236 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



ultraviolet region. Examples of response characteristics are shown 

 in Fig. (5-1)3, £-1 and £-3. 



2. Sb-M type ("alloy type"). The maximum response for these 



cathodes is between 400 and 500 m/*, the threshold wavelength 



between 600 and 700 iyiju. The thermionic dark current (see below) 



of these cathodes is small. At the response maximum, the quantum 



100 



80 



60 



40 



£0 



300 



500 



o 



700 900 



Wavelength, mju 



1,100 



1,300 



o 

 > 



c 3 



CD 0} O 



3 P = 



CC i 



Fig. (5-1)3. Relative sensitivity of different commercial 

 photocathodes {from RCA Tube Manual; by permission). 



yield is very high, about 20 per cent. Typical response charac- 

 teristics are shown in Fig. (5-1)4, S-4, S-5, and S-9. Considerable 

 increase of the sensitivity and of the threshold wavelength can be 

 obtained by combining several alkali metals with the antimony. 1 



3. Bi-O-Ag-M type. The maximum of the spectral response curve 

 is in the same wavelength region as that of the Ag-O-M type. A 

 response characteristic is shown in Fig. (5-1)5, S-S. 



The response characteristic of all photoelectric devices is in- 

 fluenced in the ultraviolet region by absorption in the glass wall of 

 the bulb. Lime glass absorbs practically all radiation of a wave- 

 length less than 300 m^. Ultraviolet transparent glass (Corex D) 

 transmits down to 280 m/u, and a specially thin window will transmit 



1 A. H. Sommer, Rev. Sci. Instr., 26, 725 (1955). 



