Evaluation of counting methods for oceanic radium‐228

Abstract

Measurement of open ocean 228Ra is difficult, typically requiring at least 200 L of seawater. The burden of collecting and processing these large‐volume samples severely limits the widespread use of this promising tracer. To use smaller‐volume samples, a more sensitive means of analysis is required. To seek out new and improved counting method(s), conventional 228Ra counting methods have been compared with some promising techniques which are currently used for other radionuclides. Of the conventional methods, α spectrometry possesses the highest efficiency (3–9%) and lowest background (0.0015 cpm), but it suffers from the need for complex chemical processing after sampling and the need to allow about 1 year for adequate ingrowth of 228Th granddaughter. The other two conventional counting methods measure the short‐lived 228Ac daughter while it remains supported by 228Ra, thereby avoiding the complex sample processing and the long delay before counting. The first of these, high‐resolution γ spectrometry, offers the simplest processing and an efficiency (4.8%) comparable to α spectrometry; yet its high background (0.16 cpm) and substantial equipment cost (∼$30,000) limit its widespread use. The second no‐wait method, β‐γ coincidence spectrometry, also offers comparable efficiency (5.3%), but it possesses both lower background (0.0054 cpm) and lower initial cost (∼$12,000). Three new (i.e., untried for 228Ra) techniques all seem to promise about a fivefold increase in efficiency over conventional methods. By employing liquid scintillation methods, both α spectrometry and β‐γ coincidence spectrometry can improve their counter efficiency while retaining low background. The third new 228Ra counting method could be adapted from a technique which measures 224Ra by 220Rn emanation. After allowing for ingrowth and then counting for the 224Ra great‐granddaughter, 228Ra could be back calculated, thereby yielding a method with high efficiency, where no sample processing is required. The efficiency and background of each of the three new methods have been estimated and are compared with those of the three methods currently employed to measure oceanic 228Ra. From efficiency and background, the relative figure of merit and the detection limit have been determined for each of the six counters. These data suggest that the new counting methods have the potential to measure most 228Ra samples with just 30 L of seawater, to better than 5% precision. Not only would this reduce the time, effort, and expense involved in sample collection, but 228Ra could then be measured on many small‐volume samples (20–30 L) previously collected with only 226Ra in mind. By measuring 228Ra quantitatively on such small‐volume samples, three analyses (large‐volume 228Ra, large‐volume 226Ra, and small‐volume 226Ra) could be reduced to one, thereby dramatically improving analytical precision.