When someone says cosmic rays, I immediately picture laser beams firing across space like in some Star Wars battle scene. Green and red beams blowing up spaceships and generally causing mayhem. It turns out that’s quite different from real Cosmic Rays. Cosmic Rays are particles like protons and electrons travelling at enormous speeds. This speed gives these particles a lot of energy. The situation is similar to what happens in a particle smasher like the Large Hadron Collider at CERN.
Protons and Electrons
Atoms are pretty small. That’s quite an understatement. An ant’s head contains around 10 quintillion atoms (that’s a one with 20 zeros after it!), so yes, they are pretty small. Atoms have a nucleus and a cloud of electrons whizzing around the nucleus (sort of). If you imagine a round stadium is an atom that would fit a rugby field, the nucleus would be about 1cm in size in the middle of the stadium. The nucleus is made of protons and neutrons. Protons have a positive charge, and neutrons don’t have any charge, so they are neutral. A proton is about 1.7 femtometers across and weighs nearly 2000 times more than an electron. In short, protons are super super tiny, and electrons are even smaller. Sometimes, protons and electrons are not in atoms, which we are interested in for cosmic rays.
What are Cosmic Rays?
Sometimes, protons and electrons get flung around in space and can go incredibly fast. Sometimes they reach such high speeds that they’re not far off the speed of light; these particles have an enormous amount of energy at that speed. Cosmic Rays are mostly comprised of protons, though there is other stuff such as electrons, Helium nuclei (called alpha particles), and nuclei of larger atoms. Something must cause a proton to accelerate to such enormous speeds. The mechanism to accelerate particles to high speeds (called relativistic speeds when approaching the speed of light) is intense magnetic fields or shock waves. These are typically supernovae, magnetic fields associated with pulsars, supermassive black holes, etc.
When they hit us
A lot of cosmic rays are constantly hitting the upper atmosphere. The Sun does help protect us from the full force of cosmic radiation by deflecting some of it, but a lot still makes it through. When a high-energy proton hits atoms in the upper atmosphere, it is like what happens in the Large Hadron Collider: particles are smashing into each other. The collision obliterates the high-speed proton and the particle(s) it hits, creating a shower of short-lived new particles called pions and kaons. These decay very quickly into muons and gamma-ray photons. We can use a specialised detector to look for the muons, which last long enough to make it through the atmosphere.
Cosmic Ray Simulator
Below is a simple simulator of what happens when a proton hits the nucleus of an oxygen atom in the upper atmosphere. You can move the slider to simulate a proton at different energies up to 100 GeV (which is at the low end of the energies of cosmic rays). When you press start, you’ll see the proton descending the window and hitting the blob of neutrons and protons in the oxygen nuclei. This shatters into what is known as a hadron shower; the green and orange particles are pions. The orange ones are charged pions, which decay into muons (the blue particles), the neutral pions (green) decay into gamma ray photons, which are shown as yellow balls flying away at high speed. The muons then turn green when they hit the ground, to show which ones are hitting the ground. This is a very simplified model of what happens, but it gives a rough idea of how a proton travelling at a super high speed produces muons that can be detected. Increasing the energy of the incoming proton (the cosmic ray) will make more particles.
Red = primary cosmic ray
Red/White Cluster = atmospheric oxygen nucleus
Orange = charged pions (π±) → muons
Green = neutral pions (π⁰) → gamma rays
Yellow = gamma rays from π⁰
Blue = muons
Green flash = detector hit
Muons
Muons are a bit weird, like electrons, except they are about 200 times heavier (still about ten times lighter than a proton). They don’t live very long either. They have a mean lifetime of 2.2 microseconds, which means some last a bit longer and some a bit shorter. The muons generated from cosmic rays travel extremely fast, more than 99% of the speed of light. Things get a bit weird because their mean lifetime should prohibit them from ever reaching the surface of the Earth. Something that dies off after 2.2 microseconds and travels at 99% of the speed of light will only go about 650m. The problem is that the muons are generated from about 20km to 40km up in the atmosphere, so how are we detecting any of them at the surface?
Relativity
Einstein figured out that if you’re travelling at the speed of light or near it, things will be pretty different from what you observe versus what someone watching you observes. So when we “watch” a muon travelling at 99% of the speed of light, it lasts on average about 15.6 microseconds, considerably longer than the 2.2 microseconds a muon at rest lasts for. A muon, travelling at 99% of the speed of light, will go about 4.6km through the atmosphere before decaying. This is still not enough for us to detect them on the surface of the Earth, so they must be going a lot faster. In fact, typically, a 3 GeV muon travels at about 99.9% of the speed of light, and this travels around 20km. It has an observed mean lifetime of around 65 microseconds at that speed!
Detecting Cosmic Rays
To detect cosmic rays, we detect the products of the collisions. The problem is that not much makes it where we can easily detect it. Muons do, as covered above. Gamma ray photons do as well, but there are a lot of sources for them, so it is hard to work out which ones are from the cosmic rays. Below is the output from our muon detector, which ran for a couple of hours.
