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 2.GQ Geiger Muller Counter
 Strange high reading on GMC 600?
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borealis

8 Posts

Posted - 02/08/2020 :  22:53:52  Show Profile  Reply with Quote
Hi folks,

I bought my GMC-600 a couple of weeks ago in part because of these really active forums, and now I'm glad I did because I got a strange high reading while literally walking down the street. I could use your help figuring out whether it looks like a software or hardware problem and I need to send my counter back for replacement.

To start off, I was walking on the sidewalk next to the grocery story and my GMC 600 started alarming. When I looked at the screen, it read "High" and the CPM was in the 400 range. I was so surprised that I just kept walking for about 20 more feet, then I turned around and went back to where the alarm started. After a little more time, the CPM suddenly went to 0 and started counting back up to the normal background rate around here, about 12 CPM. I searched a bit but never got a higher than background reading again (even spent an hour with the counter there a few days later - nothing). Image 1 is the graph of CPS around the event.

I downloaded the data and checked it for obvious software bugs at the CPS or CPM level but didn't see any suspicious power-of-two values during the spike (unless you count the one 32 CPS value). The CPS data shows two spikes with a slightly higher than average CPS in between, surrounded by normal data. The CPS of the few seconds before and after the first spike are:

0
0
0
12
23
69
82
4
1
5

The CPS for the second spike:

9
1
9
35
87
56
32
7
0
0

Before and since this, my GMC 600 has behaved totally normally. I played with some potassium salt and a smoke detector, but otherwise the CPM has stayed below 40. After this happened, I left my counter running on my desk for 2.5 days and graphed the CPM values and it looks like the expected Poisson distribution with only one outlier of 33 CPM. Image 2 is the histogram of the CPM during those 2.5 days.

So, any ideas on what could be wrong with my counter? I sincerely doubt there was anything that radioactive on the sidewalk outside the grocery store, but it could just barely be possible if... a puff of radioactive air came out of the vent in the wall and was blown away by the wind? A silent drone carrying a cesium source flew overhead? A super villain aimed their Mutation Gun at me? I can't imagine a way plausible way for this reading to be correct. Thanks in advance for any hints you can give me!

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Reply #1

Damien68

France
241 Posts

Posted - 02/09/2020 :  07:49:58  Show Profile  Reply with Quote
I do not know well the 600, I have a 500+, but the 12 CPM on average that you have on the fish diagram seems to me very low, after perhaps that it's not impossible either.
12 CPMs represent one hit every 5 seconds. In this case it's normal that there are several consecutive null cps. We must have on average 4 or 5 consecutive null cps for 1 cps to 1.
The two peaks are indeed also suspect.
what was the outside temperature during your experiment?
do you have the voltage applied to the tube?

but maybe you just passed someone who just had a medical scanner.

Mastery is acquired by studying, with it everything becomes simple

Edited by - Damien68 on 02/09/2020 08:26:15
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Reply #2

Stargazer 40

USA
370 Posts

Posted - 02/09/2020 :  15:59:02  Show Profile  Reply with Quote
The 600 uses an SBT-11/A pancake tube. I use 82CPM/uSv for it compared to the 153CPM/uSv used by GQ and what I consider a background conversion for the M4011. So about 1/2 of the CPMs of the M4011 tube is what you should see background with the 600. So not low I think. Spikes I don't know.

Stargazer 40
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Reply #3

borealis

8 Posts

Posted - 02/09/2020 :  17:54:28  Show Profile  Reply with Quote
Thank you both for your replies! Stargazer 40 is right, my GMC 600 software is using 153 CPM/uSv. When I do a histogram of the per-minute uSv/h readings from the downloaded data, it shows the average radiation is about 0.075 uSv/h which seems like normal background radiation. I just reported raw CPS because that's the most useful data for debugging this.

If you calculate uSv/h on a per second basis using 153 CPM/uSv, the peaks max out at (87*60)/153 = 34 uSv/h. Is that in the range for a medical scanner? There is also an active skyscraper construction site a few hundred feet away - could they be using a portable gauge that could cause this spike?

One of the things I notice is that the CPS is higher - not super high, but definitely higher than normal background - in between the peaks. And the peaks are only 4 seconds long and 111 seconds apart. Does that suggest anything to anyone? Thanks!
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Reply #4

borealis

8 Posts

Posted - 02/09/2020 :  17:59:03  Show Profile  Reply with Quote
Sorry, forgot to answer the temp and voltage questions!

The outside temperature was around 55F/13C.

