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 2.GQ Geiger Muller Counter
 GMC-600+ oversaturated by compass

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jlp1528 Posted - 06/07/2020 : 15:59:54
After watching a YouTube video of a vintage British army compass making Geiger counters go nuts, I decided to look for them on eBay. Sure enough, I found one, oddly (but of course very fortunately) from an American seller. Here is the video:

As it turns out, this thing is too hot for the GMC-600+ to measure at contact. It just goes to 161 million CPM, the screen locks up but it continues clicking, then it reboots after a few seconds. At first I thought this was just because the exposed strip of radium paint would be such a strong alpha emitter, but I tried putting several sheets of paper in between and all that did was slow it down - it still crashed. Ditto with a sheet of aluminum foil. So now I have some questions.

First of all, I am able to get a proper reading of over 30,000 CPM with my GMC-500+. While obviously concerning, it should be okay to handle the compass briefly to show it off... But I sure won't be carrying it around in my pocket.

That said, why is my GMC-600+ so ready to derp out on this thing? I really doubt the beta/gamma alone is that high, but I also thought 4 sheets of paper would stop alpha pretty well, so I don't know what to think.

I would also suggest a firmware update to make it detect the overload and display so in these situations, instead of 161 or 162 million CPM (why consistently that number anyway???) and crashing. It's actually very easy to "overload" pretty much any Geiger counter with a plasma lamp if you don't have any *real* sources hot enough. Just in case a developer sees this but doesn't have anything to test it with. ;)

I definitely like the very sensitive and alpha capable GMC-600+, but if I'm going to be dealing with sources this (surprisingly) hot, I just won't be able to use it. Unless of course I can rig up a reliable shield, preferably one I can do math on to calculate back up to true levels again at that. Any suggestions are welcome.
13   L A T E S T    R E P L I E S    (Newest First)
Buli Posted - 01/09/2024 : 12:30:54
EmfDev thx. and different cpm values at different death time at the same source ? should i set the parameters for highest cpm ?

source is AM241 and uranium ore - mix for highest cpm
EmfDev Posted - 01/09/2024 : 12:20:25
Hi Buli, whichever produced the more accurate result comparing to your source in terms of dose rate.

That can happen. It may be because the tube is exposed to a very high radiation that it needs some time to recover. As long as it can go back to 60 CPM, it should be ok.
Buli Posted - 01/08/2024 : 15:50:50
after removing the source (200k - 300k cpm) from before the GMC600+ cpm does not decrease to background 60cpm but stays at around 500cpm and slowly decrease to 60cpm over three hours.

Why? it is normal ?
Buli Posted - 01/08/2024 : 15:43:00
which value is more accurate ?
dead time 40ms, 290 000 cpm
dead time 80ms, 360 000 cpm

at LND7317 by gmc600+, the source was fixed
Damien68 Posted - 06/18/2020 : 07:46:46
to make CPM compensation that make sense, it's absolutely need to master rightly the dead time.
like you said, the principle of compensation is to consider for 1 minute the ratio: "total effective observation time / total effective dead time" and adjuste CPM value with it.
for that, consider that the total effective dead time during 1 minute is equal to the number of detection during 1 minute multiplyed by unity blind time (typical 50 microSeconds)
and therefore consider that the total effective observation time during this same minute is equal to 1 minutes - total effective dead time per minute

so total effective observation time during 1 minute is equal to:
1 minutes - number of detection during 1 minute multiplyed by unity blind time (typical 50 microSeconds)

the theory is good, but to apply it, it is absolutly required to underestimate the value of the blindtime, otherwise we can arrive at exponentially delusional results with times that can even become negative.

after each detection, the voltage between the anode and the cathode of the tube drops to 0 volts. The dead time is the time required to recharge the tube voltage through its anode resistance. It is the time necessary to recover the plateau voltage, and therefor its mainly fonction of the anode resistor value, HV generator voltage, tube capacitace value in pico Farad, and plateau voltage value.

