Radiation Environment and Medicine 2022 Vol.11, No.2 50–55

Regular Article

Thoron Equilibrium Factor Observed around Chhatrapur Placer
Deposit, a High Background Radiation Area in Odisha, India
Yasutaka Omori1, Ganesh Prasad2, Devulapalli Vidya Sagar3, Sarata Kumar Sahoo4,
Atsuyuki Sorimachi5, Miroslaw Janik4, Tetsuo Ishikawa6,
Rakesh Chand Ramola 2 and Shinji Tokonami1*
1
Hirosaki University, 66-1 Honcho, Hirosaki, Aomori 036-8564, Japan
H.N.B. Garhwal University, Badshahi Thaul, Tehri Garhwal 249 199, India
3
Indian Rare Earths Limited, Ganjam, Odisha 761-045, India
4
National Institutes for Quantum Science and Technology, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
5
Toyo University, 2100 Kujirai, Kawagoe, Saitama 350-8585, Japan
6
Fukushima Medical University, 1 Hikarigaoka, Fukushima 960-1295, Japan
2

Received 9 May 2022; revised 1 June 2022; accepted 6 June 2022

In thoron (220Rn)-prone areas, the contribution from thoron (including progenies) can be equal toor exceed that from radon (222Rn) during radiation exposure. The dose estimation is practically
performed using the thoron concentration and thoron equilibrium factor. In the present study,
thoron equilibrium factors were determined from direct measurements of thoron and its progeny
concentrations for dwellings in the high background radiation area of Odisha, India. The results
show that the equilibrium factor has a seasonal variation, with a minimum in winter and amaximum in summer. The frequency distributions exhibit log normality with geometric meansof 0.025, 0.044, and 0.051 in winter, rainy, and summer-seasons, respectively. The annual averageof 0.04 is observed to be in the order of the United Nations Scientific Committee on the Effects ofAtomic Radiation recommended value (i.e., 0.02).
Key words: radon, thoron, thoron progeny, equilibrium factor, inhalation dose, high backgroundradiation area

1. Introduction
Radon isotopes (radon ( 222Rn) and thoron ( 220Rn)),chemically inert radioactive gases that are generally
abundant in the living environment, have an importantrole in radiation exposure among natural radiation
sources. According to the estimation made by the*Shinji Tokonami: Hirosaki University, 66-1 Honcho, Hirosaki, Aomori 036-8564,
JapanE-mail: tokonami@hirosaki-u.ac.jp
https://doi.org/10.51083/radiatenvironmed.11.2_50
Copyright © 2022 by Hirosaki University. All rights reserved.

United Nations Scientific Committee on the Effects ofAtomic Radiation (UNSCEAR)1), the dose received frominhalation of radon, thoron, and their progenies dominates
approximately 50% of the global average annual radiationdose (1.2 mSv vs. 2.4 mSv). In particular, the contributionof the dose from radon is estimated to be up to 90% andthe dose from thoron is small as thoron concentration islower due to the difference in their half-lives (3.8 d forradon and 55.6 s for thoron). However, recent surveys
performed in the past decade have revealed the existenceof a thoron-prone area where doses from thoron can beequal to or exceed those from radon2-7). In addition, someYasutaka Omori et al./ Radiation Environment and Medicine 2022 Vol.11, No.2 50–55

researchers8-9) proposed a dose conversion factor forthoron, which was based on the latest human respiration
and bio-kinetic models and was three times higher thanthe dose conversion factor adopted by the UNSCEAR.
Thus, exposure to thoron has been recognized as animportant factor.
Thoron progenies are responsible for lung exposure tothoron in the dose estimation. However, the measurementof thoron progeny concentration is generally complex
compared to that of thoron gas concentration using thepassive technique. In sequence, the measured thoron
concentration is multiplied by the thoron equilibrium
factor to estimate the equilibrium equivalent thoron
concentration as the thoron progeny concentration1, 10-15).
It represents the degree of equilibrium between thoron
and its progeny ( 212Pb and 212Bi)16). The UNSCEAR
report determined a typical value of 0.02 for the thoron
equilibrium factor1). In contrast, thoron equilibrium
factors compiled from regional surveys with long-term
measurements differed by three orders of magnitude
among the dwellings, although representative values(arithmetic or geometric mean) were in the order of
the UNSCEAR computed value17, 18). These long-term
measurements were generally performed over several
months and were not completed within a year19-22).
We conducted a thoron and its progeny survey ofapproximately 100 dwellings in the southeastern coastal
area of India from 2010 to 20115, 23-24). The meassurements
were obtained for approximately four months andrepeated three times. In the present study, based on the
database of thoron and its progeny concentrations, thoron
equilibrium factors were calculated and their variation
over seasons was examined.
2. Materials and methods
2.1. Outline of location and conducted survey
The thoron and its progeny survey was conducted in the
Chhtrapur placer deposit area along the southeastern
coast of Odisha (Orissa), India (location map present inOmori et al.5)). The absorbed dose rate in air in the area
is up to approximately 5000 nGy h-1; therefore the area is
identified as a high background radiation area25, 26). For
the survey, 106 dwellings (100 cement constructions and6 mud constructions) were selected from 12 villages inthe study area. A pair of thoron and its progeny detectors
were installed on the wall (a few centimeters from thewall surface) or the middle of the room in each dwelling.
The exposure period was set to be approximately fourmonths, and the measurements were repeated threetimes (first period: mid-October 2010–early February 2011
(winter); second period: mid-February–early June 2011
(summer); and third period: mid-June–mid-October 2011
(rainy season)) to examine the differences throughout the51

