Long-term simultaneous measurement of indoor concentration of radon and thoron progeny in the dwellings of Una and Hamirpur District of Himachal Pradesh

Abstract

Long-term simultaneous measurement of indoor concentration of radon and thoron progeny is made using passive alpha track detectors in the dwellings of Una and Hamirpur district of Himachal Pradesh. The preliminary results of these measurements are reported, particularly regarding direct thoron progeny sensor (DTPS) and direct radon progeny sensor (DRPS) being set up in real ﬁeld conditions for the ﬁrst time in Himachal Pradesh. The concentrations of radon and thoron progenies (equilibrium equivalent radon and thoron concentration (EERC/EETC)) were measured for all the seasons of the year. A remarkable ﬁnding is the positive correlation between EERC and EETC (R²= 0.79 in summer and R²= 0.81 in winter). A comparison was made with the measurements of EERC/EETC in summer and winter season. The ratio of EETC/EERC was calculated and this agrees with worldwide range. This study contributes to the status of Himachal Pradesh in the mapping of radon in India.

Keywords

DRPS/DTPS EERC EETC Nuclear track detector Radon progeny

Introduction

Radon (²²²Rn) and thoron (²²⁰Rn) are continuously decaying and giving rise to radon and thoron progenies. These are short-lived and decay until a stable long-lived isotope is reached. Radon (²²²Rn), thoron (²²⁰Rn) and their progenies are considered to be the major contributors to human exposure from natural sources.¹ The short-lived daughters of radon (²²²Rn) and thoron (²²⁰Rn) are well established as causative agents of lung cancer.^2–5 These are mostly radioactive isotopes of Po, Pb and Bi which when produced in the air attach to the aerosol particles that are present. Although radon and its progenies are the main contributors in inhalation dose for the general public, thoron has gained increasing attention among health physicists in recent years. The short lifetime of thoron (half-life, 55.6 s) and its inhomogeneous distribution, with a strong dependence on the distance from the source, are responsible for this. However, there are some longer lifetime isotopes among the thoron progenies that are capable of spreading in the air by sticking to aerosol particles, and establishing a more or less homogeneous concentration distribution; therefore, the radiation dose contribution is not negligible. The values of radon and thoron progeny in the indoor air depends mainly on the pseudo-ventilation rates, which is the sum of the air exchange rate and the wall removal rate in the dwelling. These are expected to vary from house to house and from time to time in a given location. The nuclear track detector technique is the most reliable method for the integrated and long-term measurement of indoor radon activity.⁶ With the advent of passive detection techniques using solid state nuclear track detectors (SSNTDs), direct progeny sensor (DPS) has been developed. Direct radon/thoron progeny sensor (DRPS/DTPS) is a passive, deposition-based technique for estimating the time-integrated equilibrium equivalent radon/thoron concentration (EERC/thoron) in indoor environment. Thoron cannot be assumed to be uniformly distributed in the room due to its short half-life.⁷ Thoron decay products, being longer lived would mix more or less uniformly in the room and their activities will be fractions of a representative average thoron concentration.⁸ The concentrations of radon, thoron and progeny deviate from place to place and depend on the building material in case of indoor concentration. In India, indoor radon measurements in some dwellings of 15 towns were specially selected on the basis of high uranium content in the soil.⁹ Radon measurements are relatively simple to perform; however, to assess radon concentrations in homes, the measurements need to be based on standardized protocols to ensure accuracy and consistency. Radon and thoron progeny concentrations contribute to the natural radiation dose to general public. Due to the fact that the inhalation doses due to radon and thoron are contributed predominantly by their decay products, development of passive techniques for monitoring the decay products directly assumes considerable significance. DTPS and DRPS are developed which are deposition-based systems and hence ensure that they respond only to the decay products and not to gas concentrations.¹⁰ The lower limit of detection for the DRPS is 1 Bq m⁻³ of EERC and that for DTPS is 0.1 Bq m⁻³ of EETC.

The measurements of radon/thoron progeny concentrations were conducted for the first time with DTPS and DRPS in Hamirpur and Una district of Himachal Pradesh. In these two districts, approximately 28 houses were selected and DRPS/DTPS sensors were installed. The present work will help in understanding the status of indoor radon and thoron progeny in the environs of Una and Hamirpur districts of Himachal Pradesh. The present study was done to measure radon and thoron daughter progenies to allow a record of indoor emanations during different seasons of the year.

