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Playing with a Photomultiplier Tube

Posted <2020-11-26 Thu 22:59> by Aaron S. Jackson.

I recently purchased two photomultiplier tubes (PMT) off eBay for around £15 each or so. Not really sure what I intend to do with them, but they are very interesting things to play with. What is a photomultiplier tube, I hear you ask? Essentially a PMT is a very sensitive light sensor, so sensitive in fact, that it is able to detect individual photons.

When a photon hits the photocathode, a single electron is emitted. The photocathode is at a high negative voltage, around 1000v or so. Between the photocathode and the anode, there is a series of dynodes, which emit secondary free electrons. Each dynode is at a slightly higher (less negative) voltage, around 100v higher each time, so these electrons are attracted along the chain of dynodes towards the anode. Most photomultiplier tubes have around 10 dynodes - all these secondary electrons provide very high amounts of gain.

I've made a short (2 minute) video demonstrating (abusing, more like) a photomultiplier tube:

The question is, what am I going to do with it? I'm not sure yet. Ben Krasnow has a few interesting projects in which he's used a photomultiplier tube - most notably, a scanning electron microscope, but this requires a very expensive high vacuum system. A slightly simpler project was a spectrometer. Ben built this by attaching a diffraction grating to a potentiometer, which controls the X axis, and the output of the PMT controlling the Y axis. It seemed to work very well.

Anyway, not much else to say, just had a bit of fun with a PMT :-)

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