Ultra trace analysis of acrylic for 232Th and 238U daughters

Abstract The acrylic vessel housing 1000 tonne of D 2 O at the Sudbury Neutrino Observatory must contain pg·g −1 232 Th / 238 U , with their decay chains in secular equilibrium. (Gamma rays emitted by subsequent members of these chains will contribute most of the background signal.) The activities of the relevant isotopes have been measured by alpha spectroscopy, following chemical separation of acrylic ash, with a sensitivity of 0.5 pg·g -1 . Acrylics have been found meeting these design criteria for Th and U.