Question

Explain how the photomultiplier (PMT) works and how it can detect single photons. For an 11

stage PMT with an average multiplication of 2.5 per dynode, calculate the overall gain of the

PMT.

Answer 1

PMT working principle -

PMT working principle is based upon the photoelectric effect. Incoming photons strike a photocathode at entrance window of PMT which knocks out electrons from there. The ejected electron are accelerated by high voltage field towards anode and multiplied in number within a chain of dynode stages by using principle of secondary emission.

Now if at each stage an average of 4 secondary electrons are produced for one incoming electron, and if there are 10 dynode stages, then at the last stage; for one primary electron there will be about 410 electrons. Thus a large number of electrons reaching at the anode results in a sharp current pulse that is detected using oscilloscope, so one electron is easily detected using PMT.

Overall gain of PMT = (2.5)11 = 23841.86.