PMT Wiring

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PMT Wiring

Postby johnf » Thu Oct 28, 2010 10:08 pm

As Steven asked me in an email today on how to wire PMTs for single wire operation and some of you may want to use Neg HT and some positive HT I felt I should put up a couple of PDF's one from Saint Gobain the other from Ludlum

Voltage-Divider-Design-Considerations.pdf
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M44-17_nov04.pdf
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Re: PMT Wiring

Postby Doug Coulter » Fri Oct 29, 2010 10:19 am

Good stuff as usual, John. People should pay special attention to those bits about what happens to the later dynode voltages when there's a big signal because depending on time constants and impedances there, it can toss in some weird errors in that case, if you're caring about linearity of response. You don't want the response changing just because of radiation's normal "bursty nature".

One interesting dodge from an old RCA data sheet is to make a small C-W voltage multiplier at the volts per stage you want on your dynodes, which makes for a stiffer supply on each one, and eliminates the power losses of the resistors -- because they aren't there at all any more. This is especially helpful if you want to run off batteries, and it's not hard to make an efficient 50v or so AC source to get 100v or so per stage.

I think we can add to this the part about "how to get the signal to the rest of the gear". For the 2 wire connections, you have a choice of a series R to the supply, and capacitor coupling, or a small signal transformer (which is what I prefer). In the two wire lashups, of course one has to be ground or you pick up noise that way, and in any two wire setup, you have issues with seeing current from the divider chain and dynodes as well as the plate current -- not so great, but convenient for that one second you're just plugging in a single cable. Since that's such a small part of things, I prefer separate power and signal wiring myself. Some of the dynode currents and the currents from the R's amounts to noise otherwise and messes up the response and linearity.

For some two wire NaI:Tl heads I have, I found winding about 10-20 turns winding on a small cup core ferrite (same for both primary and secondary) strips off the HV supply nicely and lets you do the rest at ground, and it recovers much quicker from bursts than any coupling capacitor does, as well as having less noise than surface leakage across the capacitor tends to. At any rate, that's what seems to work the best for me (use good insulation in there! -- I put teflon tape between the windings).
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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