Sparking Cosmic Pixels

The new generation of silicon photomultipliers (SiPMs) is now making gamma-ray spectroscopy accessible to makers.

Alasdair Allan
2 months agoSensors
Cosmic Pixels (📷: Pierre Muth)

High-energy particles from space are constantly hitting the Earth's upper atmosphere. While they rarely make it to the ground secondary particles, called muons, ejected from these collisions do reach us. If you hold your hand, on average one muon will pass through your palm each and every second.

You can't see muons, and you won't even feel them as they sleet through your body. So to detect them you normally use a scintillating material combined with a very sensitive light sensor such as a photomultiplier tube. But such tubes are expensive and fragile, and have started to be replaced by silicon photomultipliers (SiPMs). These sensors have been around a while, but they're now starting to show up in maker projects.

Pierre Muth's Cosmic Pixels uses SiPMs to recreate something like a spark chamber, "My idea," says Muth, "is that by combining small muon detectors, we might be able to see the trajectories of the particle showers caused by cosmic rays, similar to how a spark chamber works."

Each sensor is mounted on a custom designed board and connected to a block of scintillator. "To determine if a particle has crossed several scintillators, we need to propagate the detection status to surrounding detectors," says Muth, "If one detects a flash and one or more neighboring detectors also register something simultaneously, chances are high that a particle has crossed the detector. Each board is connected to its neighbors, relaying the detection signal not only to the four directly adjacent units but also to the diagonal ones." So far Muth has built a 7-by-7 text matrix. His next steps, "…solder more boards and build the scintillator rods."

However, Muth isn't alone in making use of the new SiPMs. Mihai Cuciuc's Pomelo Gamma Spectroscopy Module is built around the same technology, and while still a work in progress it is already producing interesting results.

The all-in-one gamma-ray spectrograph by Matthias Rosezky and Sebastian D'Hyon is an affordable design built around the Raspberry Pi Pico microcontroller board which means that, as Rosezky says "…you don't strictly need a computer or even an external sound card – everything is self-contained. You can do standalone measurements using only a USB power cable and save the spectra to the Pico's flash storage or connect to your PC via the Serial-over-USB connection."

More information on Rosezky and D'Hyon's build can be found on Github.

The fallout from the smartphone war has left the maker community with sensors and processors that are almost cheap enough to throw away. But the availability of that cheap hardware relies heavily on how the smartphone and tablet market evolves, and over the last year or two the ubiquitous black rectangle we all carry with us hasn’t changed that much. Or at least not in ways that gives makers a new lever on the world. So it's always interesting when we start to see a new sensor make an appearance in projects, especially when it there is new and interesting citizen science to be done.

Alasdair Allan
Scientist, author, hacker, maker, and journalist. Building, breaking, and writing. For hire. You can reach me at 📫 alasdair@babilim.co.uk.
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