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

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. The 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. Further, 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. Finally, 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. In 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.