Low-background Beta Counters for Iodine-131 Sources

Abstract

IN a recent article¹ the use of two end-window Geiger counters (20th Century Geiger-Müller type EW2C) for scanning chromatograms containing iodine-131 was discussed. It was shown that using anti-coincidence techniques the background in the double-counter system was typically about 3.5 c.p.m. Furthermore, it was pointed out that significant fraction of this background arose from radioactive contaminants in the window material of the counters. The efficiency of detection for iodine-131 and Whatman No. 1 chromatography paper is about 50 per cent. Using the formula quoted in ref. 1 the total time (T) required to determine the presence of 2 µµc. with an accuracy of 10 per cent is 375 min. By carefully selecting Geiger counters, backgrounds lower than the typical one can be obtained, although it is doubtful if it is possible to do better than about 2.8 c.p.m. The above value of T would then be reduced to 316 min.