With the completion of its 100th surface detector at the end of October, the Pierre Auger Observatory became the largest cosmic-ray air-shower array in the world. The observatory, which aims to detect ultra-high-energy cosmic rays, so far encompasses a 175 square kilometre array of detectors, and will ultimately comprise 1600 surface detectors on 3000 square kilometres of the Argentine Pampa.
Each Auger surface unit consists of a cylindrical tank filled with 10,000 litres of pure water, a solar panel, and an antenna for wireless transmission of data. Phototubes register Cherenkov light produced in the water by charged particles in cosmic-ray showers, which are triggered at an altitude of 10 to 20 km. The particle showers strike several tanks almost simultaneously, and the slight differences in the detection times at the various tank positions allows the arrival direction of the cosmic ray to be determined. The Auger particle detectors are spaced 1.5 km apart in order to sample each air shower’s density at numerous locations on the ground.
In addition to the tanks, the observatory will also feature 24 fluorescence telescopes that can pick up the faint ultraviolet glow emitted by air showers in mid-air. The fluorescence telescopes can be operated only during dark, moonless nights, but they provide an independent means of measuring the energy in the showers, and hence of the primary cosmic ray.
The Pierre Auger collaboration is in the process of preparing a proposal for a second site for its observatory, to be located in the US. Featuring the same design as the Argentinean site, the second detector array would scan the northern sky for the sources of the most powerful cosmic rays.