The voltage is the factory default, which is 405 volts/46%.

It's all factory settings except for UI things like changing the backlight shutoff delay.
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Reply #5

borealis

8 Posts

Posted - 02/09/2020 :  18:33:07  Show Profile  Reply with Quote
I just noticed something odd... it seems like the uSv/h displayed on the device is about 1/2 of that in the downloaded data for the same CPM?

Displayed on the device:

11 CPM = 0.037 uSv/h
12 CPM = 0.040 uSv/h

In the downloaded data:

12 CPM = 0.078 uSv/h
11 CPM = 0.072 uSv/h

It looks like the conversion for the display is:

12/0.04 = 300 CPM/uSvh

And the conversion for the downloaded data is:

12/0.078 = 154 CPM/uSvh

What am I missing?
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Reply #6

Damien68

France
241 Posts

Posted - 02/10/2020 :  01:09:32  Show Profile  Reply with Quote
600 device only record CPS or CPM in his Flash memory and no have uSv/h recording, so 12 CPM = 0.078 uSv/h is certainly an interpretation made by the software you use. there must be a specific interpretation missing in it for the 600 model.
your voltage is OK and the temperature cannot be a problem.

if you have an active skyscraper construction site near to you I think it could be a residue of XRay beam to make quality control of structures. it's another possibility more likely.

but I don’t think Will Smith and the men in black who’re back with their neuralyser:-)

Mastery is acquired by studying, with it everything becomes simple

Edited by - Damien68 on 02/10/2020 12:00:21
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Reply #7

EmfDev

1243 Posts

Posted - 02/10/2020 :  12:08:13  Show Profile  Reply with Quote
Hi borealis, the spike might have been a result of the fast estimate time feature of the GMC-600. The default is in dynamic and your GMC-600 might have gotten a noise while you were walking and the CPM spiked to 400. You can turn off this feature in the User Options->Fast Estimate Time-> 60 Seconds.

When downloading, make sure the calibration in the software is the same as in the unit.
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Reply #8

borealis

8 Posts

Posted - 02/10/2020 :  13:13:41  Show Profile  Reply with Quote
Thanks, Damien68! Yes, I used the standard software provided by GQ to download the data and I've been working with the spreadsheet it exports. So the download software and the onboard software disagree. I will see if I need to upgrade the download software. I may also go hang out by the construction site or chat with the construction workers.

EmfDev, the recorded CPS is also very high, so it can't just be a problem with the CPM estimation code.

Basically I'm trying to figure out if I need to send my counter back for replacement. :)
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Reply #9

EmfDev

1243 Posts

Posted - 02/10/2020 :  13:39:37  Show Profile  Reply with Quote
Ohh I see, maybe the reading is correct. You might have passed someone under medical therapy or with some source. But not sure so you need to make sure first that that's not repeatable/noise. Then the 0 CPM is also from the dynamic CPM estimation.
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Reply #10

Damien68

France
241 Posts

Posted - 02/11/2020 :  01:13:09  Show Profile  Reply with Quote
Hi borealis,
CPS pattern doesn't look like an unstable device, it is well defined with a start a end and an incurs, and these steps don't make me think of a problem.
like say EmfDev, You have to see if it happens again but for the moment I would rather think that your counter is working well.
For software adjustment, I don't know the GQ app well, but as EmfDev says, check the calibration value in the software, this is what gives the ratio CPS/CPM <-> uSv/h

Mastery is acquired by studying, with it everything becomes simple
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Reply #11

borealis

8 Posts

Posted - 02/12/2020 :  16:54:44  Show Profile  Reply with Quote
Thank you so much everyone for replying! I guess I have a working counter. :) I checked that I had the most up to date version of the data viewer software and sent an email to support asking what's up with the two different CPM -> uSv/h conversion rates.
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Reply #12

EmfDev

1243 Posts

Posted - 02/13/2020 :  10:11:00  Show Profile  Reply with Quote
If you counter spikes for no reason, then it might be defective sensor. Just let us know if that happens randomly when it's not suppose to be.
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Reply #13

borealis

8 Posts

Posted - 02/13/2020 :  11:26:37  Show Profile  Reply with Quote
That's what I'm trying to figure out, did it spike for no reason or just for a reason I couldn't see? I couldn't find the source of radiation and I couldn't reproduce it, but the CPS data looks realistic to my untrained eye. Does the CPS data look like a defective sensor or bad electrical connection to you?
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Reply #14

