Radioactivity in Building Materials: Solving a Campus Mystery
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Radioactivity in Building Materials: Solving a Campus Mystery

By Dr. Zoomie

Deck the halls…with radioactivity?

Hi, Dr. Zoomie – I had some spare time at work the other week and, not having a very full social life, decided to survey some parts of campus I normally don’t get to. I was surprised to find some places with higher readings in places like the Liberal Arts building, a few restrooms, and even a hallway in the Psychology Department. Do I have some social scientists dabbling with radioactivity? Or is there a more benign explanation? Help! Does it help to know that most of the buildings are made of brick or stone?

First – good news – I don’t think you need to panic! I can’t give you a definitive answer without some more information (gamma spectroscopy would be helpful) – but the locations you mentioned and the building materials suggests that what you’re seeing is most likely natural radioactivity. Here’s my thinking.

Bricks are made of clay and there is clay in concrete and in porcelain. Some types of clay contain potassium and about 1 potassium atom of every 10,000 is radioactive potassium-40 (K-40). In fact, K-40 is so common that many companies that manufacture instruments designed to identify radioisotopes will use natural K-40 to automatically calibrate their instruments. So it’s certainly reasonable to assume that some of the “hot spots” you found are sue to K-40 in the clay used to make the bricks, concrete, and porcelain. And, in fact, I’ve measured elevated radiation levels from all of these – in the porcelain toilets and sinks in a men’s room at a facility that worked with a lot of depleted uranium, in the bricks lining a meeting room in a small-town city hall, in the exterior bricks of a building that once held a small test reactor, in concrete cinderblock that made up the wall of a storage room, and much more.

Something else to consider is that some of the “hits” might be from tiles – especially if they’re coated with yellow, orange, or red glaze. In this case, the culprit could well be uranium – uranium used to be used to give glass and glazed products these bright colors (think of Fiestaware). A friend told me of a corridor at CalTech that always gave high readings – the source turned out to be uranium-based glaze. I saw that myself in a load of glazed tiles that set off a radiation alarm – in this case it was a combination of high-potassium clay and uranium in the yellow glaze. So this might explain some of your readings as well.

Something else you might want to check is your Geology (or Earth Sciences) Department – especially if, like so many, they have display cases or a small museum. Uranium, thorium, and potassium all form minerals that can be moderately radioactive (see for starters) and, if your geologists have some of these minerals on display then you can expect to see elevated radiation levels near them – luckily, most of the radiation emitted by these minerals is beta radiation, which is fairly easily shielded and won’t even be able to pass through the glass cabinet front.

In addition to these minerals, some igneous rocks (especially granites) contain somewhat elevated levels of radioactive atoms…although usually not enough to show up as a hot spot. On the other hand, in some cases these elements can be concentrated to surprisingly high levels – under the right conditions a very slowly-cooling body of magma can produce amazingly large crystals with hundreds of times as much radioactivity as we normally see in granite. I’m not sure if your geology department has any of these uranium minerals or pegmatites but, if so, you might have fun surveying them.

And then there’s fossils! Because of the geochemistry of uranium, it tends to concentrate in rocks that formed in swamps or other areas with a lot of decaying organic material. The reason for this is that uranium, while soluble in oxygen-rich waters, is insoluble in water that’s oxygen-deficient (or “anoxic”). These are the conditions in which coal, petroleum, and natural gas are formed – they’re also the conditions in which fossilization can take place. One of the most-radioactive rocks I’ve seen was a piece of coal that gave dose rates of a few tens of mR/hr (a few hundred times higher than normal background radiation) – this particular piece of coal was also a beautiful fossilized trunk of a small tree. But it’s not only coal – surprisingly, dinosaur bones can also be radioactive! (

So…you might want to check out your mineral displays! And let me hasten to add that none of this is dangerous – the radioactivity can be measured, but that doesn’t make it a hazard.

Anyhow – the bottom line is that there are a number of plausible reasons behind the radioactivity you found that doesn’t involve rogue social scientists! Having said that, if you want to be absolutely sure, see if you have (or can borrow) a gamma spectroscopy instrument and make some measurements; if all you see is natural radioactivity then chances are very good that these readings are from radioactivity in the various materials you’re checking out.