Radiation Environment and Medicine 2023 Vol.12, No.1 12–24

Review

Status of Radon Traceability and Calibration Techniques for Radon Gas
Yuki Tamakuma1, Chutima Kranrod2, Masahiro Hosoda 2,3 and Shinji Tokonami2*
Center for Radiation Research and Education, Nagasaki University, 1-12-4 Sakamoto, Nagasaki, Nagasaki 852-8523, Japan
2
Institute of Radiation Emergency Medicine, Hirosaki University, 66-1 Honcho, Hirosaki, Aomori 036-8564, Japan
3
Hirosaki University Graduate School of Health Sciences, 66-1 Honcho, Hirosaki, Aomori 036-8564, Japan

1

Received 2 June 2022; revised 28 July 2022; accepted 3 October 2022

There are many calibration facilities of radon gas in the world, which are used to keep ameasurement quality of radon mainly for purpose of radiation protection. Although there is only
a limited number of techniques of a primary standard, each calibration facility employs several
different techniques to establish a secondary calibration. The article summarized a status of radon
traceability and the techniques used in the radon calibration facilities as well as the characteristics
of each technique. In terms of geographical characteristics of calibration facilities, there was abiased distribution of a primary standard; only the US and Korea have a primary standard except
in Europe. For differences in the technique used, it was found that operation modes may have an
influence on the maximum radon concentration in the chamber and the time to reach a stableconcentration. Types of radon sources may also affect the radon concentration in the chamber due
to the difference in the emanation power of radon gas. The findings are expected to be utilized forthe design and development of new radon calibration chambers in the future.
Key words: radon, calibration, standard, traceability

1. Introduction
Radon is a noble gas generated by a radioactive decay ofradium included in rocks and soils. The United Nations
Scientific Committee on the Effects of Atomic Radiation(UNSCEAR) has reported that almost half of an annual
effective dose from natural radiations is caused froman inhalation of radon isotopes and their progenies1).
According to Darby et al., a consecutive exposure ofradon of 100 Bq m-3 increased an excess relative risk oflung cancer of 16%2, 3). The World Health Organization(WHO) has also reported that radon is a secondary risk
factor of lung cancer following tobacco smoking 4). There*Shinji Tokonami: Institute of Radiation Emergency Medicine, Hirosaki University,
66-1 Honcho, Hirosaki, Aomori 036-8564, Japan
E-mail: tokonami@hirosaki-u.ac.jp
https://doi.org/10.51083/radiatenvironmed.12.1_12
Copyright © 2023 by Hirosaki University. All rights reserved.

are several areas where radon concentration naturally
become high due to high radium concentration in soil or ageographical characteristic5-7). Also, radon tends to easily
accumulate in places with a poor ventilation such as caves
and mines8); in fact, a mean annual radon concentration
was reported to be approximately 3500 Bq m-3 9). The riskof lung cancer from exposure to radon could be higher
in these environments so that the radon measurement isimportant from viewpoint of radiation protection.
Many national or regional surveys of radon, thoron
and their progenies have been conducted for dose
assessments using several types of monitors all over theworld10, 11). Besides, radon is also utilized for research infield of environmental sciences: for example, atmospherictransportation and prediction of earthquake12). However, it
has been reported that there may be significant difference
between the measurement results of the monitors13-15).
Therefore, a periodical calibration of the monitors usedYuki Tamakuma et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 12–24

13Fig. 1. Traceability of radon measurement devices

must be conducted for a quality assurance and a quality
control of measurement. Many countries developed theirown calibration facilities for radon and the facilities havebeen used not only for calibration experiments but also
for fundamental research related to radon. A limited
number of the countries own an institute or a facility tomake a reference atmosphere of radon, which is certified
by a primary standard due to its difficulty of construction.
There are also many secondary calibration facilities
and exposure systems with their own techniques which
establish a traceability to the primary standard. The
different techniques result in different conditions in acalibration or exposure chamber: a level and a stability ofradon concentration, a temperature, a humidity and an airpressure. Requirements for calibration facilities are also
different by types of radon monitors to be calibrated. For
instance, passive-type monitors do not require the stableradon atmosphere as it is based on a time-integrated
measurement while active-type monitors require it. In
addition, a humidity in the chamber must be controlled
or monitored in case of calibration for charcoal canisterbecause of the hygroscopicity of the charcoal. The
present article summarizes a status of radon traceability
and specifications of the calibration facilities of radonused in the world to clarify issues on the calibration.
2. Overview of radon calibration system
An outline of a traceability system for radon measurement
devices is shown in Figure 1. In National Metrological
Institutes (NMIs) in each country, a calibration isconducted to a radon monitor used as a secondary
standard using a primary standard of radon gas with anabsolute measurement. So far, there are several absolutemeasurement techniques of radon used in the world. The
first technique was developed by Picolo at Laboratoire

