Indoor radon measurements by SSNTD

Knowledge of concentration level of radon in air of dwellings is an important factor for radon protection. According to the published data, more than half of the dose of the natural radioactivity received by population is related to radon and its progeny. We have measured concentration of radon in air of apartments and offices of multistorey houses and also in onestorey houses with local national features, built with burnt and unburnt bricks, having wood floor and clay ceiling, and characterized by rather low ventilation (0.08-0.20). In apartments of multi-storey houses factor of ventilation was in the range of 0.08-0.45. The radon level was evaluated by long-term passive measurements using a cup detector sampler with a filtered end. A piece of the CR-39 plastic track detector of 2.5x2.5 cm2 was put on the bottom of the sampler. The cup was made of polyethylene material and had a shape of truncated cone with the height h=7 cm, the bottom diameter 4.5 cm and the open end diameter 6.0 cm. After the detector put inside the cup, the open end was closed by a paper filter in order to prevent the detector from dust and other solid particulates and also from radon progeny plate-out on the detector surface. So, the device detects only alphas from a radon gas and those daughters, which are produced inside the cup. Our previous experiments showed that this type of devices had a high reproducibility and rather high detection efficiency. The detector samplers were suspended on a ceiling on the distance of ∼200 cm from the floor. The detectors were exposed during 15-30 days. After exposition, the plastics were chemically etched in the 6N NaOH solution at 70 deg. C, for 6 hours. The produced radiographs were observed in an optical microscope equipped with a TV camera and connected with a monitor and PC. The responses of the detectors to a radon gas determined in our work were relative values. In order to estimate quantitatively a radon level in the measured rooms, the calibrated Alpha-Guard device was used along with the track detectors, and the calibration factor for the track detector device was evaluated. The result of our measurements have shown that in air of rooms of multistorey houses a radon concentration level in the first (1-4) floors was in the range of 33-97 Bq/m3, and in the higher (5-9) floors was in the range of 28-55 Bq/m3. The higher values of radon level from 120 up to 270 Bq/m3 were observed in air of separate onestorey houses. This can be explained by permeation of a radon gas through the slots in the floor and accumulation of it due to poor ventilation