An earlier prototype of Cosmic Pi (banana for scale). It's now much smaller. Image: Cosmic Pi

CERN Researchers Are Making a DIY Cosmic Ray Detector in Their Spare Time

Victoria Turk

Victoria Turk

Cosmic Pi makes each user a single pixel in an open source cosmic ray telescope.

An earlier prototype of Cosmic Pi (banana for scale). It's now much smaller. Image: Cosmic Pi

You can make all manner of things out of a Raspberry Pi but it might take a career at CERN to come up with the idea of a DIY cosmic ray detector.

A team of researchers from the Geneva-based nuclear research organisation, which is home to the Large Hadron Collider, have been spending some free time building what they hope will become "the world's largest open source distributed cosmic ray telescope." Dubbed Cosmic Pi, the idea is to design a small, low-cost device that people can build or buy, and then share data to create a crowdsourced telescope.

"The lofty aspiration of the project is to basically give people a slice of the whole of CERN in a box that they can put on their desk or on their mantelpiece or on their bookcase," said James Devine, an electrical engineer and one of the Cosmic Pi founders, in a phone call.

"Obviously we can't give people a slice of the LHC— it wouldn't work and it's too expensive and complicated—so we can use cosmic rays."

Cosmic rays are high-energy particles that constantly bombard the Earth from space. As Devine pointed out, they're actually many times more energetic than even the highest-energy particles accelerated in the LHC and they're raining down on us for free, which makes them a great subject for citizen scientists.

When I asked why someone would want to detect cosmic rays at home, Devine said, "We kind of approached it from the other way around, I guess with the typical physicist and engineer's perspective, in that of course people would want to detect cosmic rays, because they're there."

But he added that cosmic rays are interesting because they still harbour mysteries; for instance, the frequency of cosmic rays hitting the Earth fluctuates and their origins aren't all known.

"Having a cosmic ray detector really gives you a sense that the Universe is filled with things"

On a more fundamental level, it's a way of making an invisible phenomenon visible. "Every second the Earth is being bombarded by billions, trillions of cosmic rays, but most of the time you don't know they're there," said Hugo Day, a fellow CERN researcher and Cosmic Pi creator.

"Having a cosmic ray detector really gives you a sense that the Universe is filled with things; even this empty space that you see around you is filled with trillions of particles that are coming from outer space."

An earlier prototype in a hackathon. Image: Cosmic Pi

The initial idea for Cosmic Pi dates back to 2012, when a team including Devine and Day got together at CERN's first "Webfest" hackathon. At that point, they were trying to make an Android-based cosmic ray detector and were inspired by the ERGO project in the US, which works with schoolchildren to make a distributed cosmic ray observatory. But the Android-based prototype wasn't up to scratch due to a lack of USB peripherals for Android.

"It was kind of a proof of concept for everything that Cosmic Pi does, but we weren't happy with the implementation," said Devine.

They essentially shelved the project for a couple of years, but got back together when they realised Raspberry Pi, the credit-card-sized computer, could offer a better solution.

The module contains a scintillator—a block of plastic treated with a special chemical that produces light when a cosmic ray muon passes through. These photons bounce around and hit one of two detectors, which convert the light into an electrical signal. The electrical pulse is then converted into a digital bitstream. "We can encode how much energy was brought by the cosmic ray into the plastic slab by counting the number of photons," explained Devine.

Other modules in the detector include GPS to locate the recording in time and space; a pressure sensor, as cosmic rays are influenced by altitude; and a relative humidity sensor, as moisture can degrade the scintillator. An accelerometer and magnetometer give more information about the detector's positioning and orientation. The team is currently using an Arduino Due board to analyse the data.

An earlier prototype in a hackathon. Image: Cosmic Pi

It might sound complicated, but it's easy to use—plug it in, turn it on, and it'll flash a light every time a cosmic ray is detected. But the real power of the citizen science project comes from sharing results, which requires connecting the detector to the internet using a Rapsberry Pi.

Each individual module is essentially one "pixel" in an observatory consisting of all the connected Cosmic Pis; it collects data from its specific position but contributes to a broader picture when combined with others. Day compared it to a digital camera, where resolution is linked to number of pixels. "Having more pixels gives you more scientific weight behind it; it lets you get a more in-depth analysis with the network," he said.

One detector—one pixel—can tell you if it's detected a cosmic ray; but start grouping detectors and you can gather information such as where cosmic rays might be coming from, based on which ones pick up signals.

The particles detected come from showers formed when an initial cosmic ray hits the Earth's atmosphere. These showers then fall to Earth from that point of impact.

"So if we have enough detectors within one of these footprints, and we have enough events, then we can use that to reconstruct the point where the particle interacted with the atmosphere," said Devine. "For us that's really exciting, because it's effectively pointing a finger into the Universe and saying, 'The origin of this particle was here.'"

For comparison, the pixel detector in the LHC's ATLAS experiment has 80 million pixels—but it took the resources of an international effort to build it.

The Cosmic Pi is still in the prototype phase, but the team hopes to produce the whole package for under $500 and enter limited production later this year with a Kickstarter. They see it as a project for hackspace types and citizen scientists, and a potential educational tool for instructors.

Devine and Day are joined in the project by a group of researchers with a range of specialties, and both said that while they work around the world's biggest particle accelerator, building a detector was a world away from their day jobs.

"The thing that motivates me to be part of the project is I spend my whole day getting cables full and power connections to these great detectors underground, but it would be really interesting intellectually if I could understand at least the basics of how they work and how to build one," said Devine.