National Henri Becquerel (LNE-LNHB) in France16). The
technique selected consists in applying a standard definedgeometry measurement method, which is generally
used for solid sources, to a radon gas condensed on acold point. This technique has been implemented at the
Physikalisch-Technische Bundesantalt (PTB) in Germany,
the Swiss National Metrological Institute (METAS) in
Switzerland and Korean Research Institute of Standards
and Science (KRISS) in Korea17-19). Other methods are
based on internal gas proportional counting, 4 π γ -NaI(Tl)measurement, liquid scintillation counting (LSC) and highpurity germanium (HPGe) detector measurement 20-23).
The calibration by the primary standard is based on thespecification of the activity concentration by means of aradon gas activity standard and a noble gas-tight vesselof known volume24). A calibration factor to the secondary
standard is expressed by a following equation.
A
kt = ――――――――(V r−V t) ･ (Ct−Ct, bg)(1)Here, A is the activity (Bq) of the radon gas standard,
V r is a volume (m 3) of the reference chamber, V t isa displaced volume (m 3) by the secondary standard
device. Ct and Ct, bg are the radon concentration (Bq m-3)
measured by the secondary standard device and thebackground, respectively. Since establishing absolutemeasurement systems is difficult and the half-life of radonis not sufficiently long to permit a high-quality calibration
in some cases where a long-distance transportation isnecessary, secondary calibration systems are required.
In the secondary calibration facilities, radon monitors
are calibrated by exposing to a radon atmosphere ina chamber with a reference radon monitor calibrated
with the primary standard. Measurement results of the14

Yuki Tamakuma et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 12–24

radon monitors are compared with those obtained by thereference monitor and calibration factors are then given,
which establish a traceability of radon measurement toa level of secondary calibration. The calibration factor kc
can be expressed by the following equation:
Ct−Ct, bg
kc = kt ――――――Cc−Cc, bg(2)where Cc and Cc, bg are the radon concentrations (Bq m-3)
measured by the radon monitors calibrated and thebackground, respectively.
3. Outline of the calibration facilities in each country
As mentioned above, there is only a limited numberof the absolute measurement techniques. However,there are many different techniques to make a radon
atmosphere at each calibration facility. For instance,
several types of radon sources are used such as a 226Ra
source, natural soil and rocks as the facilities usually do
not require the primary standard source. There are also
several differences between the techniques used in eachcalibration facility: a volume of a chamber, an operation
type (i.e., a system providing radon into a chamber)and a range of radon concentration, temperature, andrelative humidity. In this section, the differences at eachcalibration facility and a status of traceability chain
in Europe, Asia and other regions are described. The
specifications of the radon chambers described in thissection are summarized in Table 1.
3.1. Europe
In Bulgaria, Pressyanov et al. developed an exposuresystem for radon plus thoron atmosphere at Sofia
University 25). The system consists of an exposure
chamber (0.05 m3 in volume), a 226Ra source, a 228 Th
source and a thermostat. The chamber is placed in thethermostat which can control temperature in the range
from –15 to 60 during exposure experiments. The
system can operate both in open and in closed mode. The
air flow in the system can be directed in different waysby the air valves so that atmospheres containing pureradon, thoron or their mixture can be created. To create
mixed radon/thoron atmosphere under controlled activity
concentration ratios, both sources should be consecutively
connected. In open loop mode, radon concentration in
the chamber decreases with an increase of the flow rate,
while that of thoron increases. In closed loop mode, thetrend of radon concentration is same although thoron
concentration could vary, which enables to create radon/
thoron atmosphere with different ratios of concentrationsof both radionuclides. The radon concentration can be
maintained in the range from approximately 3 to 46,000
Bq m-3.

In Czech Republic, the National Radiation Protection
Institute (NRPI) has a large walk-in type radon chamberof 45 m3 in volume where the inner walls are sprayedwith a special material for better simulation of living
condition26-28). Controllable inner chamber temperatureand relative humidity range from 8 to 45 and from 15
to 95%, respectively29). Stable radon concentration in thechamber can be achieved from around 100 to 100,000
Bq m-3 by the known air-exchange rate and radon entry
rate from various types of 226Ra sources.
In France, a primary radon standard was developed at
the LNE-LNHB based on a defined solid angle method
and updated by Sabot et al.16, 30). The Institut de Protection
et de Sûreté Nucléaire (IPSN) built a radon calibration
test chamber called BACCARA31). The chamber is a 1 m3
stainless steel cylinder where radon concentrations canvary from 40 up to 40,000 Bq m-3. The BACCARA has
recently been operated by the Institut de Radioprotection
et de Sûreté Nucléaire (IRSN) for intercomparison
exercise once in three years according to the French
Public Health Code. Recently, the LNE-LNHB constructed
an experimental facility for reference atmosphere ofradioactive gases including radon32).
I n Ger ma n y, Der sc h de v e loped a n a bsol u t emeasurement method at the PTB17). The German
reference radon chamber with a volume of 21 m3 wasconstructed at the PTB where environmental parameters
can be adjusted. Temperature and humidity can be
maintained in the range from -20 to 40 and 5–95%,
respectively33). Two typical procedures had been used
for the calibration of active radon monitors at the
PTB: (i) a primary method using a primary radon gas
activity standard, (ii) a secondary method based ona calibration via a reference monitor34). However, thePTB reference chamber was abandoned in 201635). The
Bundesamt für Strahlenschutz (BfS) also has two walkin radon chambers, five 0.4-m3 containers for calibrationand intercomparison36). The radon concentration in thechambers and containers are measured by a flow-throughscintillation cells which can be traced back to the national
standard of the PTB. After the cessation of the calibration
service of the PTB, the BfS switched the metrologicaltraceability of the radon activity to the LNE-LNHB as
of 202035). The concentration, temperature and relative
humidity can be adjusted in the range of 500–100,000
Bq m-3, -5–50 and 10–95%, respectively. The BfS recently
also uses a reference chamber with a volume of 0.168 m3
which is traced back to a standard volume provided by
the PTB, together with a secondary chamber with avolume of 0.131 m3.
In Hungary, the Government Office of the Capital City
Budapest (BFKH) has a radon calibration chamber37). A
flow-through type 226Ra source is used as a radon source
and is connected to the chamber and a buffer tank byYuki Tamakuma et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 12–24

