Uncertainties in the preparation of 224Ra Mn fiber standards

Delayed coincidence counters (RaDeCC), used for measuring Ra-223 and Ra-224 preconcentrated from water onto MnO2-impregnated acrylic fiber (Mn-fiber), require a standard Mn-fiber column that has a precisely known activity of 224Ra for calibration. This may be done by adding an aged Th-228 standard solution to adsorb both Th-228 and its daughter Ra-224 quantitatively onto a Mn fiber. We used both seawater and deionized water (DIW) for testing the adsorption efficiency of Th and Ra onto Mn fibers. Our experimental results show that more than 50% of thorium (Th-232 and Th-228) breaks through the Mn-fiber column when DIW is used as a medium. However, near quantitative recoveries are obtained if filtered (0.45 mu m) seawater is used to prepare the standard. In the case of pure DIW, the pH (initial pH similar to 5.3) rises to >10 after passing through the column while seawater (initial pH similar to 7.8) changes to similar to 7.2. Thus, the lack of thorium adsorption in DIW may be attributed to this huge increase of pH and the consequent formation of Th(OH)(4) and polyhydroxyl colloids. Based on these observations, we recommend that one should use either artificial seawater or natural seawater (which has negligible Ra-224 and Th-221) as a loading solution after 0.45 mu m filtration. In addition, the thorium adsorption efficiency should be confirmed either by thoriurn analysis of the effluent solution or long-term monitoring of the supported Ra-224 on the Mn fiber using the RaDeCC. Similar cautions are likely necessary for making Ra-223 standards by adsorption of Ac-227 onto Mn fibers. (c) 2007 Elsevier B.V All rights reserved.