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Zippotix
Poland
5 Posts |
 Posted - 03/12/2025 : 05:37:00
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Hello, I just bought GMC 800 and while I'm generally happy with it I feel beta sensitivity could be higher. There are no openings in the sensor location, how is beta particle supposed to pass through? Or does it work only for high energy betas?
Tested with radium watch hand and thorium mantle |
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| Reply #1
Zippotix
Poland
5 Posts |
Posted - 03/13/2025 : 02:56:35
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| Also, I tested my tube against sunlight (cloudy day but I saw those tubes react even on cloudy day) and it doesn't increase counts. Is it possible that I've got some other tube, and not m4011? |
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| Reply #2
EmfDev
2314 Posts |
Posted - 03/13/2025 : 13:45:45
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| do you mean that it is not sensitive to light? it is suppose to not be sensitive to light. The device should have M4011 / J321 tube. |
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| Reply #3
Zippotix
Poland
5 Posts |
Posted - 03/16/2025 : 07:25:55
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Yes, it is not, which I thought is typical for these tubes.Thats good though.
Anyhow, do you think cutting/drilling openings on the case is a good idea? Would it really increase sensitivity to beta or is it not worth the effort? |
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| Reply #4
EmfDev
2314 Posts |
Posted - 03/18/2025 : 11:39:53
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| maybe about 1% |
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| Reply #5
ullix
Germany
1189 Posts |
Posted - 03/22/2025 : 01:06:06
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@zippotix:
The crux with the betas is that the quoted decay energy of a beta decay - like 1.33 MeV for K40 - is barely seen by your counter. And while this is a rather high energy, many beta energies are in the 100keV and even below range, with accordingly lower ability to pass through matter, like the glass wall of a Geiger tube, and thus are unable to trigger a count.
Furthermore, even for high beta energy emitters, most of their betas come as low and very low energies. The reason is that in such beta decays the energy released is shared between an electron and a neutrino. And you find the electrons mostly on the low energy end. See e.g. the beta spectrum on Wiki https://en.wikipedia.org/wiki/Beta_decay with the bulge at the left end.
Electrons are strongly absorbed by matter, which includes paper, plastic and even air. Any such stuff between radioactive source and Geiger tube will significantly reduce count rate!
Drilling a few holes in the counter casing will improve the situation, but only marginally so. My solution is to remove the backside of counters completely and bring the tube in contact with the source as intimately as possible. For mechanical and electrical protection of the counters during transport, I hold the backside of the counter on the counter with a strong rubber band ;-).
For more on such counting and range of electrons in matter see my "Potty Training" article (https://sourceforge.net/projects/geigerlog/files/Articles/)
Counters witha pancake tube, like the GMC-600, have only a 2fold improvement of their gamma sensitivity, but thanks to their very thin window, they are much more sensitive to betas!
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| Reply #6
Zippotix
Poland
5 Posts |
Posted - 03/24/2025 : 07:36:39
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Thanks for replies. Ullix, I've just finished reading "potty training" and found it both informative and enjoyable. I think I had some leftover KNO3, and will test it ASAP:). Next buy will be pancake type detector, no doubt! I have also acquired americium²#8308;¹ source, and while most of it is alpha radiation it can still detect weak gammas. GMC 800 reacts to it nicely!
I will do try rubber band method, seems like only logical solution:) |
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| Reply #7
ullix
Germany
1189 Posts |
Posted - 03/25/2025 : 01:36:39
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You will have to decide whether to go for either Gamma- or Beta-sensitivity.
For Beta you need those windowed Geiger tubes. The window material is so far mostly Mica, but I have also seen tubes with a steel window, for more robustness. Electrically you can replace any current glass tube with any windowed tube - pancake or else - but the mechanical integration may be more demanding.
For Gamma sensitivity you need to go for "Crystals". The most used detectors use CsI(Tl) - Caesium Iodide doped with Thallium - crystals. Consumer oriented devices are e.g. the Radiacode devices (https://www.radiacode.com/de ), which are expensive but still cheaper than a GMC-600.
They are quite a bit more sensitive for Gamma, but not so much for beta. No surprise, because the crystals detect light sparks created by a radioactive particle being stopped inside and therefore need to be wrapped perfectly light-tight. That wrapper does hold back those low-energy electrons.
But the other huge benefit is that the light intensity of the sparks is proportional to the Gamma energy, and so you can create Gamma spectra!
The latest development versions of GeigerLog do support Gamma spectra. Here is one from Americium-241, showing its very prominent 59.54 keV line, and the near-perfect Gauss curve fitted to the peak:
Note how well it is defined, despite the low Gamma energy, being in the realm of X-ray from a doctor's office!
For a download look for the latest prereleases of GeigerLog here:
https://sourceforge.net/p/geigerlog/discussion/devel2/ |
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| Reply #8
Zippotix
Poland
5 Posts |
Posted - 03/26/2025 : 09:39:06
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I think I would go for scintillator this time,I had a pancake type before (beta1-1 gm counter) and it performed well I guess but having opportunity to be able to identify isotope is something new to me.
How would you rate GMC 800 though?
KNO3 didn't make counts increase, maybe because betas couldn't get past the dense packaging plastic |
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| Reply #9
ullix
Germany
1189 Posts |
Posted - 03/30/2025 : 03:05:51
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When you need beta sensitivity then the GMC-800 is NOT a good choice, UNLESS you remove the counter's backplate and measure with the naked tube. Then it is as good as it gets with these types of tubes!
Of course, Thin-Window tubes, like those pancake tubes, will be more sensitive to beta, but also more expensive.
... (Removed by SmartAI)
The thicker plastic and lack of holes makes the GMC-800 clearly less sensitive to beta. However, I still welcome this more sturdy design, though! By the way, design-wise it is very similar to the FNIRSI GC-01, which therefore is equally low-senstive to beta. Same solution - remove the backplate. And have a strong enough rubber band ready to hold the backplate on the counter for protection during transportation.
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GMC 800 beta sensitivity
