Muon Detector
In the field of high-energy astrophysics, cosmic rays are considered one of the essential messengers of the universe. Each messenger provides a unique perspective on astrophysical phenomena: photons offer direct images of distant objects, neutrinos pass through large volumes of matter with little interaction, and cosmic rays provide access to the study of extreme particle acceleration mechanisms.
Unlike photons, which follow straight trajectories and allow their sources to be precisely located, cosmic rays—being electrically charged—are deflected by both galactic and extragalactic magnetic fields, making it impossible to trace their origin directly. However, their energy, composition, and flux provide valuable clues about the environments where they were generated.
Ilustración de rayos cósmicos © ASPERA/Novapix/L. Bret
A significant fraction of cosmic rays reaching Earth originate from the Sun, especially during eruptive events such as coronal mass ejections. However, most come from galactic sources within our Milky Way. The most energetic cosmic rays are believed to have extra-galactic origins, possibly in regions such as galaxy clusters, relativistic jets, or even phenomena that are still unknown.
When primary cosmic rays collide with atoms in the Earth's atmosphere, unstable particles such as pions and kaons are produced. These particles decay to generate muons and neutrinos.
Muons have a very short half-life of only 2.2 microseconds, but because they travel at speeds close to the speed of light, and due to the effects of relativistic time dilation, a large proportion of them manage to reach the Earth's surface before disintegrating. Under normal conditions, an average of about 1 muon per square centimeter per minute is detected at sea level, although this value can increase with altitude and varies with the orientation of the detector.
Thanks to the international CosmicWatch project, it is possible to build a functional detector with commercially available components. The design, available at cosmicwatch.lns.mit.edu, has the following main components:
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Scintillating plastic (e.g., BC412).
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Silicon photomultiplier.
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Arduino Nano microprocessor.
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Signal conditioning electronics.
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OLED display and SD card module.
The operating principle is simple: when a muon passes through the scintillating plastic, it triggers an avalanche of photons of around 420 nm, which are detected by the photomultiplier. This device generates a signal which, when properly filtered, amplified, and converted to digital, is recorded by the Arduino microprocessor. Both the signal intensity and the associated time, along with other parameters captured by the corresponding sensors, such as temperature, pressure, and altitude, are stored on a micro SD card.
A screen displays the most relevant data, such as the cumulative muon count and the rate received per second.
Manufacturing is quite affordable, and although some components require precision soldering (especially surface mount components), the process is well documented in videos and manuals. The detector is portable, low power, and USB powered.
