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 0.60 to 0.90 for 14C and from 0.87 to 0.96 for 204Tl. In this study, the characteristics of the system are described as well as data acquisition and analysis methods.