TRUE CONSTANT FRACTION TRIGGER-CIRCUIT FOR PICOSECOND PHOTON-TIMING WITH ULTRAFAST MICROCHANNEL PLATE PHOTOMULTIPLIERS

In time correlated photon counting, the picosecond resolution offered by the reduced transit time spread of ultrafast microchannel plate (MC P) photomultipliers is fully exploited only if all other timing jitter contributions are minimized. With the ultrashort signals of MCP detec tors, the commercially available circuit modules do not provide true c onstant fraction triggering (CFT) operation. The residual amplitude de pendent time walk therefore sets the actual limit to the obtained time resolution. A new CFT scheme has been devised for obtaining true cons tant fraction triggering. A circuit has been accordingly developed and tested. The time walk is reduced to less than 8 ps for a 1:14 dynamic of the input signal (pulse amplitudes from 100 mV to 1.4 V) and to le ss than 25 ps for a 1:20 dynamic (from 70 mV to 1.4 V). It is shown th at such a performance makes it possible to exploit in actual experimen ts the 20 ps intrinsic resolution of the fastest available MCP detecto rs. (C) 1997 American Institute of Physics.