15

Table 1. Radon calibration chambers around the world and their speciﬁcations
RegionEuropeCountry

Institute

Bulgaria

Sofia University

Czech Republic
FranceNRPI
IRSN

Germany

PTB

SourceRa source

0.05

226

45

Ra source
Rn standard
sourceRn standard
source226
Ra source

1.0
21

226

BfS

30, 11, 0.4

Hungary

BFKH

0.84

226

Italy

ENEA

1.024

226

0.608

226

12.38

226

Poland

PAN
CLOR
CMI

Asia

Volume (m3)17

Ra source
Ra source
Ra source
Ra source

Ra source
Rn standard
source226
Ra source
226

Romania

IFIN-HH

RussiaRAS

0.05

Slovak Republic SMU

1

226

Spain

UPC

20

226

University of Huelva

0.22

226

Sweden

SSI

2

226

Switzerland

METAS

0.13

226

UK

PHE

43

226

1

Rn concentration(Bq m-3)

Reference

211,000-655,000(25)

100-100,000(29)Spot injection

40-40,000(16,30-32)

Spot injection-(17,33-34)

Flow through500-100,000(36)Spot injection
Accumulation
Flow through/Spot injection
Accumulation
Flow through/Spot injection-

100-5,000(37)

370-1,110(38)

500-85,000(41)

50,000 (max)(42)

100-15,000(43)Spot injection-(37,44)

Operation mode
Flow through/Accumulation
Flow throughAccumulation

31,100 (max)(23)

Ra source

Accumulation

100-100,000(45)

Ra source

Flow through3,000 (typical)(46)

Ra source

Flow through400-22,000(47)

Ra source-

10,000-1,000,000(48)

Ra source

Flow through270-150,000(37)

Ra source

Accumulation

200-8,000(29,49,50)(52)

ArabiaNajran University

0.09

Natural soil

Accumulation

100-2,600

China

NIM

12.46

226

Ra source

Flow through12-232,000

(53)

India

BARC

0.5

226

Ra source

Spot injection

1,173-9,058(54)

Mangalore University

22.7

Natural soil

Flow through500-31,000(55-57)

Iran

AUT

0.498

Uranium stone

Accumulation

28-8,958

(58)

Japan

NIRS

25

226

Ra source

Flow through894-9,617(59,60)Hirosaki University

0.54

Uranium ore

Flow through213-1,060

(61)

Korea

KRISS

25.3

226

Ra source

Flow through1,000-250,000(62,63)

Syria

AECS

0.65

226

Ra source

Accumulation

170,000 (max)(64)

Thailand

TINT

0.05

226

Ra source

Flow through5,000-15,000(65)

Chulalongkorn
University

0.1

226

Ra sourceSport injection

350,000 (max)

(66)

16.5

226
Ra source,
Uranium ore

Flow through200,000 (max)(67,68)19.4

226

Ra source

Flow through37-37,000(70,71)Other regions Australia
US

ARL
EML

two-position electronic valves. The radon gas is injected
into the chamber automatically by opening the valves via
a computer-based controller unit so that the concentration
does not fall below a desired concentration. The radon
concentration, temperature and relative humidity can be
maintained in the range of 100–5,000 Bq m-3, 15–60 and
20–80%, respectively.
In Italy, the Italian Committee for Research andDevelopment of Nuclear Energy and Alternative Energy(ENEA) developed a test chamber of 1 m3 for radon38).

Temperature and humidity can be controlled from -35 to
60 and from 10 to 95%, respectively, using a climatization
apparatus. A characteristic of the chamber is that it can be
operated in a static state and under two different dynamic
conditions: re-circulation and dynamic open circulation.
The ENEA can act as a secondary reference facility byusing a 226Ra standard solution from the National Instituteof Standards and Technology (NIST) in the US. Under
the dynamic open circulation, radon concentrations werevaried from about 370 to 1,110 Bq m-3. Also, Venoso et al.

