Measurement of 223 Ra and 224 Ra in coastal waters using a delayed coincidence counter

We describe a nuclear detector system for measuring low activities of 223 Ra and 224 Ra in natural waters based on an original design of Giffin et al. (1963). Samples are obtained by adsorbing 223 Ra and 224 Ra onto a column of MnO 2 coated fiber (Mn fiber). The short‐lived Rn daughters of 223 Ra and 224 Ra which recoil from the Mn fiber are swept into a scintillation detector where alpha decays of Rn and Po occur. Signals from the detector are sent to a delayed coincidence circuit, which discriminates decays of the 224 Ra daughters, 220 Rn and 216 Po, from decays of the 223 Ra daughters, 219 Rn and 215 Po. The system is calibrated using 232 Th and 227 Ac standards with daughters in equilibrium adsorbed on Mn fiber. Results of samples from Tampa Bay, Florida, and the Atchafalaya and Mississippi Rivers mixing zones are reported. The method is extendible to measurements of 227 Ac, 231 Pa, 228 Th, and 228 Ra.