Milagro (experiment)

Milagro, meaning "miracle" in Spanish, was a ground-based, water Cherenkov detector designed to observe very-high-energy (VHE) gamma rays and cosmic rays. Located in the Jemez Mountains near Los Alamos, New Mexico, at an altitude of 2630 meters, Milagro operated from 2000 to 2008.

The detector consisted of a large pool of water, 60 meters in diameter and 8 meters deep, lined with photomultiplier tubes (PMTs). These PMTs detected the Cherenkov light produced when energetic particles from cosmic ray air showers, or gamma rays interacting with the atmosphere, entered the water. The arrival times and intensities of the Cherenkov light allowed reconstruction of the direction and energy of the primary particle.

Milagro was notable for its large field of view, covering nearly the entire overhead sky, allowing for continuous monitoring and the discovery of transient gamma-ray sources. It was particularly sensitive to gamma rays in the TeV (tera-electronvolt) energy range.

Key scientific accomplishments of the Milagro experiment included the discovery of extended TeV gamma-ray emission from the Galactic plane, the observation of TeV gamma rays from several known supernova remnants and pulsar wind nebulae, and the search for gamma-ray bursts. Its results provided valuable insights into the acceleration and propagation of cosmic rays and the nature of high-energy astrophysical objects. Milagro was succeeded by the High-Altitude Water Cherenkov (HAWC) observatory, which has a larger detector and improved sensitivity.