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constructed a radon facility at Naples University andconducted first performance tests39). Palermo University
also developed a radon chamber40). The chamber is acylindrical shaped container made of stainless steel withvolume being about 0.4 m3. Temperature control in thechamber is achieved in the range of 5–50 by means of acirculation of water at controlled temperature in a copper
pipe wrapped around the chamber. They use radium-richsoil as a radon source in the first test to verify the stabilityof the radon concentration over time. In the test, radon
concentration ranged from around 2,000 to 3,000 Bq m-3.
In Poland, the Polish Academy of Science (PAN)
constructed a cylindrical radon chamber made of steel
with a volume of 0.608 m 3 41). Radon gas generatedusing a 226Ra source is injected into the chamber and its
concentration is controlled by an accumulation time in thesource container. The maximum achievable concentration
was reported to be 85,000 Bq m -3. For the minimum
achievable concentration, they did not mention in thereport, however, the minimum value in the report was 500
Bq m-3. The Central Laboratory for Radiological Protection(CLOR) also developed a walk-in type radon chamberusing a 226Ra source42). Temperature and humidity canbe controlled in the range from -30 to 60 and from 15
to 95% at 10-60 , respectively. The maximum radon
concentration was 50,000 Bq m-3 achieved by a series of
two sources. Lower concentrations of approximately 3,700
Bq m-3 are also available using a fork type connection
system. Skubacz et al. have also reported a walk-in
type radon chamber at the Central Mining Institutereconstructed following the first chamber43). There is thepossibility of remotely controlling the temperature andhumidity in the chamber in the range from -20 to 60
and from 20 to 90%, respectively. A 226Ra source is used asa source and the radon concentration in the chamber canbe regulated in the range of 100–15,000 Bq m-3.
In Romania, an absolute measurement system wasdeveloped based on a system of the LSC method in
Horia Hulubei National Institute for R&D in Physics andNuclear Engineering (IFIN-HH)44). The system to make asecondary standard source was also constructed using an
ionizing chamber and a gamma-ray spectrometer. In theframe of the Romanian national research project, a radon
chamber was designed and constructed at the IFIN-HH37).
The radon chamber of 1 m3 is placed inside external
chamber and polyurethane foam is filled between thetwo chambers to control environmental parameters
during the calibration. A radon standard source or a flowthrough type source were planned to be used as a radonsource for the chamber, however, the second one is not
yet implemented.
In Russia, the Institute of Industrial Ecology of theRussian Academy of Science (RAS) constructed a radon
calibration system with a primary standard based on the

HPGe detector23). The system is composed of a stainlesssteel standard chamber with a certified volume of 0.05
m3 and an emanation box. A standard 226Ra source isplaced in the emanation box and the gamma spectrum ismeasured continuously every 24 h during the calibration
period. A 137Cs standard source is also installed in thebox to ensure the stability of the gamma spectrometry.
An emanation factor can be estimated by the online
measurement of a residual 214Bi activity in the radiumsource. The radon gas emanated from the radium sourceis cycled and accumulated through the chamber by apump. A saturated radon concentration was evaluated tobe 31,100 Bq m-3 when using the 226Ra source of 3.98 kBq.
In Slovak Republic, a radon chamber was set up at the
Slovak Institute of Metrology (SMU)45). The chamber is ahorizontal cylinder made of stainless steel with a volumeof ~1.0 m3. All dimensions of the chamber are traceable
to the SMU. Radon gas generated by a 226Ra source isprovided into the chamber by a spot-injection method.
The maximum radon concentrations are achievable
within the range from 100 to 100,000 Bq m-3.
In Spain, the Technical University of Catalonia (UPC)
developed a 20 m3 radon chamber46). A control system
for radon concentration in the chamber continuously
regulates the exhalation rate of radon gas from a 226Ra
source and the chamber ventilation rate. It makes
it possible to keep the radon concentration in thechamber stable. By a refrigeration unit, a humidifier
and a dehumidifier, temperature and relative humidity
can be controlled in the range of 10–40 and 15–95%,
respectively. The controllable range of the radon
concentration was not reported although a typical radon
concentration result was ~3,000 Bq m-3. The Universityof Huelva also constructed a laboratory-made radon
calibration chamber with a volume of 0.220 m3 47). As
a result of performance tests, it was found that radon
concentration can be adjusted by the control system in
the range of 400–22,000 Bq m-3.
In Sweden, the Swedish National Institute of Radiation
Protection (SSI) constructed a radon calibration chamber
made of a stainless steel with a volume of 2 m3 48). Radongas in the chamber is produced by four 226Ra salt sourcesin a lead container and circulates continuously throughthe container from the chamber. Radon concentration,
temperature and relative humidity in the chamber can bevaried within the range of 10,000–1,000,000 Bq m-3, 5–50
and 10–95%, respectively.
In Switzerland, the Swiss radon reference test site wasestablished at the METAS37). The METAS uses a stainlesssteel cylindrical chamber with a volume of 0.13 m 3which is traceable to a standard volume provided by the
METAS. The standard operation ranges of environmental
parameters depend on environmental conditions of
the laboratory, which are 950±20 hPa of air pressureYuki Tamakuma et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 12–24

and 22±2 of the temperature. They utilize twodifferent flow-through type 226Ra sources provided by
EUROSTANDARD CZ and certificated by the Czech
Metrology Institute to create radon atmosphere inside
the chamber. The radon gas generated is fed into thechamber and the concentration is regulated by the air
flow rate through the source. Compressed air in bottles
is used as a carrier gas after storing for a few weeks.
The radon concentration can be achieved in the range of
270–150,000 Bq m-3.
In the UK, the Center for Radiation, Chemical andEnvironmental Hazards (CRCE) of the Public Health
England (PHE) has a walk-in type radon chamber of 43 m3
made of stainless steel 29, 49). The PHE (formerly, theNational Radiological Protection Board: NRPB, andthe Health Protection Agency: HPA) has conductedintercomparison of passive radon detectors annually
since 1982. The radon concentration in the chamber is
continuously monitored using ionization chamber typeradon monitors which are periodically calibrated witha primary standard by the PTB or CHUV Institut deRadiophysique in Switzerland49, 50). A dry 226Ra source isplaced in the chamber and generates radon gas without
air flowing through the chamber during operation. The
chamber contains a radon atmosphere which can bevaried and held stable from around 200 to 8,000 Bq m-3 51).
3.2. Asia
In Arabia, Abdalla et al. constructed a radon irradiationchamber with a total volume of 0.087 m 3 at Najran
University52). A natural soil sample is used as a radonsource in the irradiation system. Radon gas is circulated
by a pump and accumulated in a closed irradiation
system constructed by the radon source, the exposure
chamber and a traceable reference radon monitor. Radon
concentration and temperature can be controlled in
the range from 100 to 2,600 Bq m-3 and from 20 to 50 ,respectively.
In China, the National Institute of Metrology (NIM)
developed a calibration chamber for radon and its
progenies with a regulation capability of environmental
parameters, aerosols and radon concentrations53). The
chamber has a total volume of ~20 m 3 including an
exposure volume of 12.46 m3. Temperature and humidity
can be regulated automatically in the range of 0.66–
44.39 and 16.4–95%, respectively, according to pre-set
parameters editable via the interface of the PC software
matching the custom-built air conditioner. The chamber
employs three solid flow-through type sources made in
the Czech Metrology Institute. Each source connects witha buffer bank of 150 L, and radon generated is normallytransferred into the buffer bank. A constant activity ofradon close to the 226Ra sources is achieved in the bufferbank. The radon gas is fed into a main chamber from17

