Design of Photomultipliers for the Sub-Millimicrosecond Region

In considering the design of a photomultiplier with good time resolution characteristics, it is convenient to divide the problem into two parts, one dealing with the photocathode-to-first dynode region, the other with the secondary emission multiplier structure. The two principal causes of poor time resolution in both of these parts are: 1) the different path lengths of electrons originating from separated points on a second, and 2) the effect of initial velocities of the emitted electrons on transit times. The electron optical approach to the alleviation of the transit time spread due to path differences is discussed for both the cathode region and the structure. The focusing spiral method of analyzing the performance of a structure is of considerable value. The effects of initial velocities can be reduced by ensuring that the electron velocities within the photomultiplier are very high. It is not practical to obtain these high velocities simply by applying a high over-all voltage to the tube because of the loss of secondary emission gain and because of internal and external insulation difficulties. However, the necessary high velocities can be obtained by the use of high voltage field electrodes between successive dynodes, so arranged that electrons are rapidly accelerated to a high velocity and then decelerated before they reach the next dynode. A photomultiplier is described whose design is based upon these two principles. A high voltage central electrode system between successive dynodes serves to give electrons the high velocity required to minimize transit time spread.