Method of measurement

In the present experimental work, for indoor measurement, DRPS/DTPS sensors and wire mesh DRPS/DTPS sensors were used. These are made of passive nuclear track detectors (LR-115) mounted with absorbers of appropriate thickness.

From the model estimates, ∼70% of the total deposition ﬂux is induced by 2–4% of the 212Pb ﬁne fraction, while the remaining 30% is due to the coarse fraction. The average activity median diameter of the ﬁne fraction is ∼5 nm and that for the coarse fraction is ∼100 nm. An increase in the turbulence would directly increase the ﬁne fraction deposition velocity, but the effect would be less pronounced on the coarse fraction deposition velocity. Hence, a wire mesh-capped DTPS has been developed, in which the wire mesh will act as a ﬁne-fraction separator. When the DTPS is capped with the wire mesh, the coarse fraction penetrates the wire mesh and gets deposited on the DTPS. The tracks registered in the DTPSs are due to the alpha particles emitted by the coarse fraction-attached progeny atoms.

The basic principle of operation of these DRPS/DTPS sensors is that the LR-115 detector detects the alpha particles emitted from the deposited progeny atoms. DRPS is an absorber-mounted LR-115-type nuclear track detector tuned to respond only to the 7.67 MeV alpha particles emitted from the deposited activity of ²¹⁴Po on the absorber surface. DTPS is an absorber-mounted LR-115-type nuclear track detector tuned to respond only to the 8.78 MeV alpha particles emitted from the deposited activity of ²¹²Po on the absorber surface. DTPS element is made up of LR-115 (2.5 × 2.5 cm²) mounted with 50 µm aluminized mylar while DRPS has an absorber combination comprising of aluminized mylar and cellulose nitrate of effective thickness of 37 µm. This thickness mainly ensures that lower energy alpha emissions (from the gases and other airborne alpha emitters) do not pass through the absorber.¹¹ Since the system is intended for use in the deposition mode, it is necessary to avoid uncontrolled static charges from affecting the deposition rates and hence aluminized side of the mylar was chosen to act as the deposition surface.¹²

Sensors were hanged overhead on the ceiling at the height of minimum 1.5 m from the floor and at least 10 cm away from any surface for a minimum of 90 days. The exposed sensors were taken for analysis and replaced with new ones for the next deployment. The exposed films were then etched in an etching bath using 2.5 N NaOH solution at 60℃ for 90 min during which 4 µm thickness was etched away. Etching of the film was necessary because the tracks produced on the films would be visible for counting only after etching. The tracks recorded in this SSNTD films were then counted using a spark counter, which is an electronic counter operating on high voltage. The tracks recorded in the exposed LR-115 films related to equilibrium equivalent progeny concentration (EEC) using the sensitivity factor. To have seasonal variations in concentrations of these progeny particularly during autumn, spring, winter and summer season in a year, these sensors were exposed in each season for about three months. The number of windows, ventilations and doors were also counted; these contribute to the ventilation rate of air inside the specific room.

Indoor radon concentration is greatly influenced by the ventilation rate. During different seasons, the temperature would vary and would result in a change in ventilation rate of dwellings. Hence, seasonal variation of indoor radon as well as thoron was monitored. These detectors were employed in 28 different locations of Una and Hamirpur districts of Himachal Pradesh in bare mode and in wire mesh cup mode for a period of about 3 months. In this study, the monitoring was completed in the year 2012–2013 which was divided into four seasons viz. summer (June–Aug), autumn (September–November), winter (December–February) and spring (March–May).

