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06. THE MYSTERY OF COSMIC RAYS

THE MYSTERY OF COSMIC RAYS

Cosmic rays are particles from the cosmos that continuously bombard the Earth with very strong energies. What are these powerful accelerators? Fermi answered in 1949. Cosmic accelerators use gravitational energy. When a star of large enough mass has burned all of its nuclear fuel, it suddenly and violently collapses under its own weight. The external layers, which are expelled, contain turbulent magnetic fields which, expanding, encounter nuclei and protons, transferring to them a part of their enormous energy.

In the early 20th century, the presence of radioactivity in the air was observed using electroscopes. The electroscope is a closed cell, with a glass window, which contains two metal (gold) leaves. If the instrument is electrically charged, the leaves separate due to the appearance of charges of equal sign, which repel each other. When the air inside the cell is ionized by some agent that penetrates it, the electroscope discharges—the greater the radiation, the faster.

It was initially thought that the radioactivity in the air was due to nuclei of radioactive elements in the ground. In the years 1910–11, the Italian physicist and meteorologist Domenico Pacini measured the radioactivity of the air on the surface of the sea, away from the coast, and even three meters underwater. It was still remarkable, so it was clear that there was a source of radiation other than the nuclei in the ground. For his measurements at sea, Pacini used the destroyer Fulmine of the Royal Navy.

Cosmic rays are mostly charged particles, protons about 90%, nuclei  9%, and electrons 1%. There are also gamma rays (high-energy photons), which are few, about 1%.

Gamma rays are important to us because they come straight from their source, giving us the ability to identify it, unlike charged particles that are deflected by magnetic fields in the cosmos. The energies of cosmic rays are much greater than those of the natural radioactivity that surrounds us.

The energy spectrum of cosmic rays spans more than 32 orders of magnitude, surpassing the energy of the most powerful man-made accelerators. Their flow decreases as energy increases, reducing by about eight times for each doubling of energy.

What mechanisms accelerate them to such high energies?

Fermi LAT (Large Area Telescope), or known simply as Fermi by the operators, is a NASA satellite that contains complex instrumentation designed and built by research groups from France, Germany, Japan, Italy, Sweden, and the US. The skills of particle physicists and astrophysicists were combined to create it.

Fermi was launched on 01 Jun 2008 and continues to collect data.

Direct observation of protons cannot indicate their source because, being electrically charged, they are deflected by the galactic magnetic fields along their paths and therefore do not point to the source. Indirect observation must be used. The accelerated protons meet the supernova remnant material (SNR) and produce, among other things, electrically neutral π mesons (pions), which immediately decay into two photons. The latter travel in a straight line.

Problem: Even the electrons present in the SNR can give rise to photons. Only the accurate measurement of the energy spectrum of photons allows us to distinguish between the two. Fermi LAT did this by aiming at two well-known SNRs, namely, IC 443 and W44, finding the expected characteristics of pion decay.

A cosmic particle penetrating the atmosphere sooner or later hits a nucleus, producing secondary particles. These in turn collide and produce other particles. And so on until you have a cascade of millions or billions of particles.

The phenomenon was discovered in 1934 by Bruno Rossi, using the electronic coincidence circuit he himself invented.

He observed cosmic-ray events with detectors distant from each other. In some cases, the detectors reported simultaneous (“coincident”) events: they were due to the same primary interaction. Important contributions were made in 1938 by two German groups: Schmeiser’s and Bothe’s, who called the phenomenon “Luftschauer” (an aerial swarm), and Kolhörster and collaborators.

In 1939, Auger, Maze, and Robley at Jungfraujoch in the Swiss Alps observed incidences up to 300- m separations between detectors, i.e., evidence of the extended swarms. The largest existing cosmic ray observatory is dedicated to Auger. The detectors (1,600 in total) are water tanks in which the charged particles emit light due to the “Cherenkov effect”. Particular light detectors, called photomultipliers, transform the photon impacts into an electrical impulse.

A million protons or nuclei from the cosmos, with energy similar to that of a tennis ball served by a champion, but concentrated within a diameter a 100 billion times smaller, hit the Earth every year.

The Pierre Auger Observatory, built near Malargüe (Mendoza) in the Pampa Amarilla of Argentina, is used to study the messages that these cosmic rays bring us.

Its 1,600 detectors are distributed over an area of ​​3,000 km2. The area is surrounded by a set of 24 telescopes that capture, at night, the faint fluorescent light produced by the cosmic shower as they excite atmospheric nitrogen. The large scientific infrastructure was built between 2000 and 2008 by researchers from 17 countries, with a notable Italian contribution from the National Institute of Nuclear Physics of Italy. The Pierre Auger Observatory has opened a new window on the exploration of the universe, studying the particles of the greatest-known existing energy. At such high energies, the constitution, the origin—from outside the galaxy—and the sources of cosmic rays are still unknown. So, Auger is continuously collecting and studying valuable data on these rare particles to try to unravel their puzzles.

The EEE project consists of a special research activity on the origin of cosmic rays, conducted by the Enrico Fermi Research Center in collaboration with CERN (European Organization for Nuclear Research), INFN (National Institute of Nuclear Physics), and MIUR (Ministry of ‘Education, University and Research). The project is based on the valuable contributions of students and teachers of higher education institutions.

In each of the schools participating in the Project, a “telescope” comprising the most modern and advanced particle detectors (Multigap Resistive Plate Chambers, MRPC) is built, to be connected by GPS instrumentation with the telescopes of other schools in order to detect the cosmic muons and extended swarms, even as large as entire cities or more, produced by very high-energy primary cosmic rays.

The students are also given the very important task of building the detectors themselves, starting from basic components, so that they realize how it is possible to go from rudimentary materials to very high- precision instruments. The construction of the detectors takes place in the CERN laboratories, in the most exclusive places of the most advanced research, which are made accessible to students for this purpose.