Measurement of 224Ra:228Th disequilibrium in coastal sediments using a delayed coincidence counter

We describe a method for measuring Ra-224 and Th-228 activities in coastal sediments based on a delayed-coincidence counting system (the RaDeCC system). Milli-Q water is added to bulk sediment to form a slurry. Ra-224 in interstitial water is co-precipitated by MnO2 suspension. The MnO2 suspension and the sediment with absorbed Ra-224 and Th-228 are filtered onto a 142-mm 0.7 mu m (nominal pore size) GFF filter. The filter is placed onto a sample holder specified for sediment samples and counted in a RaDeCC system. Ra-224 and Th-228 activities are calculated from two measurements that are conducted within 6-12 h and in 8-10 days after sample collection. The RaDeCC system is calibrated with a U-232-Th-228 standard using the method of standard addition. The reproducibility and the overall accuracy of Ra-224 and Th-228 measurements based on this method are estimated to be +/- 5% and +/- 5-7%, respectively. We have applied this method to a coastal sediment and observed a significant deficit of Ra-224 with respect to Th-228 in the upper 3-4 cm. (c) 2012 Elsevier B.V. All rights reserved.