The photomultiplier is the most sensitive light detector there is. A photon knocks a single electron out of the photocathode; it is accelerated through a hundred volts and thrown at a dynode, from which 4–5 electrons emerge. A cascade of ten dynodes repeats this ten times, and an avalanche of millions of electrons reaches the anode. The gain is G = δN, where δ = A·Uk is one stage's secondary-emission ratio.
The supply is wired as it really is: the photocathode at −1000 V, the divider (10 stages of 100 k) returning to ground through pin d, and the signal taken from the anode across a 1 M load. A PMT has no heater — the photocathode is not thermionic, it is cold.
This page is a utility for simulating photomultiplier (pmt): detector for extremely weak light online with specified initial values.
Using the model. Use the light flux slider to let light in. At 0.2 nlm the photocathode delivers just 12 fA — a current no instrument could measure — yet a gain of 5·106 turns it into 64 mV across the load. Double the flux and the output doubles: the response is strictly linear.
Set the flux to zero and the dark current remains (about 1 mV) — the sensitivity limit of any PMT. The most instructive control is the high-voltage slider: the gain follows Uk·N, i.e. the seventh power. Drop the supply from 1000 to 800 V and the signal falls by almost an order of magnitude. That is why a PMT supply has to be exceptionally stable. In the valve's settings you can change the number of dynodes or switch the divider to external, which brings every dynode out on its own pin so you can build the resistor chain on the schematic yourself.
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