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 ⁸B solar neutrinos via the neutral current disintegration deuterium nuclei. This process can be mimiced by daughter isotopes of ²³²Th and ²³⁸U 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 ²³²Th (²²⁴Ra) and ²³⁸U (²²⁶Ra) were measured to be 50 ± 5% and 62 ± 7%