The Working Principles of a Photomultiplier

At this point it shonld be mentioned that, as occurs for other detectors, even in the absence of illnmination, electrons are emitted by the photocathode due to 

Taking a typical value of 5 = 5 and considering 10 dynodes. Equation (3.8) gives a gain of G = (which is of the order of 10 ). The particular value of 5 depends, of course, on the dynode material and on the voltage applied between dynodes. Similarly to the case of the photocathode, the responsivity of a photomultiplier, R , is defined as the current induced in the anode divided by the power of the light reaching the photocathode. Thus, it is very simple to show that 

Once the electrons have been accelerated and multiplied, they reach the anode. The electrons arriving at the anode produce an electrical current. This current can be measured directly, or indirectly by monitoring the voltage increment induced in a given load resistor, Rl. This load resistor is critical, as it determines the time constant of the photomultiplier. A typical time constant for a photomultiplier is 2 ns, although an adequate choice of the load resistor and anode material could lead to time constants as low as 0.5 ns. 

When time-dependent signals are to be measured by a photomultiplier, the time sensitivity is usually limited by the inhomogeneous transit time. The transit time is the time taken by electrons generated in the cathode to arrive at the anode. If all of the emitted electrons had the same transit time, then the current induced in the anode would display the same time dependence as the incoming light, but delayed in time. However, not all of the electrons have the same transit time. This produces some uncertainty in the time taken by electrons to arrive at the anode. There are two main causes of this dispersion  

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