based on LND7317 characteristiques, the maximum observable CPM on the GM tube physical connections is:
60 secondes / 50 microSeconds = 1 200 000 CPM
To show more, it can be justified by the correction algorithm.
Searinox Posted - 06/18/2020 : 06:37:12
Dead time leads to the compensated CPM used by the counter in its calculations growing exponentially higher as the actual registered CPM approaches the tube's assumed limit(which in CPM is 1 minute divided by dead time in microseconds). At 162 million CPM - with the assumed LND 7317 tube's 360 CPM for 1 uSv/h that gives you around 0.45 Sieverts per hour which is ridiculous. The 7317 tube isn't even meant to be able to go that high. Out of curiosity how much was the dead time on your unit? I'm coming up with about 370 microseconds based on your 162 million CPM.
jlp1528 Posted - 06/10/2020 : 09:36:11
Well, as I said, it's not interfering with my readings, so I'm just going to leave it off.
EmfDev Posted - 06/10/2020 : 09:06:43
Looks like the dead time default on the unit is a little high. It caused some part of the calculation to crash.
Damien68 Posted - 06/10/2020 : 09:00:44
yes I removed what I said on plasma lamps, I was thinking of industrial cutting or welding systems, sorry for my remark.
jlp1528 Posted - 06/10/2020 : 04:11:56
Thank you EmfDev! Fast estimate time was a no go, but dead time did the trick! 200,000+ CPM beta/gamma! Also, although readings are a bit slower now (despite reactivating dynamic fast estimate), disabling dead time doesn't seem to interfere with other readings either (Fiestaware, lantern mantles, radium watch...) A pleasant surprise!

I also decided to try testing the Am-241 smoke detector bead again. 100,000+ CPM, but obviously mostly alpha. Interesting comparison. Thank you again for your help.
jlp1528 Posted - 06/10/2020 : 03:07:39
I too thought it might be because of dynamic fast estimate, so I will try that. It's probably just too sudden of a change in radiation level for the device to handle properly, at least regarding that feature. Dead time also makes sense.

The plasma lamp is just a weird little interaction, though I understand the concern about potentially damaging the device and again I would only use it if I didn't have something hot to test.

As for alpha alone, there would be quite a lot coming from the exposed strip of radium paint on the mirror. But if paper isn't solving the crash, I think EmfDev is right and I need to change estimate to 60 seconds.
EmfDev Posted - 06/08/2020 : 09:45:54
Hi Jlp, it can be because of the watchdog. Can you please try to change the "CPM Estimate Time" to 60 seconds instead of "Dynamic" from the main menu? And also try to turn off the Dead Time feature it can be because of this one also.
Damien68 Posted - 06/08/2020 : 01:02:07
what you say about freezing and restarting the screen reminds me of a watchdog reset.
I do not know if these levels of radiation are sufficient to be able to disturb the electronics itself, is it a possibility?

but for high levels of radiation, I think the 500+ should be more suitable because it has a second tube much smaller and made for high radiations levels.

something else:


After 1st Decay, radium is tranformed to radon gaz which will stay partialy in the compass. after transformed to Po218 and to PB214 it emit a total of 3 alpha particules.
after, Pb 214 is decomposing to Bi214 and to PO214 and then emit 2 beta particles. and after 1 alpha 2 beta and another alpha to get stable lead.
also as long as the decay elements remain in the compass, 1 radium nucleus will emit 5 alpha and 4 beta before being transformed into stable lead. So 500+ has items to detect.

with a basic shield you will first suppress all the alpha and by strengthening the shield, the low energy beta then ... .

like that I would say that we would need an efficient shield (lead + steel or copper I don't know too much) which would only partially cover the sensor. which would lower its sensitivity in proportion to the shielded/unshielded surface, and without changing its response (with respect to the type of particle and their energies)

If you do as on the video, I do not know that there is a lot of alpha to cross the glass of the compass, so it is possible that what is detected by the 600+ is also mainly beta emited by sub decay of radium.

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