A PULSE-COINCIDENCE RN-220 MONITOR WITH 3 TIME-GATES

A pulse-coincidence Rn-220 monitor has been developed for low-level co ntinuous measurement. This method is based on delayed coincidences bet ween the alpha pulses of Rn-220 and Po-216. The monitor consists of a 300-mL flow-through scintillation cell facing a photomultiplier tube, connected to a custom-built pulse-coincidence circuit with three time- gates to distinguish Rn-220-Po-216 pairs counts from sequential alpha pulses. Each time-gate has an open duration time of 150, 300, or 450 m s, respectively. The output of the time-gates is used in statistical a nalysis to ascertain that the real Rn-220-Po-216 pair counts are corre ctly discriminated. Poisson distribution probability is introduced to correct for chance coincidence pulses. The sensitivity of the monitor is estimated at about 0.58 cph/Bq m(-3) for Rn-220 where cph is counts per hour. Therefore, this monitor is particularly useful for a low-le vel concentration. In addition, another data analysis method is also s uggested for utilization of this monitor in high-level environments. T he quality of Rn-220 concentration estimated by the monitor is guarant eed by a comparison with the value of a grab sampling method using a s cintillation cell. Copyright (C) 1996 Elsevier Science Ltd.