Low Background Thin-Wall Flow Counters for Measuring Beta Activity of Solids

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Abstract
The construction and performance of disk‐shaped thin‐walled flow counters suitable for measuring very small beta activities of solids is described. The circular faces, which serve as the cathode, consist of conducting films. Either a single pear‐shaped loop of wire whose plane is parallel to that of the faces, or a group of straight wires stretched across the midplane serves as the anode. The counters are operated in the Geiger region. Samples can be counted against both faces of the counter. Performance data are given for counters having active volumes ranging from 0.2 to 7 cc, and having active areas from 1.6 to 23 cm2. Counting efficiencies for natural potassium (mounted on Lucite) vary from 30% to 47%, depending on the relationship of sample area to counter dimensions. Inside a 20‐cm steel shield and an anticoincidence ring the background counting rates varied approximately as 1 count/hr/cm2 of counting area. Backgrounds as low as 1 count/hr were obtained. The disk geometry permits the use of a smaller metal and anticoincidence shield than is possible with cylindrical geometry. Counters shielded successively with 1.9‐cm mercury, 5‐cm steel, and 5‐cm lead, and operated with a flat multiple‐anode anticoincidence counter (15 cm2), actually gave lower background than with the 20‐cm steel and anticoincidence ring assembly. An integral counter assembly, consisting of ∼2‐cm diam disk‐shaped counter and a disk‐shaped guard counter (∼7‐cm diam), was constructed and gave satisfactory results.