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
Thank you for making this comment. I should probably have mentioned this in the video, although a remark about this was displayed with the dark current specifications. The light in the room wasn't actually that bright, I just used 3200 ISO for filming and kept the PMT away from directly pointing at windows or lights.
cone and rod cells also use a signal cascade to detect up to a single photon(in the case of rods). its an enzymatic chemical cascade but.. still cool😋😋. great dynamic range on the eyeballs
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.
@@HuygensOptics There is an interesting phenomenon related to this :- At cryogenic temperature, the dark rate in a photomultiplier is caused by single electrons, emitted spontaneously from the cathode surface. This “cryogenic” dark rate increases with decreasing temperature down to at least 4K. The average event rate is proportional to the area of the emitting surface and insensitive to the electric field at that surface. The electrons are emitted in bursts. The bursts are distributed randomly in time, but the events within a burst are highly correlated. The burst durations are distributed according to a power law. As the temperature decreases, the rate of bursts, as well as the number of events per burst, increase. The observed time distributions are indicative of a trap mechanism. So far, there is no physics explanation of the observed phenomenon.
@@primenumberbuster404 "The observed time distributions are indicative of a trap mechanism" - so I am right when I say "One go, a bunch go - and it takes time to re-load and repeat" and I have understood you correct? Very interesting - and awesome subject!
PMTs are indeed quite sensitive. I used to design downhole detectors for the oilfield and even tiny light leaks would cause the PMT to saturate to the point that the HV supply would sag. Some of them would even die an instant death if powered up in a well lit room. And as for what it could detect a physicist wanted to image a bare scintillator in operation in real time using a conventional CCD digital camera as a side experiment, but made the same mistake we all make in correlating numbers to our subjective reality. Even with the largest sources we were permitted to handle by hand and the most efficient scintillator we had (LaBr:Ce) they were still pitch black even after 20 minutes in a completely dark room to the naked eye, so no joy without a special setup and acquisition system to integrate which killed the real time part. (Edit: In hindsight the eye is quite insensitive to the peak emission, so it still might have worked with a camera) Speaking to some old timers, to satisfy their curiosity they got around this by using actual logging sources to “see” the scintillator in action. Works apparently but staring at something releasing many millicurie of radiation doesn’t strike me as very safe 😲
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.
Thanks, that is some really good info and I'll use it for the next video. I'm aware that my measurements are not ideal and some remarks were kind of simplified. Also, I'm thinking of building a dedicated box to hermetically seal the setup and which fits in my freezer ;-). About the variation in peak heights: since I'm using 520nm light, you'd expect these also in the photon pulses since a single photon can release multiple electrons. This is actually illustrated on page 232 of the hamamatsu PMT handbook, showing individual peaks in the puls height histogram at 1, 2, 3, etc. electrons of current.
@@HuygensOptics Thanks! I actually think your setup is really nice, a great start for sure. Nevertheless, a hermetically sealed freezer setup will work a lot better especially if you menage to tape the edges with aluminum tape -> you can get it to almost perfect darkness like this. Regarding one photon releasing more than one electron, I'm not too sure about this. In the photoelectric effect, the photon is absorbed by the electron, not being possible to excite another one. You are suggesting something like a Compton scattering? Or the variation in the multiplication factor at each dynode? (I looked at my copy of the handbook and page 232 is the chapter 13 cover page. Perhaps I have an old copy? (edition 3a) ) Anyway, I think your pulses during LED-on time look great. That's what I expected to see! The same for LED-off time, pretty consistent to what I usually see in a leaky setup -> dark noise + random high intensity pulses. A leaky setup is not necessarily bad. It just increases your noise floor. But if you want to measure large signals, that is usually fine :)
@brunopassarelligell1 about the figure in the PMT handbook. if you download the current version from the Hamamatsu website it's the figure 12-3 named: Output pulse height distribution in a multi-photoelectron event. Note that the pdf page and "physical" page numbers do not match. page 232 = page 245
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
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.
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.
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.
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!
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.
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. )
I love the fact that today i had my photonics exam, also about single photon detectors, small word ahah Your presentation is so engaging and interesting that it is worth actively following also at 1am 🤩
Small correction at the 9:11 mark: the current should be the charge divided by the transit time spread, not the transit time, because the current does not depend on the time delay it takes for the avalanche to hit the anode, but the interval within which the amplified charge is distributed.
Right. Although, to make good on that transit time spread, one needs a ~1GHz measurement bandwidth. That would probably require a difference socket with a 50 ohm coax output, and would definitely require a different preamp!
Quite a few years ago, Dave from EEVBlog demonstrated a photo-multiplier where he, using a micro-amp meter, tested the current at which an led emitted a tiny amount of photons. That was quite the demonstration. (20uA was enough to have some quantum-dice rolling the right way to, now and then, emit a photon in the direction of the multiplier that I remember)
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.
