H. S.
Virk
* and
Navjeet
Sharma
Department of Physics, Guru Nanak Dev University, Amritsar, 143005, India. E-mail: virkhs@yahoo.com
First published on 28th November 2001
It is well established that some areas of Himachal Pradesh (H.P.) state of India situated in the environs of the Himalayan mountains are relatively rich in uranium-bearing minerals. Some earlier studies by our group have indicated high levels of radon (>200 Bq m−3) in the dwellings. It is in this context that an indoor radon/thoron survey has been carried out in selected villages of four districts in the state of H.P. This survey has been conducted as a part of a national, coordinated project using twin chamber dosemeter cups designed by the Environmental Assessment Division (EAD), Department of Atomic Energy, Govt. of India. The track-etch technique is used for calibration of plastic detector LR-115 type-II which are employed for recording alpha tracks due to radon/thoron and their daughters. Year long radon/thoron data have been collected for seasonal correlations of indoor radon/thoron in the dwellings. The indoor radon levels have been found to vary from a minimum value of 17.4 Bq m−3 to a maximum value of 140.3 Bq m−3. The indoor thoron levels vary from a minimum value of 5.2 Bq m−3 to a maximum value of 131.9 Bq m−3. The year average dose rate for the local population varies from 0.03 µSv h−1 to 0.83 µSv h−1. The annual exposure dose to inhabitants in all the dwellings lies below the upper limit of 10 mSv given in ICRP-65.
It is well established that some areas of Himachal Pradesh (H.P.), situated in the environs of Western Himalaya are quite rich in radioactive minerals.11–12 Some of these sites in H.P. state were exploited by the Atomic Minerals Division of Department of Atomic Energy (DAE) Govt. of India. An earlier survey reported very high values of indoor radon in dwellings around some of these sites.13 High values of soil-gas radon varying from 13300 Bq m−3 to 75400 Bq m−3 were recorded by our group in a previous spot survey.14 Based on these facts, we decided to monitor radon inside dwellings in some of the villages falling in four districts of H.P., specifically Una, Hamirpur, Kullu and Kangra, as part of a national coordinated radon project sponsored by DAE.
Hamirpur and Una districts lie in the middle and lower Siwalik Himalaya while Kangra valley is enclosed between the middle Siwaliks and the Dhauladhar range of Western Himalaya. Siwalik sediments contain, in general 3–10 ppm of uranium which is much higher than the world average of 2.1 ppm in greywackes and 1.5 ppm in arkoses.15 Uranium anomalies were also reported in river waters flowing through Western Himalaya.16,17 Due to this compelling evidence of high radioactivity reported in soil-gas and water channels of H.P., it has become obligatory to undertake this survey for indoor radon monitoring for estimation of radiation inhalation dose delivered to inhabitants due to radon/thoron and their progenies.
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Fig. 1 Rn–Tn discriminating dosemeter. |
CR = T1/(dS1), | (1) |
CT = (T2 − dCRS1′)/(dS2) | (2) |
T3 = S3d[{CR + CR-A + CR-C} + {2CT + CT-C}] | (3) |
The activity fractions of the progeny are controlled by their wall loss rates for the fine fractions (λFW), coarse fractions (λCW) and ventilation rate (λV), through use of the following formulae:
For radon progeny:
FR-A = λR-A/[λR-A + fAλFW + (1 − fA)λCW + λV] | (4) |
FR-B = FR-BλR-B/[λR-B + fBλFW + (1 − fB)λCW + λV] | (5) |
FR-C = FR-BλR-C/[λR-C + fCλFW + (1 − fC)λCW + λV] | (6) |
For thoron progeny:
FT-B = λT-B/(λT-B + λCW + λV) | (7) |
FT-C = FT-BλT-B/(λT-C + λCW + λV) | (8) |
In eqns. (7)–(8), λT-B, λT-C are the decay constants for Th-B (212Pb) and Th-C respectively.
With the use of these, eqn. (3) can be written as
T3 = S3d[CR{1 + FR-A + FR-C} + {2CT′FT-C}] | (9) |
In the above equation, CT′ is the room average concentration of thoron. Since thoron has a short half life of only 55 s, its concentration cannot be uniform in the room. It will have a concentration profile while its relatively longer-lived daughters will mix more or less uniformly in the room. So it is assumed that they will be fractions of average thoron conentration in the room, CT′ rather than CT.
