Scientific Aspects

Cosmic shower

Introduction


It seems unlikely, but before 1912 (less than one hundred years ago!) Mankind did not even suspect the existence of anything like cosmic rays — of a flux of particles arriving on the Earth from the Cosmos (outer space). It makes impression, also, that their discovery was the consequence of an experiment, performed W.F.Hess (1883-1964), an Austrian scientist. One can read about this fascinating story in detail in the section dealing with References, while the task of this section is to provide general information on cosmic rays and, in particular, on unsolved problems and mysteries.

What are cosmic rays, or what is their composition.

Detailed information on cosmic rays can be found in the Internet applying any search system, for instance, in Astropedia, Wikipedia or MSU site. Here, we shall just repeat what is significant for our project.
As it was already mentioned, the flux of particles arriving on Earth from outer space is called primary cosmic rays. What are these particles? It is easy to see that, first of all, they are stable particles, since all unstable particles, if such particles are produced, decay into stable ones during their long "journey" through the expanses of outer space. Why primary? The point is that particles from outer space must cross the Earth's atmosphere before reaching its surface. And its atmosphere is not so «weightless», as it sometimes seems! The Earth's atmosphere, expressed in grams equals 1000 g/cm2Shower frontThis amount of matter is quite sufficient for cosmic particles to have to interact with nuclei present in the atmosphere before reaching the surface of the Earth. The presence of the Earth's atmosphere results in particles of low energy just «getting stuck» in it and not reaching the Earth's surface at all. Particles of sufficient energy (higher than 1013 eV will not disappear, although they do interact in the atmosphere, but will produce new particles in these collisions. These new particles (while their energies remain high) may also interact in the deeper layers of the atmosphere. And so on and so on. The figure presents an illustrative picture of this process.

As a result, the Earth's surface is not reached by the particle itself arriving from outer space, but by a whole group of all sorts of particles, produced by the first particle during its «journey». Precisely this set of particles is called a Broad Atmospheric Shower of BAS. Since the secondary particles have sufficiently high energies, they all travel with velocities close to the speed of light and reach the Earth's surface nearly simultaneously. But this «nearly» involves one more key fact relevant to our project.. The Earth's surface will be reached practically simultaneously by particles of a shower, if its axis is perpendicular to the Earth's surface. Otherwise, if the shower's axis (the direction of motion of the primary cosmic particle) is inclined, then the particles will reach the Earth's surface successively, and their succession will be strictly related to the direction of the shower's axis, which is clearly seen from the next figure. By measuring it (the sequence of arrival times of particles from different parts of the shower) it is possible to determine the direction from which the primary particle arrived. Thus, one of the most evident scientific tasks of our project, namely, determination of the distribution of directions, from which cosmic particles arrive, is quite comprehensible. Do they arrive uniformly from all directions, or do there exist identifiable sources of cosmic particles? It would seem to be very simple, and this question should have been answered during the 100 years of studies of cosmic rays, but, as it usually happens, everything is simple only at a first sight!

Cosmos (outer space) is not an absolutely empty medium. It contains various fields, including gravity and magnetic fields. And, while gravity, owing to the negligibly small masses of particles has no significant influence on the trajectories of particles, the magnetic field does affect these trajectories significantly. It is not difficult to understand or to recall from the school course in physics that the bending of the trajectory of a charged particle (down to its being totally bent into a closed curve) due to a magnetic field is the stronger the slower the motion of the particle. Calculations reveal that cosmic particles with energies lower than 1016 electron volts are prisoners of our Galaxy and their trajectories cannot go beyond its limits.

Although particles with energies between 1016 and 1018 eV do arrive to us from outer space beyond our Galaxy, but they are so strongly deflected on their way, that they no longer carry information on the location of their source. Particles with energies exceeding 1020 electron volts are already deflected insignificantly, so they do carry information on their source. Returning to the task of constructing the map of the sky in terms of cosmic rays, it becomes clear why this problem has not be resolved yet. Particles with such enormous energies (here is seems appropriate to remind that the energies of the most powerful accelerator in the world — the Large Hadron Collider (LHC), constructed by physicists and engineers in Geneva, will only amount to about the order of magnitude of 1013 eV, which is 7 orders of magnitude (10 million times!) less than this energy) encounter the Earth extremely rarely - about 1 particle per square kilometer of the Earth's surface per year(!).