The Use of Standard Alpha and Beta Surface Scintillation Contamination Monitors to Confirm the Contamination Fingerprint and to Check on Source Quality

Normally, beta and alpha surface contamination monitors are used with a simple counting threshold, i.e. any pulse over a predetermined amplitude is counted. This is very different from gamma monitoring, where the use of counting windows is very popular and the use of full multi-channel analysis is common. Many current surface contamination ratemeters have the capacity to drive dual phosphor detectors and can be set up to provide beta and alpha channels. Effectively, the beta channel is a counting window, i.e. all pulses which are bigger than the threshold and smaller than the alpha threshold are counted. Larger pulses go into the alpha channel. This paper addresses how this can be used with beta only and alpha only detectors to provide information on the source. The detector is set up conventionally to a defined point for the lowest beta energy anticipated. The instrument is then switched to alpha + beta mode and the alpha threshold set to 3 times the beta threshold. With this set up, the alpha to beta channel count rate ratio varies smoothly by a factor of 14 between Y-90 (Emax 2.27 MeV) and C-14 (Emax 0.16 MeV). Hence the instrument can be used to estimate the energy of an unknown beta contaminant or to confirm that a mixed beta fingerprint has essentially the same mix. The same approach can be used with alpha probes to confirm the source quality. The main worry with alpha monitoring is the surface condition. A poor surface condition will lead to a low count rate. Using the channel ratio method will identify grubby sources. The resulting ratio can be used either as a go/no trigger, i.e. any surface with a low ratio will be treated as untrustworthy, or alternatively the ratio can be used to correct the reading to give a better estimate of surface activity.