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

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• 作者：Pinghe Cai ; Xiangming Shi ; Willard S. Moore ; Minhan Dai
• 关键词：224Ra ; 228Th disequilibrium ; Coastal sediments ; RaDeCC system ; GEOTRACES
• 刊名：Marine Chemistry
• 出版年：2012
• 出版时间：20 July, 2012
• 年：2012
• 卷：138-139
• 期：Complete
• 页码：1-6
• 全文大小：638 K

文摘

We describe a method for measuring ²²⁴Ra and ²²⁸Th 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. ²²⁴Ra in interstitial water is co-precipitated by MnO₂ suspension. The MnO₂ suspension and the sediment with absorbed ²²⁴Ra and ²²⁸Th are filtered onto a 142-mm 0.7 ¦Ìm (nominal pore size) GFF filter. The filter is placed onto a sample holder specified for sediment samples and counted in a RaDeCC system. ²²⁴Ra and ²²⁸Th 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 ²³²U-²²⁸Th standard using the method of standard addition. The reproducibility and the overall accuracy of ²²⁴Ra and ²²⁸Th measurements based on this method are estimated to be ¡À 5 % and ¡À 5-7 % , respectively. We have applied this method to a coastal sediment and observed a significant deficit of ²²⁴Ra with respect to ²²⁸Th in the upper 3-4 cm.