Using a Photomultiplier to Detect Single Photons

Photomultiplier (PMT) principle, operation and measurements explained.

00:00 Intro and overview
00:30 The photoelectric effect
02:11 Detecting single photons
03:33 How a PMT detects a photon
10:35 How to operate a PMT
17:00 Measurements with a photomultiplier
24:59 Conclusions

The video contains 2 short clips taken from other Youtube channels:
@reps and @ElectroBOOM

[reps]

challenge accep... uh actually no, my best semiconductor current meter only has 10 aA resolution

[RichardFraser-yt]

After a long time with no observations.... The signal has returned.

[primenumberbuster]

The cascade system to amplify the signal was one of the coolest things I learned in my sophomore years.

[mikeselectricstuff]

Many years ago I had a play around with a PMT and a red LED, and it could detect the LED with 4nA forward current at room temp, and 0.5nA when cooled with a peltier cooler.
For a white LED, just visible to the eye at 50nA, it could detect with a LED current of about 20pA!
It could also very easily detect triboluminescence from rubbing sugar cubes together, peeling clear adhesive tape and rubbing two pieces of quartz together.

[twilightknight]

I know you were doing it for filming purposes, but just a heads up that it is best practice not to expose PMTs (or APDs for that matter) to room lights even without voltage applied. It won't necessarily damage the PMT, but it can cause trapped electrons in the photocathode to build up. This results in increased "dark counts" (thermionic emissions) for a period of time after light exposure as those trapped electrons work their way out. Source: I'm a quantum optical engineer and work with this kind of stuff daily

[platinum]

the abundance of detail and explanations in the video is a breath of fresh air

[brunopassarelligell]

Hi there, great video! You summarized the topic really well. I work with PMTs and SiPMs daily, collaborating directly with Hamamatsu as a PhD student in particle physics, focusing on light detection in rare event liquid noble experiments.

Just a few things I wanted to add:

- Typically, dark pulses are similar in intensity, as the vast majority of thermal electrons originate from the photocathode. If you're seeing peaks with different heights, it’s likely due to ambient light.
I totally understand the frustration—I've dealt with my share of light leakage issues. Even high-end, all-metal, ultra-high vacuum setups can have leaks. A quick calculation shows how significant diffuse light can be. For example, a 1W light, 10 meters away, shining through a 10μm x 10μm hole, produces an enormous flux of ~200301 photons per second (at 500 nm). This light is Poisson-distributed, so you can calculate the probability of two photons hitting within the same 10 ns to form a two-photon pulse. You probably have a much higher flux, and dimming the light might not make a noticeable difference to your eyes or oscilloscope. A Poisson analysis of your height distribution would likely be useful.

- Don’t rely too much on the manual for specs like dark noise or quantum efficiency. These values are often idealized and may not reflect reality, especially for used devices. Who knows what the previous owner did to the tubes? The photocathode might be damaged, which could significantly lower both dark count and efficiency.

- For photocounting, I’d recommend using the pulse's charge, as it compensates for slight differences in transit time and hit position on the PMT face.

- Have you heard of SiPMs? They’re essentially solid-state PMTs—cheaper and more powerful when it comes to photocounting. SiPMs can distinguish between an N-photon hit and an N+1-photon hit, up to around 8 photons (depending on the model). With PMTs, it becomes tricky to distinguish beyond N=2. Skipper-CCDs, on the other hand, can detect even larger N-values, going beyond 300.

[juehangqin]

Hello, another noble liquid experiment physicist here. Really cool video, I really enjoyed it!

In the lab, we once made a dark box out of a big pelican case by drilling holes for cables into the bottom half, putting some coax ports in them, and sealing them (I think with black caulk? I don't remember). It was by far the easiest setup to use, because it is possible to quickly shut off power and open it when one needs to make tweaks, I highly recommend something along those lines if you needed a dark box setup that you can reopen frequently, and so you can work with the lights on :) 300 Hz sounds reasonable to me for room temperatures, though, so I don't think you're having a serious problem with light tightness; this is in line with what I would expect.

