How to detect a single photon.  How a photo multiplier tube works. - Page 1
Forum section: electronics / general

author: BrianHG
author_rank: Super Contributor
timestamp: September 12, 2024, 03:44:06 am
content: Man, a single photon. Even in the blackest black, there has got to be some occasional detection...

author: Andy Chee
author_rank: Super Contributor
timestamp: September 12, 2024, 06:08:07 am
content: Now you know how neutrino scientists feel!

author: ebastler
author_rank: Super Contributor
timestamp: September 12, 2024, 06:13:47 am
content: Quote from: BrianHG on September 12, 2024, 03:44:06 amThere's even the occasional detection without any photon coming in."Dark counts" due to spontaneous electron emission from the photo-cathode are an issue and limit the ability to detect very weak signals. Strongly cooling the PMT helps, but makes for a more akward setup, having to avoid condensation on exposed cold parts.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 12, 2024, 06:29:54 am
content: And what´s really crazy is that this in reality is a "in average each 4th photon detector" - the overall quantum efficiency rarely exceeds 25% *.*For the experiments these devices are used for it doesn't matter. It´s important to have a stable timing correlation to single photons, not to detect each photon.

author: BrianHG
author_rank: Super Contributor
timestamp: September 12, 2024, 10:46:22 am
content: Still, that green laser diode, not even lasing, barely visible under dark conditions by a camera, at distance, going through that pin-hole opening was clearly detected.Since there are random empty junk detections, I wonder if you used 2 tubes with an and-gate to eliminate noise and a beam splitter at the input to drive the 2 tubes in parallel. With the proper spacing, maybe only entangled photon pairs will be detected.

author: Someone
author_rank: Super Contributor
timestamp: September 12, 2024, 11:17:56 am
content: Quote from: BrianHG on September 12, 2024, 10:46:22 amWell studied:https://en.wikipedia.org/wiki/Higher_order_coherence#Hanbury_Brown_and_Twiss_experiment

author: electr_peter
author_rank: Unknown
timestamp: September 12, 2024, 06:59:15 pm
content: Excellent videoScope sample rate drops at slow timebases which results in missed events or events with varying amplitude.Peak detectsampling mode would be better in this case.I have always wondered - if input light (photon) intensity is high, do output impulses start to overlap? Or are there some pulse-relax-pulse behavior with these photomultiplier tubes?

author: ebastler
author_rank: Super Contributor
timestamp: September 12, 2024, 07:32:05 pm
content: Quote from: electr_peter on September 12, 2024, 06:59:15 pmThere is a dead time while the PMT recovers (recharges its dynodes) before it can register the next photon. So thecounts/intensity ratiocount rategradually saturates.If you go much higher with the incoming light intensity you may longer see pulses at all, since instead of individual electron avalanches you get a continuous flow of electrons, with a current proportional to the incoming intensity. (Until that saturates as well eventually.)It is not uncommon to have readout electronics which are switchable between photon counting & analog measurement of photocurrent to get a larger dynamic range.EDIT: Sloppy wording fixed.

author: BrianHG
author_rank: Super Contributor
timestamp: January 02, 2025, 09:33:31 am
content: Part 2: PMT2: Photon Bunching / Hanbury Brown & Twiss effect

author: ejeffrey
author_rank: Super Contributor
timestamp: January 03, 2025, 01:30:03 am
content: Quote from: ebastler on September 12, 2024, 07:32:05 pmNot exactly. The PMT is fairly linear. Two photons at the same time produce a pulse approximately twice as high. Two photons closely spaced in time produce a smeared out double pulse. Of course at high count rates it will still saturate but there is no specific recovery time of the PMT itself. The dead time comes from the pulse discriminator circuit.This is different from an APD which, when biased for photon counting actually saturates and needs to be quenched before it can work again.

author: TimFox
author_rank: Super Contributor
timestamp: January 03, 2025, 01:45:07 am
content: The PMT will not have dead time between pulses if there are stiff voltages on the later dynodes.The “charging” to which you refer is of the capacitors on the late stages of a resistor voltage divider used to establish the dynode voltages.

author: BrianHG
author_rank: Super Contributor
timestamp: January 09, 2025, 10:15:58 pm
content: For those still interested, take a look at these photo multiplier tubes in the Super Kamiokande neutrino detector:

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 10, 2025, 12:47:40 am
content: That brings some memories, I worked for a PMT manufacturing company for five years, designed all kind of fancy circuits for these beautiful devices, from photon counting to power supplies. The maximum counting speed depends mostly on the output pulse width (which in turn depends mostly on the size of the tube). For a typical small PMT with ~4ns output pulses 100 MHz counting speed is close to the limit, with some pulse shaping trickery I’ve managed to push it to over 200 MHz. Above that you just measure the current, however that is less accurate as the gain heavily depends on the supply voltage and unless you use an active divider at least for the last few dynodes, the linearly suffers.For photon counting the photo cathode is better at a ground potential and the output sits at a high positive voltage, which makes it more difficult to have an analogue current output at the same time.CheersAlex