How do we study cosmic rays?

Cosmic rays are studied in a variety of ways depending on how much energy they have. This is illustrated in the picture below.

How cosmic rays are detected

1. Low energy cosmic rays detected by instruments carried in satellites.
2. Higher energy cosmic rays generate a small air shower. The Cerenkov radiation emitted by the shower is detected by a large telescope on the ground
3. Even higher energy cosmic rays generate very big air showers. The particles in the shower travel to the ground where they can be recorded by an array of detectors

Extensive Air Showers

When higher energy cosmic rays hit the upper atmosphere (about 20 km up) they lose about half of their energy by creating a jet of particles which carries on travelling in almost the same direction as the cosmic ray. The particles in the jet can themselves create more particles as they hit other nuclei of oxygen or nitrogen in the air. This jet is called an extensive air shower and keeps on growing until the particles in the shower run out of energy and are absorbed in the atmosphere.

We refer to the initial particle that starts the shower as the primary cosmic ray. The particles created in the air shower are known as secondary cosmic rays. Over a million of the secondary particles which are produced when primary cosmic rays hit the atmosphere pass right through your body every minute.

A single cosmic ray can generate showers with a large number of particles depending on its energy. The smaller air showers are absorbed near the top of the atmosphere and do not reach ground level. However, as the particles in the shower zip through the air they emit faint flashes of blue light known as Cerenkov radiation. Although the cosmic rays and the air showers they produce are absorbed by the atmosphere it is possible to detect the faint Cerenkov light using large telescopes but only on dark, moonless nights. The Leeds University group collaborate with scientists in the USA and Ireland at the whipple telescope in Arizona and use this technique to observe high energy gamma rays from dead stars such as the Crab nebula and the centres of very active galaxies.

Air Shower Arrays

At even higher energies the air showers contain vast numbers of secondary particles, numbering in the billions for the most energetic cosmic rays. The particles in these showers are of such high energy that they can travel all the way from the top of the atmosphere (about 20 kilometres up) down to the ground where they can be detected directly with particle detectors.

This animation is a simplified representation of an extensive air shower. A primary cosmic ray (coloured red) enters our atmosphere. At an altitude of ~20 km it collides with molecules in the air and generates a shower of secondary particles (coloured blue). These also generate further particles which travel, at almost light speed, towards the ground where some are detected by an air shower array. In this example the shower hits the detectors to the left before those on the right. This helps us to determine the direction of the primary cosmic ray. Notice there are more particles at the centre, or core, of the shower. Most of the secondary particles are absorbed in the ground but some of the higher energy particles in the core can penetrate many kilometres below ground where they can be detected by experiments such as AMANDA.

The detectors are usually arranged in a grid formation (or array) on the ground allowing measurements of each shower to be made at several points. Information from the detectors tell us how many particles struck the detector and the time that they hit. By adding up the number of particles recorded by each of the detectors we can estimate how many particles were in the shower and from that we can make a good guess as to the energy of the cosmic ray that started the shower. We can use the time that each detector was struck to measure the direction the cosmic ray was travelling when it hit the Earth's atmosphere.

Its important to realise that when we measure extensive air showers, we do not "see" the primary cosmic ray. Rather we measure the secondary particles that were generated as the cosmic ray travelled through our atmosphere.

The air showers recorded by the SPASE-2 array at the South Pole have diameters of 10's of metres at ground level and so the detectors in these arrays are spaced between 30 and 50 metres apart. The very highest energy cosmic rays produce air showers which cover many square kilometres. For this reason the planned Pierre Auger Observatory will have 1600 detectors spaced 1.5 km apart.