The tuning algorithms used by the Donner 600 crystal tomograph

The authors describe the computer algorithms used to adjust the energy thresholds and timing delays in the Donner 600 Crystal tomograph. The adjustments are made using 1890 computer controlled digital-to-analog converters (DACs) and an orbiting positron source. The energy threshold for each crystal is adjusted by measuring the counting rate of each crystal-crystal coincidence as a function of the DAC settings that control a pulse height window for the crystal in question. The DAC settings that correspond to the 511-keV photopeak are noted, thus determining the conversion from DAC setting to energy and allowing the DACs to be set to any desired energy window. The DAC settings controlling the timing delay for each channel are systematically adjusted to maximize the overall event rate. The authors discuss the convergence of the tuning algorithms and report on the photomultiplier tube gain and timing variations over a period of 18 months