EmfDev

1243 Posts

Posted - 02/13/2020 :  13:19:55  Show Profile  Reply with Quote
A common wrong reading is a high spike in 1 or maybe 2 CPS.
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Reply #15

borealis

8 Posts

Posted - 02/14/2020 :  18:30:58  Show Profile  Reply with Quote
Great, I don't think it's my counter at fault. Thanks everyone!
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Reply #16

GBG12

Canada
69 Posts

Posted - 02/17/2020 :  18:38:25  Show Profile  Reply with Quote
For X-ray imaging, it's unlikely to get a reading in a public area. The source is typically far away from the public. The crews who operate these sensors cordon aff the area and are well aware of the safety regulations. Typically the instrument is set up with the source on one side in a shielded box and an imaging plate on the other side of the thing they need to scan. They stand a safe distance away then use a cable crank to push out the source for the required imaging time then retract it back into the shielded box.

The 2 spikes could be explained by passing a person having a recent radiation imaging or therapy as mentioned by others. I once used an earbud and a GM tube in my pocket while waiting at a hospital. Some imaging injections are hot, but these have short half-lives (hours). Cancer therpies can last for days or weeks. Note, if anyone wants to try this, be discreet. You do not want to scare the patient, and the medical staff know what's being used.

I learned most of this by reading the UN NRC reports the past few years.
https://www.nrc.gov/reading-rm/doc-collections/event-status/event/2020/
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Reply #17

Damien68

France
241 Posts

Posted - 02/18/2020 :  14:54:11  Show Profile  Reply with Quote
Hi GBG12,
hello in Canada, I went there to work for 5 years.
for X-ray inspection, we can also use an X-ray tube, which is the most used for small radio,
the principle is to accelerate with 400Kvolt the electrons leaving a hot cathode (like the old TV tubes). then these electrons are projected onto the anode. at this time, 1% of these electrons will then excite an atom in the anode, this excited atom will then emit a photon (X-ray) to recover its normal (stable) energy level. the other 99% of electrons will only heat. This is what is used to control welds in the electronics industry or to make dental X-rays. but for skyscrapers it is true that it is probably easier to use a source.

Mastery is acquired by studying, with it everything becomes simple

Edited by - Damien68 on 02/18/2020 15:00:42
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Reply #18

xadamf663

USA
3 Posts

Posted - 03/17/2020 :  17:11:37  Show Profile  Reply with Quote
I also experience strange peaks.
Normally my gmc-500+ reads .20 to .40 uSv/h.
However, it'll occasionally shoot past .70 uSv/h. Just now it went to 1.20 uSv/h! (just over 200cpm). These peaks last around 10-40 seconds. I haven't been able to figure out the cause. The 0.2-0.4 background readings can be explained by my altitude in colorado.
I also live about 15 miles from a superfund cleanup site, rocky flats where plutonium a-bomb triggers were made. Bits of old radionuclides drifting in the wind? Might be a good idea to bring it on a road trip to the site to see if there's any correlation.
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Reply #19

EmfDev

1243 Posts

Posted - 03/18/2020 :  08:31:21  Show Profile  Reply with Quote
Hi xadamf663, how often does it happen?
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Reply #20

xadamf663

USA
3 Posts

Posted - 03/22/2020 :  12:16:13  Show Profile  Reply with Quote
quote:
Originally posted by EmfDev

Hi xadamf663, how often does it happen?



I've had to turn off the alarm so I am not aware of every time it goes above 0.50uSv/hr.
I'd guess it is happening about 15-30 times a week.
Just an hour ago counts went just over 200, 1.29uSv/hr.

The unit is in my bedroom, 6 doors away from the outdoors. (br door, front door, elevator doors or stairwell doors, building front door.)
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Reply #21

EmfDev

1243 Posts

Posted - 03/23/2020 :  09:07:13  Show Profile  Reply with Quote
There is a possibility that the 600 tube is not functioning properly. Can you please email support and check about the warranty? Make sure though that this is a tube fault not the background radiation.
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Reply #22

xadamf663

USA
3 Posts

Posted - 09/08/2020 :  10:34:40  Show Profile  Reply with Quote
quote:
Originally posted by EmfDev

There is a possibility that the 600 tube is not functioning properly. Can you please email support and check about the warranty? Make sure though that this is a tube fault not the background radiation.



I probably should have checked in sooner. I was relying to a similar thread, but my unit is a GMC-500+. I don't know how to check what the background radiation really is. I think it would be easier to just send it in for calibration.
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