Use of a Scintillation Gamma Counter for the Estimation of Iodine 131 in Liquid Samples

A type 1186 (A.E.R.E. Harwell) scintillation counter with 2 in. diameter sodium iodide crystal has been inverted and used for assessment of 131I in 10 cc samples placed on top of the crystal. The background is doubled for some 0·0018 μc in 10 cc compared with 0·0030 μc for the type M6 liquid GM counter tube, and the higher count rates allow shorter counting times. The method has also been used for 59Fe and 60Co, but the optimum values of the photomultiplier and bias voltages depend on the particular isotope.