Predicted responses for a particulate detection system in a continuous stack monitor

Predicted counting rate responses were developed for a particulate detection system that is used for continuous monitoring for the presence of radioactive particulates in the effluent air from a research reactor. The particulate detection system consists of a moving filter paper assembly, a plastic scintillation detector, and a rate meter output, and is part of a comprehensive stack monitoring system. A predicted response was derived for the case of a steady-state activity distribution across the surface of the moving filter paper that is in proximity to the detector and was determined to be 1.59 x 10(7) cpm per unit airborne concentration of 138Cs (expressed in units of Bq cm(-3)), where 138Cs was used as an indicator for a hypothetical fission product release. The corresponding response model provided by the manufacturer was found to underestimate airborne activity concentrations by about an order of magnitude. A predicted response also was derived for the case of a rapid change in airborne activity concentration, which was formulated based on the kinetics of the rate meter circuit and was used to establish alarm settings and detection limits for the particulate detection system.