PROPERTIES OF AN AIR-FLOW IONIZATION-CHAMBER FOR THE MEASUREMENT OF BETA-CONTAMINATION

Traditional beta or alpha contamination monitors are designed so that these charged particles are detected directly. Successful monitoring c an be difficult when the range of the particle to be detected is low a nd the surface to be monitored has a complex topology. This can be the case, for example, with contaminated tools and equipment. One means o f overcoming these problems is to draw air from over the surface to be monitored into an ionisation chamber and to detect the ions formed in air by the radiation rather than the radiation particles themselves. So-called long range alpha detectors have been developed and used for soil and surface monitoring. This paper describes the design of an air -flow ionisation chamber used to investigate the properties of air ion s formed by beta sources of various energies. The aim of this work was to study the formation of air ions, their transfer to the ionisation chamber by air flow and the loss of ions due to diffusion and recombin ation. From this data the overall detection efficiency of the air-flow chamber was calculated and an estimate of the lower detection limit m ade. The potential of this type of system for monitoring surfaces for tritium and other low energy beta emitters is discussed.