Calibration of RaDeCC systems for 223Ra measurements

• Develops a method of calibrating RaDeCC systems for 223Ra using 232Th standards
• Typical detection efficiency for 224Ra is 6–11% greater than for 223Ra.
• Differences are due to dead volume and losses in delay-coincidence circuit.

RaDeCC (or radium delayed coincidence) counters have proved to be a fast and precise way to measure 224Ra and 223Ra in water samples. However, the calibration of these counters for 223Ra has been problematic. Unlike standards prepared from 232Th to calibrate 224Ra–220Rn measurements, standards prepared from 227Ac for 223Ra–219Rn measurements decrease in the amount of 219Rn they release with time. Here we develop a way to use the 232Th 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 220Rn 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 3–8% lower predicted probability of 219Rn decaying in the counting cell relative to 220Rn. 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 219Rn–215Po pair and the 220Rn–216Po pair. These differences may increase slightly the efficiency of the 219 channel because of the higher alpha decay energies of 219Rn–215Po.