C O N C L U D I N G S U M M A R Y
From a health viewpoint, 222Rn is the most important radon isotope since its decay products, especially 218Po and 214Po, can get a so pronounced adverse effect on lung tissues, leading to lung cancer in many cases as reported recently by many epidemiologists. Radon is thus a very inescapable radiation exposure source both at home and at work. Consequently, to prevent from its incidental accumulation in human living and working areas, several detection systems and various measurement methods aiming for an accurate assessment of radon activity concentration in the ambient air, were developed in the past. In particular, continuous measurements methods (active methods) are of a fundamental relevance for the study of the time variation of radon activity concentration and related equilibrium factors, especially when correlation studies with varying environmental parameters are dealt.
Up to now, the active methods so far developed for airborne radon monitoring and based on the use of air-filled gaseous detector, are based on specially designed ionization chambers having a sensitive volume ranges from 0.5 l up to 13 litres or more. The electrical signal development is founded essentially on ionic charges collection, since the electrons realeazed within the sensitive volume are almost captured by oxygen molecules. Therefore the electron attachment effect is responsible of the observed systematic limitations. To enable the device with a spectrometry feature (separation of the alpha energy peaks) and to resolve sharp radon variations, one has to narrow drastically the dynamic range of the instrument. Extending the measurement dynamic range, does not allow to individuates such radon pulses even if they were very strong.
Concluding summary
130____________________________________________________________________________________________________________
In the present work, we propose a new active method, based on the use a specially designed multiple cell proportional counter which enables to resolve such strong radon pulses above mean radon levels extending within a 7 decades dynamic range.
The design study of such an instrument is initiated with experimental investigations of the effect of air in gas-filled detectors.
First of all, we reported in this thesis about an experimental study of the influence of the air fraction admixed to a counting gas in a specially designed gas-flow proportional counter intended for a new continuous measurement method of airborne radon activity concentration. That study has also permitted to show the feasibility of a new continuous measurement method as will be expounded. A preliminary design study of a so-called Multiple Cell Proportional Counter (MCPC) prototype are then fully described. The device is designed to use an argon-propane (1%) gas mixture to which should be admixed an appropriate fraction of ambient air, whose radon activity concentration has to be measured. In order to perform a consistent optimization of the measuring device performances, a Monte Carlo simulation program, baptized RADON-MCPC, has been written. The program takes into account all the electrical and geometrical parameters and models rather in a simple way the main physical processes that determine directly the detector performances. Using RADON-MCPC code, the main design parameters of the MCPC prototype were thus studied and optimized in order to achieve the highest possible alpha detection efficiency and a detection limit of about 15 Bq/m3 or less.
Therefore, a new proportional counter, called Multiple Cell Proportional Counter (MCPC), intended for continuous airborne radon activity concentration measurements is constructed and its operation principle presented. An appropriate fraction of ambient air is admixed to the counting gas mixture: its radon activity concentration is continuously measured through a periodic counting of the α particles emitted by 222Rn and its short-lived decay products within the well defined sensitive volume of the counter. By adopting this multiple cell pile-up design, it is possible to compensate for the electron attachment effect loss.
RADON-MCPC code has been used for improving the design optimization purposes.
The radon sensitivity, the alpha counting efficiency, the operating high voltage value and the suitable pulse height discriminator threshold are therefore determined as a function of the air sample fraction in the gas mixture. Qualitative and quantitative
Concluding summary
131____________________________________________________________________________________________________________
analyses of the wall effect, the grid opacity effect, the electron attachment effect, the gas amplification mechanism were also carried out. The simulation results show that the admixture of 10 % of ambient air is sufficient to continuously assess radon concentration levels ranging from about 15 Bq/m3 up to 1.5 105 kBq/m3 for an integral counting period of 10 minutes, when setting the energy discrimination at 250 keV.
The expected radon sensitivity is about 11.6 cpm / 100 Bq⋅m-3. This configuration therefore allows further correlation studies of radon concentration variations related to inherent environmental, climatic and/or geological parameters variations.
Finally, the simulated alpha pulse height spectra are found in a very good agreement with those recorded experimentally when using the constructed MCPC in minimal configuration, integrating only 5 effective counting cells.
Using the written simulation code, MCPC-RADON, it is possible to find out other more advantageous MCPC configurations, improving the time autonomy, regarding the necessity to reduce counting gas consuming, and thus, the economical cost of the method. Working always towards this insigt subsequent experimental tests are still in course.
HIHIHIH
