While charged cosmic rays (protons and nuclei) are affected by
galactic magnetic fields and come isotropically from all directions,
gamma rays point back to their sources. When a HE gamma ray or
charged particle enters our atmosphere,
it gives rise to a shower of secondary particles, mainly
electrons and photons. These secondary particles lie roughly
on a plane perpendicular to the incident
direction, moving down at almost the speed of light.
(In fact, some of
particles move faster than light in the atmosphere,
producing Cherenkov radiation which can be detected at ground level
using appropriate detectors).
The number of particles in the disk
grows as the shower descends until a maximum is reached
(proportional to the primary particle energy). From then on,
shower size gets smaller and smaller. It may even vanish
and never reach the ground if primary
energy was not high enough. In these kind of experiments,
the atmosphere becomes an active part of the detector, turning an
enormously energetic particle (which we are unable to
detect in a direct way) into a swarm of low energy particles (readily
detectable). In order to extract primary direction and energy,
the shower is sampled using nearly three hundred detectors. There are
three kinds of these detectors:
scintillators, Cherenkov ligth (huts and telescopes)
muon towers
(See cross sections of the huts).
Detector pictures: click icon
The more particles we detect, the more reliable our data. If we
built our experiment at sea level, we would detect
only very high enery showers (very rare). That's the reason why
these detectors are located in a high place.
At detection level, secondary particles radius can have hundreds of
meters, so detectors should cover a large extent (in our case, 200m x
200m). HEGRA experiment is located at a height of 2200 meters
in the Roque de los Muchachos at
La Palma Island (29 N, 18 W),
On analyzing data our main difficulty is establishing which
showers are produced by charged particles (the majority,
but the least interesting) and which ones are coming from photons. Separation criteria are found through
computer simulations of processes taking place in our atmosphere when a HE particle is entering it. Finding these criteria
is basic if we want to make use of HEGRA as an efficient gamma ray detector, so an important part
of our resources is invested on that.
Click HERE to get information on
papers.
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cortina@gae.ucm.es
