Delayed coincidence counter for the assay of actinon and thoron

A highly sensitive system for the detection of small quantities of Rn219 (T1/2 = 3.92 sec) and Rn220 (T1/2 = 54.5 sec) has been developed. These isotopes are continually swept from a 5-ml acid solution containing their parents into a highly efficient 2-liter scintillation counter. Through the use of a delayed coincidence technique involving the short-lived decay products of the emanation isotopes (Po215, T1/2 = 1.8 msec and Po216, T1/2 = 0.16 sec), the counter background is substantially reduced (0.03 cpm for actinon and 0.15 for thoron counting). The over-all detection efficiencies are 45 and 46 per cent, respectively. With this system as little as 4×10−12 gram Pa231 per gram (equivalent to 2×10−7 gram U238 per gram) and 2×1O−8 gram Th232 per gram can be detected in 30-gram samples of carbonate minerals.