Scintillators are materials that emit brief flashes of light, called scintillation, when charged particles pass through them and deposit energy. These flashes are extremely fast and faint, but can be detected by sensitive light sensors such as photomultiplier tubes or silicon photomultipliers.

They come in many forms: plastic scintillators are commonly used for their durability and ease of shaping, while liquid scintillators can cover large volumes and are useful in neutrino detectors. When a particle passes through a scintillator, it excites the material’s molecules. As the molecules return to their ground state, they emit photons. These photons are then captured by optical sensors and converted into electronic signals, effectively counting and tracking the particle's passage. This simple principle allows physicists to study subatomic processes invisible to the naked eye.

Cosmic rays are high-energy particles that travel through space and constantly bombard Earth. They consist mostly of protons, with some heavier atomic nuclei and electrons. Most originate from the Sun, and from our galaxy, accelerated by energetic events such as supernovae, while a tiny fraction comes from extragalactic sources.

When cosmic rays reach the Earth, they interact with the upper atmosphere, producing cascades of secondary particles called air showers. These showers include muons, electrons, and other particles that can reach the surface and even penetrate underground. At sea level, hundreds of muons pass through every square meter each second, unnoticed by humans.

The Earth’s magnetic field protects us from low-energy cosmic rays coming from the Sun, so the cosmic rays detected at the surface are mostly galactic in origin. At the highest energies, cosmic rays become extremely rare, and their sources and acceleration mechanisms are still a mystery.

The discovery of extensive air showers in the 1930s by Pierre Auger and collaborators relied on scintillator detectors to measure coincidences across multiple locations. This work revealed that cosmic rays interact in the atmosphere to produce cascades of particles, laying the foundation for modern cosmic ray observatories.

The Pierre Auger Observatory, located in Argentina, is the world’s largest cosmic ray detector. It covers over 3,000 km² in the Mendoza region and combines a network of surface detectors with fluorescence telescopes to study ultra-high-energy cosmic rays.

Each surface detector is a water Cherenkov station, which captures the Cherenkov light produced when secondary particles from cosmic ray showers pass through. A large scintillator detector sits on top of each station, counting each charged particle crossing it. Fluorescence telescopes observe the faint ultraviolet light emitted by nitrogen molecules excited during the passage of the air shower.

By combining these different detection methods, the Observatory can reconstruct the energy, direction, and composition of cosmic rays, including those with energies far beyond what human-made accelerators can reach. These measurements help scientists understand the most energetic processes in the universe.

The Pierre Auger Observatory builds directly on the discoveries of Pierre Auger, who first demonstrated that cosmic rays produce cascades of secondary particles in the atmosphere. Today, it extends that principle to the study of the rarest, highest-energy cosmic rays, mapping particle showers across the Argentine plains.