| T O P I C R E V I E W |
| ullix |
Posted - 11/23/2021 : 06:04:14
I was wondering about some background runs coming with counts significantly higher than expected. Poisson check was always perfectly ok. They look like this, here a cut for the first hour :
The counter is an GMC500+. The blue trace is recording by USB-Serial connection, the brown trace is recording by WiFi. Recording data by WiFi into GeigerLog is a feature which will be available in the next release of GeigerLog (coming soon).
The averages are:
GMC500+: Blue: CPM=41.1
GMC500+: Brown:CPM=41.5
So, they are identical. However, they are about 250% (!) of what they used to be! Wow, what's up with that?
I also have an GMC300E+ counter, which I put back-to-back with the 500 counter, the two tubes as close together as possible. The 300 was now connected by USB cable, and the 500 still by WiFi. Result:
GMC500+: Brown: CPM=42.8
GMC300E+: Blue: CPM=16.4
The 500 gives the 40+ CPM as already seen, while the 300 gives some 16CPM. The latter is fully in line with my expectations from many previous experiments. But the results are drastically different!
Both counter have the M4011 tube, the 500 a blackened one, the 300 a clear glass one. So far I have never observed any significant differences between the two.
I guess something is defect, either the GMC500+, or its tube (, or both). Has anyone seen similar results?
I also noticed that the LED at the front, which usually gives a green flash upon detecting a count, now often flashes in red! All GMC manuals are mum about the meaning of the color of the flashes. Has anyone an explanation?
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| 2 L A T E S T R E P L I E S (Newest First) |
| ullix |
Posted - 11/25/2021 : 03:58:18
I tried to swap the tubes of the two counters, but found a surprise: the tube of the GMC300E+ is NOT a M4011, but a J305, despite the data sheet still claiming this being a M4011!
That J305 has a little bit thinner glass tube diameter (by 0.5mm), and its glass tube is a little bit shorter (by 3mm). This makes its volume smaller by ~10...15%, and one would expect its sensitivity - assumed proportional to the inner volume - lower by about this value. The data do seem to confirm this.
I then replaced the GMC500+ tube M4011-blackened, with a tube M4011-clear glass, and ran again:
This time the averages are:
GMC500+: Blue: CPM=18.0 (by Serial)
GMC500+: Brown:CPM=18.1 (by WiFi)
So, again identical data for WiFi and Serial, as it should be. But this time much lower at a more reasonable CPM=18 instead of CPM=41, measured at the same spot with the same counter, but with a different M4011 tube.
These CPM=18.0 values of the M4011 clear-glass tube versus the previously measured CPM=16.4 with the J305 tube happen to be 18.0/16.4 => +10% higher, in line with the expected sensitivity differences due to different size!
If the red color of the flashing LED indicates that alarm levels of CPM=100 were reached, it sure is very strange. The recorded data were never even near this level! While it is theoretically possible that 100 are reached when 40 is the average, this should be something to occur once every week or even month, and not every minute!
Also, while the green flashes come at a leisurely pace, the red ones do seem to come in bursts of a few flashes. Perhaps a defect of the tube, where something like continuous discharges were triggered?
At least one conclusion seems to be valid: the M4011-blackened tube is defect! Which gives an opportunity to determine the live-time of the tube.
Many tubes, but not all, have Life-Time rating of pulses. Here some data I found:
SBM20: >2*10E10
SBM10: >2*10E10
LB 6500-H10: 5*10E10
M4011: >1*10E10
The M4011 is the worst of the pack, but still rated as withstanding 10000 Million counts!
How much may my tube have seen? The tube has an imprint of 2017. I obtained the counter in late 2018. About 3 years have passed since. Pulses can only occur when the tube is powered, and that was by far not all the time, but let's say it was. When it was on, it measured mostly background except for some brief experiments. But let's claim it was CPM=20 for all 3 years. This will be highly over-counting, but it makes: 3*365*24*60*20 = 32 Million.
That is a far cry away from 10000 Million!
Is it only me who gets the duds of tubes, like light-sensitive ones and under-performers? If not then I'd say the expected quality of M4011 tubes is poor!
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| Damien68 |
Posted - 11/23/2021 : 07:58:27
Hi Ullix,
on my 500+, when the displayed CPM levels exceed the programmed alert level then it flashes red and green otherwise.
indeed the difference between the 500+ and the 300E+ is confusing for tubes which should be almost identical.
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