In the present work, the time-integrated equilibrium equivalent radon/thoron progeny concentration (EERC/EETC) in bare mode and in wire mesh cup mode was calculated. In the calculation of the progeny concentrations, the track density obtained using DTPS were used directly in calculating EETC. However, in the case of EERC, since a mixture of radon and thoron progenies was expected in the environment, α energy of ²¹²Po (thoron progeny) would be higher as compared to that of ²¹⁴Po (radon progeny). So, the track density obtained using DTPS was used to eliminate the interference on the DRPS by ²¹²Po. The number of tracks per unit area per unit time (T) was correlated to the EEC in air using the sensitivity factor (S). This was done by simple subtraction method. Equation (1) was used to calculate EETC and EERC^10,11 as follows

EEC ({Bqm}^{- 3}) = \frac{T (Tracks \cdot {cm}^{- 2} \cdot d^{- 1})}{S (Tracks \cdot {cm}^{- 2} \cdot d^{- 1} / EEC (Bq \cdot m^{- 3})}

(1)

The tracks registered in DTPS were related to EETC by using a sensitivity factor of 0.94 (Tracks cm⁻² d⁻¹) (Bq m⁻³)⁻¹. Similarly the sensitivity factor, 0.09 (Tracks cm⁻² d⁻¹) (Bq m⁻³)⁻¹ of DRPS was used to determine EERC. The tracks registered in wire-capped DTPS were related to the attached fraction of EETC by using a sensitivity factor of 0.33 (Tracks cm⁻² d⁻¹) (Bq m⁻³)⁻¹. Similarly, a sensitivity factor, 0.04 (Tracks cm⁻² d⁻¹) (Bq m⁻³)⁻¹ of DRPS was used to determine EERC.

Results

Table 1 shows the calculated values of EERC/EETC in summer and winter seasons using bare DRPS/DTPS and wire mesh-capped DRPS/DTPS. Using bare DRPS/DTPS, the value of EERC varied from 3.43 Bq m⁻³ to 26.62 Bq m⁻³ and from 4.30 Bq m⁻³ to 27.12 Bq m⁻³, respectively, in summer and winter, but the value of EETC varied from 0.27 Bq m⁻³ to 1.77 Bq m⁻³ and from 0.20 Bq m⁻³ to 1.77 Bq m⁻³, respectively, in summer and winter. Using wire mesh-capped DRPS/DTPS, the value of EERC varied from 3.34 Bq m⁻³ to 42.96 Bq m⁻³ and from 4.71 Bq m⁻³ to 42.69 Bq m⁻³, respectively, in summer and winter, but the value of EETC varied from 0.27 Bq m⁻³ to 3.70 Bq m⁻³and from 0.30 Bq m⁻³ to 3.70 Bq m⁻³, respectively, in summer and winter.

Table 1.

The calculated values of EERC/EETC in summer and winter seasons.

S.No.	Village	EEC in Batch DRPS	EEC in Batch DTPS	EEC in Wire mesh DRPS	EEC in Wire mesh DTPS	EEC in Batch DRPS	EEC in Batch DTPS	EEC in Wire mesh DRPS	EEC in Wire mesh DTPS
	Hamirpur	Summer				Winter
1	Garli	17.57	0.95	18.27	0.34	18.58	1.3	11.84	1.21
2	Bijhari	10.36	0.51	7.45	0.61	9.16	0.47	10.06	0.77
3	Dhaneta	22.01	1.32	37.47	1.41	19.76	1.23	39.42	1.41
4	Barsar	18.45	1.3	23.74	1.82	21.9	1.43	26.24	1.82
5	Bhota	13.12	0.46	9.33	0.67	13.65	0.54	14.33	0.67
6	Kangoo	3.43	0.27	3.77	0.67	7.72	0.31	5.72	0.67
7	Nadaun	18.85	0.53	23.09	1.08	19.84	0.53	27.81	1.08
8	Rangas	22.95	1.74	38.23	2.32	24.12	1.68	32.68	2.32
9	Salouni	12.92	0.66	7.76	0.57	15.89	0.9	11.93	0.57
10	Taunidevi	15.89	0.78	19.49	1.35	17.09	0.93	16.99	1.35
11	Bani	10.29	0.33	11.57	1.21	7.41	0.61	7.12	1.21
12	Awahdevi	26.62	1.77	42.96	3.7	27.12	1.77	42.69	3.7
13	Galore	17.34	1.3	35.92	1.03	18.71	1.29	36.2	1.03
	Una
14	Jowar	21.21	1.75	20.86	0.81	22.98	1.71	23.58	1.14
15	Lohara	5.52	0.28	5.95	0.44	4.3	0.27	5.02	0.54
16	Daulatpur	5.14	0.41	3.34	0.27	6.58	0.46	9.04	0.4
17	Gagret	13.92	0.9	8.67	0.77	9.29	0.34	16.99	1.35
18	Oela	4.05	0.27	8.06	0.27	4.61	0.2	12.2	0.3
19	Panjawar	6.88	0.53	3.46	0.71	7.6	0.67	4.71	0.84
20	Saloh	7.37	0.53	8.78	0.94	8.06	0.58	9.23	1.04
21	Haroli	7.23	0.54	12.06	0.44	9.05	0.46	16.26	0.4
22	Kuriala	4.97	0.34	6.54	0.4	5.72	0.45	8.97	0.47
23	Kotlakala	7.21	0.57	6.75	0.47	8.97	0.66	7.83	0.51
24	Bangana	9.32	0.56	10.2	0.91	10.44	0.54	14.82	1.01
25	Chowkimanyar	12.74	0.85	16.96	1.65	13.4	0.92	20.67	1.55
26	Malangar	7.65	0.5	7.34	0.44	10.46	0.53	8.48	0.4
27	Thathal	9.09	0.54	5.16	0.67	10.46	0.53	4.75	0.81
28	Amb	10.31	0.67	7.93	0.4	10.32	0.79	10.67	0.44

