Development of calibration techniques for active and passive radon detectors

Since many years radon measurements became customary in a wide-spreading applications. Radon is used as geophysical tracer for locating buried faults and geological structures, in exploring for uranium, and for predicting earthquakes. It is also been used as tracer in the study of atmospheric transport processes and there have been many other application in meteorology, water research and medicine. In any case, the great majority of radon measurements are performed to evaluate the health risk due to radon daughters inhalation in indoor air. Indeed, for many years it is well known that radon and its decay products are the largest source of human exposure to environmental radiations. Since many epidemiological studies confirmed a correlation between lung cancer risk and radon exposure, most of the countries have introduced in their own legislation limits on the 222Rn indoor concentration in particular working places. Moreover, this specific exposition source shows peculiar mechanisms of dose assessment. In fact, the real responsible of the effective dose are the inhaled radon progeny whose behavior is strictly dependent on different air conditions. Thus, the main topics of investigation are the properties of radon progeny, its interaction with aerosol molecules and its deposition mechanism onto surfaces. In this contest, it is mandatory to carry out high-quality radon measurements which are guarantied by calibration facilities, suitable for the determination of calibration in a wide range of variability of all parameters involved and in many different experimental situations. In fact a large number methods have been developed for the radon measure in different situations and matrices and each of them requires a large variety of sampling technique and appropriate instruments. Thus, calibration procedures have to be settle for each measurements method, that account for the particular sampling and detecting processes. To calibrate correctly radon detectors, it is necessary to refer the instruments response to measurement standards, that are defined in term of different parameters, such as radon activity, radon activity concentration and so on. All of these standards have to be traceable to a reference 222Rn standard developed by the national metrology institutes. In this thesis it will be described the procedures involved in the development of a facility able to perform radon detector calibration as well as to carry out experiments for study the radon progeny behavior at controlled radon atmospheres. The radon sources are produced by emanation from a solid radium source, and an electrostatic cell is used as continuous monitor for the radon exposure determination inside a Radon chamber. This monitor, whose response is highly dependent from environmental air conditions, was characterized studying its response under different values of temperature, humidity and pressure using both an experimental and a numerical approach. The exposure chamber, equipped by a set of environmental sensors, can be used for the calibration of passive radon detector. Various tests have been carried out to study its reliability. In the first chapter the radon and its decays properties are described, including some important definitions. It follows a description of its measurements methods, related to the most used passive and active detectors. In the second chapter it will be illustrated the methods of the electrostatic collection for the active radon monitoring, that will be the technique used to control radon concentration in our facility with a suitable instrument (Ramona). In this context it will be described the physical factors that influence the 218Po+ collection as well as the experiments that have investigated the mechanism involved in its neutralizations. In this section we focus our attention on the use of the electrostatic collection technique to measure some important radon progeny physical parameters with both a theoretical and an experimental approach. Some remarks will be done about the importance of the characterization of the radon monitor against climatic parameters. In the following chapter, we will describe in detail the facility built for the Ramona calibration against temperature, humidity and pressure variation. It was also pointed out the importance of the traceability of the radon measurements to national standard and it will be described our metrology chain that refers to ENEA-INMRI primary radium source. In the fourth chapter we will illustrate the experimental results and the Monte Carlo methods used for the determination of the neutralization rates and to infer the electrostatic collection dependence on environmental parameters inside the radon monitor. The description of the radon chamber characteristics will be done in the fifth chapter. Here it will be also introduced new methods for the continuous radon monitoring in a small exposure volume. Conclusion and some perspective on the future radon studies with the developed radon facility will be outlined in the last chapter.