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Photomultiplier Tube

Photomultiplier Tube

Photomultiplier Tube

♫ What is it good for? ♫

I have been taking apart an old microscope and one of the components inside it was a strange vacuum tube device. My inclinations was that it was some manner of Spectroscope or Camera, maybe both but I'm not sure. My supervisor suggested that it might be a photomultiplier which, after looking it up, I would tend to agree.

So, what are Photomultiplier Tubes used for? (Pics related)

RE: Photomultiplier Tube

Didn't google work for you? Wiki has a very good article on PMTs
There are lots of other fact-filled articles, too.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Photomultiplier Tube

Google did and wiki did provide some answers, though I still felt it was a bit unclear.

As I can gather, in ye days of Analogue, this device gathered photons in a sort of echo chamber which focused the light onto a sensor and converted the electric field into magnetic/electrons which then could be used to record charge frequencies...? So it could be used as a spectroscope if stationary or in scanning an area to pick up a range of readings to build up an image... I think...? In television, a scanned function representing an image could then be transmitted to televisions where it was scanned onto screen...? I'm guessing some points.

RE: Photomultiplier Tube

OK. It works on a principle that actually gave A. Einstein his Nobel Prize (The relativity was too difficult for many to accept, but it was generally thought that such a smart guy should have the Prize - so he got it for the photo-electric effect, which he also discovered/explained. He WAS a real smart guy.)

The photo-electric effect means that light (or any radiation) "knocks" electrons off from a cathode and the number of electrons per second is proportional to the light intensity AND the frequency of the radiation (inverse wavelength). Planck's constant is in that formula, which isn't surprising at all.

So, there is a light quantum hitting the cathode. That quantum knocks electrons off and the resulting current is very weak. Difficult to measure at that time. So, there was another cathode that the electrons did hit. They knocked off more electrons, which were attracted by a third fourth, fifth etcetera cathode so that the current was multiplied with a certain number each time it hit another electrode. Hence the term Photo Multiplier Tube.

I used such tubes in a spectrometer back in the sixties during a summer ob at the Metallografiska Institutet at the KTH in Stockholm. The spectrometer was a large, dark room with many PMTs arranged in a semi-circle around a "Gitter" that reflected the light from an electric arc between two carbon (extremely pure) electrodes where a drop of the specimen was placed. The arc produced different spectral lines that were deflected at different angles and there was a PMT at every position where one could expect to find radiation from an element. Na (sodium) had two bright lines that were so close to each other that it was difficult to measure both. The guy that I worked with, George Carlsson, worked until he died at age 94. Fascinating guy.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Photomultiplier Tube

PMTs were and still are used for applications where conventional detectors and their amplifiers have too much noise; this problem has not and generally will not get much better. PMTs can have gains on the order of 10000 or more. Microchannel plate (MCP) technology is similar to PMTs that can have gains upwards of 1 million. So, in anycase, if you want detect low amplitude optical signals, PMTs are still excellent choices, other than size, weight, and power.

I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers

RE: Photomultiplier Tube

In physics research, PMTs are still the state-of-the-art detector to use for detecting the flash of light from Cherenkov radiation from the rare event of neutrinos interacting with ordinary matter. In these neutrino observatories, thousands of PMT are used.

At the Super-Kamiokande in Japan, over 11,000 PMTs of 50cm diameter are used - each one is larger than a beach ball!

The Ice-Cube in Antarctica is the most recently built neutrino detector, and it uses PMTs.

Specialized vacuum tubes still have their niche applications. Think about that the next time you heat your lunch in a microwave oven.

RE: Photomultiplier Tube

Thanks all! That's pretty awesome! I'm glad I kept this part whole! I might be able to re-wire it and put it somehow back to use again as a spectrometer. Could be an interesting component to supplement the the rebuilding of the microscope!

RE: Photomultiplier Tube

Do take note of the lightning bolt sticker.
Vacuum tubes in general, and PMTs in particular, use high-ish voltages to do their thing.
Keep your fingers out of their vicinity when they're on.
Make sure all the associated capacitors have discharged before assuming anything is off.

Mike Halloran
Pembroke Pines, FL, USA

RE: Photomultiplier Tube

Thanks for the tip, will do! PMT has its own casing which I decided to keep unit in, capacitors and all!

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