Reduction of the effective cathode area of photomultiplier tubes is known to yield lower dark currents and an improved signal-to-noise ratio. This can be accomplished either by electrostatic or magnetic defocusing of the electron beam between the cathode and the first dynode. Three magnetic lenses with different field geometries were tried with an EMI 9558B photomultiplier tube, and a noise reduction of more than an order of magnitude was observed. Magnetic defocusing changes the pulse height characteristies of the tube as well and improves the discrimination between signal pulses and dark pulses when using simple pulse height discrimination techniques. If the tube is operated as a photon counter, the light-to-dark count ratio may be improved by nearly two orders of magnitude with the application of a suitable magnetic lens.
W. Knight, Y. Kohanzadeh, and G. Lengyel, "Evaluation of Magnetic Defocusing for a Photomultiplier Tube with Large Area Semitransparent Photocathode," Appl. Opt. 7, 1115-1120 (1968)