A good place to start is that counting labs rarely (read: I have never seen one) end up on the top floor with beautiful views of your surroundings. No, you get the dungeon labs where sunlight & windows are a rumor, but the radon down there is quite real.

[The eleventh in an ongoing series of my compiled explainers for my CHOOSE YOUR OWN RADIATION ADVENTURE quizzes. There’s never really a right answer but some might work out better under the constraints of the scenario. It’s like poetry, really.]

How much radon you have in your subterranean science lair is very much a function of where you are and what your local geology is like. But even in the newest, most freshly heaved from ocean sedimentary formations you’re going to have some. GOOD NEWS! Your building HVAC takes care of this. Well, it should take care of it. If the HVAC is balanced to actually move air through your room. Have you made friends with the Facilities folks yet? You should really do that. 

If you live around some really nice old cracked granite, you’ll have extra fans to blow it clear. In a fine parsing of the language of the scenario and the explainer tweets so far, you’ll notice I just said “radon”. I didn’t give you any specific radionuclides, like Rn-222. This is because for naturally occurring radon, you get all of the them, all the time. But with a half-life of just shy of 4 days, Rn-222 is the only one that really gets a chance to accumulate. The next longest lived is measured in hours. But they’re all there. But assuming your HVAC is working properly, this isn’t a issue. You know the radon evolution rate in your area and you ventilate appropriately. In most places, the typical air changes to blow your human stink out of the building is sufficient. In Wisconsin, you might want extra. 

Also, we don’t *really* detect radon. Radon is an annoying noble gas which means it doesn’t stick to anything, so it’s hard to get enough in any one place to detect. But it’s decay products when it spontaneously stops being radon? Oh yeah, we can work with those.

NOTE: for anyone about to share radon immersion dose stories and calculations, please smugly keep your edge case to yourself.

But what your instrumentation is telling you is that you are experiencing events where you are getting WAY more radon than normal and that’s weird. The question is from where. The game “What the hell is this signal?” is the unofficial hobby of all counting labs. Because you put your sample in the counter, you know what you expect to see and then when you a huge signal of something extra, well, that’s like a big wet fart from the man in front of you playing his Brown Note Solo during the quietest part of a symphony. In the CYORA: Surprise Positrons, an inconsiderate researcher managed to throw off pretty much every counting experiment in the entire building with their insufficiently shielded Na-22 source. Hooboy, those other researchers were hunting for that source.

Counting labs having anomalous signals, even if they’re far away, are how we’ve detected pretty much every incident that has happened in the Soviet Union and its successor states when they don’t feel particularly forthcoming at the time of the incident. If a reactor does the bad burp, you WILL notice it downwind. If it’s particularly bad, that signal will make its way all the way around the planet to show up on your detector from the other direction. Labs in Minsk detected Chernobyl before Sweden did but eventually everyone could.  But what reactor leaks don’t look like is radon. Depending on the particularly kind of leak, you’re going to have fission product signals that show up in your counts. As there’s only so many ways you can get those, you should probably call someone about that. 
If someone was rude enough to set up a SURPRISE ACCELERATOR next to your counting lab without so much as an Employee Right-To-Know chat over coffee, they’re probably using the other side of the wall from your detector as their backstop to be Maximally Inconsiderate Colleague. But again, an accelerator, even one operating in a mode/power that can cause activation, isn’t going to give you a radon signal. It’ll give you a big honking Bremsstrahlung curve to absolutely wipe out your detector, but not radon. Seriously, SURPRISE ACCELERATORS are rude, but they aren’t subtle. You tend to notice when one shows up before they turn it on and can have very productive discussions about shared spaces, resources, and institutional research priorities. It’s also a super great time to make new enemies for the rest of your respective careers.
Which means you’re now looking for the things that are subtle. Changes that might have happened that you can’t see. Perhaps changes that happened to the built environment that no one would think are an issue. Changes like someone getting a fancy new smaller counterweight for the elevator. Elevator counterweights come in a lot of flavors, but the key is that your space is limited in the shaft. Concrete is cheap, but very bulky. Junk steel? It’ll work. Lead? Now we’re talking to get the size down of the counterweight down and you already have a CA Prop 65 warning on the building anyway. Tungsten? You are superfancy and must have a lot of budget to burn because that’s expensive. How about a depleted uranium one?

Oh dear. 

As several of you identified, an elevator shaft is a lovely low space where you could collect radon but the pumping action of the elevator tends to flush it out regularly. Admittedly, you’re flushing it into the rest of the building but that’s what your HVAC is for. Having a DU counterweight means there’s a chance to evolve a teensy extra bit of radon in the elevator shaft from the decay of the U-238 as it heads toward equilibrium with its daughters. Mind you, hitting equilibrium is gonna take about a million years so it’s a teensy amount of radon. There are, however, several amusing gammas coming off the 2000kg+ slab of DU regularly going up and down the shaft. If your counting lab is near the elevator, you’re going to see it every damn time it goes by, but it’s not radon.
To get a radon spike large enough to effect your instruments you are going need, technically speaking, a shit ton of radon, far more than your build HVAC can handle. Where the heck are you gonna to get that? Why, the Earth of course!  But how to get it? Radon is constantly evolving out of the soil but must of it decays away before it ever gets a chance hit the surface. The rate of radon evolution is not only a function of the soil composition but also of weather. Depending on the barometric pressure, radon gets tamped down into the soil during highs and when it’s low, like a thunderstorm or blizzard, that lid comes off.

For very sensitive counting labs, watch the weather closely. 

For the events that inspire the scenario, there was a counting experiment that was getting pronounced Ra-226 lines showing up in their overnight runs at least once a month over the course of a year. There was no rhyme or reason other than “only overnight runs”. I was asked to help find the source of this mysterious source because one researcher had had their experiments ruined three times in a row and it was driving them crazy.

Keeping in mind what they told me, I started by looking at the experimental setup. Experiment looked solid and I found no signs of stray contamination leftover from previous experiments. All sources were accounted for and secure when I performed an inventory. I took a step back, sort of cleared my mind, and took in the whole space. That’s when I figured it out and moved one item. The random radium peaks vanished. 

They were very thankful. Then, six months later, it happened again. I got an angry phone call saying the peaks were back and I hadn’t fixed it after all. 

Me: Did you move your trashcan next to your experiment again?
Them: Uhh [clearly looking to check], yes. What’s that got to do with anything?
Me: As long as it was by the door, the janitor with a SPICY radium watch didn’t have to walk into your lab and near your incredibly sensitive experiment to empty the trashcan.
Me: Why didn’t you leave your trashcans out in the hallway like they were supposed to be in the first place. The janitor was doing you a favor by even entering your space to collect up your trash.

MORAL: Don’t blame the janitor for your fuck up.