Introduction

Radon (222Rn) and thoron (220Rn) are the decay products of radium (226Ra, 224Ra) which are present throughout the earth crust as the members of natural U-238 and Th-232 series. 222Rn, 220Rn and their decay products contribute to almost 52% of the total dose resulting from ionizing radiation thus affecting the inhabitants.(UNSCEAR 2000, 2010). Epidemiological investigations have also furnished persuasive confirmation of a relationship between indoor 222Rn vulnerability and lung cancer (Darby et al. 2004, Krewski et al. 2005). The parent gases and the associated decay product are found inside the dwellings because of the occurrence of their parent radionuclides in building materials and soil. 222Rn and 220Rn concentration levels in dwellings primarily depend upon their exhalation from building material, soil, etc., and secondarily, on ecological constraints like ventilation rate, temperature, pressure, humidity, etc. 222Rn/220Rn exhalation and emanation rate (i.e. source term) depends on the parent radionuclide’s content, terrain, rock layer composition, soil permeability, other parameters particular to geology and the region’s environment (Johner and Surbeck 2001; Prasad et al. 2008).

The 222Rn and 220Rn exhalation rate signifies the emission of 222Rn or 220Rn gas from a unit mass or surface area with respect to time from the soil, which is used as a stable source term for calculating the indoor 222Rn concentration employing various models (Stoulos et al. 2003; Sahoo et al. 2011; Kumar et al. 2014). Several survey-based studies have been carried out reporting the 222Rn, 220Rn concentration levels in dwellings and occupational environment (Bossew and Lettner 2007; Eappen et al. 2006, 2007; Jelle 2012; Kant et al. 2009; Kumar et al. 2014; Ramola et al. 2010, 2013, 2016; Teras et al. 2016). Relatively lesser investigations have been conducted in order to study and examine the source term (Chau et al. 2005; Kumar et al. 2014, Mayya 2004; Prasad et al. 2008, Ryzhakova 2014; Tripathi et al. 2008). In our previous study, Singh et al. 2016, 222Rn/220Rn and decay products were measured in Almora district for winter season only. Relatively higher values of their concentration were reported in this region than the normal. This study conducted in the same region (as chosen for this work), winter time concentration levels of 222Rn/220Rn and decay products were observed to be high and the need for addressing the contributory factors (source terms and/or contributing parameters) responsible for maintaining indoor levels.

In the present work, 222Rn, 220Rn exhalation rate was determined from the soil samples for the estimation of source term. These soil samples collected from diverse locations of Almora district, situated in Kumaun, Himalaya, one of the coldest regions in India, were used for the study. Standard measurement techniques and protocols were used for measurement of exhalation rates and a correlation study was performed as a follow-up.

Materials and methods

Location and geology of study area

Almora is situated in Kumaun hills of Indian Himalayan belt (Latitude: 29º36′N, Longitude: 79º 30′E). The focused area contains extremely mylonitised porphyritic, schist phylites slates, quartz porphyry, granite and quartzites (Prasad et al. 2008). The crystalline zone of Almora is divided into two distinct lithologic units: (i) Sarju formation, by the porphyries, migmatities, schists and micaceousquartzites, (ii) Duram formation represented by quartzites. The rocks of Almora mainly consists of garnetiferous mica-schists, and within these occur layers of quartzites, lenses of graphitics schists and bands of gneissic rocks, and metamorphically belonging to the staurolite-almandine subfacies of the almandine-amphibolite facies (Valdiya 1980). Twenty-four soil samples were collected from diverse sites of Almora district which were selected on the basis of measurement of background gamma levels. Twenty-four soil samples were collected from various locations for carrying out 222Rn/220Rn exhalation studies.

Measurement of exhalation rate of222Rn and 220Rn

The measurement of 222Rn and 220Rn exhalation rate was carried out by accumulator technique assisted with continuous 222Rn/220Rn monitor (SMART RnDuo). The SMART RnDuo is a commercially available portable continuous monitor for measurement 222Rn, 220Rn and gross alpha in air, soil and water. It is based on the principle of detection of alpha particles by scintillations with ZnS:Ag. The details of detection principle and measurement protocols are discussed elsewhere (Gaware et al. 2011).

