PMT degradation in air - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 26, 2021, 04:54:03 pm
content: Hi!I'm in need of an electron detector, of which the channeltron is the most common.Discrete dynode electron multipliers do exist as well.Could one remove the envelope of a photomultiplier tube (keep the base), and use it as an electron multiplier in a vacuum chamber?Or will the dynode material degrade on exposure to air?Thanks!

author: daqq
author_rank: Super Contributor
timestamp: January 26, 2021, 05:17:44 pm
content: Take a look at this guy:He uses a butchered PMT in a DIY electron microscope, could work.Though my gut tells me that there might be some issues in the long run due to the dynode coating materials, some of which are doped with cesium. See:https://www.hamamatsu.com/resources/pdf/etd/PMT_handbook_v3aE.pdf, page 32.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 26, 2021, 05:29:35 pm
content: I have never heard of the dynodes being coated, the alkali coated element is typically the photocathode (window) into the tube. Not sure what's on p.32.Thanks for the video, I had forgotten that! I'll give it a re-watch!

author: jonpaul
author_rank: Super Contributor
timestamp: January 26, 2021, 05:49:31 pm
content: They are fabricated in clean dry air, before pulling vacuum.I would use a dessicant and minimize time of air exposure.Jon

author: daqq
author_rank: Super Contributor
timestamp: January 26, 2021, 07:47:04 pm
content: QuoteI think that they are coated. From:https://www2.chemistry.msu.edu/courses/CEM988Nuclear/lectures/Chem988_S09-Ch9.pdfQuote

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 26, 2021, 10:39:11 pm
content: Ah yeah, so they are. I wonder how stable the coating is. Most dynodes look metallic, so it's likely a very thin layer that could be removed in dilute acid.Unless of course it's stable enough for general use.Maybe I should just buy a channeltron..

author: penfold
author_rank: Frequent Contributor
timestamp: January 28, 2021, 12:57:49 am
content: Hi,There shouldn't be a problem with 90% of available PMTs, just check the data sheets for any unique selling points that allude to special geometries or coatings or exotic electrode materials, avoid miniaturised versions and anything strange. And just be conscious of the fact you don't have the front grid electrode and need to be aware of your angle of incidence especially for particularly low ion/electron currents.For the purely physical and storage at ambient conditions problem, you're actually probably better off with the broken PMT since the electrode supports are normally glass, mica or glazed ceramics compared with the electron multipliers I've used which are supported with a surprisingly porous ceramic which really need baking out thoroughly after long periods in atmospheric conditions... first time you power up, ramp the voltage slowly and keep an eye on the current, if the current's too high, switch off and give a bit longer for moisture and stuff to come outFYI, if I remember correctly the electron multipliers I used were guaranteed for 24 hours exposure to atmosphere before use without special baking out requirements, but we typically got away with much longer