Optimizing Measurement Sensitivity to Facilitate Monitoring Environmental Levels of Rn-DAUGHTER Concentrations

Abstract

• In the measurement of environmental levels of radioactivity, the primary problem is the accumulation of a statistically meaningful number of counts within a reasonable period of time. In the case of measurements of airborne 222 Rn-daughter concentrations, the problem is further complicated by the particularly short halflife, 3.05 minutes, of RaA (Po 21°). Since three Rndaughters-RaA, RaB (Pb214) and RaC (Bi 2 ") _ar e of Interest, the equations interrelating these Rn-daughter concentrations were derived from the laws of radioactive-series decay. These equations, although straightforward, are cumbersome to solve. To facilitate the efficient use of these equations, a computer program has been written which permits the calculation of Rn-daughter concentrations or expected counts for a given set of measurement parameters (flow rate and detector efficiencies). A subroutine then calculates the optimum pumping and counting times required to provide the number of counts necessary for acceptable statistics at environmental levels of 222 Rn-daughter concentrations. This subroutine contains a set of parameters, flow rate and efficiencies, that are fixed using realistic restrictions. The use of these optimized pumping and counting times results in maximum measurement sensitivity under realistic constraints.

Use of Gross Filter Activities in a Continuous Working Level Monitor

“…At the end of sampling, the filter paper was advanced to a counting position and counting started 17 sec after the end of sampling. Such short sampling and waiting periods are necessary for an accurate measurement of RaA concentrations (Ke78). The average sampling flowrate was 17 l./min.…”