Dear Photomultiplier Doctor,
In the optimisation of discriminator bias setting, we have found that if the bias is set just
beyond the knee of the integral dark pulse-height distribution, it gives better stability. Do you
agree?
Dr. Ganesan Ramanathan
National Physical Laboratory
Dear Dr. Ramanathan,
The meaning of the term" knee voltage" is not universally agreed but I can tell you about Electron Tubes' approach. Taking a bent human leg as an example, where the knee corresponds to the highest rate of change of slope, should we really operate at this point on the HV vs count rate curve. Certainly not, because this is such an unstable point on the curve. A small shift in HV or a gain change will bring about a considerable change in count rate. It is preferable to operate to the right of this point where the slope of the curve is much lower.
Our recommendation is to operate at a point on the plateau where the slope first attains 0.1% per volt. This is typically 100V higher than the knee. For more information on this subject please refer to our technical paper "Optimal use of photomultipliers for chemiluminescence and bioluminescence applications" RP090 (colour version 523K, mono version 556K).
Regards,
Photomultiplier Doctor.
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