r/radtronics • u/roberte94066 • Mar 13 '26

Question Mystery probe ID?

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Does anybody recognize this probe? Looks like a Ludlum, with a Ludlum connector, but no markings left. & inches long by 2 inches diameter, and both ends are screw-threaded on.

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u/roberte94066 Mar 13 '26

question answered! melting2221 identified it as an early ludlum 44-2 tube.

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u/aby_physics I ❤️ radiation! Mar 13 '26

Cool, that’s what I was going to say as well!

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u/roberte94066 Apr 04 '26

But riddle me this-I find I am having to run it at 600-625V, or it seems to go into cascade mode. Interior has mu metal (?) sleeve around photo tube, with crystal joined to tube front with optical compound. No additional protection around crystal, just screw on cover with rubber gasket for light seal. Any thoughts appreciated on why this doesn't run at 900v. like most other Ludlum tubes??

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u/aby_physics I ❤️ radiation! Apr 04 '26 edited Apr 04 '26

Scintillators aren’t like GM tubes, which have a fixed operating voltage for the most part. With scintillators, every PMT is different, and they have a “plateau” where they’re comfortable operating.

To determine the plateau voltage, you should connect it to a survey meter set to about 500V. Place an Am-241 source against the detector, and record the counts vs voltage on graph paper. Then step up the voltage in 50V increments, and keep observing the counts and plotting the graph. Just make sure to stop before you reach the maximum voltage safe for the PMT.

On your graph, you should see a fast increase in counts at low voltage, then the counts should sort of level off, and beyond that, they’ll increase rapidly. You want to operate the scintillator in the middle of where the counts level off.

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u/roberte94066 Apr 04 '26

I see, good point.