CosmicWatch: A $100 Muon Detector Opens Particle Physics

A pocket-sized detector costing roughly $100 can reveal the invisible stream of cosmic particles constantly passing through Earth. Developed by University of Delaware physics professor Spencer Axani, the CosmicWatch device flashes and records a count whenever a muon passes through it.

Muons are secondary particles created when high-energy cosmic rays from events like supernovae strike Earth's atmosphere. They travel through the atmosphere and can penetrate solid materials, including our bodies, without causing damage. Studying them provides clues about the original cosmic rays' energy, mass, and direction.

From Student Project to Research Tool

Axani first created CosmicWatch in 2017 as a graduate student at MIT, aiming to build a compact muon detector for the IceCube neutrino observatory in Antarctica. He realized the technology could be made portable and inexpensive enough for educational use. The device, roughly the size of a box of animal crackers, is built from electronic components. Its third version can monitor its surroundings, tolerate high radiation, and gather data more quickly.

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, Axani said.

Applications in Research and Imaging

The latest CosmicWatch design is now used to calibrate large-scale detectors. It is part of the NuDot experiment at the University of Delaware and the Coherent CAPTAIN-Mills dark matter detector in Los Alamos, New Mexico. Researchers are also developing a version for measuring primary cosmic rays aboard rockets and spacecraft.

Muons are useful for imaging hidden structures because they leave a detectable energy trail as they pass through matter. In 2016, muon technology uncovered an unknown corridor in the Great Pyramid of Giza. Historically, muon flux measurements in the early 1940s provided an early confirmation of Einstein's theory of special relativity.

Educational Impact and Student Experiments

At the University of Delaware, students assemble CosmicWatch detectors themselves, gaining experience with high-speed electronics. Doctoral student Musarate Shams modified his detector with temperature and pressure sensors. In May, it traveled on a high-altitude balloon to 100,000 feet, measuring how cosmic ray flow changes with altitude.

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.

The detector is also used in introductory physics courses at Cornell University. Natasha Holmes, the Ann S. Bowers Associate Professor of Physics there, said the hands-on work gives students an experience closer to professional experimental physics. Students have reported feeling like they are doing 'real science' after using it.

Future Visions: A Global Network

Axani estimates thousands of CosmicWatch detectors have been built in the eight years since its debut. He envisions a worldwide 'citizen science' network where people measure local muon rates and share data online to create a global picture of particle activity. He is also developing a related detector system that could allow satellites to communicate about space environment changes, like solar flares.

Doctoral student Masooma Sarfraz, primary author on the recent journal article, highlighted the project's value for theory-focused students. "For a student like me who has been working on theoretical ideas, this was a perfect opportunity to dive into the experimental side," she said. What began as an educational tool has unexpectedly spread into several areas of advanced physics.