248 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



primarily by the method of operation of the photomnltiplier (see 

 below). The lower limit is set by a dark current and by noise. By 

 counting the pulses caused by individual photoelectrons, the lower 

 limit in a 5-min period of observation is 2 x 10~ 16 lumen. 1 At high 

 levels of illumination, the useful range of a photomultiplier is limited 

 by fatigue and lifetime considerations. This limit is, in general, of 

 the order of 10~ 6 to 10~ 4 lumen for standard photomultipliers and 

 continuous operation but can be raised appreciably for intermittent 

 operation. 



An exposure to light in excess of the specified level can cause a 

 reversible and, under extreme conditions, irreversible reduction of 

 the sensitivity (fatigue). The fatigue effect can be caused, even with- 

 out the application of a voltage to the photomultiplier, by an attach- 

 ment of electronegative gases that remain in the tube after pumping. 2 

 With voltage applied to the photomultiplier, the impact of positive 

 ions tends to deteriorate the cathode surface or the dynode surfaces. 

 Also, the exposure to high levels of illumination causes electrons to be 

 emitted from the cathode at a high rate. These electrons are not 

 replenished fast enough from internal layers, and a number of posi- 

 tive ions remain unneutralized. 3 The result is a decrease of sensitivity 

 and may be a shift of spectral distribution, in particular a reduction 

 of the photoelectric threshold to shorter wavelengths. Frequently, 

 the photocathode recovers after a period of idleness (several minutes 

 to hours) in darkness at room temperature; recovery may also be 

 achieved by short heating or by irradiation with red or infrared light 

 of a wavelength longer than the threshold wavelength. A fatigue 

 effect also occurs at the last dynode stage or stages under the in- 

 fluence of high current. If the continuous anode current is less than 

 1 mA, the tube usually recovers after storing in darkness for a day. 

 At higher currents, the sensitivity can be irreversibly lost. 4 The 

 effect is probably caused by a dissociation of surface molecules at the 

 last dynode and a subsequent diffusion of the cesium off the surface. 5 



Commercial photomultipliers are available with opaque or trans- 

 parent photocathodes. Tubes with transparent photocathodes are 

 usually built for end-on observation so that the cathode can be 

 brought close to the illuminating source (e.g., scintillation crystal). 



1 R. W. Engstrom, J. Opt. Soc. Am., 37, 420 (1947). 



2 J. H. de Boer, "Electron Emission and Adsorption Phenomena," chap. 6, 

 The Macmillan Company, New York, and Cambridge University Press, London, 

 1935. 



3 de Boer, ibid., chap. 8, sec. 96. 



4 Engstrom, loc. cit., 



5 Report on a paper by L. Cathey, Nucleonics, 16, 59 (June, 1958). 