Measurement of 222Rn mass exhalation rate

For the measurement of 222Rn mass exhalation rate, soil samples (each measuring ≈ 1000 g) were collected from nearby areas of dwellings/workplaces. Due precautions were taken to prevent any change in the matrix during the soil sample collection process. The Smart RnDuo is connected to a closed accumulation chamber with a sample capacity of approximate 500 gm as shown in Fig. 1. The chamber is made up of stainless steel cylinder with dimensions 8 cm height and 10 cm diameter connected to Lucas cell along with photomultiplier tube at the top. For 222Rn measurement, the SMART Duo was used in diffusion mode to avoid the 220Rn interferences (if present in the soil). The build-up of 222Rn concentration in accumulation chamber was measured after every 1 h and continued until it gets saturated. The 222Rn concentration data with time elapsed was plotted. The radon growth with time in Bq/m3 inside the accumulator chamber is given by the exponential equation (Aldenkamp et al. 1992).

$$C(t) = \frac{{J_{m} M}}{{V\lambda_{e} }}\left[ {1 - e^{{ - \lambda_{e} t}} } \right] + C_{\text{o}} e^{{ - \lambda_{e} t}}$$
(1)
Fig. 1
figure 1

A schematic experimental setup measurement of 222Rn mass exhalation rate

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Where, C(t) = 222Rn concentration at any instant t in Bq m−3, Jm = 222Rn mass exhalation rates (Bq/kg/h), M = Mass of sample (kg), V = Volume of accumulator chamber and the scintillation cell (m3). λe = Effective decay constant (h−1) which is the summation of 222Rn leakage rates and decay constant of accumulator in case it exits and C0 = Initial 222Rn concentration in the chamber at t = 0.

Figure 2 shows the typical plot for the 222Rn growth in the accumulator. Exponentially obtained 222Rn build up data can be fitted against exponential decay equation.

Fig. 2
figure 2

Typical graph between 222Rn activity concentrations versus time

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Equation:

$$Y = Y_{0} + Ae^{{ - x/t_{1} }}$$
(2)

By comparing Eqs. 1 and 2, 222Rn mass exhalation rate can be deduced as

$$J_{m} = \frac{{Y_{0}^{{V\lambda_{e} }} }}{M}.$$

Wherein \(\lambda_{e} = {1 \mathord{\left/ {\vphantom {1 {t_{1} }}} \right. \kern-0pt} {t_{1} }}\)

Measurement of220Rn surface exhalation rate

The same technique (as used for measuring 222Rn mass exhalation rate) has also been employed for measuring 220Rn surface exhalation rate from the collected soil samples. The 220Rn is not uniformly distributed in the accumulator because of its small diffusion length in atmosphere (2–3 cm).However, it is possible to maintain a uniformity of 220Rn distribution using flow mode. 220Rn sampling from the accumulator was done in flow mode in a closed loop set up. Schematic diagram of closed loop set up is given in Fig. 3. In the flow mode operation, Smart Rn Duo having in-built function, sets pump automatically ON for 5 min in 15 min cycle or for 15 min in 60 min cycle. During the 220Rn exhalation rates measurement, the air volume was kept minimum in the chamber to accomplish the proper mixing of air within the closed loop. The detailed measurement principle has been elaborately discussed in literature (Gaware et al. 2011). The averages of four values were considered for calculating the steady state 220Rn concentration CT (Bq/m3).The value of CT was then made use of estimate the 220Rn surface exhalation rate using Eq. (3) based on mass balance (Sahoo and Mayya 2010).

Fig. 3
figure 3

A schematic experimental setup for measurement of 220Rn surface exhalation rate

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$$J_{s} = \frac{{C_{T} V\lambda }}{A}.$$
(3)

Where, Js = Surface exhalation rate of 220Rn from the soil in Bq/m2/h, V = Volume enclosed in the closed loop in m3, λ (0.012464 s−1) = Decay constant of 220Rn and A = Cross section area of the chamber (m2).

