A simple low background photon coincidence detector for tritium liquid scintillation counting

The electronic system consists of two 13-stage Venetian blind photomultipliers coupled to a simple DeBenedettiRichings coincidence circuit, followed by a medium-gain amplifier, pulse selector, and scaler. The main feature of the system is its simplicity, chiefly realized by utilizing the quasi- linear transfer properties of the take-the-lesser type coincidence -circuit. Balance-point operation permits the use of multiplier high voltage supplies and pulse amplifiers not possessing the high stability usually required for scintillation counting. The counter is operated at 26 deg C with a tritium counting efficiency of 16% at a background of 1OO cpm. The minimum detectable tritium concentration in water is 1.7 x 1O/sup -//sup 8/ c/liter. However, the time needed for the counter to ""quiet down"" after a change of sample is prohibitively long, 2 to 3 days. Of the background, nearly 50% seems to be caused by light interaction between the photomultipliers. Some suggestions for future improve- . ments are given. (auth)