Calibration of RaDeCC systems for 223Ra measurements

RaDeCC (or radium delayed coincidence) counters have proved to be a fast and precise way to measure Ra-224 and Ra-223 in water samples. However, the calibration of these counters for Ra-223 has been problematic. Unlike standards prepared from Th-232 to calibrate Ra-224-Rn-220 measurements, standards prepared from (227)AC for Ra-223-Rn-219 measurements decrease in the amount of Rn-219 they release with time. Here we develop a way to use the Th-232 calibration of the 220 channel efficiency to calibrate the 219 channel. We first consider the slightly different effects of delay and window opening time constants in the default RaDeCC system. These differences favor Rn-220 detection by about 3%. Next, we consider the effect of dead volume on the relative efficiencies. In a typical system, with a source volume in the range 100-300 ml, there is about a 3 similar to 8% lower predicted probability of Rn-219 decaying in the counting cell relative to Rn-220. Thus, the absolute efficiency of the 219 channel should be about 6-11% lower than the efficiency of the 220 channel, depending on the dead volume of the system. We tested the effects of window time constants and dead volumes by altering these parameters in a known manner and correcting for the alterations. In this analysis we neglect the differences in alpha decay energy between the Rn-219-Po-215 pair and the Rn-220-Po-216 pair. These differences may increase slightly the efficiency of the 219 channel because of the higher alpha decay energies of Rn-219-(215)po. (C) 2013 Elsevier B.V. All rights reserved.