Abstract
The activities of
228Ra in natural waters were determined by the Cherenkov counting of the daughter nuclide
228Ac. The radium was pre-concentrated on MnO
2 and the radium purified via ion exchange and, after a 2-day period of incubation to allow for secular equilibrium between the parent–daughter
228Ra(
228Ac), the daughter nuclide
228Ac was isolated by ion exchange according to the method of Nour et al. [2004. Radium-228 determination of natural waters via concentration on manganese dioxide and separation using Diphonix ion exchange resin. Appl. Radiat. Isot. 61, 1173–1178]. The Cherenkov photons produced by
228Ac were counted directly without the addition of any scintillation reagents. The optimum Cherenkov counting window, sample volume, and vial type were determined experimentally to achieve optimum Cherenkov photon detection efficiency and lowest background count rates. An optimum detection efficiency of 10.9±0.1% was measured for
228Ac by Cherenkov counting with a very low Cherenkov photon background of 0.317±0.013
cpm. The addition of sodium salicylate into the sample counting vial at a concentration of 0.1
g/mL yielded a more than 3-fold increase in the Cherenkov detection efficiency of
228Ac to 38%. Tests of the Cherenkov counting technique were conducted with several water standards of known activity and the results obtained compared closely with a conventional liquid scintillation counting technique. The advantages and disadvantages of Cherenkov counting compared to liquid scintillation counting methods are discussed. Advantages include much lower Cherenkov background count rates and consequently lower minimal detectable activities for
228Ra and no need for expensive environmentally unfriendly liquid scintillation cocktails. The disadvantages of the Cherenkov counting method include the need to measure
228Ac Cherenkov photon detection efficiency and optimum Cherenkov counting volume, which are not at all required when liquid scintillation analysis is used.