One of the world’s largest and most sensitive cosmic-ray facilities has begun operation with its first set of detectors. Located about 4410 m above sea level in the Haizi Mountain in Sichuan Province in southwest China, the 1.2 billion yuan ($180m) Large High Altitude Air Shower Observatory (LHAASO) will attempt to understand the origins of high-energy cosmic rays.
Cosmic rays are particles that originate in outer space and are accelerated to energies higher than those that can be achieved in even the largest man-made particle accelerators. Composed mainly of high-energy protons and atomic nuclei, cosmic rays create an air shower of particles such as photons and muons when they hit the atmosphere. Where cosmic rays come from, however, has remained a mystery since they were first spotted some 100 years ago.
LHAASO aims to detect cosmic rays over a range of energies from 1011-1018 eV. The first phase of the project, which is now complete, involves 900 Cherenkov water detectors that are each equipped with a 20cm- and 4 cm-diameter photomultiplier tube. The 900 units are located in a single “pool” containing 100 000 tonnes of water that is 4.5 m deep and has an area of 22 500 m2.