Choice of alpha-probe operating voltage to suit a wide range of conditions

Alpha probes, consisting of a ZnS(Ag) scintillator and a photo-multiplier tube, are commonly used throughout the nuclear industry for radiation protection and clearance of materials during decommissioning. The success in achieving these purposes is dependent on a number of factors including the counting efficiency of the probe, the condition of the material being monitored, the speed of monitoring and the distance between the probe and material. The efficiency of the probe is dependent on the operating voltage and is the only factor that is under the control of the calibration facility. As the calibration laboratory may not be aware of the specific environment in which the probe will be used, an operating voltage to suite a wide range of conditions must be chosen. In the past, it has frequently been assumed that it is necessary to set as high an operating voltage as possible in order to maximise the counting efficiency to low-energy alpha particles. However, the response to gamma rays, particularly those having low energies, also increases with operating voltage and will therefore limit the upper operating voltage that can be set. The efficiency of a scintillation-type probe (NE Technology AP2) in measuring contamination levels on a number of typical surfaces using different operating voltages has been investigated. It has been found that the surface characteristics of the material being monitored have far more effect on the results of alpha monitoring than the choice of operating voltage. Thus the calibration laboratory can set the operating voltage below the level at which there is a risk of response to low-energy gamma rays without significantly affecting the overall counting efficiency for low-energy alpha particles.