ULTRAVIOLET SPECTROSCOPIC TECHNIQUE 137 



which are sensitive only within well-defined spectral regions; the upper 

 wave-length limit is defined by the quantum threshold and the lower by 

 the absorption properties of the photocell envelope. 



Thus a cell with a cadmium-magnesium surface and a Corex-D window 

 has a spectral response curve closely paralleling the action spectrum for 

 erythema production, and is of considerable utility in the measurement of 

 the erythemal effectiveness of various sources (KoUer and Taylor, 1935; 

 Kerr, 1947; Taylor, 1944). Other cells with magnesium (Coblentz and 

 Cashman, 1940), titanium (Coblentz and Stair, 1935; Kuper et al., 1941), 

 or uranium (Rentschler, 1930) surfaces have found use for the measure- 

 ment of the intensity of extreme ultraviolet radiation from the sun. 



Other metals such as zirconium, thorium, tantalum, platinum, and 

 alloys such as beryllium-copper have threshold wave lengths at various 

 places in the ultraviolet spectrum and might be used to provide photocells 

 with specific spectral response characteristics (Rentschler et al., 1932; 

 Glover, 1941; Andrews, 1945; Morrish et al., 1950; Piore et al., 1951). 

 The precise threshold wave length and spectral response curves of these 

 metallic surfaces depend considerably on the particular method of prepa- 

 ration (Dejardin, 1933). 



Most metallic surfaces, however, have relatively low quantum effi- 

 ciency, emitting one electron per lO'^-lO^ incident quanta (Sommer, 1947). 

 Hence, most modern photocells are made with composite surfaces, such 

 as the cesium-antimony surface which has a quantum yield of approxi- 

 mately 0.1-0.3 at the wave length of maximum response (400 m/x) (Janes 

 and Glover, 1941; Sommer, 1947; Morton, 1949; Zworykin and Ramberg, 

 1949, Chaps. 5 and 6) and maintain a high yield well into the ultraviolet 

 (Fig. 4-4). 



Glass-jacketed photocells begin to decline in sensitivity at wave lengths 

 less than 3500 A. Commercially available ultraviolet-sensitive photo- 

 cells have envelopes of Corex-D, Corning 9741, or Vycor glass. The 

 Vycor glass provides good transmission to approximately 210 m/x, but 

 begins to absorb appreciably at shorter wave lengths (Nordberg, 1947). 

 Some response may be obtained to wave lengths as short as 160 m^ 

 (Dunkelman and Lock, 1951), owing in part to fluorescence of the glass. 

 Special quartz-jacketed photocells have high sensitivity to 175 mu and 

 will respond to wave lengths as short as 155 m^t. 



In the vacuum-type photoemissive cell, the current developed is directly 

 proportional to radiation intensity over several decades of intensity range. 

 The current developed, obtained in a typical photoemissive cell as a func- 

 tion of anode voltage, is shown for several radiation intensities in Fig. 4-5. 

 Evidently at any anode voltage greater than 25 volts, the full photocur- 

 rent is collected, and the current is thus substantially independent of 

 anode voltage. The total current which may be drawn from a photo- 

 surface without damage is limited to values of 5-10 ^a/cm-. 



