A twin channel device for the continuous spectrometry of radon daughters.

The continuous registration of radon daughter concentrations in room air is often needed, e.g. for measuring the variation of the equilibrium factor. Usually a moving filter system is used for this purpose. Such an instrument is quite large and insensitive. The new instrument is an arrangement of two filter detector assemblies for alpha spectroscopy with a data processor. The on-line processor collects the alpha spectra of both filters and calculates the concentration of the different radon daughters in the air with respect to time. Both filter devices sample the air alternatively with a cycle time of two hours while the alpha detectors are running continuously. The alternating sampling technique is used to reset the 218Po and 214Pb activities of the filters by radioactive decay each hour. The time resolution is in the order of five minutes for each daughter. With decreasing activity concentration the time resolution will also decrease. In the worst case the instrument operates in the long-term integration mode with a lower detection limit of 10-4 WL.h. The whole direct reading instrument is of light weight, portable, and battery powered. A similar set-up is suitable for measuring the fraction of unattached radon daughters