Calibration system for portable alpha and beta contamination monitors

Evaluation of contamination is done by various contamination monitoring instruments. The calibration of these instruments is essential for evaluating their performance over time. Generally, calibration of contamination monitors is performed using small-area disc sources. It has several challenges, such as irradiate only a small portion of the detector’s sensitive area, which may not accurately represent the monitor’s overall response, especially for large-area detectors. In addition, maintaining a fixed and reproducible geometry between the point source and the detector is difficult. This paper describes a calibration system for portable alpha and beta surface contamination monitors with automated source handling. This system features large area standard sources such as Americium-241 and Strontium (+Yttrium)-90 sources with active area 250 mm × 170 mm with activity uncertainty of ±3%. In addition, a fixed and reproducible distance ensures consistent irradiation across the entire sensitive area of the detector. This paper discusses in detail the calibration methodology, efficiency determination, and conversion factor calculations using this system. The system provides high reproducibility and repeatability due to the fixed distance between the source and detector and the use of a large-area planar source.