Hi & a question about CPM

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2 years 11 months ago - 2 years 11 months ago #4597 by docsquee
Hi all, greetings from wet and windy Cumbria, UK.

I'm a novice whose lovely partner bought me this kit , it's using a SMB20 tube.

I've hooked up the kit to a RasPi model 2b via a serial connection and I'm using PyRadmon to upload the CPM data.

I've placed both pieces of hardware in a clear plastic (lock n lock) food container and placed the tube on the outide of the box. See here.

The box is sat on my bedroom windowsill and is powered via the mains.

Question:

The kit has not been up and running long but for around 48 hours the counter settled on an average somewhere around 33 CPM. Last night I was awoken by HEAVY rain (approx 4am BST), I was looking out of the window (easily excited) and noticed the counter flashing, it was reading over 70 CPM ( see here ). I restarted the kit and Pi, tried again and got the same reading. I moved the kit to a different room (same floor) and got a similar (high 50s) count. The kit is back in its original location. The average now is sitting around 50 CPM.

Note that I sleep in the same room as the counter. My instinct tells me this is a hardware 'issue'. Does anyone have any ideas what has caused this upward trend?

Many thanks
Doc
Last edit: 2 years 11 months ago by docsquee. Reason: SPaG

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2 years 11 months ago - 2 years 11 months ago #4598 by mw0uzo
Replied by mw0uzo on topic Hi & a question about CPM
Leave your counter running while we discuss what it can be. There is small but non-zero chance that you may have caught a real event, due to recent events in Russia and at an unknown event at Heysham near to you in the UK. Heavy rain washes out gases in the atmosphere above and concentrates them at ground level. Sometimes after heavy rain, there is radon washout, which is detectable with SBM-20 when significant.

However, as the hardware is new, there is a good chance that there is a problem with it, or that environmental conditions at the high voltage circuits are damp, e.g. being by a window at night.

Steps to take:
1. Clean all solder connections around the HV tube and pcb circuitry, flux deposits can cause problems
2. Ensure no dampness can affect the PCB or tube
3 ....

<got to go - can someone take over from me here, thankyou!!>
Last edit: 2 years 11 months ago by mw0uzo.
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2 years 11 months ago #4599 by docsquee
Replied by docsquee on topic Hi & a question about CPM


...However, as the hardware is new, there is a good chance that there is a problem with it, or that environmental conditions at the high voltage circuits are damp, e.g. being by a window at night.

Steps to take:
1. Clean all solder connections around the HV tube and pcb circuitry, flux deposits can cause problems
2. Ensure no dampness can affect the PCB or tube


1. I've cleaned the contacts and solder connecting the tube to the PCB and I've wrapped them in shrink tube.
2. Windowsill is completely dry; I've placed a humidity meter to check. The box is vented (drilled holes on each side).

Thanks
Doc

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2 years 11 months ago #4600 by Alpha01
Replied by Alpha01 on topic Hi & a question about CPM
Hello,
Good also to check if your HV generator isn't over-driving the SBM20. I had myself this kind of trouble with a former tube. Check if the voltage, before the anode resistor, isn't too much over 400V, reaching the end of the plateau region (always use a high impedance probe, at least over than 100M, 1G the best).
Also check the anode resistor, it should ideally be a 5.6M for a SBM20, best if connected as close as possible next to the anode clamp (if your kit allows that).
According to your installation picture, it could be worth reducing the length of the connection to the tube. The only advantage to put your tube outside the box is to render it a little bit more sensitive to hard beta radiation, but nothing more with gamma. Thin plastic enclosure wouldn't stop that much hard beta too... So to my opinion, it's better to put the tube inside the box and shorten the connections as much as possible.
Hope this could be of any help...
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2 years 11 months ago #4601 by FSM19
Replied by FSM19 on topic Hi & a question about CPM
Looks like your counts are back to normal, latest counts are about 30cpm. I have experienced odd spikes like that myself in the past, and still don't know what caused them, except on rare occasions. I have raised the sbm20 resistor to 10M, it doesn't seem to have made any difference. I have the advantage of having a 1G impedance voltmeter, and have checked the tube voltage many times, and it seems stable at 400V, but short spikes wouldn't show in this kind of test.
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2 years 11 months ago #4607 by Simomax
Replied by Simomax on topic Hi & a question about CPM
What everyone else said. There is a small chance it could be interference from something electrical. Cheap nasty Chinese switch mode power supplies can often emit noise. Make sure you have a decent PSU running your Pi and counter. As an example, running a plasma ball near a Geiger counter will make it go crazy!

As mw0uzo mentions, leave it running for a few days to see if there is a pattern of it counting higher at nights. Also, do you any kind of check source such as Uranium glass, fiestaware pottery (Uranium glaze) or something else you could use? If so then place it right at the tube to get the highest count then slowly move it away from the tube. If you see the counts going down steadily as it gets further away the tube voltage is probably set OK. If the counts stay high then drop in sort of chunks then the voltage may be too high causing the tube to avalanche. Don't rely on the CPM display though as this takes ~30 seconds to settle. Instead listen to the clicks it makes.

I Have just had a look at the kit. Make sure the jumper is removed for correct 400v. With the jumper fitted it will be set to 500v which is too much for the SBM-20 tube. Also if this was a kit you have built then check that the correct 10M ohm resistor is correct and not mixed up with another value resistor.

If all seems well then jet it run a few days and we can have a look at the data. Times/counts etc. There is a possibility this was an event, though at 70CPM it's nothing to worry about if it has come and gone. I have had a couple of times when it has rained hard and my counts have gone up due to the rain trapping radioactive particles in the air and bringing them down to ground level.

Don't worry about it though. There are a lot of clever people on this site so we'll get you up and running properly :)
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