Amazing content; I can only imagine what you could achieve in a dedicated facility and with a decent budget! The mechanics of the photo-multiplier was explained beautifully, in particular the amplification avalanche but I was wondering how we know there is a 1:1 relationship between an incident photon and a photo cathode electron in the first place? Is this postulated or demonstrable? I look forward to the follow-up video with anticipation.
Actually, this isn't the case, it's way more complicated than that. A photon with sufficient energy can release way more than one electron from the photocathode. I will talk about that in the follow-up video.
Great question Mike. The Hamamatsu PMT handbook lists 6 different reasons for this, among which are: leakage currents (I guess due to charge accumulation on the insulators), scintillation in the glass envelope caused by stray electrons on the glass surface, and ionization of residual gasses inside the tube. All these take some time to fade away and after 30 min. the performance approches the best attainable value again.
Well you could invert the negative voltages and maybe "purge" the electrons out, then invert back to negative, like a sort of electrostatic degaussing Much more complicated power supply though
This is a great resource to have for an introduction to PMTs. I have an old 90s high end film scanner that is based around three PMTs and I have to do some maintainance on it but was always a bit squeamish about the high voltage portion that drives the PMTs. I'll try calibrating it again in the future with this info.
Dont forget, a photo multipler is very similar to a geiger counter tube. Some strikes of background radiation on the charged plates with knock-out and electron that then can rush down the pipe and create an event - despite the tube being in the dark.
PMT's are really interesting ! They were used in very high-end drum scanners to digitize film. The film was mounted to a transparent tube with a light inside, and it would spin very quickly and scan the frame pixel by pixel using three PMT's behind dichroic filters to separate R, G and B signal. These scanners were the best ever made in terms of dynamic range, which is unsurprising when PMT's can count photons, as well as in terms of resolution, with some reaching 11k pixels per inch ! The optics used in these is also quite interesting, as the enlarged/projected light is pretty much non-image-forming as it consists of a single pixel. I have been looking into SiPM's as well when exploring this topic, and while they work very differently, I feel like it may be interesting to see a proper comparison between regular PMT's and SiPM's !
So interesting to see how commercial stuff is made. I made a device with photomultiplier but I had to make everything, including a custom socket to all the electronics. Making a suitable custom socket is always a pain.
Some will in fact do that, depending on the local electric field vector and their kinetic energy. However, the construction is designed such that the electrostatic field gradient can create as many secondary electrons as possible during an avalanche.
Awesome stuff as always. You have so much to teach. I hope to see collaboration with some of these channels setting out to make awesome things, even if just for spectacle.
How can we ever be sure that it is indeed only single photons that are responsible for the observations of events? Just assuming that the minimum detection level of any setup equates to single photons seems unsafe. I've read many accounts of the double slit experiment that say they reduced the light level to single photons but never any justification or verification of single photons.
That is a good question and indeed this is not the case: many of the pulses observed are the result of more than 1 electron emitted simultaneously (either by a single photon or multiple). If you record the surface / height of many pulses and then put them in a histogram with the number of pulses having a specific surface area, you will find that there are several peaks in the histogram that correspond to the amplification of initially 1,2,3,4 etc. electrons. The effect is described on page 232 of the Hamamatsu PMT Handbook. I did not want to go there in the current video, because it complicates the discussion quite a bit. But it is actually the reason why we observe large height differences between individual pulses.
You can do it easily with a Poisson approach in light emission. Just do a small enough Poisson distribution so the chance of getting 2 photons is basically 0. For that you can pulse an LED with lower and lower voltages, until you see only a photopulse every 10 or so LED pulses. That gives you lambda=1/10, which has a change of 2 photons smaller than 10^-5. Now do the same with 1:100 or 1:10000, you will see the pulses remain the same height, just more sparse
Sleep can wait. Whew, good video. I’d be excited about immersing the tube in liquid nitrogen, but I’m not sure how the hermetic seals would handle it. Another thing to test would be if the energy of the photon corresponds to the average size of each charge pulse. Don’t suppose you’ve got a low-amplitude tunable light-source, do you? It’s never too late to build yourself a visible light free electron laser!
For this type of photomultiplier (plastic base), you should not cool below -30C. At this temperature, the thermionic emission will be lower than 1/100th of the room temperature value. The avalanche gain of the PMT will also go up with decreasing temperature.
I have 4 931s in the office that were used for research, biggest use were pre 2010, until the detectors counters decided to fail and out unit could no longer be repaired (chip was impossible to replace). Many a Grad paper and research paper used the tubes. I was even working on a new unit until we switched over to using CCD’s 100% of the time. Getting the tubes to sub-0C will help immensely. We used ours with either a thermoelectric cooler or dry-ice coolers.
