Large Volume Scintillation Chamber for Radon Counting

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Abstract
A 450‐cc cylindrical Lucite chamber covered internally with ZnS(Ag) and viewed at each end by 5‐in. photo‐multiplier tubes with summed outputs has been used routinely as a low background (∼0.8 counts per min for a new chamber) Rn scintillation counter. The counter is simple to construct and has operated reliably for about two years. The counting efficiency for Rn in transient equilibrium with its two alpha‐emitting daughters is 2.1 counts per Rn decay. Thin lines of silver conducting paint applied externally on the ends of the chamber, together with surrounding Al foil, form a complete electrostatic shield which has assured reproducible counting efficiencies by suppressing randomly occurring internal fields. Results of experiments, in which external potentials were applied to the end plate electrodes of an experimental chamber, are presented in detail. It was found that as little as 22 V applied across the chamber can change the counting efficiency by 8%. A back voltage effect caused by the deposition of ionization‐induced charge on the internal surfaces during voltage application was observed.

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