year. Details of the survey can be found in Omori et al.5)
2.2. Measurement of thoron concentration
The thoron concentrations were measured using solidstate nuclear track detectors enclosed in diffusion
chambers, RADUET (Radosys Co. Ltd., Hungary). A
combination of chambers with different air exchange
rates can measure radon and thoron concentrations
separately27). A solid-state nuclear track detector CR-39
with dimensions of 10 mm×10 mm (Radosys Co. Ltd.,
Hungary) was used to record tracks of alpha particles
from radon, thoron, and their progeny. The RADUETs
were calibrated using radon and thoron chambers
established by the National Institute of Radiological
Sciences (NIRS; currently National Institutes for Quantum
Science and Technology)28, 29).
The RADUET was positioned on the wall or in the
middle of the room in each dwelling to measure theaverage thoron concentration over approximately fourmonths indoors. After the exposure was completed,
the RADUETs were processed in NIRS for chemical
etching of the CR-39 chips, counting the number oftracks registered on the chips, and evaluating thoron
concentrations. The CR-39 chips were etched in 6.25 M
NaOH solutions at 90 °C for 6 h. The number of tracks
formed on the chips was counted using an automatic
reading system, Radometer2000 version 1.43 (Radosys Co.
Ltd., Hungary), and it was divided by the analyzed area todetermine the track density. Thoron concentration was
estimated from the exposure period, track density, andcalibration factors for thoron and radon exposure5).
2.3. Measurement of thoron progeny concentration
Thoron progeny concentrations were measured using
a deposition-based technique30, 31). The thoron progeny
detector used in the present study has the function ofalpha-energy discrimination to detect alpha particles
from one of the thoron progenies 212Po (8.8 MeV) only.
CR-39 chips with dimensions of 10 mm×10 mm (Nagase
Landauer, Ltd., Japan) were mounted on a stainless steel
plate and covered with two-layered Mylar films. The
thickness of the Mylar films was adjusted such that alphaparticles from 212Po could pass through the film and reachCR-39.
The thoron progeny detector was positioned on thewall or in the middle of the room together with RADUET
over approximately four months. After the exposure
was completed, the thoron progeny detectors were
processed in NIRS for chemical etching of the CR-39
chips, counting the number of tracks registered on thechips, and evaluating the thoron progeny concentrations.
The CR-39 chips were etched in 6 M NaOH solutions at
60 °C for 24 h. The number of tracks formed on the chips
was counted using image capture and analysis. In the52

Yasutaka Omori et al./ Radiation Environment and Medicine 2022 Vol.11, No.2 50–55
Table 1. Statistical values of thoron equilibrium factor during the measurement periodsab

SeasonNa

Winter62

Summer

76

Rainy

41

Thoron equilibrium factor
AM±SDb

GM (GSD)b

Range

0.030±0.019

0.025 (1.89)

0.006 - 0.087

0.066±0.059

0.051 (1.99)

0.016 - 0.42

0.056±0.044

0.044 (2.05)

0.012 - 0.24

N means the number of dwellings for analysis.
AM, SD, GM and GSD mean arithmetic mean, standard deviation, geometric mean
and geometric standard deviation (no unit), respectively.

image capture, ten photographs (approximately 17.7 mm2in total) of the formed tracks were taken manually forevery etched chip at 100×magnification with an optical
microscope and camera (Model LBV-A570, KENIS Ltd.,
Japan). Image analysis was performed to count the tracks
captured on the digital photographs using CellProfiler 2.0,
software developed by Carpenter et al.32) The equilibrium
equivalent thoron concentration was estimated from theexposure period, track density, and a calibration factor forthoron progeny exposure5).
2.4. Analysis
Thoron equilibrium factor (F Tn) was calculated from thethoron concentration (C Tn) and equilibrium equivalentthoron concentration (E ETC ) determined by the
RADUET and thoron progeny detector, respectively, as
follows:
EETC
F Tn = ———— .
C Tn

