. Electronic apparatus for biological research . J Collector *' connection mm Photocathode Figure Arrangement of electrodes of , 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 and 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— mumetal— shield. Any metal cl
. Electronic apparatus for biological research . J Collector *' connection mm Photocathode Figure Arrangement of electrodes of , 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 and 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— 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 within it. It is the cell used in the familiar photographic exposure meter. Its structure is shown in Figure 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 met
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