Brief Overview of the Science of Atmospheric Cherenkov Detection
VERITAS is an array of four Imaging Atmospheric Cherenkov 12m Telescopes in southern Arizona. Images of the same shower can be recorded from different view points. This permits to reconstruct the primary particle with great precision and select preferentially gamma rays against the much more numerous cosmic rays. In 2003-2004, we constructed and operated a prototype telescope with which we detected gammarays from the Crab nebula and from a distant galaxy. We also recorded what is probably the fastest motion picture ever, one frame every 2 billionth of a second! We can see atmospheric shower fronts traveling at the speed of light. Starting in 2007, we have 3 telescopes in operation and the construction of the 4th telescope has completed. We are currently in the process of implenting the 4th telescope into the array.
The gamma ray sky brings us information about the most violent phenomenon in the universe, from super-novae and their possible role in cosmic ray acceleration to the accretion of material onto super massive black holes in distant galaxies. It is also a window through which the most intriguing questions of astrophysics can be addressed like the nature of dark matter and dark energy in the universe.
The diagram above shows the general setup of the VERITAS array. First, a gammaray from somewhere in the universe enters the upper atmosphere. At this point, it may collide with a nucleus, break it and transfer its energy to the various subatomic particles. These energized particles will then collide with other atmospheric atoms, breaking them and transfering energy to more particles. This process creates an atmospheric shower of energetic particles. Any of these particles that are charged and have enough energy will emmit small amounts of light. This light is referred to as Cherenkov Radiation. The telescopes on the ground have very sensitive cameras, equipped to detect this Cherenkov Radiation. The advantage of having more than one telescope is the same reason it's better to have two eyes than one. With multiple telescopes it is easier to determine the height of the particle shower, and ultimately the gammaray's universal origin.