(1)

At three of the 106 dwellings investigated, the RADUET
and thoron progeny detector were deployed in the middleof the room. Data obtained in these cases were excluded
from the analysis.
Statistical and regression analyses were performed
using the software OriginPro 2020 (OriginLab Corporation).
Statistical significance was set at a level of 0.05.
3. Results and discussion
Thoron equilibrium factors were evaluated successfullyin 62, 76, and 41 dwellings in the first (winter), second(summer), and third (rainy season) periods, respectively.
In 24 dwellings, the total measurement period covered ayear. Thoron equilibrium factors for some dwellings were
not evaluated because the thoron concentrations werelower than the detection limits for the first and second
periods. The smaller number of samples in the third
period was because of the smaller number of investigateddwellings.
Figure 1 shows the distributions of the thoron
equilibrium factor in the form of cumulative probability

Fig. 1. Distribution of thoron equilibrium factor in the form of
cumulative probability on a logarithmic scale. Fitted (dotted, dashed,and solid) lines are drawn by estimating ideal log-normal distributions
from the geometric mean and geometric standard deviation of thethoron equilibrium factor.

on a logarithmic scale. The Shapiro–Wilk test for theequilibrium factor after logarithmic transformationsuggests that it had a lognormal distribution in all
periods (p -values were 0.79, 0.14, and 0.59 for datasets inwinter, summer, and rainy seasons, respectively). Thiswas evident from the regression analysis for the plots in
Figure 1, which shows that a line drawn under an ideal
lognormal distribution fitted well.
The statistic results for the thoron equilibrium factors
are summarized in Table 1. In the summer period, thethoron equilibrium factors were distributed in a range of
0.016–0.42 with arithmetic and geometric means of 0.066
and 0.051, respectively. The thoron equilibrium factors
differed by one order of magnitude in the minimum andmaximum values. A similar characteristic was found in
the dataset for the rainy period, where the equilibrium
factors were distributed in the range of 0.012–0.24 witharithmetic and geometric means of 0.056 and 0.044,
respectively. In contrast, all the equilibrium factors in
the winter period were in the range of 0.006–0.087, witharithmetic and geometric means of 0.030 and 0.025,

Yasutaka Omori et al./ Radiation Environment and Medicine 2022 Vol.11, No.2 50–55

53

Table 2. Summary of seasonal variations in thoron equilibrium factors
Location

Thoron equilibrium factor
Arithmetic mean±standard deviation (range)
Dry season

Rainy season

0.25±0.05 (0.02 - 0.85)

Lolodorf, Cameroon

6)Detector position(distance from wall)
0.09±0.01 (0.01 - 0.21)

WinterSummerRainy

Uttarkhand, north India33)

0.04 (0.01 - 0.80)

0.03 (0.01 - 0.27)

0.04 (0.01 - 0.63) 0.04 (0.01 - 0.53)

WinterSummerRainy

Uttarkhand, north India34)

0.06±0.05 (0.01 - 0.21)

0.06±0.05 (0.01 - 0.31)

0.09±0.06 (0.02 - 0.36)

20 cm

Autumn
10 cm
10 cm

Uttar Pradesh, north India

0.01±0.01 (0.005 - 0.04) 0.01±0.01 (0.005 - 0.036) 0.01±0.01 (0.005 - 0.032)

30 cm

Punjab, north India36)

0.04 (0.01 - 0.11)

30 - 50 cm

Punjab, north India

0.05±0.03

35)

37)

0.04 (0.01 - 0.11)

0.05±0.04

0.04±0.03

20 cm

0.04±0.08 (0.005 - 0.09)

0.03±0.02 (0.01 - 0.07)

10 - 20 cm

0.03±0.02 (0.006 - 0.09) 0.07±0.06 (0.02 - 0.42)

0.06±0.04 (0.01 - 0.24)

On wall

Jammu and Kashmir, north India38) 0.03±0.02 (0.01 - 0.09)
Odisha, east India (Present study)

0.03 (0.01 - 0.08)

Fig. 2. Distributions of thoron equilibrium factor ratios amongsummer, rainy, and winter seasons. Boxes are bounded at 25
percentile value, median value, and 75 percentile value of the ratiosfrom lower to upper, respectively. Open squares represent thearithmetic mean.