The spatial distribution of thoron near any emitting surface can be approximated by a one dimensional profile given by eqn. (10).19
C(x) = C0exp(−x/ξ) | (10) |
KC″(x) − vC′(x) − λTC(x) = 0. | (11) |
ξ(λV) = (L/2λT)[λV + {λV2 + 4KλT/L2}1/2] | (12) |
The average thoron concentration CT′ in the room is calculated using
CT′ = (1/L)∫L0CT(x) dx = C0(ξ(λV)/L)[1 − exp( − L/ξ(λV))] | (13) |
CT = CT(Y) = C(L/z) = C0 exp(−L/[zξ(λV)]) | (14) |
CT′ = [CTξ(λV)/L] [exp(L/[zξ(λV)]) − exp(L(1 − z)/[zξ(λV)])] | (15) |
Using the value of CT′ from eqn. (15) in eqn. (9) and using the subsequent equation and eqn. (12), the value of λV can be found out.
Using the value of λV in eqns. (4)–(8), FR-A, FR-Betc. can be calculated which lead to equilibrium factors for radon and thoron.
The progeny working levels are determined using equations:
WLR = CRFR/3700 = CR(0.104FR-A + 0.518FR-B + 0.37FR-C)/3700 | (16) |
WLT = CT′FT/275 = CT′(0.908FT-B + 0.092FT-C)/275, | (17) |
The dose rate is calculated by use of a formula given in UNSCEAR(1993):20
D/μSv h−1 = 10−3[(0.17 + 9FR)CR + (0.11 + 32FT)CT′] | (18) |
Sample no. | C R/Bq m−3a | C T/Bq m−3b | C T′/Bq m−3c | F R d | F T e | mWLR/mWLf | mWLT/mWLg | D/μSv h−1h |
---|---|---|---|---|---|---|---|---|
a Radon concentration. b Thoron concentration. c Room averaged thoron concentration. d Equilibrium factor for radon. e Equilibrium factor for thoron. f Radon progeny concentration. g Thoron progeny concentration. h Inhalation dose rate. | ||||||||
1 | 50.9 ± 4.6 | 17.6 ± 1.9 | 3.2 | 0.53 | 0.24 | 7.2 | 2.8 | 0.27 |
2 | 137.7 ± 9.4 | 73.7 ± 11.6 | 26.9 | 0.48 | 0.16 | 18.0 | 17.1 | 0.77 |
3 | 47.1 ± 3.2 | 24.6 ± 1.9 | 4.8 | 0.53 | 0.24 | 6.7 | 4.2 | 0.27 |
4 | 140.3 ± 13.6 | 65.3 ± 7.2 | 20.3 | 0.32 | 0.13 | 11.6 | 6.8 | 0.47 |
5 | 89.5 ± 7.6 | 72.9 ± 4.9 | 49.9 | 0.08 | 0.01 | 2.5 | 0.5 | 0.11 |
6 | 128.1 ± 16.1 | 53.9 ± 8.1 | 19.4 | 0.17 | 0.06 | 6.6 | 8.8 | 0.33 |
7 | 113.2 ± 10.0 | 20.7 ± 2.4 | 9.0 | 0.24 | 0.09 | 10.5 | 7.0 | 0.42 |
8 | 61.2 ± 2.4 | 36.8 ± 2.7 | 6.0 | 0.43 | 0.13 | 7.3 | 2.4 | 0.27 |
9 | 68.3 ± 4.7 | 48.7 ± 6.1 | 54.9 | 0.44 | 0.15 | 8.3 | 32.2 | 0.59 |
10 | 62.8 ± 6.2 | 92.4 ± 10.0 | 20.3 | 0.44 | 0.12 | 7.6 | 8.3 | 0.34 |
11 | 85.5 ± 4.4 | 61.1 ± 6.7 | 33.6 | 0.31 | 0.13 | 5.4 | 3.0 | 0.22 |
12 | 41.3 ± 2.4 | 54.1 ± 5.0 | 22.2 | 0.14 | 0.01 | 1.3 | 0.3 | 0.06 |
13 | 37.2 ± 2.4 | 27.4 ± 2.7 | 5.4 | 0.42 | 0.18 | 4.2 | 2.1 | 0.17 |
14 | 45.4 ± 1.8 | 26.9 ± 2.6 | 9.7 | 0.53 | 0.24 | 6.4 | 8.5 | 0.30 |
15 | 56.1 ± 5.7 | 66.9 ± 5.2 | 11.6 | 0.32 | 0.05 | 4.7 | 1.6 | 0.18 |
16 | 36.2 ± 1.2 | 27.0 ± 3.1 | 5.3 | 0.49 | 0.18 | 4.8 | 3.7 | 0.20 |
17 | 36.3 ± 2.0 | 16.5 ± 1.2 | 6.5 | 0.53 | 0.24 | 5.2 | 5.7 | 0.23 |
18 | 37.5 ± 1.5 | 29.9 ± 1.6 | 11.2 | 0.51 | 0.21 | 5.2 | 7.8 | 0.25 |
19 | 39.0 ± 2.5 | 20.3 ± 3.2 | 7.6 | 0.53 | 0.24 | 5.6 | 6.7 | 0.25 |