the buffer banks by a constant flow sampling pumpand flows to exhaust gas treatment system. The radon
concentration in the main chamber is maintained to asetting value in the range between 12 and 232,000 Bq m-3
by control of sampling time of the constant flow pumps.
In India, a calibration facility has been set up at the
Bhabha Atomic Research Center (BARC)54). The facility
is composed mainly of a calibration chamber, an aerosol
generator, an online radon gas monitor, a radon andthoron progeny monitor, standard sources and equipment
for aerosol characterization. A cubic chamber madeof stainless steel having a volume of 0.5 m3 is used.
Radon/thoron gas and aerosols are injected directly into
the chamber through inlets and mixed by fans in thechamber. Radon concentration can be controlled at leastin the range of around 1,000–9,000 Bq m-3 according to
Eappen and Mayya54). Recently, Mangalore University
also constructed a calibration facility using soil gas
as a radon source55). The facility consists of a walk-in
type chamber (22.7 m3), a radon generation system, anenvironmental system and an exhaust system. They
adapted an innovative technique of harvesting soil gas
as an efficient source of radon56, 57). It is referred to as“semi-dynamic mode” in which soil gas is injected into
the calibration chamber at regular intervals. Using
the technique, an ultrasonic humidifying system anda refrigerated coil type dehumidifier, the facility canmaintain radon concentration in the chamber in therange from 500 to 31,000 Bq m-3 as well as temperatureand relative humidity in the range of 5–50 and 30–95%,
respectively.
In Iran, Heidary et al. developed a radon calibrationchamber at Amirkabir University of Technology (AUT) 58).
The chamber is cubic with two trapezoid sides witha total volume of 0.498 m3 and made of stainless steel.
Temperature and relative humidity are controlled by aheater and a sprayer in the range of 20–45 and 10–70%,
respectively. Radon gas generated from uranium stone
samples is accumulated with time in the chamber,therefore radon concentration cannot be kept in stablecondition. A minimum and maximum radon concentration
were reported to be around 30 and 9,000 Bq m-3.
In Japan, the National Institute of Radiological
Sciences (NIRS) developed a radon/aerosol calibrationchamber59, 60). The chamber is made using a stainlesssteel for inner walls and has an inner volume of 22.45
m 3. It is designed to control temperature, relative
humidity, aerosol concentration, radon concentrationand pressure differentials between inside and outside of
the chamber. Radon gas generated from eight ceramicsources of 226Ra is fed into the chamber through anaging tank, and then flows out via a radon gas eliminatorsystem. The temperature and relative humidity can becontrolled using an air conditioner in the range of 5–

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30 and 30–90%, respectively. The controllable range ofradon concentration is between around 900 and 10,000
Bq m-3. Hirosaki University has also developed a radon
exposure system using natural uranium ores as a radonsource61). Radon gas is fed into an acrylic chamber of
0.54 m3 through a stainless-steel chamber to mix radongas with aerosols. The radon concentration is controlled
in the range of around 200–1,000 Bq m-3 by a flow rateof the whole exposure system. Hirosaki University alsouses another exposure technique using a stainlesssteel chamber of 0.15 m3. In this technique, radon gas
accumulated in a source chamber for a certain period of
more than 24 h is collected into a radon-proof bag madeof polyethylene by a sampling pump. The collected radongas is injected into the stainless-steel chamber which
includes radon monitors to be calibrated. The initial radon
concentration in the chamber can be controlled fromaround 2,000 to 10,000 Bq m-3 by a period of accumulation
and a volume of radon gas collected. Note that theradon concentration in the chamber with a spot-injection
method decreases with time according to a half-life ofradon because there is no additional source of radon.
In Korea, a radon calibration chamber has been
constructed to test and calibrate radon and radon
progeny at the KRISS62). The volume of the main roomin the chamber is 25.3 m3. There are ten sampling portsof 8 cm in diameter allowing users to collect radon andits progeny and to continuously measure the activity
concentrations in the main room. Temperature andhumidity can be controlled between 10 and 40 , 30%
and 80%, respectively, by an equipped environmental
control unit. They use three flow-through type 226Ra
sources independently or simultaneously depending upona desired radon concentration. Radon gas is injected into
the chamber by putting a filtered dry air into a containerof the sources located outside of the chamber. The radon
concentration is monitored by a radon monitor calibratedusing a primary standard source certified by the KRISS63)
and it can be maintained in the range from 1,000 to
250,000 Bq m-3.
In Syria, Shweikani et al. constructed a radon
calibration chamber at the Atomic Energy Commissionof Syria (AECS)64). The chamber is a cubic one made of a
pixy glass with a volume of 0.65 m3. A solid 226Ra source
is installed in the chamber and generates radon gas.
Radon concentration in the chamber was measured bypassive radon monitors calibrated at another institute, andestimated to be 170,000 Bq m-3.
In Thailand, a small chamber for an improvement ofradon calibration measurement of the solid-state nuclear
track detector, was established in 2009 at Thailand
Institute of Nuclear Technology (TINT)65). The chamber
is a cylindrical stainless-steel pot with a diameter andheight of 40 cm, and an inside volume of 50 L. There

