Alessandro Bettini discusses cosmic rays
In the first years of the last century, physicists and meteorologists studied, among other things, how radioactivity influences the air and atmosphere; with electroscopes they measured ionization induced by radioactivity in the molecules of air. In 1910-11, an Italian meteorologist named Domenico Pacini thought of distancing himself from the land because he expected that the radioactivity in the air would thus diminish. He took a ship and sailed off the coast of Liguria a few hundred meters where his calculations showed that the radioactivity of the rocks on land would be practically zero—but he still found radioactivity.
He then descended 3 meters underwater where he expected to find that radioactivity would diminish but he still found it. So he concluded that there must be a source of radioactivity that did not come from rocks on land. The problem was resolved in 1912 by Victor Hess who began to take measurements of radioactivity by flying in aerostatic balloons and going progressively higher: it was assumed that since radioactivity came from rock, moving higher and higher it should diminish. And essentially it did—up to about 1,000 meters. But going higher, up to 5,000 meters, the levels began to rise again. He thus concluded that this radiation must come from out in the cosmos and gave it the name of cosmic rays. Up until the Fifties, cosmic rays were the source for studying elementary particles before the advent of accelerators. Even today they’re important because we still don’t completely understand them. They arrive on Earth with all levels of energy, both very low and very high, even higher than those produced by the most powerful accelerator, the LHC. But as their energy grows they become more and more rare so to track them, a network of monitors that cover a vast surface area is needed, such as in the case of the Auger experiment, built with international cooperation including Italy, in the Argentinian pampas and which covers 3,000 square chilometers with a series of monitors spread over the entire terrain in the form of a matrix.
Fermi was not only a great theoretical physicist but a great experimental physicist as well, a quality that is practically unique in this century and the last one. In the case of cosmic rays, Fermi didn’t conduct any experiments but gave an enormous theoretical contribution. In 1949, he wrote an article in which he proposed a theory of how these particles, which are either protons or nuclei, become accelerated to such high energies: what are these powerful super accelerators? Throughout the cosmos there exist “clouds” that are lightyears wide and that contain charged particles. As they move, these charged clouds produce magnetic fields that change rapidly from point to point. Let’s suppose that there’s an electrically charged cloud that moves in a certain direction: a proton moving in the opposite direction enters the cloud and finds some magnetic fields that deflect it and make it turn around and go back. Millions of years are necessary for this to happen but eventually the proton comes back with higher energy. But if it bumps into another cloud moving in the same direction and perhaps more slowly, it would bounce back with less energy. We can imagine tossing a tennis ball against a moving car: if we throw it from in front of the car it will bounce back with more energy but if we throw it against the rear of the car it will bounce with less energy. Fermi quickly calculated that the probability of a frontal impact is much greater than a rear impact because it depends on the relative speed of the two objects. And in fact, a couple of years ago there was an experiment on a NASA satellite that was called (surprise, surprise!) Fermi LAT (Large Area Telescope), demonstrating for the first time that Fermi’s mechanism actually functions in the majority of cases, if in a slightly different form. Clouds are shells of exploded stars: the supernovas explode and release layers of charged particles that move away from the mother star at high speed. Bumping into these layers, protons are accelerated.
Cosmic rays are everywhere, they’re a sort of continuous shower that comes from the atmosphere and reaches the ground. Each square meter of ground receives 10,000-20,000 particles per second with energy of over a billion electronvolts.
Cosmic rays can help us comprehend many energetic phenomena and even give us information about the first moments of life in the universe; they have widely varying energy levels, both low and enormously high. For example, there are particles, nuclei, protons that come to us with all the energy of a tennis ball served by a champion player at more than 100 chilometers per hour, concentrated within their miniscule dimensions. But the flow of these high energy particles is very low: that’s why it takes square chilometers of land covered with monitors to be able to track just a few every year, like the Auger observatory in Argentina, for example. For very delicate experiments that observe natural phenomena such as neutrinos coming from the sun or rare radioactive decay, cosmic rays cause problems because they constitute a sort of background noise. As an example, we can say that stars exist during the day including those we see at night, but we don’t see them because the light of the sun is too intense. So we need to be in a place where there is a sort of cosmic silence, which means building laboratories deep underground, under mountains or inside mines. The main lab of this kind, the laboratory of the Gran Sasso in Italy, is beneath almost 1,500 meters of rock. So the shower of cosmic rays is diminished by a factor of one million and, consequently, delicate experiments of this kind can be carried out there.