I'm a retired EE, and have played with PMTs, on the cheap. Mostly because my junk parts bin is better stocked than most electronics parts stores. When those stores still existed. I use them to detect very dim light sources.
Very helpful clear explanation of the photomultiplyer and the temperature effects on the dark current. I wish to see videos with other detectors like InGaAs for near infrared but I could probably try to do it myself seeing your experiment here. Also you are excellent with the electronics tinkering here.
I used to work at a place that manufactured PET scanner detectors, they were basically a multi-anode PMT with a scintillator crystal on top (all wrapped up in opaque material so only high energy photons could reach the assembly)
This is a very useful video that might work well for QuarkNet students (high school) exploring the technology we use in our cosmic ray muon detectors. Thank you!
I'm wondering, have you worked through the math of the superposition of the photon? Have you landed on virtual photons as a force or are you advocating for something like irradiated electric field with probability amplitudes? And might I say, just wow. The presentation is outstanding. Miles beyond anything else on TH-cam. It's all here. This is the end of science. The technical jargon with the demo to boot really scratches that quest for knowledge itch I haven't been able to quite reach. thanks!
If you are looking for an easy way to block light - use tin foil or foil tape. The metallization does a great job blocking light when compared to fabric or similar (cardboard), and the stiffness makes for it staying in place without too much extra effort. Worth mentioning that a grounding clip to earth ground could help too - when measuring such small currents, it's not so difficult for a floating metal piece to act as an antenna and couple in some stray EMF to your sensing lines, so having it grounded dramatically reduces any chance of electrical interference.
Thank you for the excellent explanation of how these work! I use them at work in scanning electron microscopes where they amplify the faint flashes of light created when secondary electrons emitted from a sample strike a scintillator plate. This is amplified and used to form the image we see. Also used in scanning laser confocal microscopes to also amplify faint fluorescent light from a sample.
This is fascinating. Have you thought about setting up some compact split experiments to show effects of laser light (I remember from your older video, lasers at low energy are not stimulated emission right? I would love to see a series on all the quantum experiments done in depths, setup and explained, maybe get @SabineHossenfelder involved?
In the follow up video, I'm going to discuss the difference between detecting coherent and incoherent radiation quanta using multiple photomultipliers.
One supercool commercial application with PMT was the 1953 thru early 1959 Autronic Eye from General Motors. This device would automa6tic control your headlights (high beam/ low beam)! But the type PMT is unknown, it was an RCA or Delco tube.
As always you switched on my brain, love it, thank you again. The links to the other potentialshifters made my day😆and I'm not in any way jealous of your guys equipment parks, nooooooooooo. 😂 Was a more relaxing time with current than lights "preferences". Btw. did I mention I hate dust?
In the early 1970s the group of Werner Reichardt at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany showed that a rhodopsin molecule in the rhabdomere of the Dipteran eye could detect a single photon.
I love how this old tech vacuum tube is superior to semiconductor sensors. A great example for electronic engineers that newer isn't always better. The amplifier circuit you mentioned is more a current to voltage converter. We don't realy speak of amplification here because we can't compare voltage with current even though the power gets amplified.
This makes me understand why those Night Vision goggles that uses photomultipliers is so extremely expensive. Since actually getting lots of "pixels" with this is difficult.
Absolutely fascinating. Now Marco Reps is gonna make an amplifier circuit. I wonder how this thing would behave with different frequencies, right up to hard UV or soft X-rays?
It's actually insensitive to those, for one because the quartz envelope will become opaque below160nm in the deep UV and X-rays will partly pass the complete device unnoticed because they are not absorbed.
@@HuygensOptics I see. I still look forward to your next video about this nifty thing. I'm into particle physics, and they use fancy ones in neutrino detectors.
Excellent explanation. Just curious how is it detecting single photons, isn't the laser diode emitting lots of coherent photons? Only one photon makes it through the aperture of the photomultiplier? What happens if a large sum of photons goes in there?
A Gamma camera uses an array of the Hamamatsu miniature square PMTs with Cesium iodide crystals made into an X-Y array. They are used in CAT scan medical instruments. The company I worked for built a 16x16 gamma camera to be used in oilfield pipe inspection. This was back when the 3/8 diameter PMTs were just coming out.
In the past, to develop compact gamma cameras I used position-sensitive PMTs like Hamamatsu's H8500 or H9500, but recently, I've switched to SiPM arrays coupled to CsI(Tl).
Try detecting cosmic ray Cherenkov light. You’ll need a focusing mirror, it for a table top sized thing, you should get around six photons all at once.
Detecting single photon events in a thermal background of a few 100 counts per seconds will be a challenge. However, if the laser is pulsed and you gate the amplifier with the laser pulse the background may be reduced. The amplitude of the amplifier output then should vary with the number of primary photoelectrons. I am curious if this will show up. Eagerly awaiting your next video.
