Quick way to kill a PMT

Scintillation crystals, PMTs, voltage dividers etc...
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Steven Sesselmann
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Quick way to kill a PMT

Post by Steven Sesselmann » 07 Jun 2019, 08:10

Hi All,

A few days ago I did something really silly, and I should have known better. I was using a brand new 3" PMT to test the resolution of some new crystals, and I was temporarily mounting the crystals to the pmt and running resolution tests, somehow in my absentmindedness I forgot to disconnect the power while removing the black sticky tape on one of the tests, and as soon I removed the crystal I realised what a silly thing I had just done, but it was already too late, the photo-cathode was already stripped off.

Never powering up a PMT in daylight is something I have been quick to point out to others, and then I go and do it myself, so I feel pretty silly !!

For anyone reading this who is not familiar with this problem, PMT's are sensitive devices designed to measure single photons, they operate by the photoelectric effect where the energy from a single photon of light can dislodge an electron in a material. Normally photons don't have enough energy to strip electrons from solid materials, but by biasing the PMT with a high voltage the odds are in our favour.

The cathode (negatively charged part) of a PMT is normally the glass window, which is why it has that brown colour, the brown coating, on the inside of the glass is transparent as well as conductive. This coating depletes over time, but with proper use it lasts a very long time, because the number of photons emitted by a crystal are vanishingly few compared to ordinary daylight. If you look carefully inside a PMT you will see that the cathode pin actually makes contact with the glass on the inside.

I lost a good PMT, but I hope this post serves to save many PMT's in the future, so maybe it's a win !

Steven
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Steven Sesselmann | Sydney | Australia | gammaspectacular.com | groundpotential.org | beejewel.com.au |

luuk
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Re: Quick way to kill a PMT

Post by luuk » 08 Jun 2019, 04:18

Hi Steven,

I am surprised to see that the cathode on our pmt is completely gone how long did the pmt stayed in full light with the hv turned on?
It happened a few times that I while testing a crystal I opened the door of the testbox(blackbox) and removed the crystal so the cathode was uncovered with hv on.
Of course the pmt does not like that but what happened is that the gain drops dramatically and the pmt gives a lot of noise but will recover after some time(hours) and most of the times the pmt is like it was before.
So not all exposure to daylight from a pmt has to end in a complete dead pmt.

Luuk

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Steven Sesselmann
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Re: Quick way to kill a PMT

Post by Steven Sesselmann » 08 Jun 2019, 11:10

Luuk,

I don't know exactly how long it was exposed, but my guess is less than 30 seconds. Despite the photocathode appearing to be stripped off, it still works, but the efficiency is lower.

I think a fail safe solution would be to build a black box with an SHV connector that protrudes through a hole in the door, that way you can't open the door without first disconnecting the plug.

Steven
Steven Sesselmann | Sydney | Australia | gammaspectacular.com | groundpotential.org | beejewel.com.au |

luuk
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Re: Quick way to kill a PMT

Post by luuk » 09 Jun 2019, 00:25

Hi Steven,
Yes a safety maybe a good idea I have thought of it to, but while testing a lots of detectors every day it is to much to work for me so I just have to pay attention what I am doing and up till now I never killed a pmt so the chance that is goes completely wrong is really small, I think you just had bad luck.
Luuk

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