The dark current and signal/noise characteristics of an RCA 1P21 and an E.M.I. VX5031 photomultiplier have been compared from the point of view of their suitability for photoelectric Raman spectroscopy. The construction of a simple apparatus for the refrigeration of the RCA tube to liquid oxygen temperature is described. It is found that at low temperatures the noise associated with the dark current of the photomultiplier is negligible compared with the fluctuations (shot effect) in the photocurrent obtained when recording Raman spectra. Under these conditions the signal-to-noise ratio obtainable for a given rate of scanning of the spectrum depends solely on the square root of the light flux incident upon the photomultiplier, and cannot be improved by further refrigeration of the detector.
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