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Someone call Superman and tell him we don't need his X-ray vision anymore. I mean, sure, X-rays can be used to see through all sorts of solid objects, like people, paintings that are hiding other paintings, lead-lined containers of nuclear waste... Wait, no. You got me there. But what if I told you there's a subatomic particle out there that might do an even better job?

It's called the "muon." It's safe to use, free to access, and raining down on us every second of every day. Over the past few decades, scientists have developed their own muon-vision superpower.

[intro]

The universe is constantly shooting subatomic particles across the void of space, be it from a supermassive black hole jet or the Sun just casually living it's life. These particles are collectively called "cosmic rays," and they often run smack-dab into the earth. And when cosmic rays collide with the molecules in our atmosphere, they create showers of muons. These little guys are 200 times heavier than their electron siblings and are moving so fast, they're packing about 10,000 times the energy of a typical X-ray. That might sound a little scary, especially when I tell you 10,000 muons hit every square meter of the Earth every minute. But don't worry; you're totally safe. All those muons pass right on through you. And then, they keep going. They can penetrate up to 2.5 km into the ground before breaking down into other, more stable subatomic particles.

So muons are everywhere, freely available, and great at getting through solid objects. And in the 1950s, scientists started using them as a tool to peer inside secret places. This newfangled tech is known as "muon radiography." Even though muons can pass through solid objects much more easily than X-rays, they do still lose a very tiny bit of speed as they bump into atoms along the way. And the slower the muon goes, the sooner it'll decay. So the denser the material, the fewer muons will make it through all the way to the detector to create a 2D image called a "muograph." It's just like how an X-ray machine works, where stuff that's denser and made up of heavier atoms will absorb more X-ray particles, like bones or that wallet your dog ate.

But muons can make it through *way* more matter, so they're used to image way bigger things than your dog, like a building or mountain. Scientists can also set up multiple muon detectors in different positions, stitching together to make a 3D model and mapping the interior. In some cases, a spot that comes out less dense is actually a big patch of underground air. Back in 2017, scientists used muography so discover a hidden chamber inside the great pyramid at Giza. This technique has also been used to analyze the inner plumbing of Mount Vesuvius and Mount Aetna in Italy at a resolution that's 10x higher than what any other modern volcano-scanning technologies can offer. Because liquid magma has a different density than solid rock, we can track its movement while it's still hiding beneath the surface. Scientists can throw in some other survey data, like from seismographs, to get an even clearer picture. One day, we might be able to use muography to accurately predict when  the next volcanic eruption will take place.

But muon radiography isn't the only way humanities muon-vision works these days. Back in 2003, researchers developed what's sometimes called "muon tomography." This technique tends to be used on things that are a lot smaller than natural landmarks, and it maps interiors not just by density, but chemical composition. It does this by measuring how much muons are deflected during their travels. This requires at least two detectors: one sits on one side of an object and records the angle that all the muons will enter, and around the back, a second detector measures the angle that the muons exit. If that angle is different, it means something is deflecting the muons along the way.

Now, when muons interact with atoms that are either more energetic or more massive, they'll deflect at a sharper angle. So by studying how muons scatter, scientists can estimate what they're chemically made of. For example, muon tomography has been used to scan shipping containers for contraband without anyone having to actually dig through supplies. But what's really cool is that scientists can use this technique to scan nuclear sites. A lot of important, but radioactive, things - like fortified reactors and nuclear waste - are shielded to protect the environment. And I, for one, am in favor of protecting the environment from dangerous amounts of nuclear radiation.

But we can't use X-rays to look through that shielding and make sure everything is going great or very wrong inside because that's that the exact kind of radiation all that shielding is designed to block. It'd be like sending Superman to check out what's going on inside a lead box. But with scientists with muon-vision, it's no problem at all.

The X-rays we use for medical imaging are thwarted by less than a millimeter of lead shielding, but muons can peek through a lead wall that's two meters thick. And since radioactive elements like plutonium and uranium are really massive, muon tomography is great at picking them out of the proverbial lineup. It's been used to make 3D models of entire nuclear reactors and nuclear waste storage sites. And at the Fukushima Daiichi Nuclear Power Station in Japan, it's helping scientists locate the remaining nuclear fuel that still needs to be cleaned up after the nuclear accident there in 2011.

Every year brings new and exciting uses for muons, like modeling the inner structure of cyclones, ocean tides, and tsunamis, to better predict their behavior, or safely monitoring a patient's lung health without the need to keep them in an MRI all day. There are so many applications, we can't talk about all of them here. They may not give us superpowers like we see in the comics, but muons are certainly helping save the day.

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