Secondary-Electron Emission
The secondary-emission process, which is fundamental to photomultipliers used in scintillation counters, can be considered to consist of three physical processes: (1) excitation of secondary electrons, (2) transport of these electrons through the solid, and (3) transport through the vacuum-solid interface. Based upon these processes, a model can be derived which explains the shape and magnitude of the secondary-emission yield curves as well as the velocity distribution of the emitted electrons. Furthermore, such a model can be useful in the search for better secondary emitters. The application of certain solid-state concepts such as band bending to the preparation and processing of appropriate semiconductors has led to enormous improvement in the transport and escape of secondary electrons and has made possible the development of a new type of secondary emitter with a maximum yield which is an order of magnitude or more greater than the yields of the best previously described materials. These secondary-electron emitters may be useful in producing multipliers with fewer stages, better statistics, and higher speeds than those presently available.