Those amongst us, who, in the last year of secondary school, studied the Second World War, probably remember a rather dark and extremely controversial event: the flight of Hess (Rudolf Hess), in May 1941, from Germany to the United Kingdom, on board his Messerschmitt. The story of this flight is told in a book, The Flight of Rudolf Hess, published many years ago and before his death, in 1987, at the age of 93, in Spandau prison in suspicious circumstances. Fewer of us would have heard talk of yet another “flight of Hess”, this time, in a hot air balloon, which occurred many years previously, in 1912, in light of a different Hess, Victor; this time, we must appreciate the significance and results. It is studied as part of some astronomy, astrophysics and physical cosmology courses. With this flight, which marks its hundredth birthday this year, Hess confirmed the existence of cosmic rays and was awarded the 1936 Nobel Prize for Physics (alongside Carl Anderson, who was awarded the prize for the experimental discovery of the positron, the antiparticle of the electron).
It goes back as far as Coulomb, who in the late 18th Century, observed that a metal sphere, which was electrically charged and isolated, with time, gradually lost its charge. Subsequent quantitative studies of the discharge process, carried out with the support of ever increasingly sensitive and accurate electroscopes in the following century – those overseeing the discovery of electrons, X-rays, natural radioactivity – were able to conclude that there was ionizing radiation present in the atmosphere. Initially, it was thought that this was owing to gamma rays produced from the natural decay of radioactive elements present in the earth's crust. According to this theory, the radiation should decrease in intensity gradually as it rises above the earth's surface and it was therefore possible to calculate the decrease curve. It was simple a case of obtaining the necessary measurements to confirm the hypothesis. In 1910, an electroscope was bought to the top of the Eiffel Tower to measure, at approximately 300 meters from the ground, the ionization levels in the atmosphere. The result, although it demonstrated slight degrowth when compared to the measurement taken on the ground, indicated an ionization level which was significantly higher than that estimated in the hypothesis where radiation was owing to the earth's natural radioactivity. The idea that there was another source of ionizing radiation came to light. In this context, many experiments were carried out using various types of electroscopes on board hot air balloons and at various altitudes. The flight with which Hess, on the 7th August 1912, went higher than 5,000 meters of altitude, where he noted, instead of a decrease, an increase to the factor of 4 of the intensity of the ionizing radiation proved significant. Hess concluded that the radiation was not owing to natural radioactivity, but it entered the atmosphere externally. This result was confirmed subsequently by Werner Kolhöster, who achieved measurements from up to 9,000 meters of altitude. Then Millikan, once convinced of the extraterrestrial origin of this radiation, coined in 1920, the expression “cosmic rays”. Initially the scientific community believed that these cosmic rays were electromagnetic radiation, gamma rays, to be precise.
Subsequent studies proved that they felt the effects of the earth's magnetic field and hence, at least in part, they must have been formed of charged particles. Gradually, it became understood that there were primary cosmic rays (those externally reaching the atmosphere and are therefore extraterrestrial) and secondary cosmic rays (produced from the interaction between the primary rays and this same atmosphere), and that the primary rays are primarily formed of protons with energy reaching up to 1020 eV or slightly higher. There are are also small quantities of alpha particles (helium nuclei) and nuclei with heavier elements. The strong magnetic fields typical of pulsars, neutron stars and black holes and violent shocks produced in supernova explosions are able to accelerate charged particles until they reach these high energy levels and are therefore primarily responsible for the production of cosmic rays.
The cosmic rays, which formed a natural source of high-energy particles, from which the study of interaction with matter, was possible, were extremely important for elementary particle physics up until the time of systematic creation and usage, towards the middle of the last century, of quite powerful particle accelerators. The positron, the muon, the pion and other particles, were all discovered thanks to the study of cosmic rays (secondary). At present, and for more than fifty years, physics has been using, for the purpose of studying elementary particles and nuclear reactions, accelerators, instruments able to provide - upon command - much greater flows of accelerated particles. The advantages for the cosmic rays and their properties lies exclusively with the element of astrophysics. Many Italian scientists have contributed in developing this discipline, beginning with Domenico Pacini who, at more or less the same time as Hess, subsequent to measuring the intensity of the ionizing radiation taken at various sea depths, added to these same conclusions (see “le Stelle” no. 101, pgs. 36-43). Pacini's results remained concealed form the public at the time, probably because they were published in Italian and not published in compliance with International standards, however, they were later recognized by Hess himself. Then Bruno Rossi, who developed the electronic circuits using the technique of "coincidence" between various detectors, enabling the creation of “telescopes” for cosmic rays and to optimize (owing to the work of Occhialini) the use of cloud chambers for his study. Rossi also foresaw the “east-west” effect, which could be explained where the cosmic rays were positively charged. Giuseppe “Beppo” Occhialini also played, without a doubt, a significant part in terms of the physics of cosmic rays: he contributed to the discovery and study of pions and the confirmation of the discovery of the positron. In addition, Conversi, Pancini and Piccioni, who demonstrated how the “mesotron”, the meson discovered in cosmic rays, was not the mediator of nuclear forces as judged by Yukawa.
At present, one hundred years since their discovery and awaiting the identification of gravitational waves, the cosmic rays represent the only non-electromagnetic window open to the Universe and their relative study forms part, in whole, of that big science which involves lots of international cooperation and expensive and complex instruments. There are many fundamental questions without answers; amongst which, those relative to the origin of those particles with higher energy and on the mechanisms which provoke acceleration. Verification of an upper energy limit must also be provided. The Auger Observatory, in the Argentinian pampas, has been, for some years now, searching for the most energetic events, the interactions between cosmic energy rays at or higher than 1020 eV and the nuclei of elements forming part of our atmosphere, interactions which produce intense extended particle clusters and radiation which when it reaches the ground, spreads over an area of a few square kilometers. These events are quite rare (approximately one per year is expected per hundred square meters) which the Auger Observatory, in order to gather sufficient statistics, extends, with its detectors, over an area slightly smaller than Luxembourg! You can take a tour of the Observatory using the relevant plug-in provided by Google Earth. The main objective of the experiment (and others worldwide, such as the Telescope Array Project or WALTA) is to understand the mechanisms governing extreme acceleration able to transfer huge amounts of energy to charged particles.
However, the questions in the field of the physics of cosmic rays are not limited to acceleration mechanisms: they also relate to the composition of the matter within the Universe. In space, PAMELA (see “le Stelle” no. 73, pgs. 34-41) and AMS (see “le Stelle” no. 89, pg. 61) detectors look to clarify the nature of dark matter and the absence of antimatter, whilst IceCube, in Antarctica, and ANTARES, in the Mediterranean Sea, are looking for the neutral component of cosmic rays, that made up of neutrinos. A hundred years from its birth, the physics of cosmic rays appears healthy and boasts advantageous prospectives, and integrates well with other astrophysics branches in terms of contributing and facing the fundamental questions on the nature of the Universe.
Extracted from: Le Stelle no.108, July 2012