Table 2 shows the calculated values of EERC/EETC in autumn and spring season using bare DRPS/DTPS and wire mesh capped DRPS/DTPS. Using bare DRPS/DTPS, the value of EERC varied from 3.71 Bq m⁻³ to 23.77 Bq m⁻³ and from 2.93 Bq m⁻³ to 20.69 Bq m⁻³, respectively in autumn and spring, but the value of EETC varied from 0.21 Bq m⁻³ to 1.58 Bq m⁻³ and from 0.12 Bq m⁻³ to 1.52 Bq m⁻³, respectively, in autumn and spring. Using wire mesh-capped DRPS/DTPS, the value of EERC varied from 2.66 Bq m⁻³ to 37.10 Bq m⁻³ and from 2.74 Bq m⁻³ to 26.99 Bq m⁻³, respectively, in autumn and spring, but the value of EETC varied from 0.24 Bq m⁻³ to 2.63 Bq m⁻³ and from 0.20 Bq m⁻³ to 1.65 Bq m⁻³, respectively, in autumn and spring. Thus, the calculated value of EERC was higher than the value of EETC in bare and wire mesh mode. As the progeny can be fine or coarse in nature, the progeny (coarse) could attach the aerosols and this is known as attached progeny, whereas some of the progeny (fine) could not be attached to the aerosols and so is less harmful. This type of progeny is known as unattached progeny. In order to estimate the effect of unattached progeny of radon and thoron, the sensors in wire mesh cup mode was used. As per the measurements of the radon progeny, the radon progeny concentration of the studied area was lower than the maximum value of 134.40 Bq m⁻³.¹³ When compared with the study of UK, the present mean equilibrium equivalent ²²⁰Rn (EETC) concentration was higher and the mean equilibrium equivalent ²²²Rn (EERC) concentration was lower than the UK measurements.¹⁴ Also, the reported concentrations (Tables 1 and 2) were far lower than the arithmetic mean obtained in autumn–winter period of 117 Bq m⁻³ found in Italy (Friuli-Venezia Guilia).¹⁵ The high EERC values were observed in houses located at higher altitude. This could be because of possible higher aerosol loading coupled with decreased ventilation in these houses. The calculated values of ratio of EETC and EERC in summer and winter season were calculated and are shown in Table 3. Using bare DRPS/DTPS, the measurements ranged from 0.028 to 0.082 and from 0.027 to 0.089 with an arithmetic mean of 0.063 and 0.061 in summer and winter, respectively. Using wire mesh-capped DRPS/DTPS, the measurements ranged from 0.018 to 0.204 and from 0.025 to 0.179 with an arithmetic mean of 0.077 and 0.075 in summer and winter, respectively. The result agrees with worldwide range of 0.01–0.5.¹

Table 2.

The calculated values of EERC/EETC in autumn and spring seasons.

