Hi, Dr. Zoomie! So the other day I heard about some type of particle called a “muon” and found out that we actually get radiation dose from them. Which made me wonder – what in the world is a muon, where do they come from, how do they give us dose, and why don’t most people seem to have heard of them?
Yeah – muons are everywhere, but they’re one of those subatomic particles that people just don’t think about all that often. Although…I am informed by a non-scientist colleague that the category of “subatomic particles people just don’t think about all that often” encompasses almost every particle known to science. So let me try again….
Yeah – muons are everywhere! But even many scientists don’t think of them all that much…in fact, even I don’t think about muon radiation all that much, and I’ve spent 40 years thinking about exposure to natural radiation. So let’s learn about muons together!
The best way to think of muons is that they’re sort of like the electron’s heavier cousins – their electrical charge and other quantum properties are similar to electrons, but they weigh more than 200 times as much (which, for a bunch of physics reasons, means that they can travel farther into matter). There are other differences, too – while electrons can only penetrate about a half inch into human tissue, muons can penetrate hundreds of meters of rock; muons have been used to discover unknown chambers in the Egyptian pyramids and have been detected in mines deep underground. Another difference is that muons are unstable, with a half-life of only about 2.2 microseconds, meaning that they can only travel about 660 meters in one half-life and less than 10 % of our atmosphere’s 100 km depth before virtually all would decay away. And this – the combination of a very short half-life and our ability to detect them deep underground – is really important, but it’s going to take a paragraph or two to explain why, starting with how they’re formed.
Muons result from slamming heavy particles (including atoms) into each other at high speeds; on Earth those particles tend to include a lot of cosmic rays slamming into atoms in our atmosphere. Atoms are pretty small, and cosmic ray particles are even smaller; such collisions would seem to be very infrequent. But even so, enough muons are formed and make their way to the Earth’s surface that there are about 10,000 muons passing through each square meter every minute. And the fact that so many muons reach the surface – let alone penetrate tens or hundreds of meters into the rock – is seemingly miraculous, considering their very short half-lives; they should all decay away while in flight from the top of the atmosphere to the bottom…but they clearly don’t, and it’s reasonable to wonder why.
The reason for muons’ remarkable longevity turns out to be relativity – Einstein’s theory that says, among other things, that when objects are moving very quickly time (for them) moves very slowly. In the case of cosmic ray-induced muons, they are moving within a whisker of the speed of light (99.97%, to be more exact), a velocity at which their subjective time stretches out long enough for them to penetrate 60 miles of atmosphere instead of the quarter mile or so that would otherwise be expected. Muons are also produced in the laboratory by physicists shooting beams of heavy particles into each other. But, however, they’re formed, when muons decay they produce an electron and two neutrinos.
I’ve already hinted at one of the uses of muons – since they can penetrate deeply into matter the paths they follow can be used to probe the structures through which they pass. More specifically, when muons interact with atoms they’re deflected (called scattering) through an angle that’s related to the mass of the atoms, so a heavy uranium atom will deflect muons more than a light carbon atom. So muons passing through stone that contains an empty space can be analyzed to determine their paths through the stone; those passing through the empty space will scatter less than those passing through the surrounding stone. And the same technique has been used to use cosmic ray-induced neutrons to probe luggage and cargo for materials that shouldn’t be present…including smuggled radioactive sources or worse.
This brings up an interesting question – if a muon can penetrate through hundreds of meters of rock, how in the world do enough of them interact in our bodies to give us radiation exposure? Mostly, the thing is that there are so many of them, and because their high velocities give them a lot of energy (remember, radiation dose is a measure of energy deposition per gram or kg of tissue). According to a 2024 report by the United Nations Science Committee on the Effects of Atomic Radiation (UNSCEAR), we receive about 35-40 mrem annually from muons, which is comparable to the dose we receive from our internal radioactivity.
And another fun thing about muons! It turns out even the flood of particles from an exploding star (supernova) isn’t going to give us much additional radiation exposure! It’s measurable – but even one of the universe’s most energetic events doesn’t produce many individual particles or photons with enough energy to make many muons. Which, to be honest, is sort of cool (to me, anyways) – that even a star exploding relatively close to Earth isn’t likely to give a huge dose of radiation to the creatures on our planet. I mean, it’ll still destroy our ozone layer so we’ll all need SPF 400 or 1000 sunscreen – but sunburn is only skin deep, right? It’s nice to know that the parts of our bodies that are harder to see won’t receive all that much exposure.
And that’s about it on muons! They’re sort of like heavy electrons that can penetrate through several hundred meters of rock, can be used to “xray” cargo containers and pyramids, and don’t give us much radiation exposure. Easy!
A muon “picture” of a c-clamp (https://www.science.org/content/article/muons-might-betray-uranium-smugglers)
Muon image of tunnels in the Great Pyramid of Giza (Page 7) (https://lss.fnal.gov/archive/2022/pub/fermilab-pub-22-055-nd.pdf)