CONTAMINATION CONTROL OF PLUTONIUM-241 BY MEASUREMENT OF THE LOW ENERGY BETA RADIATION.

Plutonium-241 (a β-emitter of maximum energy 20 KeV) occurs with other plutonium isotopes (α-emitters) in irradiated reactor fuel. and in laboratories using material enriched in plutonium-241, contamination control must be based on β-ray measurement. Direct monitoring of contaminated surfaces is impracticable, but smears from such surfaces, and samples of airborne dust collected on filter paper may be measured either with an internal gas-flow proportional counter or a thin plastic scintillation phosphor. To detect the 20 KeV beta radiations high amplification has to be used and accompanying alpha radiation then gives rise to several interfering effects which have been investigated. The gas-flow proportional counter cannot measure filter papers directly, and sample preparation is necessary; a concentration in air of about 02 of the maximum permissible value is detectable. To avoid the delay in ashing the scintillation system may be usde, in which the limit of detection is about one maximum permissible concentration in air. Smear sampling of clean surfaces can enable contamination of about 02 of a maximum permissible level to be detected with the scintillation counter, and somewhat greater sensitivity with the less convenient proportional counter.