High speed photomultipliers

Experimental results on two 8-stage photomultipliers exhibiting flat baseband response to Gc will be discussed. One photomultiplier employs crossed electric and magnetic fields, the other is purely electrostatic. At present both devices employ Cu-Be dynodes and an internally situated, opaque-backed photocathode. Frequency response measurements have used heterodyning between the modes of the Zeeman-split doppler profiles of a 6328 A He-Ne laser and also shot noise in the current from a steadily illuminated photocathode as sources of microwave modulated current. Both photomultipliers have been used to observe sub-nanosecond pulses from phase-locked He-Ne and argon ion lasers. An estimate of the frequency response to be expected from identical stages of secondary emission, when only the secondary electron emission velocities contribute to electron transit time differences, gives results in agreement with experiment, assuming that the most probable secondary electron emission energy from BeO is about 3 e.v. This value is consistent with the direct measurements of others.