Development of an electronic monitor for the determination of individual radon and thoron exposure

摘要

The carcinogenic effect of the radio isotope Rn-222 of the noble gas radon and its progeny, as well as its residential distribution, are well studied. In contrast, the knowledge about the effects and average dwelling concentration levels of its radio isotope Rn-220 (thoron) is still limited. Generally, this isotope has been assumed to be a negligible contributor to the effective annual dose. However, only recently it has been pointed out in several international studies, that the dose due to thoron exceeds the one from Rn-222 under certain conditions. Additionally, radon monitors may show a considerable sensitivity towards thoron which was also not accounted for in general. Therefore a reliable, inexpensive exposimeter, which allows to distinguish between decays of either radon and thoron, is required to conduct further studies.udThe scope of this thesis was to develop an electronic radon/thoron exposimeter which features small size, low weight and minimal power consumption. The design is based on the diffusion chamber principle and employs state-of-the-art alpha particle spectroscopy to measure activity concentrations. The device was optimized via inlet layout and filter selection for high thoron diffusion. Calibration measurements showed a similar sensitivity of the monitor towards radon and thoron, with a calibration factor of cfRn-222 = 16.2±0.9 Bq×m-3/cph and cfRn-220 = 14.4±0.8 Bq×m-3/cph, respectively. Thus, the radon sensitivity of the device was enhanced by a factor two compared to a previous prototype. The evaluation method developed in this work, in accordance with ISO 11665 standards, was validated by intercomparison measurements. The detection limits for radon and thoron were determined to be C#Rn-222 = 44.0 Bq/m3 and C#Rn-220 = 40.0 Bq/m3, respectively, in case of a low radon environment, a one-hour measurement interval, and a background count rate of zero. In contrast, in mixed radon/thoron concentrations where the Po-212 peak must be used for thoron concentration determination, a calibration factor of cfRn-220 = 100±10 Bq×m-3/cph was measured, yielding a detection limit of C#Rn-220 = 280 Bq/m3.udFurther, Monte Carlo (MC) simulations were performed by means of various codes including Geant4, to study the effect of the variation of parameters influencing the calibration factors. The results showed reasonable agreement between simulated and acquired spectra, with differences being below 8%, thus validating the employed simulation model. The simulations indicated a significant impact of environmental parameters, such as temperature and pressure, on the measured spectra and accordingly on the calibration factor. Therefore the calibration factor was quantified as a function of temperature, relative humidity and pressure as well as chamber volume. For devices with increased detection volume a considerable influence of air density changes, corresponding to altitudes from 0-5,000 m, and temperatures from -25 to 35 °C, on the calibration factor of up to 32% was observed. In contrast, for devices with standard housing the calibration factor changed only up to 4%. When increasing the detection volume compared to the employed standard housing by at least a factor of four, a maximum increase of the sensitivity of about 20% was found, at the expense of device portability. On the contrary, when reducing the height of the housing by 10~$mm$, which yields 40% less volume, a decrease of sensitivity by 30% and 41% for radon and thoron was observed, respectively.udFinally, devices were used and tested at different realistic conditions, such as mines, radon spas, and dwellings with mixed Rn-222 and Rn-220 environments. Measurements in a salt mine with the device developed within the framework of this thesis revealed maximum radon concentrations of up to 1.0 kBq/m3.udIn the Bad Gastein Heilstollen, Rn-222 concentrations up to 24.3 kBq/m3 were found, in agreement with an AlphaGuard reference device. First measurements in radon/thoron environments of about 200 Bq/m3 each, in a clay model house at the Helmholtz Center Munich, showed reasonable agreement with reference devices, thus validating the introduced evaluation method. First measurements in a private Bavarian clay house revealed a low thoron concentration of about CRn-220 = 13.0±3.0 Bq/m3, in comparison to a high radon concentration of CRn-222 = 200±70 Bq/m3.