What is CosmicWatch? How a $100 detector can spot invisible particles 'muons' raining down from space

Imagine having a small device that can detect particles from space passing through your body - all for just around $100 worth of parts. This is what CosmicWatch is. It is a particle detector designed by physics professor Spencer Axani from the University of Delaware. It measures 'muons', which are invisible particles generated by high energy cosmic rays hitting atoms in the Earth's atmosphere. Muons are constantly moving through our planet, and they even pass through us. What makes CosmicWatch stand out is the fact that it is quite simple. It was initially designed as a student physics experiment but now serves as a tool in physics research projects, student projects, and balloon projects.

WHAT IS COSMICWATCH AND HOW DOES IT WORK?

The CosmicWatch is a handheld muon detector that is about the size of a pack of animal crackers. It has an electronic circuit that costs only $100, which is much cheaper and portable than most detectors for muons. Whenever a muon strikes the detector, the CosmicWatch generates a flash along with a counter count. Data from this detector can be analyzed later by downloading. This instrument was first designed in order to make learning particle physics easier for students. This detector was first made by Axani back in 2017 when he was a graduate student at MIT.

WHY ARE INVISIBLE MUONS IMPORTANT?

Muons are created when the cosmic rays collide with the atoms found high up in Earth’s atmosphere. Cosmic rays may come from very high-energy events such as explosions from stars and other such occurrences far outside our solar system. Muons, although invisible, are capable of passing through the atmosphere and even into materials underground. Information about the cosmic rays that create muons, such as the energy, mass, and direction, may be gathered through measurements of the muons. According to the ScienceDaily article, muon measurement has been an essential tool for physics as well. In the 1940s, measurements of muons provided early experimental evidence of Einstein’s special theory of relativity. Muons are also useful for studying structures concealed by vast amounts of material because they produce energy trails.

FROM A STUDENT PROJECT TO REAL-WORLD RESEARCH

But CosmicWatch has come much further than just an educational application. The latest version is currently being used for calibration purposes in larger detectors such as the NuDot experiment at the University of Delaware and the Coherent CAPTAIN-Mills dark matter detector in Los Alamos, New Mexico. The science behind the latest version of CosmicWatch was published in the Journal of Instrumentation. The 2025 study 'CosmicWatch: The Desktop Muon Detector (v3X)' was authored by Spencer N Axani, Masooma Sarfraz, Miles Garcia, Collin Owens, Katarzyna Frankiewicz and Janet M Conrad. In this article the authors discuss the improvements, which help CosmicWatch to monitor its environment, endure high level of radiation, and increase the speed of data collection. CosmicWatch has even gone into the skies. A special version of CosmicWatch created by doctoral student Musarate Shams went on board of the high altitude balloon reaching 100,000 feet in order to study the changes in cosmic-ray fluxes at different heights. Axani believes that there are thousands of CosmicWatch detectors available now.

It is not just the capability of CosmicWatch to detect the muons but the fact that a device worth about $100 will allow students and scientists to experiment with something that cannot be detected by the human senses. Students can assemble the detector, gather data and design their own experiments. According to Axani, as quoted by ScienceDaily, it might be possible to create a global citizen science network where individuals will be able to measure muon counts in their vicinity and share the information with each other.

From this perspective, CosmicWatch converts an invisible flow of particles into something that can be measured. The machine does not 'see' space as we normally understand it. Rather, it measures the slight evidence left behind from particles passing through the Earth’s atmosphere - giving ordinary students and researchers an extraordinarily direct means to investigate the physics of the universe.