Selection of Filter Media in Alpha Air Monitors for Emergency Environmental Monitoring

INTRODUCTION We have developed an alpha air monitor which is possible to measure rapidly and sensitively the concentrations of airborne alpha-emitting particles, such as plutonium, for the environmental monitoring at an accident of nuclear reprocessing plant. The monitor is designed to collect airborne alpha-emitting particles by drawing the ambient air through a filter and to detect the activity by alpha spectroscopy. In order to achieve high-sensitive measurements, selection of a suitable filter used in the monitor is considerably important (1, 2). The most important requirement for the filter is that it has a high surface collection efficiency to obtain the sharpness of the alpha energy spectrum. This makes it easy to distinguish the alpha-ray peak of plutonium from the alpha spectrum of naturally occurring radon decay products in the environment. And the filter is also desired to have low resistance of the air flow so that particles can be collected at a high flowrate. We have examined the surface collection efficiency and pressure drop for the various filters. Test filters were glass fiber, cellulose-glass fiber, membrane and so on. The surface collection efficiency has been evaluated by the sharpness of alpha-ray energy peaks of thoron decay products generated in a laboratory and collected on the filters. The pressure drops were measured at the various face velocities. Furthermore, for the test filters which showed good performance at the above two experiments, we have examined the effect of the radon decay products simultaneously collected on the filter by air sampling in the atmosphere. The present report describes the experimental method and the results obtained.