A compact automatic low-level liquid scintillation system for radon-in water measurement by pulse pair counting

A novel compact liquid scintillation counting system with automatic changing of seven samples, dedicated to the assay of low levels of 222Rn in water, is described. Its has a single phototube, and its operating mode is 222Rn series selective, as only the close pulse pairs given by 214Bi and its short lived decay product 214Po (half life 0.16 ms), are counted. Its 214Bi/214Po counting efficiency is 90.4% and the background, with an unshielded detector, is 3 pulse pairs per hour. 75% of the radon in 200 ml water samples is transferred to the scintillator (15 ml toluene) by circulating air, which is bubbled through the two liquids for 4 min. With a counting time of 3 h, the minimum detection level is 8 mBq l(-1). The method is also suitable for various environmental and geophysical studies of 222Rn and 226Ra.