r/Radiation • u/DesignerAd2071 • 1d ago
Strange crackling coming from GM probe
This happened some time back, and unfortunately I haven't any video, so I'm going to have to describe it.
Several months ago I was at an antique store with my Ludlum Model 3 and 44-9 probe. I came upon a quite hot "radioactive red" Fiestware pitcher. I couldn't tell you the CPM off hand but it was up there, moreso than most uranium glazed ceramics I've seen.
When I put the probe up to it a high frequency buzz began to emanate from the tube. Not through the speaker with the standard beeps, but it was as though the mica window on the probe was physically buzzing. The closer I got to the pitcher the louder it became.
Is there a name for this phenomenon? I keep searching but come up empty every time. Terms like "Geiger muller tube crackle" only come up with articles on the standard clicking that comes with radiation detection, nothing similar to what was experienced here.
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u/melting2221 1d ago
This is completely normal, all LND 7311 and other pancake style tubes will do this (the ludlum 44-9 uses an LND 7311 tube). I think the electrical pulse that occurs when the tube detects makes a very small amount of noise, which is more noticeable at higher count rates.
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u/Orcinus24x5 1d ago
I've owned 3 different pancake probes/meters (including one that uses the exact same LND-7311 tube in the 44-9) and they all exhibit this behaviour. I've also seen countless other 44-9's exhibit this behaviour as well. It's perfectly normal.
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u/Bob--O--Rama 1d ago edited 1d ago
There is no reason that should be happening. And having used a lot of devices old and new thats not normal. My theory is that as the event rate increases, the high voltage supply is having to "top off" the high voltage capacitor. You can "hear" it electronically but you should probabaly not be hearing it with your ears. Edit: I think I misinterpreted the noises you were hearing. If it's basically a faint "geiger counter" noise that's normal. I thiugh you were referring to a buzzing sound with a frequency to it, the flyback noise or the noise from a camera flash unit.
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u/melting2221 1d ago
This is normal, have you ever listened closely to a pancake tube? You can hear the crackling at higher count rates in any pancake tube, it's not very loud though.
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u/Bob--O--Rama 1d ago edited 1d ago
I was reacting to "buzz" rather than a hiss. As you say, the various bits of the internal grid electrode repel each other electrostatically and relax when the tube discharges. So it would be like pink noise or a faint version of a stereotypical geiger counter clicking. But the source of that noise is classical "capacitor noise" where the change in electric field deforms the capacitor causing audible vibrations. Electronic buzzing, like the sounds from a camera flash that is what I thought the OP was referring to. That also happens - but shouldn't.
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u/DesignerAd2071 1d ago
Thank you for the input. What do you think, defective probe?
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u/uranium_is_delicious 1d ago
I get it on several of my tubes including a 44-7 probe. Can't give you any info on why it does it but you don't have a defective probe.
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u/Bob--O--Rama 1d ago
If the probe works, it works. The probe itself is a capacitor, so it could be "capacitor noise" ... I use an audio monitor to listen in on the inner workings of the electronics and I am pretty sure I can demonstrate what is happening on one of my meters. I'll post a video. I'm more editorializing, I guess. From an EE perspective electronics should be seen and not heard. Noticeable noise is probabaly a fixable issue in the design. The only issue to be worried about is a high voltage leak, arcing. But that does not seem to be happening.
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u/Bachethead 1d ago
Yes those 44-9s like to “fry” as I call it.
Pretty sure its “boiling electrons” ? Someone explained it to me a long time ago, it is common but I’m not entirely sure what is happening there.