

CFRn � NETCPMRn=CRn* and

CFTn � NETCPMTn=CTn* �cpm=Bq=m3�: �4�

These have been experimentally found to be

CFRn=0.263 cpm/Bq/m3 and CFTn=0.182 cpm/Bq/m3 for u = 7� 10ÿ3 m3/min.

Sample Measurements

Sample measurements of Rn and Tn concen-trations have been made in a laboratory and a resi-

dence. Due to the short half life of Tn, itsconcentration will decrease rapidly with the distancefrom walls (Katase et al., 1988). Therefore,

sampling has been ®xed at a position 30 cm fromthe wall and 1 m from the ¯oor. The sampling timehas been ®xed at 30 min while the measurement

lasts for 120 min. The results are listed in Table 1.The largest uncertainties are of the order of 2%and 3% for Rn and Tn respectively. The minimumdetection limits are around 2.0 Bq/m3 for both

gases. If only Rn concentration is required, themeasurement time can be signi®cantly shortened.With present detection limits, this method would

also be suitable for measurements of outdoor Rnconcentrations.

AcknowledgementsÐThe authors would like to thank M.Li and M. M. Lau for their assistance with the measure-ments and G. H. Tan for assistance with the calibrationsof the systems.

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