Cosmic rays are raining down over Earth all day, every day, but you'd never know it – unless of course you had the right tools. For the most part, those tools take the form of bulky, expensive lab equipment, but thanks to an MIT team, you can now build your own detector for US$100, which is small enough to take on a plane or the subway.

Forged in the crucible of supernovae and other astronomical events, cosmic rays hurtle through the universe until they collide with something. When that something happens to be the Earth, our atmosphere causes these rays to decay into muons – charged particles that only last a few fractions of a second. Although extremely short-lived, these particles can be found throughout the atmosphere and even several kilometers into the Earth's crust, since their high energy lets them pass through most matter.

This constant cosmic rain is happening all around us, but detecting the rays is usually an intricate process performed with large equipment. To spot these spaceborn transients, muon detectors are designed to amplify the signal of even a single photon. They do so by using "photomultiplying tubes," lined with a material called a scintillator that emits light when struck by charged particles. As a particle, such as a muon, ricochets through the tube, the current it creates is amplified to the point that the detector can pick it up.

The MIT handheld muon detector started life as an add-on for the South Pole neutrino detector, IceCube. An array of small muon detectors was suggested as a way to improve the device's ability to pick up neutrinos. Muons can muddy up the readings, so by knowing exactly where they are, the scientists can better ignore them while they search for the much "quieter" neutrinos, which have no electric charge.

From there, MIT graduate student Spencer Axani shrunk the system down even further to develop a portable prototype. The pocket-sized muon detector is much thinner than usual, requires very little power and can be assembled relatively easily. Other attempts at handheld detectors have taken advantage of the tech and cameras in a smartphone, but this appears to be the smallest, cheapest dedicated device.

Axani and his team have since started a program called CosmicWatch, where people can buy kits and follow instructions to put their own detectors together in a matter of hours, with parts costing just $100. The kits include a plastic scintillator, a silicon photomultiplier, an Arduino Nano computer, a readout screen, a custom circuit board and a 3D-printed casing.

These kits are aimed at high school and university students, as a DIY electronics project and a kind of physics field trip. Different amounts of muons will be detected at different altitudes and depths underground, opening up a range of possible experiments to run.

According to feedback on CosmicWatch, teams have already taken detectors up in planes and weather balloons, as well as down onto the subway. The concentration of muons at those levels can actually be used to determine how high up, or how deep underground, the detector was taken.

"You get funny looks when you take particle detectors into the subway, but we did that in Boston," says Axani. "Since the muon rate will decrease the further down you go, we put the detectors in a subway station to measure how far underground we were. At sea level, you might see one count every two seconds, but on a plane at cruising altitude, that rate increases by about a factor of 50 — a dramatic change. From the measured rate you can back-calculate what the actual altitude of the plane was."

In future, the researchers plan to use their mini muon detectors for muon tomography, a process that creates 3D images of structures by measuring the muon distribution in the matter around the detector. This kind of method was recently used to uncover a huge void in the Great Pyramid of Giza.

"That's something I'd like to try out at some point, maybe to map out the office on the floor above me," says Axani. "For now I like to take these detectors in my briefcase and measure the muon rate when I'm traveling."

A paper describing the portable muon detector design was published in the American Journal of Physics.

Sources: MIT, CosmicWatch

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