ULTRAVIOLET SPECTUOSCOPIC TECHNIQUE 



141 



variations in time of transit of electrons from photosurface to anode, which 

 are of the order of 6 X 10~^ sec (Morton, 1949). As a consequence, the 

 photomultiplier tube will faithfully respond to very brief pulses of light, 

 as short as 10~^ sec. 



The amplified photocurrent from the photomultiplier tube easily over- 

 whelms the random fluctuations in electric current arising in the external 

 circuit, as a result of thermal agitation, so that the only limitations on the 

 sensitivity of a photomultiplier detector are those arising from the ran- 

 dom fluctuation of the "dark current" which is actually the thermal 

 emission of electrons from the photosurface, and from those inherent in 



1.000,000 



125 



150 



75 100 



VOLTS PER STAGE 



Fig. 4-9. Amplification characteristic of focused type photomultiplier. (Ent/strom, 

 1947a; Journal Optical Society of America.) 



the statistical nature of the radiation intensity itself. The thermal- 

 emission dark current and its corresponding fluctuations may be reduced 

 by choice of a photocell with a small photosensitive surface, or it may 

 be minimized by refrigeration of the photocell, without appreciably 

 influencing the sensitivity to radiation (Engstrom, 1947a, b). 



It should be recognized that photoelectric detectors vary considerably 

 from tube to tube (of the same design) with regard to sensitivity, to 

 variation of sensitivity with wave length, and to the dark current. 

 Because of these variations, if it is desired to use photoelectric detectors 

 for the comparison of two beams of radiation, one of two courses is neces- 

 sary: (1) some artifice whereby one detector may be used must be 

 employed, or (2) if two detectors are used, either matched tubes must be 

 found, or some means of compensating for their differences (which may be 

 expected to be reasonably stable) must be provided. A single detector 

 may be used, if it is alternately exposed to the two beam.s in time, or if 

 the electrical signal arising from each beam can be distijiguished by virtue 

 of a frequency or phase modulation (Wright and Herscher, 1947; Savit- 

 zky and Halford, 1950; Wyckoff, 1952). 