Measurement of220Rn mass emanation rate

The 220Rn mass emanation rate as obtained from the soil can be calculated utilizing Eq. (4) (Celikovi et al. 2008).

$$J_{m} = \frac{{J_{s} }}{{\rho L_{s} }}\tanh \frac{{Z_{s} }}{{L_{s} }}.$$
(4)

Where, Zs = Height of sample, Ls = 220Rn diffusion length within soil and ρ = Density of the soil sample. The diffusion length of 220Rn in soil is more or less 1 cm (Mayya 2004), then the ratio of Zs/Ls can be much larger than 1. Considering the approximation, Eq. (4) can be written as

$$J_{m} = \frac{{J_{s} }}{{\rho L_{s} }}$$
(5)

Equation (5) is employed to estimate the 220Rn mass emanation (Jm) rate. More details for this procedure can be found elsewhere (Chauhan et al. 2015).

Measurement of Gamma radiation level

The measurement of outdoor gamma radiation level was performed by ATOMTEX Gamma Radiation Survey Meter. The variation in the background gamma radiation is helpful in tracing the origin of such terrestrial radiations. The background gamma radiation levels have been taken at 1 m height from the ground surface.

Measurement of natural radionuclides activity concentration (Bq/kg)

In the present study, radionuclides (226Ra, 232Th and 40K) activity concentrations (Bq/kg) in soil samples were measured using gamma ray spectrometry. The Gamma Detection Unit (GDU) consists of a NaI(Tl) scintillation detector of size 63 × 63 mm with a multichannel analyzer. The activities of 226Ra and 232Th were evaluated from the 1764 keV gamma line of 214Bi and 2610 keV gamma line of 208Tl, respectively. Similarly, the activity of 40K was evaluated from its own 1460 keV photo peak. The energy calibration of the spectrometer has been done at 661 keV of 137Cs point source which was made available from the manufacturer and energy resolution was limited till 9.5%. After energy calibration, the system calibration was done by using three reference materials, obtained from the International Atomic Energy Agency (IAEA) for U-238 (RGU-1 is U-ore), Th-232 (RGTh-1 is Th-ore) and K-40 (RGK-1 is K-ore) activity measurements. These 24 soil samples were stored (for secular equilibrium) and analyzed in the containers of same geometry (Marinelli beakers) as was used for standardization of detector. In order to reduce the contribution of background radiation, the spectrum was also analyzed without any source (or sample) and background counts (activity) were eliminated from the recorded sample counts to get the net sample counts (activity). The detector system was shielded by lead absorbers to reduce the background radiation coming from the environment. The spectral analysis was done with the help of gamma radiation computer software SPTR–ATC (AT-1315).

Result and discussion

222Rn and 220Rn exhalation rate from soil

The mean and standard deviation of gamma radiation level, 222Rn mass exhalation rate, 220Rn surface exhalation rate and 220Rn mass emanation rate is presented in Table 1. 222Rn mass exhalation rate varied from 16 to 54 mBq/kg/h with arithmetic mean (standard deviation) of 30 ± 10 mBq/kg/h while 220Rn surface exhalation rate was ranging from 0.66 to 6.43 Bq/m2/s with arithmetic mean (standard deviation) of 2.18 ± 1.36 Bq/m2/s. Higher value of 222Rn mass exhalation (denoted by location code S3, S4, S8 and S18) can be linked to high radium (226Ra) content in the soil as shown in Table 2. Similarly, higher value of 220Rn exhalation rate was found at locations code S3, S4, S15 and S21 which is due to high thorium (232Th) content as compared to other selected locations. 226Ra and 232Th content in the soil samples, measured using gamma ray spectrometry are presented in Table 2. The obtained values of 226Ra, 232Th at these locations are found relatively higher than the global average value (35, 30 and 400 Bq/kg for 226Ra, 232Th and 40K). Potassium (40K) found somewhat higher than mentioned global value because of Indian geological formation and topography (Kumar et al. 2015a). The variation in 222Rn and 220Rn exhalation rate can also be attributed to the difference in geological distribution and soil composition. As, the area is divided in different (Valdiya 1980) geological formations like krol formation, Saryu formation (granite, gneisses), Siwalik formation comprising sandstone inter-bedded with mudstone along the main boundary thrust, etc. (Prasad et al. 2008; Ramola et al. 2006) of this area are responsible for such variations. These formations of thrust and local scale faults are expected to significantly affect the exhalation rate of study area. For visualizing the distribution pattern, frequency distribution of measured 22Rn and 220Rn exhalation rate are also plotted on Figs. 4 and 5 respectively.

