Cosmic rays from the sun constantly bombard earth and it’s magnetosphere

Even though we may not feel it, every hour of every day, we are bombarded by all sort of radiation from space. A lot of the worst of it is filtered out by Earth’s magnetic field and the atmosphere, but plenty still gets through. We call this radiation cosmic radiation. Cosmic rays can be made of any number of different particles, from gamma rays to heavy atomic nuclei, but most of it comes in the form of high energy protons (hydrogen atoms). When these super fast particles crash into an atom in Earth’s atmosphere they explode producing a shower of secondary particles and radiation called an air shower.

A simulation of an air shower from a proton collision. Many of those paths are particles that only exist for a fraction of a second. This image was made by the COSMUS group at the university of Chicago

Since many of these collisions have more energy than we can produce artificially in things like particle accelerators, scientists can study them as a way to see how physics behaves at incredibly high energy levels. In 1991, a single proton moving nearly the speed of light crashed into the atmosphere and had an energy of 3×1⁰²⁰ eV (3 zetta-electronvolts) or about 48 joules. It was called the “oh-my-god particle” because that’s about the same energy as a baseball moving 98 km/h, all crammed into a single proton. We’ve since detected more than a dozen more of these sorts of events so it’s a fairly regular phenomena. For comparison the highest energy the LHC can manage is 7 x 10 ^12 (7 Tera-electronvolts) or about 0.00000017 joules.

The most common ways scientist measure these particles is with radiation detectors. There’s lots of different options when it comes to radiation detection. The most common ones are traditional geiger muller tubes, or scintillation detectors.

Geiger tubes come in a variety of size, wall thicknesses, gas mixtures, and whether or not they have a window. All of these things change what the tube is sensitive to. When a particle of radiation either strikes the geiger tube or passes through it, it ionizes some molecules allowing them to emit a high energy electron. The two electrodes of the tube (a wire going down the centre and the wall of the tube) are charged with a high voltage so when the electron is emitted, it gets accelerated towards the middle wire. As the electron speeds up and gets closer to the wire, it begins to ionize other molecules in the gas that fills the tube. This is called an avalanche event and the whole event lasts a few microseconds. It also multiplies the strength of the signal of that first energized electron by a factor of several million making it easier to detect. As all the newly energized electrons crash into the middle wire electrode it produces a voltage difference that can be measured as a brief pulse of electricity. With some simple electronics you can count the number of pulses and their strength, and therefore the number of particles hitting the detector.

An example avalanche event started by a collision with the tube wall. Normally several avalanches are started, not just the one, but this provides a simplified picture of what’s going on. Photo credit to Doug Sim

If the tube is a pancake style and has a window, you can also cover it with different materials to gauge the level of various kinds of radiation. Normal geiger tubes can detect most forms of ionizing radiation but are blind to things like neutrons that don’t have a charge. However there are some tubes that are modified with an isotope of boron, b-10, that are able to absorb neutrons and emit charged particles. This allows them to detect neutrons in a similar way to the way ionizing radiation is detected.

My pancake style detector. In this case the electrode isn’t the wall of the tube. Instead there are a set of concentric circular electrodes. The top of the tube is made of a special mica window that alpha particles can pass through, making my detector sensitive to them.

Individually they’re good at determining the general level of radiation. However if you set up 15–20 of them in a grid, you can determine the direction particles are moving based on which detectors fire quickly one after the other

How a scintillation detector works

Scintillation detectors operate on a very different principle than geiger tubes. All scintillation detectors are composed of two main parts; a scintillation material and a photomultiplier tube. A material capable of scintillation will emit a small amount of light proportional to the energy of the radiation that hits it. So if it’s hit by a 100MeV (mega electronvolts) gamma ray, it will emit less light than if it’s hit by a 400 MeV gamma ray. Scintillation materials come in all shapes and sizes and range from specially doped(contaminated on purpose) crystals, to plastics. They all behave a little differently when exposed to different sorts of radiation, so you choose your material based on what you want to detect. The photomultiplier tube (PMT), is a special type of vacuum tube that can detect single photons of light. By pairing a PMT with a scintillating material, you can not only count how many particles are going through your detector, but also what energy the individual particles have based on the number of photons in each flash of light. This can give you all sorts of useful information about the source of the radiation. As with the geiger tubes, lining up these detectors in various patterns can give you even more information about the particles passing through them.

My scintillation crystals with a lighter for scale. These are LYSO (Lutetium-yttrium oxyorthosilicate) which are sensitive to gamma rays. The white coating is optional but reflects light going the wrong direction back towards the PMT so you get a more accurate reading.

Scientists and amateurs use both of these sorts of detectors to observe cosmic events and to try and better understand our universe. However there is another way to detect the millions of particles streaming through our atmosphere from outer space. Since many of the particles have large amounts of charge they interact strongly with the Earth’s magnetic field. When they interact a small blast of radio is produced. Some of this radio is in what’s called the very low frequency (VLF) range. VLF is interesting in that if you directly convert a VLF radio signal into audio, the frequencies are within the range of human hearing.

There are a lot of sources of VLF radio from the natural world, including lightning, and cosmic rays, but what’s cool is the different sources sound different. For example, lightning sounds like a loud whistle and as such their signals are called whistlers. Particles hitting the atmosphere sound more like birds chirping. And at times of strong auroral activity (when the northern lights are brightest), the combined signals from all the different particles hitting the magnetosphere sound like the morning chatter of a swamp full of animals. These signals are called a dawn chorus for this reason.

All it really takes to pick up VLF signals is a long wire , so humans have been hearing the whistles that come from lightning strikes since some of the first telephone lines were installed. With modern equipment it’s trivial to listen to VLF signals, and it can even be done using a computer sound card and a long speaker wire. I made a video exploring this idea as well as testing out one of the kits sold online specifically for receiving VLF transmissions. For those who want to experience cosmic rays from the comfort of their bedroom, this is probably the best way to do it on a budget. Ever since I first listened in, I’ve wanted to build a better detector to get a clearer signal. The sounds really are enchanting, so it’s no wonder that early radio pioneers thought some of these transmission were coming from the spirit world.

My personal collection of various radiation detection equipment now includes one of everything I’ve mentioned in this article. My collection started as a way to stay safe during one of my other projects, but now I can use these devices to take a small peak into the invisible world. I hope some day soon to use these devices to detect more exotic particles like muons and pions, particles that exist only for a moment as the fallout of cosmic ray collisions. But for now, I’ll just keep my eyes on the stars, and listen to the birds.