Tiny, invisible particles from the depths of space are constantly passing through our planet, slipping through the ground, buildings, and human bodies alike without leaving any physical sensation behind. These elusive visitors are often the byproduct of cosmic rays—extremely energetic particles launched across the universe by violent astronomical events, including exploding stars and distant active galaxies. When these high-speed cosmic rays slam into atoms high in Earth’s atmosphere, they trigger cascading showers of secondary particles. Among them are muons, heavy cousins of the electron that can travel all the way through the atmosphere and plunge deep underground.
For decades, studying these atmospheric messengers required cumbersome, expensive equipment far out of reach for most secondary schools and undergraduate labs. But that barrier is rapidly changing thanks to an ingenious, low-cost device developed by University of Delaware physics professor Spencer Axani. Known as CosmicWatch, the portable device makes it remarkably easy to detect these otherwise invisible particles. The technology has quietly bridged the gap between advanced astrophysics and hands-on learning, finding a home everywhere from high school classrooms to major professional research facilities.
A $100 Detector for Invisible Cosmic Particles
Roughly the size of a standard box of animal crackers, CosmicWatch is built using ordinary electronic components that cost around $100 in total. Despite its modest budget and diminutive footprint, the detector is remarkably functional. Whenever a high-energy muon passes through the device, it flashes and records a numerical count, securely storing the data so that users can later download and analyze the information on a computer.
The device was initially conceived as an inexpensive, accessible way to introduce students to the foundational principles of particle physics. Since its inception, however, CosmicWatch has far outgrown its original educational mandate, carving out an unexpected niche in international astrophysics research.
"CosmicWatch detectors allow us to do far more physics at a dramatically lower cost, in a compact and portable form, opening the door to many new kinds of experiments and outreach opportunities," said Axani.
What Muons Can Reveal About the Universe
Scientists maintain a keen interest in studying muons because these particles serve as direct proxies for some of the most powerful and violent phenomena in the cosmos, ranging from supernovae and gamma-ray bursts to supermassive black hole systems known as blazars. By carefully measuring the properties of arriving muons, researchers can reverse-engineer characteristics of the original cosmic ray, estimating its energy, mass, and even the precise direction from which it originated across the cosmos.
The study of muon flux also holds an esteemed place in the history of modern physics. In the early 1940s, precise measurements of these atmospheric particles provided one of the earliest experimental confirmations of Albert Einstein’s theory of special relativity, demonstrating how relativistic time dilation allows short-lived subatomic particles to reach Earth’s surface before decaying.
Beyond astronomy, muons are extraordinarily useful for examining structures right here on Earth. Because the particles can travel through dense materials like solid rock, thick concrete, or massive masonry without causing internal damage, they leave behind a detectable energy trail that allows scientists to "image" hidden architecture. By tracking how muons scatter or pass through dense matter, researchers can peer inside otherwise impenetrable objects. In 2016, this exact muon-imaging technology made global headlines when researchers used it to uncover a previously unknown, massive internal corridor hidden deep within the Great Pyramid of Giza.
The primary hurdle has always been accessibility. Conventional muon detectors are notoriously bulky, fragile, and expensive, severely limiting both the scale of experiments researchers can perform and the number of educational institutions that can offer students direct, hands-on access to the technology.
"A typical undergraduate physics lab course uses a rack of electronics about the size of a small bookshelf to measure muons," Axani noted, highlighting the stark contrast with modern portable alternatives.
How CosmicWatch Began
The story of CosmicWatch dates back to 2017, when Axani was working as a graduate student at the Massachusetts Institute of Technology. His initial objective was purely practical: he needed to build a compact, highly energy-efficient muon detector that could be deployed at the IceCube Neutrino Observatory buried deep beneath the Antarctic ice sheet.
IceCube is a massive scientific installation designed to search for neutrinos—elusive, ghostlike subatomic particles passing through the Earth. In such extreme environments, a reliable muon detector is essential because it helps researchers distinguish between background atmospheric muons and the rare, high-energy neutrinos they are actually trying to identify.
As the engineering work progressed, Axani realized that the underlying technology could easily be modified, made portable, and manufactured cheaply enough for broader educational use. What started as an instrument for Antarctic research quickly evolved into a powerful outreach tool for teaching particle physics.
