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 2.GQ Geiger Muller Counter
 GMC-600+ detects tritium?!
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jlp1528

USA
19 Posts

Posted - 05/23/2020 :  07:34:36  Show Profile  Reply with Quote
A month or two ago I purchased a tritium vial keychain from www.tritiumkeychains.com" target="_blank">h p://www.tritiumkeychains.com to replace the zipper pull on my favorite hoodie and add to my collection of radioactive things. As expected, my GMC-500+ doesn't detect it, but the 600+ shows 300 CPM! Background is about 40 CPM.

I know that the 600+ is supposed to be a very sensitive model, but the manual lists "detectable radiation level" of beta as 50 KeV. Tritium decay energy is well below this threshold at 18.6 KeV, and the electron (beta particle) itself averages only 5.7 KeV. Sure, Wikipedia isn't always accurate, but I did check other sources too.

So is the manual just being extremely conservative when it comes to detectable energy levels, or is there something else going on here? Again, the 500+ didn't show an elevated reading from this keychain, so I doubt it's contaminated... This is just really weird to me and some clarification would be nice.
Reply #1

kotarak

USA
19 Posts

Posted - 05/23/2020 :  14:41:14  Show Profile  Reply with Quote
Remember that the low energy beta (electrons) coming out of the tritium nuclei, must be able to go thru the phosphorous compound on the inside wall of the vial,then the glass of the vial itself, (then plastic of the keychain enclosure), then must travel thru air, then thru the plastic of the counter enclosure, then the glass of the M4011 tube...that's quite a journey for a low-energy electron.
I have put one of these tritium vials in my cloud chamber and you can actually observe the beta tracks, zig-zagging thru space as the electron bumps in gas molecules and gets deflected.
You do need a really sensitive GM tube to pickup the increase of CPM at these energies - my Gamma Scout with a mica end-window tube (LND712) shows a slight increase but it is not as nearly as dramatic as the pancake of 600+.
Again - it has to do (as Damien pointed out in your other post) - the geometry and sensitivity of the tube are factors.
When it comes to low-level activity measurements, you do need either a pancake GM tube or even better - a scintillator type detector.
These small glass GM tubes as M4011 are better suited for general dosimetry rather than a sensitive survey work. Forget what the tube specs say - I highly doubt them and who knows if these were just lab testing specs or actual real-world conditions.
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Reply #2

jlp1528

USA
19 Posts

Posted - 05/23/2020 :  16:38:12  Show Profile  Reply with Quote
I suppose I'd rather have a (much) more sensitive detector than what the manual says, considering the alternative... Also, clearly I need to throw everything I've heard about shielding out the window and figure it out myself. I already knew that alpha particles are in fact not always stopped by paper, or even skin for that matter. The things I keep reading about tritium are along the lines of "maybe those electrons will travel 1cm in air" and "it's safe because the betas can't even get through the glass". Obviously these are both false statements, though tritium is still safe, right???

Not sure why we *ever* use absolute statements like this when referring to radioactivity. It is a spectrum, after all...
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Reply #3

kotarak

USA
19 Posts

Posted - 05/23/2020 :  17:38:28  Show Profile  Reply with Quote
The betas (or at least some) for sure are making thru the glass of the tritium vials - I have *seen* them in my Cloud Chamber.
Check out the last clip on the link bellow (use full screen to see it better) - I just realized - I have posted a video of a tritium vial inside my CC.
h**p://blog.kotarak.net/2020/04/continuously-sensitive-diffusion-cloud_56.html

They are also detectable by a GM counter - you just need a sensitive one - as I mentioned - unless you want to venture in the world of scintillators - your best bet is GMC-600+ (it has become my favorite Geiger Counter).
As for how safe are these vials - they should be safe enough to warrant their use when you need to find something important in the dark (gun-sight, flashlight, knife, compass, watch..etc) using them for a zipper pull on a tent - I can see that - on a hoodie, just for the cool factor - I personally wouldn't do it - it seems a bit of a frivolous use but that's me... as you said - it is a spectrum...you decide where you want to situate an exposure vs. need

Edited by - kotarak on 05/23/2020 17:52:47
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Reply #4

Searinox

Romania
34 Posts

Posted - 06/18/2020 :  07:28:19  Show Profile  Reply with Quote
I was able to get a reading of 1.5 uSv/h on a GMC-600(non-plus) but with the GMC-320+ I had to open the battery cover and put the tritium vial inside directly up against the tube and only got 0.36 uSv/h. The 320+ likely never got any beta, just x-rays from the electrons coming to a halt as they hit the tube's glass. I don't think 50 KeV is a hard cut-off rather the detection rate just becomes worse and worse at the lower end.
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Reply #5

Damien68

France
241 Posts

Posted - 06/18/2020 :  08:06:52  Show Profile  Reply with Quote
0.36ÁSv/h vs 1.5ÁSv/h is a ratio of 1/4, maybe 1/6...1/8 if we remove the background offset, I think the difference is mainly due to the shape of the detector, pancakes are much more suitable for this kind of measurement.

Mastery is acquired by studying, with it everything becomes simple

Edited by - Damien68 on 06/18/2020 08:09:10
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