The HAWC Instrument
Site: Sierra Negra, Mexico
The HAWC site is inside the Parque Nacional Pico de Orizaba, a Mexican national park. The park contains Citlaltepetl (or Pico de Orizaba), the highest peak in Mexico at 5610 meters, and Sierra Negra, a 4600-meter volcano 7 km southwest of Citlaltepetl. A satellite view of the site is shown here.
The main HAWC array was completed March 19, 2015. It resides on a 200m×450m plateau near the saddle between the two peaks. The geographical coordinates of the site are approximately (97.3°W, 19.0°N), and the altitude of the plateau is 4100 meters. The latitude and longitude of the site provides a good overlap with other observatories in the US, Mexico, and Chile. The high altitude of the site means that the detector will be sensitive to gamma rays as low as 100 GeV in energy.
The Water Cherenkov Technique
The HAWC design builds upon our experience with the Milagro detector. Milagro was the first large uniformly-instrumented gamma-ray air shower array to use water Cherenkov technology. In this method, an artificial body of water is used to sample the charged particles created in the air showers produced by TeV gamma rays. As the charged particles pass through the water, they emit Cherenkov radiation which can be observed with photomultipliers. Using these measurements, the properties of the initial gamma rays can be reconstructed.
The Milagro detector was built around a shallow geothermal pond near the Los Alamos National Laboratory in New Mexico. The HAWC design uses densely-spaced steel water tanks to observe particles from air showers. Each tank contains four photomultipliers, and the entire detector comprises 300 tanks in total. This design represents several improvements over Milagro and makes HAWC 15 times more sensitive than its predecessor even with identical photomultipliers and similar electronics.