How will CTA work ?

Detecting gamma-rays is incredibly difficult. Unlike normal light, gamma-rays can’t be focussed by lenses, they just past right through them, and they are rare – the number hitting the top of the Earth’s atmosphere is only a few per square metre per month.  However, luckily for astronomers, when energetic gamma-rays enter the Earth’s upper atmosphere they don’t make it to the ground. Instead as the gamma-rays smash into the Earth’s atmosphere, they give up their energy, producing pairs of sub-atomic particles known as electrons and positrons. These electrons and positrons then start to decelerate, and as they do they produce additional gamma-rays. These in turn go onto produce additional electrons and positrons. This process continues, so from a single gamma-ray one can end up with a “shower” of many millions of gamma-rays, electrons and positrons, as shown in the movie below.

Movie showing formation of particles produced as gamma-ray interacts with Earth’s atmosphere. Source: PAO

The electrons and positrons possess electric charge, and give off a blue glow known as Cherenkov radiation. This light propagates to ground level where it is imaged by ground-based telescopes. This whole process from gamma-ray through electrons and positrons to Cherenkov light signal spreads the light from a single gamma-ray over an area equivalent to around 100 football pitches.

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