S.No.	Village	EEC in Batch DRPS	EEC in Batch DTPS	EEC in Wire mesh DRPS	EEC in Wire mesh DTPS	EEC in Batch DRPS	EEC in Batch DTPS	EEC in Wire mesh DRPS	EEC in Wire mesh DTPS
	Hamirpur	Autumn				Spring
1	Garli	17.06	1.09	12.2	2.53	13.79	1.03	8.2	1.25
2	Bijhari	8.24	0.4	13.99	1.01	7.25	0.4	11.89	0.61
3	Dhaneta	18.1	1.16	30.88	1.35	17.11	0.79	24.15	0.57
4	Barsar	20.66	1.31	20.12	1.55	17.47	0.92	15.79	0.88
5	Bhota	12.43	0.53	11.73	0.77	11.69	0.41	6.65	0.57
6	Kangoo	7.79	0.24	10.25	0.3	6.7	0.21	7.82	0.24
7	Nadaun	19.38	0.5	17.94	0.67	17.48	0.54	11.83	0.67
8	Rangas	21.77	1.56	30.71	2.63	20.2	1.52	22.52	1.65
9	Salouni	13.17	0.66	6.54	0.4	11.82	0.4	5.35	0.2
10	Taunidevi	15.75	0.79	15.52	1.14	14.54	0.77	11.79	0.71
11	Bani	7.74	0.53	7.42	0.91	5.65	0.27	2.74	0.88
12	Awahdevi	23.77	1.42	37.1	2.63	20.69	1.29	26.99	1.35
13	Galore	18.15	1.11	29.58	0.98	17.23	0.79	26.58	0.64
	Una
14	Jowar	21.13	1.58	17.94	0.67	19.2	1.42	18.21	0.4
15	Lohara	3.71	0.24	3.76	0.4	3.22	0.12	3.93	0.24
16	Daulatpur	6.38	0.41	3.03	0.3	5.26	0.3	3.13	0.2
17	Gagret	7.96	0.31	7.66	0.67	7.11	0.3	7.73	0.61
18	Oela	3.98	0.21	6.71	0.24	5.32	0.24	6.71	0.24
19	Panjawar	7.12	0.53	2.66	0.67	7.54	0.48	2.83	0.51
20	Saloh	7.62	0.53	6.04	0.91	5.18	0.38	6.37	0.57
21	Haroli	8.73	0.4	7.52	0.54	6.62	0.3	7.65	0.4
22	Kuriala	5.11	0.45	4.88	0.67	2.93	0.4	5.15	0.4
23	Kotlakala	7.01	0.27	5.4	0.44	5.33	0.22	5.4	0.44
24	Bangana	9.03	0.47	7.9	0.71	6.58	0.33	8.04	0.57
25	Chowkimanyar	13.04	0.79	14.68	0.88	10.3	0.44	14.68	0.88
26	Malangar	10.64	0.47	5.25	0.3	9.09	0.3	5.29	0.27
27	Thathal	10.96	0.4	4.49	0.51	10.53	0.21	4.66	0.34
28	Amb	8.8	0.71	5.22	0.34	6.99	0.41	5.32	0.24

Table 3.

The calculated value of ratio of EETC and EERC in summer and winter seasons.

S.No.	Village	EETC/EERC in summer using bare DRPS/DTPS	EETC/EERC in winter using bare DRPS/DTPS	EETC/EERC in summer using wire mesh-capped DRPS/DTPS	EETC/EERC in winter using wire mesh-capped DRPS/DTPS
	Hamirpur
1	Garli	0.054	0.07	0.018	0.102
2	Bijhari	0.049	0.052	0.081	0.077
3	Dhaneta	0.06	0.062	0.038	0.036
4	Barsar	0.07	0.065	0.077	0.069
5	Bhota	0.035	0.04	0.072	0.047
6	Kangoo	0.079	0.04	0.179	0.118
7	Nadaun	0.028	0.027	0.047	0.039
8	Rangas	0.076	0.07	0.061	0.071
9	Salouni	0.051	0.057	0.074	0.048
10	Taunidevi	0.049	0.055	0.069	0.079
11	Bani	0.032	0.083	0.105	0.17
12	Awahdevi	0.067	0.065	0.086	0.087
13	Galore	0.075	0.069	0.029	0.028
	Una
14	Jowar	0.082	0.075	0.039	0.049
15	Lohara	0.051	0.063	0.074	0.107
16	Daulatpur	0.08	0.07	0.081	0.045
17	Gagret	0.065	0.037	0.089	0.079
18	Oela	0.067	0.044	0.033	0.025
19	Panjawar	0.077	0.089	0.204	0.179
20	Saloh	0.072	0.072	0.107	0.113
21	Haroli	0.075	0.051	0.036	0.025
22	Kuriala	0.069	0.078	0.062	0.053
23	Kotlakala	0.079	0.074	0.07	0.065
24	Bangana	0.06	0.052	0.089	0.068
25	Chowkimanyar	0.066	0.069	0.097	0.075
26	Malangar	0.065	0.051	0.06	0.048
27	Thathal	0.06	0.051	0.131	0.17
28	Amb	0.065	0.077	0.051	0.041