When we run experiments, we often find that there are major variances between PMTs, even from the same batch, so they're rarely run at the exact manufacturer-specified voltage. Reducing the voltage can often reduce the dark current significantly, so if your circuit is capable of that that's an easier approach than cooling them. It seems like you are getting nice and clean waveforms well within what your oscilloscope can pick up, so there's probably quite a bit of margin to reduce the voltage (and hence gain). Coincidence triggers with multiple PMTs are also another way to get around dark currents, though that's a whole thing...if you are interested in that, Leo's Nuclear Methods and Techniques is a go-to for such techniques, and also gives an introduction of the NIM modules that one would use to implement these typically in a particle physics setting. They're often available cheap-ish (high tens, low hundreds) because some of these modules, such as various LeCroys, have been in production since the cold war era and there's tons of oldstock sitting in Physics department attics.

I also recommend looking into SiPMs if you're interested, they are very easy to use and also much cheaper if you count in the fact that you don't need a HV supply; I personally haven't used them but I know they're much easier to use, and the only reason I haven't used them is because the experiments I work on really care about low dark current per area, which is where traditional PMTs shine (though SiPMs are making strides).


Finally, here's a fun effect you might be interested in: did you know you can use PMTs to measure the frequency of light? Essentially, if the photon energy is greater than twice the workfunction of the photocathode metal, there's a non-zero probability for one photon to result in the release of two electrons. This essentially means that as long as the light is weak enough that pulses don't overlap too often, we can measure the frequency of light by looking at the mean pulse height and/or integrated area! The effect depends on the photocathode material, but in my experience starts to become visible <300nm, which is still within commercial UV LED range.

[davidg]

Throwback to my undergraduate work at university, calibrating photomultipliers for use in a neutrino experiment.
The first thing we discovered was that our initial version of an isolation booth wasn't good enough! The temperature in the lab was about 1-1.5°C degrees higher by late afternoon than morning, and the difference in thermionic emission showed up in our testing.

[dennisk]

I was product manager for a company that made photographic darkroom equipment. We had a color analyzer that utilized the PMT. When testing, we had a radioactive element that was contained in a steel, light tight container. The PMT was put in a fixture that was also light tight then a special valve was opened exposing the PMT to emissions from this radioactive target. The PMT was then adjusted to the constant we determined for our product.

[themrworf]

My friend and I made gamma spectrometer from scratch using PMT. We had to figure out low noise amp by ourslefs because PCBs with amp and PMT was way expensive back then. It was fun project. PMT are fun!

[projectartichoke]

These are always one of my favorite types of tubes! I find it amazing that such a thing could be conceived of in the first place, let alone actually constructed.
And it all flows from the work of some of the most brilliant physicists who ever lived. It's truly an amazing story of what's practically achievable by the physical sciences.

[supercompooper]

Is very kind of you to try and detect single photons because photons deserve to be paired with someone else and not be lonely.

[mikeselectricstuff]

The datasheet mentions a 30 minute settle-down time after exposure to high light levels - what is the mechanism that causes this ?

[vincei]

"Even yours, Marco" made me laugh more than I should have.

[erikziak]

This video is beyond what words can capture. I paused several times to think about things. Any questions that I had were answered in later stages of the video. I highly value the effort you put into this. Practically on the same level, as I would, if I were not too lazy to actually do this. Please take this comment as praise from my side. I am looking forward to see other videos. Thank you very much for doing this, even if only a handful of people acknowledge and recognize the effort you put into it. Respect. Best wishes, Erik.

[BartKus]

You are enriching the world, Mr.Huygens!

[MrCarlsonsLab]

Great video! Some possibly helpful hints: A cheaper choice for a photomultiplier tube is the 931A, and there are tons out there. They are just fine in normal light, and not going to get damaged. One example: The B&K 1076 used one in front of a CRT (video section) with louvers in the case exposing the multiplier tube to outside light while the device was in use, and it performed fine, in fact they still work today. Have fun, don't get zapped! :^)

[slavayorish]

Many years ago, I took part in the TOXICHIP project. The goal was to measure single photons from genetacally modified Ecoli bacteria that started polluting light in the presence of toxins. We picked an avalange diods instead of photomultipliers because of high voltage. Plus, semiconductor design is more compact. Tyndall institute in Cork Ireland built for us such diods. I designed a platform that included a chassis, a chip, an array of avalange diods, and electronics. It was 20 years ago.

[RobertCraft-resf]

At the South pole is the Ice Cube neutrino observatory. There is 6km of pure clear ice there. They made a 1x1 km observatory there by sinking (melting) strings of photo multiplier detectors there. Really amazing. They detect neutrinos that pass through the earth. They can also sometimes tell where they came from. Crazy. (The neutrinos cause a very small amount of light in the ice when they hit something, very very rarely, which the observatory detects. )