Increase in CPM during downpour

Yes, thanks for the input! The rainout off course also contain isotopes from the nuclear tests performed from the mid 60s an forward since these bombs were able to push the dust up into the stratosphere.
The material in the stratosphere don't gladly mix with the material in the troposphere, but during spring and summer the two layers mix, injecting isotopes into the stratosphere.
But the isotopes from the radon is still the main part of the radioactive material in a rain out as this radon constantly is being added from the bedrock.

I recorded a spectrum of the cloth but I couldn't make out any pekas, just slightly higher readings compared to the background. The background was not recorded the same day even though the date says so, it should be the 24:th.
I will reproduce this next time it rains and make some shielding around the detector.

The death of the fish in the the river I personally have doubts about. The radioactive rain being the single cause of the deaths seems very far-fetched. But one can never be sure uh

Agreed. Far more likely to be due to run-off from some farmer's fields treated with pesticides or fertiliser.. There simply isn't enough radiation to do the job.

At the time a colleague was primarily looking for acid rain as hand pH measurements with low pH that had been taken regularly on the North Yorkshire moors had closely correlated with the occasions washout into the river was causing significant fish kills, were being reported. It so happened we were continuously monitoring the gamma background at the same time and were also observing increases that closely correlated with the increases in the acidity.

I agree the acid rain washout events may well have simply been responsible for leaching the offending components from the soil that killed the fish, while the acidity in the rain could well have come with a washout of what may have been noticeably radioactive. I do recall a few folk scratching their heads on this one, with mention of the output from the not too distant coal fired power stations, like Drax and others in the region, being responsible for both the acidity and gamma increases. And, I also do agree with the radon washout explanation generally, as for me it has stood well since the mid 80's when we first started monitoring continuous gamma background and were asking anyone who had a clue .

What we did find intriguing in our very early days, was seeing an inverse relationship between gamma and air pressure. Now I have not looked into this in recent years, but it has been a bit of a puzzle that remains unanswered. So any thoughts would be very welcome on this one. I should add, for what it's worth, that when this was observed we were originally using the Mullard ZP1221/01 as used within the Mini-680 counter's probe. Later it's manufacture was taken over by Phillips and then Centronic, followed by Saint-Gobain after which Thermo-Nuclear provided them for us. During this period we found out the hard way, that all tubes bought between the mid-80s through to the mid-90s, were destined to fail by becoming temperature sensitive !! - They would after months and at time, years, but in the 90s they were beginning to fail within hours of powering them up. The then NRPB got very interested in this fault condition but had no explanation. Anyway there are many strands to this story, so back to the curiosity of why we were seeing the gamma to pressure inverse relationship. Maybe it was an aspect of the tube fault condition, albeit they were seeming to work correctly at the time. Oh, and these tubes have an intrinsically low background count with a 24cm sensitive length as well as being energy compensated. They have glass seals too. (In 1995 we switched to using the American LND equivalent tube, which have ceramic seals, and have never looked back - not one has failed or even drifted in the case of those that went for lab re-calibration.)

Graham.

I think the inverse relationship between air pressure and gamma background could be due to Radon movement through the soil. A high pressure weather system would tend to push some air down into the soil pores, and a low pressure system would result in soil gas rising more. Since the decay of Uranium into Radon is constant, surface readings would rise as more gas is released from the soil. Some have noticed higher readings during drought conditions as well, which could be explained by deeper gas movement due to less water in the upper soil layers. This is supported by seasonal variations that can be seen on sites that have multi-year graphs.

Yes, some time back I have heard the suggestion of the variation being as a result of changes in the radon emanating from the surrounding ground. I doubt anyone has a monitor over an expanse of water, maybe a large lake with another on adjacent land? - It would be interesting to compare such readings. I had even considered the marginally increased gamma at times of low atmospheric pressure may occur when there is less dense atmosphere above the probe resulting in an increase in the cosmic induced gamma scatter. On the other hand, I had also considered the tube could have been at fault, considering I do know it will have been one that failed catastrophically when it inevitably went temperature sensitive, as all the UK manufactured ones did !

I'm fascinated by the finding of elevated readings in drought conditions you talk of, but have no experience of this.

Graham.

I think it's important to be able to differenetiate between isotopes contributing to normal background radiation and those which are man-made. Things like Radon and Uranium (and daughters) have been around for 4.5 billion years and are still being produced, they are generally harmless unless locally concentrated to much higher levels (e.g., in your granite basement ). However man-made fission products have much longer half-lives and are both persistant and cumulative... and toxic if concentrated via living organisms. Levels of these isotopes are creeping up globally and it is those which should concern us most. Although still at trace levels (generally speaking) it is exactly these which we should be monitoring via a global network, we cannot rely on government sources for such information. However to measure such trace levels (assuming you don't live next door to a failed nuclear reactor) you need a sensitive gamma spectrometer and a way of concentrating your sample. An HPA filter connected to a vacuum line is a good substitute for professional sampling equipment, then maybe your NaI scintillaton detector will pick it up. It has been done, using a filter from an air conditioner unit.

Our crude background monitoring will tell us something about the next major nuclear disaster that hits the headlines perhaps, particularly if you happen to live within 50 miles of it, but beyond that it is the insidious creeping increase in fission products that we should be monitoring.