were four Swagelok connectors installed on the lid;
two connectors were for the circulation of radon gas,
and the other two were for connecting with a radon
monitor. The radon concentration levels (5,000–15,000
Bq m-3) were controlled by accumulation radon gas in thesource container for a period and injected to calibrationchamber via mini-pump at flow rate of 5 L min-1 for 60
seconds. However, there was no report on environmental
conditions of radon calibration such as relative humidity,
pressure, and temperature for this calibration chamber.
In addition, the radon chamber has been constructed
in 2011 at Chulalongkorn University for calibration ofradon detectors in simulated environmental conditions66).
The chamber is a cylindrical shape made of stainlesssteel with an inner volume of 100 L. It could adjust thetemperature (15–50 ), control humidity (10–80%), andcould equip radon detectors at various vertical positions.
The source material for radon gas was a dry powder of226
Ra. The maximum radon concentration was ~ 350,000
Bq m-3 by a spot injection of radon gas after accumulatedradon in the source container for a period. Environmental
parameters such as temperature and relative humidity
are also controlled by chiller, heater, and independent
humidifiers.
3.3. Other regions
In Australia, the Australian Radiation Laboratory (ARL)
designed a radon chamber with an effective volume of
7.2 m 3 67). By a closed cycle air conditioning unit,
temperature and relative humidity in the chamber are
maintained at ±1 and ± 5% of the standard valuesof 20 and 50%, respectively. Radon concentration canbe increased and held stable at the level of up to 60,000
Bq m -3 by flushing radon gas from a 226Ra source atpredetermined intervals. The original ARL chamber has
been updated a few times and the current version of the
facility has been operated by the Australian Radiation
Protection and Nuclear Safety Agency (ARPANSA)68).
After the update, the volume of the chamber is 16.5 m3.
Discrete radon sources are available (uranium ore andradium sources) and various radon concentration can beachieved up to greater than 20,000 Bq m-3.
In Canada, the Radiation Safety Institute of Canada(RSIC) have a walk-in type national radon chamberwith a volume of 12 m3 69). The chamber is an approved
chamber for the Canadian National Radon Proficiency
Program and is certified secondary reference chamber
under the American Association of Radon Scientists andTechnologists National Radon Proficiency Program. The
chamber utilizes two commercial flow-through type 226Ra
sources, which is traceable to the primary standard of
the NIST in the US. The radon gas is injected into thechamber and the concentration is controlled via mass flow
meters. Environmental parameters are also regulated by

Yuki Tamakuma et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 12–24

independent heaters and humidifiers.
In the US, the Environmental Measurement Laboratory(EML) developed a radon calibration chamber with avolume of 19.4 m3 70). A dry 226Ra source was used for aradon source. The air flow through a drum, where thesource is housed, is fed into the chamber at a constantrate. The radon concentration can be controlled over arange of 37 to 3,700 Bq m-3. Temperature and humidity
in the chamber can be adjusted over the range of 2 to
45 and 15% to 100%, respectively. In 1987, the EML radon
chamber was used for the first three radon proficiency
exercises by the Environmental Protection Agency(EPA)71). The EPA began to conduct the radon proficiency
exercises in the EPA radon facility in Montgomery
Alabama in the following year. Later this service wastransferred to the EPA radon calibration facility in Las
Vegas until it ceased to provide the services in 2014.
Since the primary calibration facility ceased its operation,
the second calibration chamber at the NIST traceablefacility at Bowser Morner Lab is used for calibration andintercomparison.
Some Latin American countries also have a radon
calibration chamber: Mexico, Argentina, Peru andVenezuela72). However, there were no detail informationon the chambers as well as on a primary standardof radon. In case of the Mexican National Instituteof Nuclear Research (ININ), their passive monitors
were calibrated every year at the Oak Ridge National
Laboratory in the US 73). Radon chambers at otherinstitutes in Latin America might also be certified andtraceable to the national standard of the US.
Among African countries, only Egypt has a radon
calibration facility using the NIST 226Ra source74). The
National Institute of Standards of Egypt (NIS) constructed
a radon calibration chamber made of stainless steel witha volume of 0.05 m3. Temperature in the chamber can becontrolled in the range of 0–70 by a temperature control
system. However, they did not mention the range ofachievable radon concentration.
4. Discussion
4.1. Geographical characteristics
There are many calibration facilities in Europe, andsome facilities own a primary standard of radon, which
establish a robust traceability chain in Europe. It isbecause many countries in Europe have a national
regulation or a reference level for radon gas concentration
due to a relatively high concentration compared withother countries75). Under such circumstances, there wasa project to enable the traceable monitoring of radon at
low radon activity concentrations including calibration
and radon mapping which is named MetroRADON. They
conducted an interlaboratory comparison for European

