Study of the background on a ZnS(Ag) alpha counter with a plastic veto detector

Alpha counters based on the scintillation of ZnS(Ag) have been used widely to measure total alpha activity in environmental samples. The main difficulties for this kind of detectors consist of having a reasonable low background, i.e., around 10−3 counts/s. It is assumed that the background comes from natural sources (mainly radon) and there is no contribution from cosmic, beta or gamma rays since the efficiency for these sources is extremely low. However, the study of the background using a thin plastic veto detector shows that the cosmic ray contribution to the alpha background is about 5–50%, whereas the estimated gamma contribution is below 10%. Therefore, this anti-coincidence set-up can be useful to know and select the optimum conditions for operation, reducing significantly the background due to these sources, and, consistently, needing less time for the measurements.