You could (if your wife allows it) stick the whole multiplier setup in the freezer. That should also be pretty well sealed so very little to light should get in.
Really cool little units with all the needed parts integrated, and at a very reasonable price. What do you think they came from - a university undergrad optics lab being cleared out, or is there a commercial use?
Some kind of classified apparatus: the seller could not disclose the application because of a confidentiality agreement with the company that sold the apparatus to him that contained these items.
I am a long time follower of you fantastic channel. Hamamatsu builds also qcmos cameras, that can do photon counting in each pixel of the sensor. It means the camera can measure if a pixel has detected 1,2,3 etc photons. Are you interested?
This is interesting because I've only ever seen videos about photomultipliers used for TV production to enhance the picture on their cameras. It stands to reason they would have more scientific uses, having been born of science, but I just got so bogged down in the TV stuff, I forgot to consider it.
I was just talking to one of my coworkers about these. We use a Nuclear Moisture Density gauge like a Troxler or Instrotek. These devices use these photocathodes.
Thanks for the video! Great as always. I have two questions: 1. Why is there always 1 photon that gets absorbed by the electrons? Why not two, for instance? 2. By the time Einstein explained the photo effect, did we know about the quanta of energy of light? Or not yet?
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After a long time with no observations.... The signal has returned.
Checks notes: Wow! (Underlined)
challenge accep... uh actually no, my best semiconductor current meter only has 10 aA resolution
Dang, I need to design you some new stickers for that resolution level. PPMs seems like a bulldozer by comparison.
Well, that is still pretty damn impressive Marco, especially if these last digits are significant.
I'm sure you could make an amplifier circuit similar to the one used here. Between that and your multimeter, single electrons my be in reach!
well, still a challenge, go ahead and build one.
Glad to see we watch the same stuff, marco
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
Thank you for making this comment. I should probably have mentioned this in the video, although a remark about this was displayed with the dark current specifications. The light in the room wasn't actually that bright, I just used 3200 ISO for filming and kept the PMT away from directly pointing at windows or lights.
The specifications show measurements taken 30 minutes after startup. This allows time for almost all of the trapped electron buildup to dissipate.
Gratitude for this window ♥️
The cascade system to amplify the signal was one of the coolest things I learned in my sophomore years.
That's not that cool tbh
cone and rod cells also use a signal cascade to detect up to a single photon(in the case of rods). its an enzymatic chemical cascade but.. still cool😋😋. great dynamic range on the eyeballs
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.
They are indeed incredibly sensitive, which can be a disadvantage because you have to do all of that in absolut darkness.
@@HuygensOptics There is an interesting phenomenon related to this :-
At cryogenic temperature, the dark rate in a photomultiplier is caused by single electrons, emitted spontaneously from the cathode surface. This “cryogenic” dark rate increases
with decreasing temperature down to at least 4K. The average event rate is proportional to the
area of the emitting surface and insensitive to the electric field at that surface. The electrons are emitted in bursts. The bursts are distributed randomly in time, but the events within a burst are highly correlated. The burst durations are distributed according to a power law. As the temperature decreases, the rate of bursts, as well as the number of events per burst, increase.
The observed time distributions are indicative of a trap mechanism. So far, there is no physics explanation of the observed phenomenon.
@@primenumberbuster404 "The observed time distributions are indicative of a trap mechanism"
- so I am right when I say "One go, a bunch go - and it takes time to re-load and repeat" and I have understood you correct?
Very interesting - and awesome subject!
PMTs are indeed quite sensitive. I used to design downhole detectors for the oilfield and even tiny light leaks would cause the PMT to saturate to the point that the HV supply would sag. Some of them would even die an instant death if powered up in a well lit room. And as for what it could detect a physicist wanted to image a bare scintillator in operation in real time using a conventional CCD digital camera as a side experiment, but made the same mistake we all make in correlating numbers to our subjective reality. Even with the largest sources we were permitted to handle by hand and the most efficient scintillator we had (LaBr:Ce) they were still pitch black even after 20 minutes in a completely dark room to the naked eye, so no joy without a special setup and acquisition system to integrate which killed the real time part. (Edit: In hindsight the eye is quite insensitive to the peak emission, so it still might have worked with a camera) Speaking to some old timers, to satisfy their curiosity they got around this by using actual logging sources to “see” the scintillator in action. Works apparently but staring at something releasing many millicurie of radiation doesn’t strike me as very safe 😲
@@primenumberbuster404 Interesting, I have never heard about this! Thanks!
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.
