# Why do photomultiplier tubes have such a stable gain?

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?

• 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. – garyp Oct 16 '16 at 21:07