Photomultiplier Johnson noise

As was mentioned before, noise is a term used to describe any random output signal that has no relationship with the incoming signal (the incoming light). In photomultipliers, noise can be classified, depending on its origin, into three types dark current, shot noise, and Johnson noise. The differences between these three classes are explained next ... 

In analog detection methods (See Section 4.B.2) the photomultiplier output is treated as a continuous variable. At low light-scattering levels, the signal-to-noise ratio in these methods may become small because of various sources of ftostdetection noise in the system (thermal or Johnson noise, PM dark current, etc.). Under these conditions it becomes advantageous to use the digital or photocount autocorrelation method. 

For GRi iO 0.05 V, the Johnson noise can be neglected. With the gain factor G = 10 and the load resistor of R= 10 Q, this implies that the anode current /a should be larger than 5 x 10 A. Since the anode dark current is already much larger than this limit, we see that the Johnson noise does not contribute to the total noise of photomultipliers. 

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