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 Rn-222 in water, is described. Its has a single phototube, and its operating mode is Rn-222 series selective, as only the close pulse pairs given by Bi-214 and its short lived decay product Po-214 (half life 0.16 ms), are counted. Its Bi-214/Po-214 counting efficiency is 90.4% and the background, with an unshielded detector, is 3 pulse pairs per hour. 75% of the radon in 200 mi water samples is transferred to the scintillator (15 mi 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 Rn-222 and Ra-226. (C) 2000 Elsevier Science Ltd. All rights reserved.