PHOTOELECTRIC DETECTORS 



oblique slats which permit the secondary electrons to pass from one dynode 

 to the next (Figure 28.34). Tubes with 11, 13 or 14 dynodes are available. 

 Gain varies approximately as the eighth power of the overall voltage supply 

 for an 1 1 stage tube and a highly stabilized supply is necessary for consistent 

 results. A 14 stage tube with a 160 V per stage may have a sensitivity of 

 between 10,000 and 30,000 A/lm with a dark current of about 5 ^A. 



Photocathode 

 connection 



-7 7/8 'It 1/8" 



75/i6"±V8" 



-53/16"+ 1/16" 



J Collector 

 *' connection 



mm 



Photocathode 

 Figure 28.34 Arrangement of electrodes of E.M.I, type 5311 photomultiplier 



Table 5 gives the main characteristics of this range of cells. Their spectral 

 sensitivity differs slightly from the sensitivity curves shown in Figures 28.23 

 and 28.24 because of the form of photocathode used. Refer to Rodda^^ and 

 Sommer^ for design theory for these tubes. 



Photomultiplier tubes should not be operated in the presence of strong 

 magnetic fields unless protected by a highly permeable — e.g. mumetal — 

 shield. Any metal clamps around the body of the tube should be at cathode 

 potential to prevent small electrical leaks which may give rise to hght 

 emission and to prevent deflection of the electron beams within. 



In selecting a photocell for a particular task it is important to bear in 

 mind that the thermionic noise from the photocathode is proportional to its 

 area. If it is possible to focus the light signal optically by means of lenses 

 or mirrors so that it forms a small image, a photocathode about the size of 

 the image should be used as this will give the best signal-to-noise ratio. If it 

 is not possible to focus the light on to a small area, as in, for example, a 

 scintillation counter, then a large area photocathode will be preferable. 



Selenium barrier-layer cell 



This type of cell has the great advantage over other light detectors that it 

 does not require an external source of voltage; the action of the absorbed 

 radiation is to generate an e.m.f. within it. It is the cell used in the familiar 

 photographic exposure meter. 



Its structure is shown in Figure 28.35. The cell consists of an iron or steel 

 base plate upon which is deposited a specially prepared selenium compound 

 in a crystalline state. On top of this is deposited a thin transparent, electri- 

 cally conducting layer. This layer and the base plate are the two electrodes. 

 To facilitate electrical contact with the upper layer a robust metal ring is 

 sprayed on to it. The front surface, excluding the collecting ring, is then 

 covered with a protective lacquer whilst the exposed iron is protected by a 

 rustproof metal coating. Potted cells, cast in Araldite resin behind a glass 



367 