After transitioning to the faculty at the University of Delaware in 2022, Axani continued refining and improving the design. He recently rolled out the third generation of CosmicWatch detectors. Details of these engineering improvements were published in the Journal of Instrumentation, outlining how the updated version can successfully monitor its local environment, withstand high radiation levels, and aggregate data at a much faster rate.
"Even though I had studied cosmic rays, I didn’t fully appreciate the rich physics behind the working of these detectors to actually ‘see’ the world and atmospheric particle production," said Masooma Sarfraz, a doctoral student in Axani’s lab and the primary author of the journal article. "For a student like me who has been working on theoretical ideas, this was a perfect opportunity to dive into the experimental side. It also connects beautifully to my current broader research work with particle physics."
From Dark Matter Research to Spaceflight
The latest iteration of the CosmicWatch design is uniquely suited for calibrating large-scale physics detectors. The technology is currently being deployed in active experiments, including the NuDot experiment at the University of Delaware and the Coherent CAPTAIN-Mills dark matter detector located in Los Alamos, New Mexico. Meanwhile, researchers are actively developing an even more specialized version designed to measure primary cosmic rays directly aboard rockets and spacecraft operating beyond the protective cocoon of Earth’s atmosphere.
Despite its expanding footprint in advanced research, education remains a core pillar of the CosmicWatch project. At the University of Delaware, Axani incorporates the detectors directly into his coursework to teach particle, nuclear, and astrophysics. Students physically solder and assemble the devices themselves, gaining practical, hands-on experience with high-speed electronics before deploying their finished detectors in independent experiments.
Musarate Shams, a doctoral student in the university’s quantum science and engineering program, took the project a step further by modifying his own CosmicWatch unit to include integrated temperature and pressure sensors, aiming to study cosmic rays in the upper reaches of Earth’s atmosphere.
In May, that customized detector hitched a ride aboard a high-altitude research balloon, ascending to an astonishing 100,000 feet near the edge of space. After retrieving and analyzing the telemetry data, Shams was able to clearly demonstrate how the flux and behavior of cosmic rays change as altitude increases.
"It’s a very cool thing to build something in the lab in a couple of days that’s able to detect these cool particles from hundreds of light-years away," Shams said.
Giving Students a Taste of Real Particle Physics
The reach of CosmicWatch extends well beyond Delaware. Natasha Holmes, the Ann S. Bowers Associate Professor of Physics at Cornell University, integrates the detectors into her introductory physics courses, having students build the units from scratch and design their own experiments.
According to Holmes, working directly with tangible hardware gives students an academic experience that mirrors professional experimental physics far more closely than traditional, textbook-driven lab assignments.
"The students seem really excited about doing this thing that is more like what particle physicists and experimental physicists actually do," Holmes explained. "They get to learn some coding with it, and sometimes they break the devices, and then we have to talk to them about being careful with your equipment. It’s very different from a typical physics lab. We’ve had students say they’re doing ‘real science’ after using it."
A Possible Global Network of Cosmic Ray Detectors
Axani estimates that thousands of individual CosmicWatch detectors have been successfully built and deployed across the globe since the first prototype appeared eight years ago. He harbors a long-term vision for that number to grow exponentially through a worldwide citizen science initiative.
Under this proposed model, everyday people scattered across different continents could measure local muon rates using their own low-cost detectors and upload their observations to a centralized online database. Combined, those decentralized measurements could generate an unprecedented, real-time picture of high-energy particle activity and space weather surrounding the entire planet.
At the same time, Axani is developing complementary detector systems that could eventually help constellations of commercial and scientific satellites respond more intelligently to dynamic shifts in their space environment. Such technology could allow orbiting satellites to autonomously communicate with one another regarding local radiation conditions. If a sudden solar flare were detected, for instance, the networked system could automatically alert neighboring spacecraft, allowing vulnerable electronics to safely power down until the radiation wave passes.
What began nearly a decade ago as an inexpensive educational experiment has gradually blossomed into a versatile tool utilized across multiple branches of advanced physics—an evolution that even its creator did not initially foresee.
"Although it started as an educational program, it’s found a use in a lot of different areas of physics," Axani said. "It’s pretty cool."
