Distribution pattern of indoor thoron concentrations in a dwelling

Author(s): L.A.Sathish, K.Nagaraja, S.Shoba, H.C.Ramanna, V.Nagesh, S.Sundareshan

Atest roomof volume 31m3 was selected and walls are classified asW, N, S, E, C and F for west, north, south, east, ceiling and flooring respectively. Nine dosimeters consisted of LR 115 type II plastic track detectors were installed at a constant distance from the west (74cm), east (533cm), ceiling (105cm) and floorings (191cm) with all the four windows and one door closed for 90 days to observe the explicit variations of 220Rn and their progeny levels with respect to the distance from the north to south wall, south to north wall, ceiling to floorings and flooring to ceiling of the room. This process is repeated for one year to see the seasonal variations also. After the exposure period of 90 days the films were removed and chemically etched in 2.5N Na OH solution at 60 ºC for one hour and counted for alpha track densities using spark counter.Aspecific trend of indoor 220Rn concentrations for bare, filter and membrane films were observed. It is viewed that as the dosimeter is moved away from the north to south wall or south to north wall the concentrations were found to decrease exponentially and becomes minimum at the central dosimeter, which gives the structure of parabola. Higher concentrations were observed at the ceiling and flooring of the room and it reduces as the detector is moved away from them. 220Rn progeny concentrations did not show any typical variations with the distance from the wall. The 220Rn concentration in the studied area is found to vary from1.37 to 69.44 Bqm- 3 where as their progeny concentrations ranges between 0.012 to 12.93mWLrespectively. The arithmetic mean concentration of 220Rn and their progenywith standard error are 23.57 ± 0.67 and 0.43 ± 0.07 respectively. About 85 % of the dwellings have shown the concentrations below 30 Bq m-3 and 15 % of the dwellings show the concentrations above 50 Bq m-3. The geometrical mean concentration of 220Rn and their progeny of all the seasons in all the locations are 14.02 Bqm-3 and 0.12mWL respectively.

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