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 2.GQ Geiger Muller Counter
 GQ GMC-600+ Reads CPM too high?
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Zugstar

USA
6 Posts

Posted - 04/07/2022 :  17:51:04  Show Profile  Reply with Quote
I just bought a new 600+ from Amazon and I think it's defective, so I need to decide quick to send it back or not. Software is v2.24.
The background reads between 30-50 CPM indoors and outside at my home. It also reads the same at work in the middle of a large 4 story building. It stays the same if I put it in a metal filing cabinet drawer and close it. A Uranium calibration sample from "Images SI" is certified at 2579 CPM but the 600+ reads about 11,500 CPM. I tried a range of tube voltages from 450V-550V but it makes no difference. Should I send it back? It otherwise looks like a nice product. I realize itís hard to buy a counter right now due to the war, so if I return it I wonít be able to get a replacement for a long time.

Edited by - Zugstar on 04/21/2022 20:36:37
Reply #1

EmfDev

1700 Posts

Posted - 04/07/2022 :  17:56:13  Show Profile  Reply with Quote
Hi Zugstar, I'm not quite sure what you mean by defective? It seems to work fine for me. Do you mean it is defective because it reads the same everywhere? Or because the Uranium sample read 11,500 CPM? For the 2579 CPM reading, did it say from which counter it was detected from?
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Reply #2

Zugstar

USA
6 Posts

Posted - 04/07/2022 :  18:22:22  Show Profile  Reply with Quote
Hi EmfDev, The sample's cert sheet says the 2579 CPM reading was obtained using a Model GCA-07W counter. That's possible that it will would read lower than the 600+, but the discrepancy seems quite high.
Certainly, the 600+ unit is not defective in terms of being dead: It does read samples. But I was most concerned that the background stays at 30-50 CPM inside a large building and also being inside a metal filing cabinet in that building (San Jose, CA). I'd think the readings would be closer to zero in that case, and so this could mean something is wrong with the unit that could get worse over time, and such that I would not be able to return it.

Edited by - Zugstar on 04/07/2022 18:55:29
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Reply #3

Damien68

France
695 Posts

Posted - 04/07/2022 :  22:47:24  Show Profile  Reply with Quote
quote:
Originally posted by Zugstar
The sample's cert sheet says the 2579 CPM reading was obtained using a Model GCA-07W counter. That's possible that it will would read lower than the 600+, but the discrepancy seems quite high.



the GCA-07W sensor is a LND712, on the GMC600+ there is a LND7317
https://www.imagesco.com/geiger/digital-geiger-counter.html

https://www.lndinc.com/products/geiger-mueller-tubes/712/
https://www.lndinc.com/products/geiger-mueller-tubes/7317/

the window of the 7317 is much wider than that of the 712 which explains why it is much more sensitive.

the CPM values are not standard and depend on the sensor used, they must be converted for example into uS/h with a conversion factor. each sensor has its own conversion factor.

the background you are measuring is mostly strong gamma and therefore passes through metal sheets.
alpha rays don't travel very far through the air and betas are easily stopped.
your background levels are quite low for this type of sensor which is very sensitive.
I don't think you have to worry about that, your counter seems to be working normally.

Mastery is acquired by studying, with it everything becomes simple

Edited by - Damien68 on 04/07/2022 23:57:38
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Reply #4

Damien68

France
695 Posts

Posted - 04/07/2022 :  23:24:21  Show Profile  Reply with Quote
quote:
Originally posted by Zugstar
I realize itís hard to buy a counter right now due to the war.


war or shortage of semiconductors or both?

Mastery is acquired by studying, with it everything becomes simple
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Reply #5

ullix

Germany
775 Posts

Posted - 04/07/2022 :  23:46:44  Show Profile  Reply with Quote
quote:
A Uranium calibration sample from "Images SI" is certified at 2579 CPM
I have serious doubts that you can get any meaningful "certification" on a uranium sample in terms of CPM. What was the distance of the sample to the detector? Perhaps the orientation of the sample also plays a role? what was the material of the case of the counter, and what its thickness? Can you publish or link to that certificate?

Chances are that this sample is also a good beta emitter, and many of the betas may have been absorbed in the counter's case, while the 600 has only a thin membrane, allowing almost all betas to pass, and thereby create much higher counts.

The background may have a good contribution from cosmic rays, and they may not care too much about the metal sheet of your filing cabinet. After all, they managed to get through all the atmosphere and into your house.

From all I can see so far, the counter works fine!

If you want to be sure, you need to do some tests:

First, put the sample on the table, and position the counter 2...3 cm above it, with the tube opening facing the sample. Record data for a few minutes.

Then put a sandwich of ~5mm of plastic plus ~2 mm of metal (steel, copper, aluminum) between sample and counter, with the plastic facing the sample. If you have no suitable plastic, take same thickness of copy paper.

Record data for a few minutes, and compare with first measurement. Should have given significantly fewer counts.

Even more good data possible from using Potassium as source. You may have it in your house already; check your kitchen for cocoa powder, or your garden for fertilizer! Details in my "Potty Training" article, GeigerLog-Potty Training for Your Geiger Counter, download from https://sourceforge.net/projects/geigerlog/files/Articles/
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Reply #6

Zugstar

USA
6 Posts

Posted - 04/08/2022 :  20:12:48  Show Profile  Reply with Quote
quote:
Originally posted by Damien68

quote:
Originally posted by Zugstar
I realize itís hard to buy a counter right now due to the war.


war or shortage of semiconductors or both?



