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Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
Stella, no need to worry about that, all it needs is a power supply and a network connector plugged in, and it does the job by itself. Just let me know if you have a router, with a permanent internet connection.
Conundrum, which tubes? SBM-19's or the SI-39's? For both there's just one answer: ex communist block :)
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
The second anomaly, for a total of 8 minutes, and now radiation levels are slightly higher:
Highest CPM reading was 5175, considering an average of 81cpm .
Similar to last time, no indication of system malfunction: everything is in scale, and running continuously, except radiation data. Debug data posted here:
Probably it is time to add a backup/redundant detector to the system.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
This first circuit is a GM coincidence detector I found on a cosmic ray site a few years ago. I haven't tried it, but I think you'll agree that the cathode resistors look too high at 1:100 against the anode resistor. But at least they have taken the signal off the cathode, so it can't be all bad.
This here is a commercial anti-coincidence detector I found in a geophysics database of radiation tools.
Used together in a simple array of four GM tubes, these two circuits - or variations or improvements of them - would be powerful tools to identify your anomaly.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
Yes, I will probably have to add that eventually, but I am also considering connecting a scintillation probe to the monitor.
While I do that, here is some more data. I have attached the maximum resolution charts, built with the data I have. What's interesting is that temperature has increased during the event with 0.5 degrees. Keep in mind that the peak happened during a cold winter night, with no sources for additional heat: except perhaps a little extra radiation, passing a bit of its energy to the air, as heat? All three temperature sensors running in parallel have recorded an increase:
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I believe the rate of release of Rn from rocks, and its rise and fall as an Rn layer can be weather-related. I have no personal experience of this, but can't see how it could produce such a large and distinct effect.
If the tube voltage is set too high, right at the top end of the plateau slope, then even a very small increase of Va would trigger a storm of false counts. Perhaps you could try pulling the anode voltage right down to the threshold level, so the tube will still be operating on the plateau if there is a sudden increase of anode voltage (providing it is not too large).
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
This was my first thought, and I already checked that the first time it happened. The tube voltage is monitored constantly, so what this means is that each time I take a reading, that is every minute, I get the CPM, but also the voltage on tube for that minute.
In both cases, the tube voltage has been in normal parameters (that is +-5V + 400V). The microcontroller unit carefully regulates the voltage on the tube. Here are the tube voltage measurements: January 31: The log shows: 402 / 401 / 400 / 399 V values during the abnormal radiation event
February 8: The log shows: 400 / 399 V values during the abnormal radiation event
Also the duty cycle of the 400V inverter PWM is monitored. Also in normal parameters. And the voltage and duty cycle has been kept constant all the way since this monitor first became operational.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
We see in the SBM-19 datasheet that the threshold is in the range 260V - 320V, and operating range is between 350V and 475V.
I suggest thoroughly cleaning the anode insulator with alcohol etc, baking it in a cool oven to boil off the solvent, and then painting it with dielectric varnish.
When this is done, re-set the power supply to 350V. This will give you the whole length of the plateau before random avalanches kick off in the tube, and also reduce the possibility of false triggering by micro surface discharges by reducing the voltage.
You must eliminate every possible error before deciding to accept this anomalous reading as true.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
I cannot do that. Dropping the voltage would also decrease the number of registered pulses.
I just tried it with 350V, and the immediate change was a drop from an average of 80 cpm to about 65cpm. Since I already accumulated a few months of readings, changing the voltage at this point would break the data into two , incomparable sets.
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