A High Current Coaxial Photomultiplier

A photomultiplier structure capable of high current output and relatively fast rise time has been developed and several prototypes have been built. The design incorporates eight separate photocathode elements spaced around the cylindrical structure. A five or six-stage multiplier focuses the electrons onto a central coaxial collector-output structure matched directly to a coaxial line. Each stage of the multiplier consists of a ring of eight dynodes, giving effectively eight parallel current channels from photocathode to collector. The structure is approximately two and one-half inches in diameter by four and one-half inches long. Saturated pulse output currents of nine amperes have been obtained in initial tests. Linearity within approximately 10 per cent has been obtained with output currents of four amperes. Calculations and measurements indicate a transit time of approximately 5 millimicroseconds with a spread of the order of 1 mμsec. Current gains of 4000 at 2500 volts over all hate been obtained with a five-stage multiplier with antimony-cesium dynodes. The geometry of this structure and the operating characteristics are discussed. A psuedo-logarithmic response capability, inherent in this design of tube, is expected to be useful for covering an extended dynamic input range.