Table 1 Gamma level, 222Rn and 220Rn exhalation rates and 220Rn mass emanation rate measured from soil samples
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Table 2 226Ra, 232Th and 40K radionuclides measured from soil samples
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Fig. 4
figure 4

Frequency distribution of 222Rn mass exhalation rate

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Fig. 5
figure 5

Frequency distribution of 220Rn surface exhalation rate

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Correlation studies

222Rn and 220Rn exhalation rate

In order to further dwell into the observations, correlation graph between 222Rn and 220Rn exhalation rates was plotted (as shown in Fig. 6). As noticeable it presents weak positive correlation between them with correlation coefficient of 0.21. This is due to distribution of their parent source term (226Ra and 232Th, respectively) and soil composition which are expected to be uncorrelated.

Fig. 6
figure 6

Correlation between 222Rn and 220Rn exhalation rate

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Gamma exposure and 222R/220Rn exhalation rate

Figure 7 representing the correlation between gamma level and 22Rn mass exhalation rate also shows weak correlation (correlation coefficient of 0.45). Similarly, Fig. 8 also signifies a weak positive correlation (correlation coefficient of 0.33) between gamma level and 220Rn exhalation rate. This is because of high mobility of 222Rn/220Rn in nearby area than the 226Ra and 232Th content in soil. Besides, collected samples representing diverse locations have dissimilar geometries and size of grain (soil composition), that determine and effect the 222Rn/220Rn exhalation rate from the soil.

Fig. 7
figure 7

Correlation between 222Rn exhalation rate and Gamma level

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Fig. 8
figure 8

Correlation between 220Rn surface exhalation and Gamma level

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220Rn mass emanation and exhalation rate

The Table 1 presents the 220Rn mass emanation rates (mBq/kg/s) from the soil samples of selected locations. The emanation coefficient varied from 78 to 769 mBq/kg/s with average 251 ± 161 mBq/kg/s. The 220Rn mass emanation and exhalation rate are correlated with each other and the graph is shown in Fig. 9.

Fig. 9
figure 9

Correlation between surface exhalation and emanation rate of 220Rn

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220Rn mass emanation and exhalation rates when correlated, a strongly positive (R = 1) relationship is observed. An obvious direct relationship is found between the 220Rn mass emanation rate and 220Rn exhalation rate (Table 1).

Comparison with the other Indian studies

These values of 222Rn and 220Rn exhalation rates are compared with results from other studies conducted in recent times for different north Indian regions in Table 3. It is observed from that the average data of 222Rn mass exhalation rate measured for the study region is found comparable (except Punjab) to those obtained from other studies in recent times. Significantly high values for 220Rn surface exhalation rate are observed than the other reporting studies. These values may explain the higher concentration values reported for this region (Table 1).

Table 3 Comparison of results of present study with recent studies conducted in north Indian regions
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Conclusion

In the present work, 222Rn and 220Rn exhalation measurements were performed in order to estimate the stable source term. Chamber technique assisted with Smart RnDuo portable monitor was employed for determining 222Rn and 220Rn exhalation rates. Fundamentally 222Rn and 220Rn exhalation rate depends on thetwo factors (1) 226Ra and 232Th content in soil and (2) geographical distribution and soil composition. Higher 226Ra, 232Th and 40K content in soil (except at few locations) than the global average and relatively higher values of 222Rn and 220Rn exhalation rates, corroborate with the observed indoor 222Rn and 220Rn concentration values reported for this region. Correlations between 222Rn and 220Rn exhalation rate and gamma level with 222Rn and 220Rn exhalation rate were found weak due to random source term distribution and soil composition. Results of the present study will be helpful in bridging the source terms, contributing factors and the concentration values for the study area important for future research expeditions.