Located 13,500 feet above sea level on the slopes of Mexico's Volcán Sierra Negra, a new scientific tool comprised of 300 huge tanks holding 55 million liters of water, will soon detect the highest energy photons ever observed.
The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory was inaugurated a few days ago, March 20, and will begin collecting data at full capacity soon. The goal is to study explosive events like supernovae, neutron star collisions, and active galactic nuclei, to learn more about the nature of high-energy radiation.
Each of HAWC's detectors is a huge tank containing 50,000 gallons of ultrapure water with four light sensors anchored to the floor. When gamma rays or cosmic rays reach Earth's atmosphere they set off a cascade of charged particles, and when these particles reach the water in HAWC's detectors, they produce a cone-shaped flash of light known as Cherenkov radiation. The effect is much like a sonic boom produced by a supersonic jet, because the particles are traveling slightly faster than the speed of light in ultrapure water.
The light sensors record each flash of Cherenkov radiation inside the detector tanks. By comparing nanosecond differences in arrival times at each light sensor, scientists can reconstruct the angle of travel for each particle cascade. The intensity of the light indicates the primary particle's energy, and the pattern of detector hits can distinguish between gamma rays and cosmic rays. With 300 detectors spread over nearly three football fields, HAWC is able to "see" these events in relatively high resolution.
Below is a cool timelapse video of the construction of the first 250 tanks in the HAWC array, taken between April 2013 and May 2014.