Faulty Ludlum Model 44-21?

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Emma Black
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Faulty Ludlum Model 44-21?

Post by Emma Black »

I have a Ludlum Model 44-21 based NaI-Tl scintillator, that I have connected to my model 3. However there is a condensable amount of noise at all times, despite playing around with the voltage. Upon investigation it seems to be very light sensitive, making me wonder if the film window on the end is leaking light. Covering the end in tape improved things a lot but there is still noise there.

Am I doing something fundamentally wrong here and does anybody know if a more permanent fix is possible?
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Liam David
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Re: Faulty Ludlum Model 44-21?

Post by Liam David »

I think your synopsis is correct. Even a pinprick hole can swamp a detector like that in noise. The noise should start at some threshold voltage and then grow considerably as you increase the voltage.

Are you sure the tape makes it light-tight? Unless it's black and completely sealed around the edges, there's a good chance light is still leaking through. How do you know that the counts are still noise after covering the end in tape? Scintillators have much higher background count rates than e.g. a GM tube. Ludlum's website gives a background count rate of 450 cpm.
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Richard Hull
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Re: Faulty Ludlum Model 44-21?

Post by Richard Hull »

Even if you seal the thing against light, the scintillator/PMT may be noisy for a day or two with ever lowering noise level. You said "film" might be leaking. Most scintillators have a full metal aluminum cup that is forever light tight. A film on a scintillator typically means an alpha sensitive probe.
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Liam David
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Re: Faulty Ludlum Model 44-21?

Post by Liam David »

Yes, I forgot to mention that there will be an afterglow. If you monitor the count rate with time after it is re-sealed, it should be exponential.

Based on the description of the probe found in the manual, it seems that the film is used as both the light barrier and the hermetic seal for the NaI(Tl) crystal, which is below a 0.1" slab of plastic scintillator for betas. If the film is punctured and indeed the only hermetic seal, then the NaI(Tl) crystal is likely compromised since it is very hygroscopic.

https://ludlums.com/products/all-produc ... odel-44-21
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Emma Black
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Re: Faulty Ludlum Model 44-21?

Post by Emma Black »

Thanks for the really helpful responses. I think the phrase "you get what you pay for" applies here. It was £30 from ebay.

The film does have some marks but they don't look like holes more like its been stretched. It just looks like its all weirdly aligned as if it's been taken apart and reassembled badly.

I didn't know that an afterglow on scintillation crystals was a thing, thats really interesting and obvious in retrospect! Maybe also why it didn't immediately calm down.

Some foil and better tape later and the noise is noticeably better already.
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Alex Aitken
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Re: Faulty Ludlum Model 44-21?

Post by Alex Aitken »

The wrinkling might be a sign that moisture is getting into the crystal. Google suggests this is an NaI(Tl) with a thin window for low energy beta and gamma.

iradinc on ebay sells some very thin alumiinised film 0.8mg/scm. The shipping cost is unreasonable, but something like that might be a route.

PMTs also get very noisy after being exposed to daylight and should be kept in the dark for a day(?) before even being powered. I've not heard this about the usual scintillators but I imagine ZnS is a nightmare.
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