Thanks, that is some really good info and I'll use it for the next video. I'm aware that my measurements are not ideal and some remarks were kind of simplified. Also, I'm thinking of building a dedicated box to hermetically seal the setup and which fits in my freezer ;-). About the variation in peak heights: since I'm using 520nm light, you'd expect these also in the photon pulses since a single photon can release multiple electrons. This is actually illustrated on page 232 of the hamamatsu PMT handbook, showing individual peaks in the puls height histogram at 1, 2, 3, etc. electrons of current.
@@HuygensOptics Thanks! I actually think your setup is really nice, a great start for sure. Nevertheless, a hermetically sealed freezer setup will work a lot better especially if you menage to tape the edges with aluminum tape -> you can get it to almost perfect darkness like this.
Regarding one photon releasing more than one electron, I'm not too sure about this. In the photoelectric effect, the photon is absorbed by the electron, not being possible to excite another one. You are suggesting something like a Compton scattering? Or the variation in the multiplication factor at each dynode? (I looked at my copy of the handbook and page 232 is the chapter 13 cover page. Perhaps I have an old copy? (edition 3a) )
Anyway, I think your pulses during LED-on time look great. That's what I expected to see! The same for LED-off time, pretty consistent to what I usually see in a leaky setup -> dark noise + random high intensity pulses.
A leaky setup is not necessarily bad. It just increases your noise floor. But if you want to measure large signals, that is usually fine :)
@@HuygensOpticsBe careful with cooling and HV! Condensation, high voltage and sensitive op amps do not mix well.
@brunopassarelligell1 about the figure in the PMT handbook. if you download the current version from the Hamamatsu website it's the figure 12-3 named: Output pulse height distribution in a multi-photoelectron event. Note that the pdf page and "physical" page numbers do not match. page 232 = page 245
@@HuygensOptics So you can finally answer the age old question if the light really goes out if you close the fridge!
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
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.
Yes, I guess that 5% difference should be clearly detectable, especially if you test a lot of them.
"Even yours, Marco" made me laugh more than I should have.
That was awesome. Loved the tie-in.
the abundance of detail and explanations in the video is a breath of fresh air
For a chemist, you know more physics, electronics and SW than 90% of people working on those fiields
Is very kind of you to try and detect single photons because photons deserve to be paired with someone else and not be lonely.
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.
True PMTs are old school, but they are still interesting and beautiful technology.
Can you recommend some materials, which would help build such decice? I would like to measure 100 photon/second flux for hobby astronomy project
@Woloszow try to search for Dr. Hadar Ben-Yoav. He published a few papers about this project.
@@HuygensOptics I think Hammamatsu would challenge that! They come out with new PMT designs each year.
@@glasslinger like the TO-8 packaged R7400U PMTs! Super neat: PMT the size of a fingertip.
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.
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!
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.
2:40 19.5 Digit Multimeter. Impressive!
Now let's see Paul Allen's Multimeter
"This bad boy can keep it's cal for a solid 3 femtoseconds"
Marco Reps eat your heart out😅
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. )
Placing them by sinking and melting is a certified "during a shower " idea.
I love the fact that today i had my photonics exam, also about single photon detectors, small word ahah
Your presentation is so engaging and interesting that it is worth actively following also at 1am 🤩
No need to wait to watch, instant thumbs up for any Huygens Optics vid.
Small correction at the 9:11 mark: the current should be the charge divided by the transit time spread, not the transit time, because the current does not depend on the time delay it takes for the avalanche to hit the anode, but the interval within which the amplified charge is distributed.
Right. Although, to make good on that transit time spread, one needs a ~1GHz measurement bandwidth. That would probably require a difference socket with a 50 ohm coax output, and would definitely require a different preamp!
Yes you are correct, this is an error that I overlooked. I will set this straight in the follow-up!
You left us with a "cliff hangar"! I cant wait for the next episode.
Ooh, I designed some bases for some Hamamatsu PMTs a few years ago. Very cool stuff!
Quite a few years ago, Dave from EEVBlog demonstrated a photo-multiplier where he, using a micro-amp meter, tested the current at which an led emitted a tiny amount of photons. That was quite the demonstration. (20uA was enough to have some quantum-dice rolling the right way to, now and then, emit a photon in the direction of the multiplier that I remember)
th-cam.com/video/TJcMgoJ4DOE/w-d-xo.html
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.
New video - new portion of the greatest scientific information I probably never need in my life. But it is so interesting! Thank you
Amazing content; I can only imagine what you could achieve in a dedicated facility and with a decent budget!
The mechanics of the photo-multiplier was explained beautifully, in particular the amplification avalanche but I was wondering how we know there is a 1:1 relationship between an incident photon and a photo cathode electron in the first place? Is this postulated or demonstrable?
I look forward to the follow-up video with anticipation.
Actually, this isn't the case, it's way more complicated than that. A photon with sufficient energy can release way more than one electron from the photocathode. I will talk about that in the follow-up video.