Discussion

Figures 1 and 2 show the correlation between EERC and EETC for two seasons (summer and winter season) in each dwelling of Hamirpur and Una district of Himachal Pradesh. A remarkable ﬁnding is the positive correlation between EERC and EETC (R²= 0.79 in summer and R²= 0.81 in winter). This study is seen as a contribution to exploring whether and how the simple method based on track etch detectors can be used for regional surveys of indoor radon and thoron progenies. It was observed that the strongest influence on indoor radon and thoron levels is that of ventilation. To minimize the health hazards, it is necessary to characterize the sources. This requires attention to the rate at which radon and thoron are generated, the modes of transport and how they actually enter the indoor atmosphere. It is necessary to provide adequate ventilation to avoid building up of the daughter products in the dwelling. Both the bare and the wire mesh-capped DRPS/DTPS have the potential to be used as an easy to wear personal dosimeter for population dosimetry in indoor and occupational environments. Figures 3 and 4 show EERC and EETC in each dwelling of Una and Hamirpur District (attached and unattached progeny) in summer and winter seasons, respectively. As evident from the data, among all the selected locations, the maximum concentration of EERC and EETC is recorded in Awahdevi. Figure 5 shows the comparison of EEC of attached and unattached radon (Bq m⁻³) in summer and winter. In general, it is seen that the maximum concentration is found in winter season. The reason for higher values in winter may be due to poor ventilation, use of electric devices, coal fire, etc. Figure 6 shows the comparison of EEC of attached and unattached thoron (Bq m⁻³) in summer and winter seasons. Figures 7 and 8 show the EEC of radon and thoron in Una and Hamirpur District (attached progeny) in summer and winter seasons.

Figure 1.

Correlation between EEC of radon and thoron in summer season.

Figure 2.

Correlation between EEC of radon and thoron in winter season.

Figure 3.

EEC of radon and thoron in Una and Hamirpur district (attached and unattached progeny) in summer season.

Figure 4.

EEC of radon and thoron in Una and Hamirpur district (attached and unattached progeny) in winter season.

Figure 5.

Comparison of EEC of attached or unattached radon (Bq m⁻³) in summer and winter seasons.

Figure 6.

Comparison of EEC of attached or unattached thoron (Bq m⁻³) in summer and winter seasons.

Figure 7.

EEC of radon and thoron in Una and Hamirpur District (attached progeny) in summer season.

Figure 8.

EEC of radon and thoron in Una and Hamirpur district (attached progeny) in winter season.

Conclusion

The time-integrated equilibrium equivalent radon/thoron progeny concentration (EERC/EETC) in bare mode and wire mesh cup mode was calculated. The calculated value of EERC was higher than the value of EETC in bare mode. The high EERC values were observed in houses located at higher altitude. In general, the maximum concentration was found in winter. A remarkable ﬁnding was the positive correlation between EERC and EETC (R²= 0.79 in summer and R²= 0.81 in winter). This study illustrates the feasibility of using simple method based on track etch detectors for the measurement of indoor radon and thoron progenies.

Footnotes

Authors' contribution

All authors contributed equally in the preparation of this manuscript.

Acknowledgements

The authors thank the residents of the study area for their cooperation during the ﬁeld work and Department of Physics, Dr B. R. Ambedkar National Institute of Technology, Jalandhar, India for providing experimental facilities. The authors would also like to thanks Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai for providing sensors in the experimental work.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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