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My professor mentioned that a PMT has very high internal gain, on the order of $10^6$. He also mentioned that for a given PMT, the gain is surprisingly constant, that is, one photon will always give you a million electrons at the end, give or take a surprisingly small number of electrons.

What causes this small standard deviation in the gain? Is there something in the avalanche process that is so stable?

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Here's a figure from the Hammamatsu PMT handbook. It shows the distribution in the number of electrons detected at the anode for a single photon at the cathode.

enter image description here

To me, the plus-or-minus does not look surprisingly small. In fact, it is the rather large spread that makes it difficult to distinguish between one photon hitting the cathode and two.

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  • $\begingroup$ Does Hammamatsu supply any numbers to accompany these graphs? Or are these supposed to be zero scaled? It's difficult to determine how large the spread is without numbers on these graphs. $\endgroup$ – Jonathan Wheeler Oct 16 '16 at 19:54
  • $\begingroup$ Pulse height is zero at the left end of the $x$-axis. The ("modal") gain is the location on the $x$-axis where the distribution peaks. ($\approx 10^6$) The rise at the left end are small pulses due to electrons thermally generated from the dynodes. These electrons suffer fewer gain stages, and so are smaller. $\endgroup$ – garyp Oct 16 '16 at 21:07

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