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!!>

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