Cosmic rays are high-energy particles that rain down on Earth from outer space. When cosmic rays enter Earth’s atmosphere, they collide with atomic nuclei in the air and produce secondary particles. Of these, muons are the representative particles that reach the surface.
Types of Cosmic Rays # Primary Cosmic Rays # Particles traveling directly from space, observed above the atmosphere.
Properties of Muons # Muons (μ particles) are elementary particles belonging to the same family as electrons (leptons).
Property Value Mass ~207 times the electron mass (105.7 MeV/c²) Charge −1 (antimuon: +1) Spin 1/2 Mean lifetime ~2.2 microseconds (rest frame) Muons are far heavier than electrons and have high penetrating power, passing through most matter with ease.
Why Do They Reach the Ground? # Muons are created in the upper atmosphere (about 15 km altitude). With a rest-frame lifetime of ~2.2 μs, they should only travel about 660 m at light speed. Yet they reach the surface thanks to special relativity.
The amount of cosmic rays reaching Earth isn’t constant — it changes over time. One major reason is the activity of the Sun.
The Solar Wind Shields Galactic Cosmic Rays # The Sun constantly emits the solar wind, a stream of charged particles (protons, electrons, and others) that carries magnetic field throughout the solar system. Cosmic rays arriving from outside the galaxy (galactic cosmic rays) are partly deflected by this field, making it harder for them to reach the inner solar system.
A scintillator is a material that emits visible or ultraviolet light when radiation — a charged particle or gamma ray — passes through it. This light emission is called scintillation.
Why It Glows # When a charged particle passes through a scintillator, it temporarily excites the atoms it passes near, bumping electrons into a higher energy state. As the electrons return to their original (ground) state, the energy difference is released as light. Charged particles like muons continuously trigger this emission as they travel through matter.
The light emitted by a scintillator is extremely faint — far too dim to see with the naked eye. A photodetector amplifies this faint light into a measurable electrical signal.
Types of Photodetectors # There are two broad approaches to converting and amplifying light into an electrical signal: vacuum-tube based and semiconductor based.
Photomultiplier Tubes (PMTs) # A Photo Multiplier Tube (PMT) is a vacuum tube that converts and amplifies light into an electrical signal.
Even with a scintillator and photodetector working together, not every signal that comes out is caused by a muon. Thermal and electrical noise can produce signals that look a lot like the real thing. Coincidence is a technique that combines multiple detector layers to filter out this noise and select only the events where a muon truly passed through.
Threshold # A photodetector’s signal always contains a mix of genuine muon signals and faint background noise. The boundary value used to distinguish noise from signal is called the threshold; an event is only recorded when the signal exceeds it.
When a primary cosmic ray enters the atmosphere, the single particle branches out again and again, producing a swarm of countless secondary particles. This phenomenon is called an air shower. By the time it reaches the ground, the shower has become a group of particles arriving simultaneously across a wide area.
How a Shower Spreads # When a primary cosmic ray collides with a nucleus in the upper atmosphere, it produces secondary particles such as pions. These particles go on to react with other nuclei or decay, multiplying the particle count in a cascading chain reaction.
Muography uses the high penetrating power of muons to image the internal structure of large objects. Just as X-rays image the human body, muography “sees through” buildings, volcanoes, and pyramids.
How Muography Works # As cosmic-ray muons pass through matter, they travel in a straight line while gradually losing energy. Denser material absorbs more muons, so the number of muons observed on the far side of an object varies with the density of the material along the path they traveled.
Cosmic ray research once required large, dedicated equipment found only at universities and research institutes. As detectors have become smaller and cheaper in recent years, the field has grown into a form of citizen science, where schools and individuals can take part in genuine measurement.
What is Citizen Science? # Citizen science is a style of research where members of the public — not just professional researchers — take part in observation, data collection, and analysis. It has produced results across many fields, from amateur astronomers discovering new celestial objects to volunteers logging biodiversity sightings.
Cosmic rays have been studied for over a century since their discovery in 1912, yet some fundamental questions remain unanswered. Here are a few of the open problems researchers around the world are still working on.
Where Do the Highest-Energy Cosmic Rays Come From? # Observations have confirmed that, on rare occasions, cosmic rays reach Earth carrying an almost unbelievable amount of energy for a single particle — these are called ultra-high-energy cosmic rays. The highest energies ever recorded far exceed what humanity’s largest particle accelerator, the LHC, can produce.