respectively. Compared with the summer and rainy
periods, the maximum thoron equilibrium factor was
smaller by one order of magnitude in the winter period.
The annual means for the 24 dwellings were in the rangeof 0.01–0.16 with an arithmetic mean of 0.04.
The thoron equilibrium factors reported in the present
study agreed with those summarized from the results ofregional surveys by Hosoda et al.17) and Chen et al.18) They
reported wide ranges of thoron equilibrium factors from
0.0001 to 0.1. Similarly, the present study reports a wide
range of thoron equilibrium factors. However, they wereless than or in the order of the value recommended by the
UNSCEAR (0.02)1) for 100%, 80%, and 90% of the cases inwhich thoron equilibrium factors could be determined in
winter, summer, and rainy seasons, respectively (Fig. 1).

The thoron equilibrium factors showed seasonalvariation. In other words, those in winter appeared lower
than those in the other seasons. To confirm the seasonalvariation, ratios of the equilibrium factors were calculated
between the seasons for the 24 samples, that is, ratiosfrom summer to winter, rainy to winter, and summer
to rainy. Figure 2 shows the distributions of the ratiosof the thoron equilibrium factors between the seasons.
The thoron equilibrium factors in the summer and rainyseasons relative to those in the winter season were higher
than unity for most samples. In other words, the thoron
equilibrium factors in winter were generally lower thanthose in the other seasons, which is expected from theresults in Figure 1. Similarly, the thoron equilibrium
factors in the rainy season were slightly lower than thosein the summer.
Table 2 summarizes the seasonal variations in thethoron equilibrium factor reported in previous studies33-38)
and the present study. This clearly shows that thethoron equilibrium factors do not exhibit typical seasonalvariations. In detail, the equilibrium factors were higher
in the colder seasons in some cases, higher in the warmer
seasons in other cases, and constant throughout theseasons in other cases. Ideally, the thoron equilibrium
factor decreases with an increase in the ventilation rate39, 40).
The equilibrium factor is expected to be higher in colder
seasons due to the poor ventilation of the room air.
However, such seasonal variation was rarely observedin previous studies33-38) and in the present study. Thisis partly because the behavior of thoron in room air isvery complicated due to its short half-life. In particular,
in the colder season, room air does not mix well due topoor ventilation. In this situation, the spatial distributionof thoron concentration becomes more heterogeneous,and the concentration gradient increases significantly
at locations close to the thoron source (i.e., walls, floors,

54

Yasutaka Omori et al./ Radiation Environment and Medicine 2022 Vol.11, No.2 50–55

and ceilings). This phenomenon reduces the thoron
equilibrium factor if the thoron progeny concentrationis constant. To better understand seasonal variation in
the thoron equilibrium factor, together with thoron andits progeny concentrations, aerosol concentration shouldbe measured, which has a role in changes in the thoron
equilibrium factor.

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17.

Hosoda M, Kudo H, Iwaoka K, Yamada R, Suzuki T, Tamakuma Y,
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18.

Chen J, Harley NH. A review of indoor and outdoor radon
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19.

Chen J, Moir D, Sorimachi A, Tokonami S. Characteristics ofthoron and thoron progeny in Canadian homes. Radiat Environ
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Chen J, Moir D, Sorimachi A, Janik M, Tokonami S. Determinationof thoron equilibrium factor from simultaneous long-term
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21.

Janik M, Tokonami S, Kranrod C, Sorimachi A, Ishikawa T,
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22.

Omori Y, Tokonami S, Sahoo SK, Ishikawa T, Sorimachi A,
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4. Conclusion
The main findings of the present study are summarized
as follows:
Thoron equilibrium factors followed a log-normal
distribution with geometric means of 0.025, 0.044,
and 0.051 in the winter, rainy, and summer seasons,respectively;
In 80–100% of the cases for each of the three seasons,thoron equilibrium factors showed values lower than orin the order of the value recommended by UNSCEAR(0.02)1);
Thoron equilibrium factors showed seasonal variation
with a minimum in winter and maximum in summer;
Typical seasonal variation for the thoron equilibrium
factor could not be determined based on comparisonswith previous studies and the present study.
Acknowledgement
The authors would like to thank the reviewers for theirvaluable comments in improving the manuscript.
Funding
This work was partly supported by the grant-in-aid“Construction of Natural Radiation Exposure Study
Network” from the Special Coordination Funds for
Promoting Science and Technology of the Ministry of
Education, Culture, Sports, Science and Technology,
Japan.
Conflict of Interest
The authors declare that they have no conflict of interest.
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