19radon calibration facilities to evaluate the establishment ofa harmonized quality level for the activity concentrationof radon in air and to demonstrate the performance of the
facilities.
In Asia, there are many calibration facilities, however,
only Korea has a primary standard. Many facilities
periodically calibrated their secondary standard
measuring device to establish the traceability or
conducted an intercomparison experiment among Asian
countries76). Janik reported that the NIRS in Japan has
calibrated the secondary standard device at the KRISS as
well as the BfS29).
In North America, there is only one primary standard,
which is the NIST 226Ra standard in the US. The
secondary standard measuring devices of the facilities
in the US as well as in Canada are traceable to the NIST
standard. On the other hand, there is a limited number ofthe calibration facilities among Latin American countries
while a number of mines where radon concentration tends
to increase are in operation72). The detail information on a
traceability are not available, however, it was reported atleast that the traceability to the NIST standard was kept
at the Instituto Nacional de Investigaciones Nucleares(ININ) in Mexico73). In Africa, there is only one reportof the calibration facility at the National Institute forStandard (NIS) in Egypt74).
Overall, two problems could be pointed out from viewof the geographical characteristic. First, a distribution offacilities owning a primary standard is biased. There are
only two primary standard facilities in areas other than
Europe: the NIST in the US and the KRISS in Korea. Ifthey cease the service, many facilities in the areas must
depend on the primary standard in Europe to keep the
metrological traceability, which sometimes requires
a long-distance transportation. Second, there is only alimited number of calibration facilities in Latin Americanand African countries although they have active mines
and caves which might make a large contribution to theradiation dose due to inhalation of radon isotopes andtheir progenies. Especially in Africa, there is only one
calibration facility in Egypt. A traceability chain does
not appear to be established in these areas although thenumber of radon survey areas are recently increasing.
4.2. Dif ferences in techniques used
Ma n y di f f erent t echniques w ere used a nd t hespecifications varied widely at each facility. In thissection, the differences in types of operation and radonsources are briefly discussed for a better understandingof characteristics of radon calibration.
Three different operation systems to provide radongas were used in each facility: an accumulation, a flow
through and a spot injection. Figure 2 illustrates typical
expected trends of radon concentration for each operation

20

Yuki Tamakuma et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 12–24

the loop, λd and λv are a decay constant of radon and aleakage rate from the chamber or the loop, respectively.
It is assumed that there is no radon gas at the beginning(t = 0). As seen in Figure 2(a), it takes long time
(approximately 25 d) to reach a stable concentration. The
equation also shows that only the decay constant andthe leakage rate have an influence on the time to reach amaximum concentration.
In the case of the flow through mode where radon gas
flows into the chamber at a constant entry rate I and the
air go out at a certain flow rate v, the radon concentrationin the chamber are expressed as the equation (2).
Iv
C = ―――――――(1−e −(λd +λv + −v) t)v(λd + λv + ―) V
V

Fig. 2. Typical expected radon concentrations for each operation
mode: (a) an accumulation, (b) a flow-through, and (c) a spot injection.
The concentrations were calculated using simplified models. Cmax is a
maximum radon concentration.

Figure 2(b) illustrates the radon concentration in thechamber for the flow through mode. It is also assumed
that there is no radon gas at the beginning (t = 0). In
addition to the decay constant and the leakage rate, aratio of the flow rate to the volume of the chamber is
include as the influential factor on the time until reaching
the stable concentration. In general, the time strongly
depends on the ratio because the decay constant and theleakage rate are usually small enough compared with theratio. In case of the University of Huelva, it takes around 3
h according to their model calculation. Also, it was several
hours to reach stable in case of Hirosaki University47, 61).
At last, in the case of the spot injection mode, a radongas accumulated in a certain volume of chamber would
be injected once at the beginning of the exposure. Since
there is no additional supply of radon gas, the radon
concentration exponentially decreases with time by
the radioactive decay and the leakage, as shown in thefollowing equation.
C = C o e −(λd +λv) tmode calculated using simplified models. In the case of
the accumulation method, a radon source is placed in thesame chamber as monitors installed or in a closed loopwith monitors. If the emission rate of radon gas fromthe source I is constant, the radon concentration in thechamber or the closed loop C can be expressed as thefollowing equation:I
C = ―――――― (1−e −(λd +λv) t)(λd + λv) V(3)

where t is the elapsed time from the beginning of theaccumulation, and V is a volume of the chamber or

(4)

(5)

Here, Co is an initial radon concentration in the chamber.
The radon concentration by the spot injection method isshown in Figure 2(c).
As seen in Table 1, a relationship between theoperation mode and the radon concentration was unclear
due partly to the difference of activity of radon sources
used. However, Pornnumpa et al. have reported the radon
exposure systems of flow through and spot injection using
the same radon source, and the maximum concentration
for flow through method and the initial concentrationof spot injection method were around 1,000 and 10,000
Bq m-3, respectively61). Note that this initial concentration
was observed when radon gas accumulates for one
day. Even if the accumulation period was quite short