You are enriching the world, Mr.Huygens!
The datasheet mentions a 30 minute settle-down time after exposure to high light levels - what is the mechanism that causes this ?
Great question Mike. The Hamamatsu PMT handbook lists 6 different reasons for this, among which are: leakage currents (I guess due to charge accumulation on the insulators), scintillation in the glass envelope caused by stray electrons on the glass surface, and ionization of residual gasses inside the tube. All these take some time to fade away and after 30 min. the performance approches the best attainable value again.
Various trapped electrons can be released over time resulting in additional dark counts.
Well you could invert the negative voltages and maybe "purge" the electrons out, then invert back to negative, like a sort of electrostatic degaussing
Much more complicated power supply though
Always so excited to see a new video from this channel!!
Great video - very informative. Thanks for taking the time to put all of this together.
This is a great resource to have for an introduction to PMTs. I have an old 90s high end film scanner that is based around three PMTs and I have to do some maintainance on it but was always a bit squeamish about the high voltage portion that drives the PMTs. I'll try calibrating it again in the future with this info.
Excellent example of using science to study more science. I love that kind of advancement. Every device is an experiment in this perspective.
Have you figured out what the measurement output labeled "video" is for? Thanks for the very detailed explanation on this interesting topic!
Oooo! Greatly looking forward to part 2.
Nice video. A step by step guide about how to make a Photomultiplier Tube work and how it works. Thank you.
Dont forget, a photo multipler is very similar to a geiger counter tube. Some strikes of background radiation on the charged plates with knock-out and electron that then can rush down the pipe and create an event - despite the tube being in the dark.
Shout out to Marco reps!!!! Sir, my hat is off.
PMT's are really interesting ! They were used in very high-end drum scanners to digitize film. The film was mounted to a transparent tube with a light inside, and it would spin very quickly and scan the frame pixel by pixel using three PMT's behind dichroic filters to separate R, G and B signal. These scanners were the best ever made in terms of dynamic range, which is unsurprising when PMT's can count photons, as well as in terms of resolution, with some reaching 11k pixels per inch ! The optics used in these is also quite interesting, as the enlarged/projected light is pretty much non-image-forming as it consists of a single pixel.
I have been looking into SiPM's as well when exploring this topic, and while they work very differently, I feel like it may be interesting to see a proper comparison between regular PMT's and SiPM's !
So interesting to see how commercial stuff is made. I made a device with photomultiplier but I had to make everything, including a custom socket to all the electronics. Making a suitable custom socket is always a pain.
Flow Cytometry specialist here: Without this we wouldn't be doing ANYTHING>.
Why do the electrons from the -900V dynode go to the -800V dynode across from it rather than the -700V dynode right next to it?
Some will in fact do that, depending on the local electric field vector and their kinetic energy. However, the construction is designed such that the electrostatic field gradient can create as many secondary electrons as possible during an avalanche.
Love this video and look forward to the next one!
Awesome stuff as always.
You have so much to teach. I hope to see collaboration with some of these channels setting out to make awesome things, even if just for spectacle.
How can we ever be sure that it is indeed only single photons that are responsible for the observations of events? Just assuming that the minimum detection level of any setup equates to single photons seems unsafe. I've read many accounts of the double slit experiment that say they reduced the light level to single photons but never any justification or verification of single photons.
That is a good question and indeed this is not the case: many of the pulses observed are the result of more than 1 electron emitted simultaneously (either by a single photon or multiple). If you record the surface / height of many pulses and then put them in a histogram with the number of pulses having a specific surface area, you will find that there are several peaks in the histogram that correspond to the amplification of initially 1,2,3,4 etc. electrons. The effect is described on page 232 of the Hamamatsu PMT Handbook. I did not want to go there in the current video, because it complicates the discussion quite a bit. But it is actually the reason why we observe large height differences between individual pulses.
You can do it easily with a Poisson approach in light emission. Just do a small enough Poisson distribution so the chance of getting 2 photons is basically 0.
For that you can pulse an LED with lower and lower voltages, until you see only a photopulse every 10 or so LED pulses. That gives you lambda=1/10, which has a change of 2 photons smaller than 10^-5. Now do the same with 1:100 or 1:10000, you will see the pulses remain the same height, just more sparse
@@brunopassarelligell1 if you could elaborate on this procedure, I'd be very grateful 🙏
amazing video 🎉🎉
can't wait for the second video
Sleep can wait.
Whew, good video. I’d be excited about immersing the tube in liquid nitrogen, but I’m not sure how the hermetic seals would handle it. Another thing to test would be if the energy of the photon corresponds to the average size of each charge pulse. Don’t suppose you’ve got a low-amplitude tunable light-source, do you? It’s never too late to build yourself a visible light free electron laser!
