The methods currently known to analyse the activity of a gaseous sample containing the various isotopes of radon and their daughter products relate to two major categories, but neither of these categories makes it possible to carry out this measurement in a discriminatory way, that is, by allocating to each isotope and its own daughter products the share of activity returning to them in the examined gaseous atmosphere sample.
The known methods for measuring the activity of radon atoms, these methods being overall nondiscriminatory measurements, essentially rely on two different physical principles, namely:
1) ionization of the air produced in a circulation chamber by radiations emitted by the isotopes of radon and their daughter products;
2) the action of the particles .alpha. emitted by the daughter products of the radon on a surface rendered sensitive for this purpose.
Neither of these two methods makes it possible to easily and precisely determine the isotopic distribution of the various radons Rn.sup.222, Rn.sup.220 and Rn.sup.219.
In fact, in the measurements which use the air ionization method, this ionization is evaluated with the aid of the current it creates between the conductive armature of a cylindrical ionization chamber and the axial electrode between which a suitable potential difference is applied. The main drawback of this method is that a current is used which is the result of ionizing the ambient air by all the radioactive transmitters contained in the taken sample. Now, for a given volume activity of a certain atmosphere sample, the ionization current varies with the energy of the radiation which has provoked ionization. As, by definition, the isotopic distribution of the various radons is not known, the measurement of the volume activity may not be strictly accurate.
In those methods using the interaction of particles on a sensitive surface, the measurement of the volume activity of the radon is often carried out by means of scintillating flasks previously placed in a vacuum. These flasks are glass vessels whose inner wall, except for the bottom, is coated with a scintillating material, such as zinc sulphide activated with silver. The gas corresponding to the atmosphere to be analysed is then introduced through the stopper of the flask by means of a hypodermic needle.
The measurement itself is only carried out after a certain time for placing the radon in equilibrium with its various solid daughter products, the latter then being secured to the walls of the vessel. The interaction of particles .alpha. with the scintillating material by these same solid daughter products produces photons which are taken into account and measured by a photoelectron multiplier placed under the bottom of the flask so as to then be analysed by a suitable electronic device. This routine laboratory practical technique also has two major drawbacks.
Firstly, no more than the previous method, this technique does not make it possible to dissociate the respective activities of the isotopes Rn.sup.222, Rn.sup.220 and Rn.sup.219 of the radon, since the pulses of the three isotopic daughter products cannot be discriminated by the photoelectron multiplier.
Secondly, in order to arrive at the value of the volume activity of the isotopes of the radon, it is necessary to assume that the solid daughter products are distributed homogeneously over the internal wall of the flask so as to embody a geometry for measuring this activity able to be reproduced from one experiment to another. This introduces an additional element of uncertainty concerning the measurement of the volume activity of the isotopes of the radon.