Older 14-pin Bicron PMT help / reality check needed.

[Bob-O-Rama]

Hi,

A colleague "gifted" me some larger / older Bicron sensors, like a 8x3" and 4x4" etc. - these all seems to have the same type of 3" 14-pin PMT. I suspect they have moisture issues, as they are old, and still seem to work-ish with g-m counter meters at 700 or 900 volts.

But before I give up, and harvest their PMT organs, I note that the dynode strings in the original caps I have are like 5M total, so made of 490K resistors. Will these lower resistance one work with things like a GS-PRO? They all seem to function properly from an electronics standpoint and an electrostatic volt meter shows reasonable staircase of voltages, caps are good, etc. I have a spare socket and some resistors / HV ceramic caps I can use to make a higher impedance one. Just not sure if its worth the effort. If so, can someone point me at a schematic I might try.

-- Bob
Bob Mahar

[Sesselmann]

Bob,

The lower impedance divider should still work with your GS-PRO, you just have to be aware that the voltmeter will overstate the actual voltage. This is because the voltmeter measures on the high side of the 1M load resistor.

So if you are using a 5M divider the actual bias will be less by this amount;

\[ \frac{5*100}{6}=83.3 \% \]

** Remember guys that this forum can handle LaTex just start and end your expression like this...

"backslash start squarebracket" \frac{5*100}{6}=83.3 \% "backslash end squarebracket"

[Bob-O-Rama]

The issue is I cannot handle LaTex, even though I manage our R and Jupyter servers. So based on that, and and a poorly Google translated Italian forum post that claimed the crystal was supposedly fully encapsulated and not just bare NaI(Tl) - I popped the PMT's off. I didn't spend a lot of time inspecting - and sheepishly sealed it up just in case. But then noted the "gaskets" on the PMT fixtures were made of black felt. LOL!

The optical mating goop was very discolored - like dark honey - and cloudy - so I am not sure how much of the yellowing is the crystal, as any light in through the windows also went through the goop. Under one of the PMT windows there was a faint, iridescent conchoidal fracture, seemingly near the surface about 50% the diameter of the window. I'm somewhat optimistic that replacing the optical goop will get it back to pulse counting at least. It was not anywhere as bad as many of the horror shows people have posted.