Artist’s conception of cosmic rays. A charged particle from space hits the atmosphere and interacts with an air molecule, causing a shower of particles and interactions that gets bigger as it gets closer to the ground.

I love the ways that science lets us see through seeming impenetrable walls. Let’s say, for instance, that you have a nuclear fission plant that has suffered some catastrophe. Let’s get more specific: you’re responsible for the clean-up for the Fukushima power plant in Japan, which was badly damaged in the 2011 earthquake. You know that somewhere in this large building is some highly dangerous uranium, but you’re not sure exactly where. You need to figure out where it is so that you can clean it up without sacrificing more lives.

You might try to snake some robots in there with cameras so you could look around, but the radiation would destroy them, and you wouldn’t be able to see anything. You really need to figure out where this stuff is without sending anything in to stumble over it. You know that the Uranium is way denser than anything else in there. Could we take a picture of it somehow from the outside?

What about X-rays? They can let us see through things, right? Except they probably can’t penetrate the building with all its layers, and it’s incredibly expensive to get an X-ray of something the size of a building. But the scientists in Japan have found something that looks like it will work. You need to be able to shoot high-energy particles of some kind through that building. They will get stopped by high-density stuff, but not by low-density stuff, so what comes out (compared to what went in) will tell you where the uranium is. But producing high-energy particles and radiation is really energy intense and very expensive. Is there some source of this stuff we could use that is cheap?

Yes! Enter supernovas. The huge explosions that happen when huge, massive stars run out of fuel to burn. They make high energy particles for free. High energy particles, mostly protons, accelerate to huge speeds in the shock waves of these explosions and travel throughout space until they run into something, and that’s usually a really long way. Space is mostly… well…. space. But then some of them hit the top of Earth’s atmosphere. They interact with the gas up there and cause a bunch of reactions and ultimately create muons. Muons are heavy cousins of electrons, small, charged particles, and because of these supernova particles – we call them cosmic rays — we have muons showering down on us all the time. If you hold your hand out, palm up, there’s about 1 cosmic ray muon per second passing through your hand. You can’t feel them, but they’re there. Most of them just plow through, but every once in a while, they get stopped. If they go through really dense material, they get stopped more often. They’re perfect.

Muons are the stand-ins for X-rays that the universe provides for us. They’re already raining down on us naturally. So let’s use them to take pictures of things. There are companies already using these things to see through shipping containers and identify heavy radioactive material. The shipping container is pulled forward by truck between two sensors, one above and one below it. The universe provides the muons showering down at various angles, but we need a detector above to know what’s going in and a detector below to see what’s coming out. Then we can work out the densities of materials in the shipping container.

Using muons to search for radioactive material in a shipping container. The muons are supplied at random angles by the universe, and a computer analyzes where extra-dense material (which also tends to be radioactive– bad!) is absorbing muons between the top detector and the bottom detector.

That’s great, but how does that work at Fukushima? Most of those muons are showering down. Are we going to tunnel under the power plant to put a detector in? No way! Too dangerous.

The muons are mostly coming down, but they’re coming at crazy angles, too. They’re coming from all directions. There are a lot of them, and a few of them, a minority, are going sideways. Using this idea, scientists are working out how to take pictures of Fukushima using sideways-going muons – one detector to the east, say, and one to the west, to image what’s between them. That should allow them to figure out where the uranium is, so that they can come up with the plan to clean it up.  It’s going to take patience, but it won’t endanger anyone’s life to get a picture of what’s in there. It turns out that the universe has provided us with a natural X-ray vision, and we’re just learning how to use it.