20 | 60.7 ± 1.5 | 49.7 ± 4.6 | 18.0 | 0.45 | 0.17 | 7.2 | 9.5 | 0.33 |
21 | 47.0 ± 2.6 | 40.2 ± 3.1 | 6.6 | 0.20 | 0.02 | 2.7 | 0.5 | 0.10 |
22 | 49.3 ± 2.5 | 17.1 ± 2.1 | 3.6 | 0.49 | 0.18 | 6.6 | 1.8 | 0.25 |
Mean | 66.8 ± 4.8 | 42.9 ± 4.3 | 16.2 | 0.39 | 0.14 | 6.6 | 6.4 | 0.29 |
s | 33.9 ± 4.0 | 22.2 ± 2.8 | 14.4 | 0.14 | 0.08 | 3.5 | 7.0 | 0.16 |
The radon/thoron concentrations, equilibrium factors, progeny concentrations and dose rates for 25 dwellings in Kullu and Kangra districts are summarized in Table 2. Radon concentration varies from 17.4 Bq m−3 to 75.1 Bq m−3 while the thoron concentration varies from 5.2 Bq m−3 to 57.5 Bq m−3. The equilibrium factors for radon and thoron vary from 0.12 to 0.53 and from 0.01 to 0.24 respectively. Radon/thoron progeny concentrations vary from 0.5 mWL to 8.3 mWL and from 0.4 mWL to 60.0 mWL, respectively. Due to large variations in equilibrium factors and ventilation rates of dwellings, the indoor thoron progeny levels show wider fluctuations compared with radon. The inhalation dose rates vary from 0.03 µSv h−1 to a maximum value of 0.83 µSv h−1 which corresponds to an annual dose rate of 0.21 mSv a−1 and 5.81 mSv a−1, respectively. Hence, for all the dwellings surveyed in Kullu and Kangra districts, the total annual dose delivered to inhabitants lies within safe limits (3–10 mSv) as recommended by ICRP-65.
Sample no. | C R/Bq m−3 | C T/Bq m−3 | C T′/Bq m−3 | F R | F T | mWLR/mWL | mWLT/mWL | D/μSv h−1 |
---|---|---|---|---|---|---|---|---|
1 | 29.4 ± 2.5 | 11.8 ± 1.8 | 56.8 | 0.28 | 0.09 | 2.2 | 45.6 | 0.49 |
2 | 31.7 ± 2.9 | 13.1 ± 2.0 | 9.3 | 0.39 | 0.09 | 3.6 | 3.9 | 0.16 |
3 | 17.4 ± 2.2 | 6.5 ± 6.3 | 28.9 | 0.27 | 0.12 | 1.2 | 8.5 | 0.12 |
4 | 54.8 ± 3.3 | 57.5 ± 6.3 | 83.2 | 0.18 | 0.02 | 2.9 | 7.5 | 0.18 |
5 | 28.9 ± 2.6 | 26.8 ± 3.8 | 31.6 | 0.19 | 0.02 | 1.5 | 4.0 | 0.09 |
6 | 38.0 ± 3.3 | 7.4 ± 1.0 | 3.1 | 0.27 | 0.09 | 2.9 | 0.8 | 0.11 |
7 | 41.7 ± 2.3 | 10.2 ± 1.0 | 7.9 | 0.53 | 0.24 | 5.9 | 6.9 | 0.27 |
8 | 63.3 ± 3.6 | 35.3 ± 4.2 | 25.5 | 0.23 | 0.03 | 3.6 | 2.3 | 0.15 |
9 | 18.7 ± 1.0 | 22.2 ± 1.5 | 14.6 | 0.12 | 0.01 | 0.7 | 0.4 | 0.03 |
10 | 75.1 ± 2.5 | 19.7 ± 1.5 | 92.2 | 0.41 | 0.17 | 8.3 | 60.0 | 0.83 |
11 | 60.2 ± 4.2 | 13.2 ± 1.7 | 6.0 | 0.30 | 0.06 | 4.0 | 1.2 | 0.15 |
12 | 43.2 ± 3.6 | 12.3 ± 1.4 | 86.7 | 0.42 | 0.12 | 4.9 | 49.3 | 0.61 |
13 | 26.3 ± 2.2 | 7.4 ± 1.2 | 2.0 | 0.36 | 0.10 | 2.8 | 0.8 | 0.10 |
14 | 32.2 ± 3.1 | 33.2 ± 3.9 | 55.5 | 0.28 | 0.07 | 2.6 | 15.5 | 0.24 |
15 | 50.6 ± 3.0 | 14.3 ± 1.7 | 9.2 | 0.21 | 0.03 | 2.7 | 0.9 | 0.11 |
16 | 43.7 ± 2.2 | 36.7 ± 2.9 | 24.4 | 0.20 | 0.03 | 2.3 | 1.3 | 0.10 |
17 | 23.2 ± 1.8 | 5.2 ± 0.8 | 3.8 | 0.47 | 0.15 | 2.9 | 1.9 | 0.12 |
18 | 27.1 ± 1.3 | 20.4 ± 2.0 | 14.7 | 0.19 | 0.08 | 0.5 | 3.0 | 0.05 |
19 | 28.8 ± 3.0 | 25.8 ± 4.2 | 13.0 | 0.37 | 0.12 | 3.1 | 5.7 | 0.16 |
