A highly selective monitor for air-borne iodine-125

A description is given of an instrument which achieves high specificity for /sup 125/I with a standard sodium iodide scintillation detector, by using matched foils on indium and cadmium. An iodine-absorbing air filter is counted alternately through indium and cadmium. Only for /sup 125/I is there a significant difference in counting rate, due to the difference in k-edge energy for the two metals. The differential rate is averaged by a digital filter, and the /sup 125/I activity present is calculated and displayed. The air concentration of /sup 125/I is also estimated from the rate of change of the counting rate.<
>