Yuki Tamakuma et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 12–24

compared with that to reach stable (~25 d), the radon
concentration of spot injection method was higher thanthat of flow through method. In addition, the equation (1)and (2) indicate that the maximum radon concentrationof accumulation method is obviously higher thanthat of flow through method. To reveal a relationshipbetween maximum concentrations for accumulation andspot injection, a simple gas transfer model was madeunder following assumptions; radon gas accumulates
at a constant rate I in the chamber with a volume ofV 1 for a period enough to reach stable, and a certain
volume V S of radon gas is collected and injected into the
exposure chamber with a volume of V 2. The maximum
radon concentration in the accumulation chamber C1 isexpressed as the equation (6) from the equation (3).
IC1 = ――――――(λd + λv) V 1(6)

Thus, the initial radon concentration in the exposure
chamber C2 can be expressed as following:VS
C2 = ―― C1.
V2(7)

Note that V S cannot exceed V 1. C 2 would become
maximum value at V 1 = V S = V 2, however, the volume
of exposure chamber may generally exceed that ofaccumulation chamber. In fact, there are many largevolume walk-in type chambers26, 36, 49). Thus, the maximum
concentration for the accumulation method would be
usually higher than that for the spot injection method.
Therefore, it was found that the maximum concentrationsof each opera t ion mode were high in order ofaccumulation mode, spot injection mode and flow throughmode when the same radon source was used. On theother hand, it was also found that it takes much longer
time to reach stable concentration for the accumulation
method (~25 d) than for the flow through method (several
hours) using the simplified models.
For types of radon sources, many institutes anduniversities use a 226Ra source while a limited numberof institutes use natural soils or uranium ores assummarized in Table 1. Most of the 226Ra sources
used are the dry flow-through type 226Ra source which
generate radon gas with almost 100% of emanation
power. The dry source made at the Czech Metrology
Institute was used in Bulgaria, China, Czech Republicand Slovak Republic. The Pylon radium source wasalso used in many countries such as Australia, Korea,
Poland, Spain and Syria. The facilities in these countries

21

are capable of making a high radon concentration up to655,000 Bq m-3 whereas the others using natural soils or
uranium ores as a source can make the concentration
up to only 31,000 Bq m-3 56, 68). It depends on the activityof 226Ra included in such soils and ores. Also, it is partly
attributed to the emanation power from the source.
The flow-through type 226Ra source is optimized togenerate radon gas, however, radon gas cannot beeasily exhaled from soils or ores into the air. Moisture
content also affects the amount of exhalation becauseradon released from grains and trapped in water at porecan relatively easily emanate while high water content
prevents radon from flowing to the atmosphere 77, 78).
There are many factors such as a porosity, a moisture
content, a density and a 226Ra activity related to theexhalation of radon, which is why radon concentration in
the chamber using soils or ores are relatively lower thanthat of the 226Ra source.
Table 1 also shows that many facilities can expose tohigh radon concentration, however, only a few facilities
can make a low level of radon atmosphere which ismost likely to exist in living environments. A world
average indoor radon concentration has been reported
to be 40 Bq m-3 while the Japanese mean value was 15.5
Bq m-3 79, 80). Also, low radon concentrations in outdoor
environments are measured using a high sensitivity radon
monitor such as an electrostatic collection type monitor
and a two-filter detector so as to use radon as a tracer foratmospheric transportation81). Although a lower level ofdetection of these monitors are several tens mBq m-3, thecalibration facilities cannot serve such low concentrations.
For indoor radon measurements, the calibration at low
concentration is not required because a passive timeintegrated radon monitor is generally used. The qualityof measurand of the monitors employed for atmospherictransportation is thus assured by intercomparisons. Under
such circumstances, a new metrology for radon at theenvironmental level was developed using a new source
integrated with a detector to establish the traceability
chain and ensure high quality radon concentrations andradon flux measurements82, 83). Although the metrology
is innovative and useful, it requires a research-based
technology to make the source so far. Therefore, othermetrologies of radon at a low-level concentration should
also be studied for establishing a more robust quality
assurance in the future.
5. Conclusion
This article summarized a status of traceability of radonmeasurement, characteristics and techniques of radon
calibration facilities around the world. It was found thatthe distribution of facilities owning a primary standard
was quite biased. There were many calibration facilities

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Yuki Tamakuma et al. / Radiation Environment and Medicine 2023 Vol.12, No.1 12–24

in Europe and some projects were launched for arobust traceability chain, although a traceability chain
in other regions was relatively weak. Three different
techniques for operation of radon chamber were used: an
accumulation method, a flow through method and a spot
injection method. Using simplified models, it was foundthat radon concentration in the chamber may be highin order of the accumulation method, the spot injection
method and the flow through method when the same
source was used. Also, duration until reaching stable wasquite shorter in the flow through method than that in theaccumulation method. For types of sources, it was foundthat most of the facilities use a dry 226Ra source optimized
for radon generation whereas others use natural soils
or uranium ores. Maximum radon concentrations in theradon chamber were observed in facilities using the226
Ra source due partly to the difference of emanation
power. The radon calibration at a low-level concentration
should be studied in the future for high sensitivity
radon monitors which is employed for tracer studies ofatmospheric transportation. The authors hope that thesefindings are utilized for design and development of new
radon chambers.

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Conflict of interest
The authors declare that they have no conflict of interest.
Author contributions
Conceptualization, S.T.; Data acquisition and interpretation,
Y.T.; Supervision, C.K., M.H., and S.T.; Validation, Y.T.,
M.H., C.K. and S.T.; Writing—original draft, Y.T. and C.K.;
Writing—review and editing, Y.T., M.H., C.K., and S.T. All
authors have read and agreed to the published version of themanuscript.
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