I think both: Semi + war.
There is the well known semiconductor shortage which needs no discussion, and I also think that folks are concerned the war may kick up active dust from Chernobyl that could spread via Jetstream. This would be of curious academic interest as the background activity levels, though possibly elevated will probably remain safe. Folks may also worry in the extreme case that leaders will start pressing buttons, but really that's not a concern a Geiger counter will help with. My location, in the center of Silicon Valley, will probably cease to exist.
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Reply #7

Zugstar

USA
6 Posts

Posted - 04/08/2022 :  21:18:20  Show Profile  Reply with Quote
quote:
Originally posted by ullix

quote:
A Uranium calibration sample from "Images SI" is certified at 2579 CPM
I have serious doubts that you can get any meaningful "certification" on a uranium sample in terms of CPM. What was the distance of the sample to the detector? Perhaps the orientation of the sample also plays a role? what was the material of the case of the counter, and what its thickness? Can you publish or link to that certificate?

Chances are that this sample is also a good beta emitter, and many of the betas may have been absorbed in the counter's case, while the 600 has only a thin membrane, allowing almost all betas to pass, and thereby create much higher counts.

The background may have a good contribution from cosmic rays, and they may not care too much about the metal sheet of your filing cabinet. After all, they managed to get through all the atmosphere and into your house.

From all I can see so far, the counter works fine!

If you want to be sure, you need to do some tests:

First, put the sample on the table, and position the counter 2...3 cm above it, with the tube opening facing the sample. Record data for a few minutes.

Then put a sandwich of ~5mm of plastic plus ~2 mm of metal (steel, copper, aluminum) between sample and counter, with the plastic facing the sample. If you have no suitable plastic, take same thickness of copy paper.

Record data for a few minutes, and compare with first measurement. Should have given significantly fewer counts.

Even more good data possible from using Potassium as source. You may have it in your house already; check your kitchen for cocoa powder, or your garden for fertilizer! Details in my "Potty Training" article, GeigerLog-Potty Training for Your Geiger Counter, download from https://sourceforge.net/projects/geigerlog/files/Articles/



Hi ullix, I ran the tests: Background = 42 CPM, unit elevated 5cm over the sample with plastic/aluminum sheets inserted, = 69 CPM, elevated 5cm direct exposure (no plastic/aluminum sheets) = 2281CPM. So yes, the unit works as expected, assuming the 42CPM background is normal for this unit. My concern is that, suppose the background is actually about 20 CPM, but that the tube is having self-trigger events that adds another 20CPM. If so, a +20CPM offset is not significant when actually measuring samples, but if such a a hypothetical offset is present, it could indicate a defect (e.g. poor tube processing-contaminated backfill) get worse over time. Nonetheless, based on your comment that background 40-50CPM is normal for the pancake tube in this unit, I am thinking that a tube defect likelihood is very low, and so I'll keep my unit. BTW, the "Geiger Potty Training" paper is very well researched/written...thanks for posting it.

I wonder what background levels other people read with their 600+ units?
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Reply #8

ullix

Germany
775 Posts

Posted - 04/08/2022 :  23:58:21  Show Profile  Reply with Quote
Nice results!

First, you confirmed having have a strong beta emitter.

quote:

My concern is that, suppose the background is actually about 20 CPM ...
Second, the background.

Tubes like M4011, J305, SBM20 do get a background at my place of CPM=15 ... 25. Many others also report similar background levels.

Let's look at the sensitivities of these tubes. Find a listing of some tube sensitivities in the GeigerLog manual in chapter Appendix G. The M4011, J305, SBM20 are shown at somewhere near 150 CPM /(uSv/h).

There you also find the tube LND7317, which is used in the 600 counter, and it is shown as having 348 CPM/(uSv/h) for Co60 according to the tube manufacturer.

Let's pretend those sensitivities are appropriate also for background, then we calculate the ratio 348/150 = 2.3. So, expect the background for the 600 being 2.3 times that of a 300 or 500 counter, which, with my data, becomes CPM=35 ... 58.

It all fits; it is fully in line with expectations! Your CPM=42 seems to be on the low side.

If you want to get a good handle on background, then do this: Keep in mind, that your counter does have a significant directional dependency for betas (and alphas, but not relevant here) due to its tube with a membrane opening!

Put the counter on top of a wooden table in the middle of the living room, both face-up and face-down. I expect no difference.

Then put the counter very close to a brick wall, face-to-wall and face-away. May give a significant difference, depending on wall material.

Same thing with the bathroom tiles. Depending on the age of the house you could be up for a surprise!

Any granite somewhere? Same testing.


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Reply #9

Damien68

France
695 Posts

Posted - 04/09/2022 :  01:50:32  Show Profile  Reply with Quote
with strong shield the background of LND7317 is specified to 30 CPM max
https://www.lndinc.com/products/geiger-mueller-tubes/7317/
but strong shield is 50mm PB + 3mm AL
some go into deep underground tunnels to test this, but it can be polluted by the nature of the soil or by the materials used. also it is an experience that can be interesting but not necessarily relevant.
ullix is right and the LND7317 is one of the best choices in this product range.

Mastery is acquired by studying, with it everything becomes simple

Edited by - Damien68 on 04/09/2022 03:08:53
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Reply #10

Irmidjusz

Poland
1 Posts

Posted - 05/06/2022 :  16:21:25  Show Profile  Reply with Quote
quote:
Originally posted by Zugstar
I wonder what background levels other people read with their 600+ units?



Hi,

I'm new in the world of gm-counters :D very happy owner of GMC-600+ from 5th of May :) And I confirm that my gmc also shows background's CPM around 50 and at the beginning I was also a bit worried why.

But it is all OK as we know now.

This forum and all topics related to measurement of radiation is something quite new for me and I see there is a lot of enthusiasts, what is great!
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