IMPROVEMENT OF SIGNAL-TO-NOISE RATIO OF PHOTOMULTIPLIERS FOR VERY WEAK SIGNALS

For the photoelectric detection of Raman spectra excited with a He-Ne laser, the dark current of an EMI 9558 QA photomultiplier is reduced by magnetic defocusing of the electrons originating from the inefficient part of the photocathode. A lightpipe cone concentrates the light signal to a spot of 2.7 mm diameter and enhances the quantum efficiency of the photocathode by multiple reflection. A pulse counting system with a deadtime of less than 100 nsec is described and compared with dc detection. It is now possible to detect signals with a power of less than 10-17 W or less than 35 photons/sec at 650 nm with a time constant of 1 sec. The improvement in signal-to-noise ratio is demonstrated with the Raman spectrum of polycrystalline azobenzene. © 1969 The American Institute of Physics.