254 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



of the temperature from room temperature to that of liquid air 

 reduces the noise level by a factor of 100. 



The equivalent noise input, i.e., that light intensity at the input of 

 the photomultiplier which furnishes at its output a current equal to 

 the rms noise output over a band width of 1 cycle is, for commercial 

 photomultipliers operated at room temperature, between 5 x 10 -13 

 and 10 -11 lumen. 



The noise spectrum is flat over a frequency range from to 10 8 cps, 

 but falls off between 10 8 and 10 9 cps. 1 Gordon and Hodgson 2 have 

 observed a reduction of the noise level by a factor of 7 in a photo- 

 multiplier that was kept in darkness for five weeks. The effect seems 

 to be due to a decay of glass fluorescence. 



Comparison of a photomultiplier with a photoelectric cell plus 

 amplifier furnishes the following result: Both the photoelectric cell 

 and the photomultiplier exhibit thermionic emission noise and shot 

 noise, which appear amplified at the output. However, the ampli- 

 fier following a photoelectric cell and the coupling resistor between 

 the photoelectric cell and the amplifier also cause a noise component 

 (Johnson and shot noise) in the output. This noise from the amplifier 

 is constant; it does not diminish if the light intensity decreases. 

 Consequently, the signal-to-noise ratio for a photocell plus ampli- 

 fier decreases for small light intensities. On the other hand, the shot 



noise from a photomultiplier is proportional to vi p , therefore pro- 

 portional to the root of the incident light intensity, so that the shot 

 noise decreases more and more with a decrease of the incident light 

 intensity and only thermionic emission remains as a limiting factor. 

 However, the superiority of the photomultiplier exists only at small 

 photoelectric currents. At higher levels of light, when the photo- 

 current from the cathode reaches the order of 10 -6 amp, the signal-to - 

 .noise ratio is almost the same for both systems. 3 



Investigations of the influence of temperature upon the perform- 

 ance of photomultiplier tubes have led to contradictory results. 

 While some authors find a slight increase of amplification with in- 

 creasing temperature, as shown, for instance, in Fig. (5-1)19, others 

 have observed a decrease of amplification, sometimes up to 40 per 

 cent for a change of temperature from —15 to — 50°C. 4 



The movement of the electrons can be strongly influenced by 



1 Sard, loc. cit. 



2 B. E. Gordon and T. S. Hodgson, Nucleonics, 14, 64 (1956). 



3 J. A. Rajchman and R. L. Snyder, Electronics, 13, 20 (December, 1940). 



4 F. E. Kinard, Nucleonics, 15, 92 (April, 1957); further references in the 

 same paper. 



