Aspects of an instrument for in situ measurements of radon exhalation rates

The properties of an instrument for in situ measurements of radon-exhalation rates were investigated. Rather than enclosing the sample in a measuring volume this instrument consists of a hood which is to be placed with its open end onto the sample. By applying an electric field positively charged radon-decay products are collected on an aluminised mylar foil in front of an alpha-particle ZnS(Ag) detector. It is shown in this paper that for porous materials like gypsum, an accurate determination of the exhalation rate requires a measurement of the α-activity for several days until the count rate reaches its equilibrium value. The increase of the count rate with time (growth curve) is expected to be smooth; in the present investigation, however, structures on this growth curve were observed. Such structures indicate that the operation of the instrument, and hence the deduced results, may depend on various parameters. Experiments are described from which it could be deduced that important parameters are: leak rate, presence of light, shape and strength of the electric field, temperature and aerosol conditions. Since only the first three of these parameters may be controlled in actual measurements, a further study of the dependence on temperature, aerosol conditions and consequently, also relative humidity, will be necessary.