Some promising experiments were performed at Frascati in 1961-62. Accelerated electrons were extracted from the accelerator and let into a storage ring, where they kept circulating with energies of several hundred Mev for about 2 days. By the same means it was also possible to store up fast positrons and even to create for the first time opposed beams of electrons and positrons. The resultant number of particles, however, was much too small to produce observable collisions, whose analysis is so important for determining the size and structure of the electron and the positron. