Development of 3-PM liquid scintillation counting system with geometrical efficiency variation.

An improved triple-to-double coincidence ratio system has been developed to obtain efficiency variation by means of geometrical displacement, that is, three photomultiplier (PM) tubes move back and forth direction from a vial source. The detection unit of the system is located in a cylindrical chamber for light shielding, and a stepping motor connected to the three PM tubes controls the source-to-detector distance up to 50 mm with 1 mm resolution. In activity measurements of 14C and 204Tl using this measurement technique, the efficiencies varied from 0.