For this type of photomultiplier (plastic base), you should not cool below -30C. At this temperature, the thermionic emission will be lower than 1/100th of the room temperature value. The avalanche gain of the PMT will also go up with decreasing temperature.
What a cliffhanger! Can't wait for part 2 :)
I have 4 931s in the office that were used for research, biggest use were pre 2010, until the detectors counters decided to fail and out unit could no longer be repaired (chip was impossible to replace). Many a Grad paper and research paper used the tubes. I was even working on a new unit until we switched over to using CCD’s 100% of the time.
Getting the tubes to sub-0C will help immensely. We used ours with either a thermoelectric cooler or dry-ice coolers.
It's fascinating to see how well that works on such low voltage.
I'm a retired EE, and have played with PMTs, on the cheap. Mostly because my junk parts bin is better stocked than most electronics parts stores. When those stores still existed. I use them to detect very dim light sources.
Very helpful clear explanation of the photomultiplyer and the temperature effects on the dark current. I wish to see videos with other detectors like InGaAs for near infrared but I could probably try to do it myself seeing your experiment here. Also you are excellent with the electronics tinkering here.
It's very impressive to be able to detect single photons.
I used to work at a place that manufactured PET scanner detectors, they were basically a multi-anode PMT with a scintillator crystal on top (all wrapped up in opaque material so only high energy photons could reach the assembly)
This is a very useful video that might work well for QuarkNet students (high school) exploring the technology we use in our cosmic ray muon detectors. Thank you!
I'm wondering, have you worked through the math of the superposition of the photon? Have you landed on virtual photons as a force or are you advocating for something like irradiated electric field with probability amplitudes?
And might I say, just wow. The presentation is outstanding. Miles beyond anything else on TH-cam. It's all here. This is the end of science. The technical jargon with the demo to boot really scratches that quest for knowledge itch I haven't been able to quite reach. thanks!
Shots fired @Marco Reps 2:56
We all seem to have the same interests
@LuxmasterCZ No shots, just a few photons in his direction.
@@HuygensOptics He gonna be sad when he discover that he cannot measure those with signal to noise ratio on the 120db range.
Marco Reps mentioned!
If you are looking for an easy way to block light - use tin foil or foil tape. The metallization does a great job blocking light when compared to fabric or similar (cardboard), and the stiffness makes for it staying in place without too much extra effort.
Worth mentioning that a grounding clip to earth ground could help too - when measuring such small currents, it's not so difficult for a floating metal piece to act as an antenna and couple in some stray EMF to your sensing lines, so having it grounded dramatically reduces any chance of electrical interference.
Would love to see a breakdown on a streak tube next 😅
Thank you for the excellent explanation of how these work! I use them at work in scanning electron microscopes where they amplify the faint flashes of light created when secondary electrons emitted from a sample strike a scintillator plate. This is amplified and used to form the image we see. Also used in scanning laser confocal microscopes to also amplify faint fluorescent light from a sample.
This is fascinating. Have you thought about setting up some compact split experiments to show effects of laser light (I remember from your older video, lasers at low energy are not stimulated emission right?
I would love to see a series on all the quantum experiments done in depths, setup and explained, maybe get @SabineHossenfelder involved?
In the follow up video, I'm going to discuss the difference between detecting coherent and incoherent radiation quanta using multiple photomultipliers.
@@HuygensOptics This is going to be fantastic, this is exactly what we're waiting for, this is some amazing stuff, thank you!
One supercool commercial application with PMT was the 1953 thru early 1959 Autronic Eye from General Motors. This device would automa6tic control your headlights (high beam/ low beam)! But the type PMT is unknown, it was an RCA or Delco tube.
Looking at spontaneous emission with two photo multipliers will be cool, haven't heard of the term photon bunching but i bet it does what it says
As always you switched on my brain, love it, thank you again. The links to the other potentialshifters made my day😆and I'm not in any way jealous of your guys equipment parks, nooooooooooo. 😂
Was a more relaxing time with current than lights "preferences". Btw. did I mention I hate dust?
I've been looking for a video like this for the past two months
This is awesome... the video, the content and way of expressing 👍👍:))
Insane!! Thank you for making such cool stuff... Looking forward to part II!
In the early 1970s the group of Werner Reichardt at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany showed that a rhodopsin molecule in the rhabdomere of the Dipteran eye could detect a single photon.
I love how this old tech vacuum tube is superior to semiconductor sensors. A great example for electronic engineers that newer isn't always better. The amplifier circuit you mentioned is more a current to voltage converter. We don't realy speak of amplification here because we can't compare voltage with current even though the power gets amplified.
Semiconductors are pretty close. They have a small edge on QE and aren't affected by magnetic fields. However they have their own problems.
use those every day in the microscope at uni! cant wait for the photn bun nching ep!
vacuum tubes are massively underappreciated
This makes me understand why those Night Vision goggles that uses photomultipliers is so extremely expensive.
