A beta-alpha coincidence counting system for measurement of trace quantities of 238U and 232Th in aqueous samples at the Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory experiment was built to measure the total flux of B-8 solar neutrinos via the neutral current disintegration deuterium nuclei. This process can be mimiced by daughter isotopes of Th-232 and U-238 which can photodisintegrate the deuterium nucleus. Measurement of the concentration of such radioisotopes in the heavy water was critical to the success of the experiment. A radium assay technique using Hydrous Titanium Oxide coated filters was developed for this purpose and it was used in conjunction with a delayed beta-alpha coincidence counting system. The design, calibration and operation of this counting system are described in this paper. The counting efficiency for Th-232 (Ra-224) and U-238 (Ra-226) were measured to be 50 +/- 5% and 62 +/- 7%