Detecting the Universe

How do we study cosmic rays?

The atmosphere protects us against primary cosmic rays, the particles coming directly from space. These primaries interact with particles in the atmosphere, making it impossible to detect them directly without putting a detector outside the atmosphere. A few such experiments have been sent up but it's a difficult task since the detectors have to be lighter and more compact than detectors on earth. It's these experiments that have looked, among other things, for anti-atoms in space as discussed in the "Accelerators in the Universe" page.

The development of a cosmic ray shower in the Earth's atmosphere. Click for a bigger version.

Nevertheless there is a way of learning about primary cosmic rays from earth-based laboratories. The primary cosmic particles interact with nuclei in the upper atmospere initiating a so-called cosmic shower. The secondary particles produced in the first interaction continue to interact with nuclei as they descend through the atmosphere giving rise to a cascade of particles. These cosmic showers can consist of many millions of particles and for the highest energy primaries may cover an area of 10 square kilometres. By distributing detectors across a large area to catch all or part of these large showers, scientists can determine the direction the primary cosmic ray came from and how much energy it had. They can even work out what kind of particle it might have been.

The size of the showers is one reason why earth-borne cosmic ray detectors must cover a large area. The rareness of the highest energy events is another. With just one particle of 1020 eV or more falling on one square kilometre every hundred years, the more square kilometres a detector covers, the more high energy cosmic rays it will catch. A 10 kilometre by 10 kilometre array, for example, could reasonably expect to see one such event every year.

The experiments usually consist of large passive volumes such as tanks of water. The particles of the shower collide with the nuclei of the water molecules and give rise to reaction products which can be detected by surrounding detectors covering the interior of the walls of the tank. Other experiments have several layers of flat gaseous detectors to detect the shower particles directly through the ionization they cause in the gas. By combining several experimental installations located at a certain distance from each other in a grid, a detector with a very large effective area can be obtained. By measuring the total amount of reaction products or ionization and its distribution a lot can be learned about the shower and thereby also about the particle that initiated the shower and its origin.

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Introduction to detectors

Particle Physics Education CD-ROM 1999 CERN