Modification in ZnS(Ag) scintillation monitor for detection of interference due to stray light

Portable α-scintillation monitor that uses ZnS(Ag) as a scintillator is the most common detector for detection and measurement of α contamination on floor, equipment surfaces etc. The scintillator is coupled to a photomultiplier tube to detect light pulses from the scintillator and the scintillator is covered with thin aluminized mylar film (on the face opposite to that facing the photomultiplier tube) to prevent stray light falling onto the photomultiplier tube. It is observed that the response of the detector goes down and there are errors in measurement if the mylar film permits passage of stray light onto the photomultiplier tube. This can happen even with a few pinholes in the mylar film which may not be easily visible. In order to avoid this, the instrument was modified using discriminator circuitry for detecting light leakage and also to indicate saturation of the photomultiplier tube that may occur in the event of leakage of large amount of stray light. Incorporation of a discriminator and a saturation indicator in an α-scintillation monitor has helped to improve error free measurement of α-contamination. α scintillation monitor modified for indication of stray light leakage and photomultiplier tube saturation is in use at AFFF, Tarapur. The paper describes the details of the studies and the experience gained. (author)