Since actually getting lots of "pixels" with this is difficult.
Very nice. I worked once with a gamma ray log for oil rock samples that used a large photomultiplier, very expensive machine and very sensitive.
excellent description
Absolutely fascinating.
Now Marco Reps is gonna make an amplifier circuit.
I wonder how this thing would behave with different frequencies, right up to hard UV or soft X-rays?
It's actually insensitive to those, for one because the quartz envelope will become opaque below160nm in the deep UV and X-rays will partly pass the complete device unnoticed because they are not absorbed.
@@HuygensOptics I see.
I still look forward to your next video about this nifty thing. I'm into particle physics, and they use fancy ones in neutrino detectors.
Excellent explanation. Just curious how is it detecting single photons, isn't the laser diode emitting lots of coherent photons? Only one photon makes it through the aperture of the photomultiplier? What happens if a large sum of photons goes in there?
Thank you! Very interesting, but how do we know that it is just a single photon that ejects the electron?
Excellent video as usual, thank you.
fun fact: human eye is responsive to single photons but neural filtration limits light detection to a few photons per 100ms.
The QE for rods in the eye is what, 5%?
Awesome explanation. Thanks.
Today is a good day...
As side note, I have wondered if is possible to use one of those as a camera.
In that case you need to add an XY scanner.
A Gamma camera uses an array of the Hamamatsu miniature square PMTs with Cesium iodide crystals made into an X-Y array. They are used in CAT scan medical instruments. The company I worked for built a 16x16 gamma camera to be used in oilfield pipe inspection. This was back when the 3/8 diameter PMTs were just coming out.
In the past, to develop compact gamma cameras I used position-sensitive PMTs like Hamamatsu's H8500 or H9500, but recently, I've switched to SiPM arrays coupled to CsI(Tl).
Awesome video! Feels like direct response to my question
Try detecting cosmic ray Cherenkov light. You’ll need a focusing mirror, it for a table top sized thing, you should get around six photons all at once.
Detecting single photon events in a thermal background of a few 100 counts per seconds will be a challenge. However, if the laser is pulsed and you gate the amplifier with the laser pulse the background may be reduced. The amplitude of the amplifier output then should vary with the number of primary photoelectrons.
I am curious if this will show up. Eagerly awaiting your next video.
Cool experiment!
12:18 you're a chemist, not a phycicist by education? That's new to me 😅
Yep. But with fireworks and explosives being a hobby when I was younger, I guess experimenting with electrons an light is much safer.
@@HuygensOptics Fireworks and explosives, I'm sure you enjoyed reading "Ignition!" (Clark, 1972), a LOT. :D
You could (if your wife allows it) stick the whole multiplier setup in the freezer. That should also be pretty well sealed so very little to light should get in.
Yep, I'm already constructing a light-tight box that fits our freezer!
@@HuygensOptics cool!
Really cool little units with all the needed parts integrated, and at a very reasonable price. What do you think they came from - a university undergrad optics lab being cleared out, or is there a commercial use?
Some kind of classified apparatus: the seller could not disclose the application because of a confidentiality agreement with the company that sold the apparatus to him that contained these items.
@@HuygensOptics Maybe check it with a radioactivity detector!!
You could try making DIY drum scanner with that. If you had machine controlled XY stage with negative on it, you can scan the film with sensor.
Thank you for this excellent tutorial! 👏👏👏
I am a long time follower of you fantastic channel. Hamamatsu builds also qcmos cameras, that can do photon counting in each pixel of the sensor. It means the camera can measure if a pixel has detected 1,2,3 etc photons. Are you interested?
Sure, but I think these will be even more expensive than the tubes that I have now, meaning they are a bit above my pay grade (;-).
let me check what I can do. Maybe other viewers have nice ideas what you should do with such a camera?
This is interesting because I've only ever seen videos about photomultipliers used for TV production to enhance the picture on their cameras. It stands to reason they would have more scientific uses, having been born of science, but I just got so bogged down in the TV stuff, I forgot to consider it.
Next video I expect the PMT under LN2
I was just talking to one of my coworkers about these. We use a Nuclear Moisture Density gauge like a Troxler or Instrotek. These devices use these photocathodes.
Thanks for the video! Great as always.
I have two questions:
1. Why is there always 1 photon that gets absorbed by the electrons? Why not two, for instance?
2. By the time Einstein explained the photo effect, did we know about the quanta of energy of light? Or not yet?
This is exactly what i wanna know! Thank you!
gold like this really sets you apart. amazing
this channel is so freaking cool
How did I understand most of that? It might be because you’re an excellent communicator but I’ll go with my mum’s explanation that I’m really smart