I've been noticing increased background readings over the past few weeks here in my lab, which I'm having trouble explaining -- none of our sources have been "out loose" and this extends to more than one building we can carry our portable counters to. The increase is hard to quantify without better record-keeping than I've been doing, and inside the normal variation range, but for one thing - in the normal variation, we also see the odd low count (ten second periods) and those are gone entirely.
For example, in ten second counting interval, we'd see a range of about 60 to about 120 cpm. The extreme numbers would be more rare, but in a given day, you'd see some of both, with a lot in the middle. Now it seems I never see less than 100, and see 150+ a lot more often. Obviously this is somewhat disturbing. We did see a count increase on collected snow-melt just a few days after Fukishima, which went away quick, sadly before we got the gamma spec going to see if it was iodine -- it's gone now from our rain collection anyway (I think, next time we get rain, I'll check again).
So, we normally see a diurnal variation in cosmics - after dawn and before dusk it's hottest here. And of course, some we've traced back to solar weather.
Anybody else been running detectors and notice this, or am I perhaps looking at some local phenomenon? It's not just in the fusor lab, but all over my land - which is large and mostly forest and fields.
I'll try to get a sample and check it with the gamma spec to identify what it might be, but this is complicated by the fact that the bedrock around here is hotter than "normal", and to get low backgrounds in the lab I use the second floor for measuring to get away from that. This is one reason we look at rainwater - it didn't come from here, but surely does wind up in the soil.
Edit:
I just recalibrated the gamma spec and took a background upstairs, to compare with one from a couple weeks ago. They line up within the usual errors I think I have due to cal drift, but I'll now do it more carefully and try to get errors to nearly zero -- it's a real sensitive function of just how I do that, but I'm seeing nothing important different -- in gamma, which isn't what my pancake geiger is most sensitive to. The usual high end stuff from cosmics -- check. The usual mid band background which is pretty random (no peaks per se) check. There is some difference in the real low energy range, sub 100kv, which is going to require another calibration to really see -- but that stuff is below where a geiger sees well anyway.
No un-encapsulated sources have been anywhere near that pancake detector, so I really doubt something fell into it, but will try with another identical tube that's been in storage and see if I can track this down better.