This arrangement eliminates switching of dynodes in the high impedance grid circuit. Voltage regulator tubes, V-3 and V-4+ are selected, and the gain controls R-9 and R-19 adjusted, so that the gains of the individual photomultiplier tubes are matched into the single amplifier circuit. Ligtt excited in the phosphor by gamma radiation illuminates the photocathode of the photomuitiplier tube which produces electrons that are multiplied by secondary emission of electrons from the dynodes. The electron current pulses appearing on dynode 10 are integrated by capacitor C-2 and averaged by tke effective grid to cathode resistance of the amplifier tubes. The value of this RC product is kept small to keep the time constant of the instrument fast, The amplifier consists of two Raytheon CK-533AX tubes, which are connected in parallel to secure the power output necessary to drive a recorder. When these tubes are driven from a high-impedance current source, as provided by the dynode of a photomultipiier tube, the plate current is a logarithmic fimction of the input grid current. Individual tubes are selected for this characteristic. Since the electrical characteristics of photomultiplier *tubes differ, individual bias adjustments are required. inserted under the grid resistor, R-25. This bias is Because of the 107 ohm value of R-25, the grid sees a constant current bias. The slope of the logarithmic characteristic (at the high end of the scale) is set by the gain controls, this slope at the low end. The bias controis are used to match Over a 4-1/2 decade range the plate current change is 180 microampers per decade of radiation. -ll- This current is