20 | 23.9 ± 1.0 | 17.3 ± 1.3 | 44.7 | 0.53 | 0.24 | 3.4 | 39.1 | 0.47 |
21 | 27.7 ± 1.0 | 34.1 ± 3.2 | 36.2 | 0.43 | 0.17 | 3.4 | 23.5 | 0.33 |
22 | 31.9 ± 1.6 | 24.5 ± 2.8 | 6.8 | 0.53 | 0.24 | 4.5 | 6.0 | 0.21 |
23 | 53.7 ± 3.3 | 33.4 ± 3.7 | 9.2 | 0.40 | 0.11 | 5.7 | 5.0 | 0.24 |
24 | 33.1 ± 1.6 | 17.8 ± 2.0 | 10.2 | 0.29 | 0.12 | 1.9 | 2.1 | 0.09 |
25 | 51.7 ± 2.0 | 11.2 ± 0.8 | 5.4 | 0.07 | 0.01 | 1.0 | 3.1 | 0.04 |
Mean | 38.3 ± 2.4 | 20.7 ± 2.5 | 27.2 | 0.32 | 0.10 | 3.1 | 11.9 | 0.22 |
s | 15.0 ± 0.9 | 12.5 ± 1.6 | 27.6 | 0.13 | 0.08 | 1.8 | 17.3 | 0.19 |
Radon/thoron concentrations will depend upon the type of dwelling, particularly the building materials used for construction. Most of the dwellings are constructed using local sandstone and mud-mortar as the main building materials. Some kutcha houses have mud flooring while others have wooden or cemented flooring. Another factor that influences the radon/thoron concentration inside the dwellings is the nature of the soil at the plinth level and the nature of the crustal rocks underneath. High radon/thoron levels are recorded in the dwellings constructed near the vicinity of uranium/thorium-bearing soils in H.P. state.
From the point of view of construction, we have divided the dwellings into two categories, those with mud floor as type I and others as type II. The concentration of both radon and thoron in both types of the dwellings is given in Table 3. The average concentration of radon is higher in type I dwellings as compared with type II dwellings, indicating that subsurface soil may be the predominant source of indoor radon. However in case of room averaged thoron, the values are very nearly equal for the two types of dwelling. Since the dosemeter was generally placed at a height of about 2.5 m from the floor, any effect of subsurface soil on indoor thoron concentration could not be detected. The main sources of indoor thoron at this level are the walls of the room.
Type of house | No. of houses | Average concentration/Bq m−3 | |||
---|---|---|---|---|---|
Radon | s | Thoron | s | ||
I (mud floor) | 21 | 57.7 | 28.4 | 23.0 | 25.5 |
II (concrete or other floor) | 26 | 47.0 | 19.5 | 21.3 | 20.9 |
2. Thoron concentrations are significant and a predominant source of inhalation dose inside dwellings.
3. The indoor radon levels have been found to vary from 17.4 Bq m−3 to 140.3 Bq m−3 and thoron levels from 5.2 Bq m−3 to 92.4 Bq m−3 inside dwellings.
4. The integrated annual exposure dose rate varies from 0.03 µSv h−1 to 0.83 µSv h−1 which corresponds to an annual dose rate of 0.21 mSv a−1 and 5.81 mSv a−1, respectively.
5. This is the first systematic survey in dwellings located in Western Himalaya where radon and thoron contributions are taken into consideration separately. Evidence for thorium bearing rocks is established.
This journal is © The Royal Society of Chemistry 2002 |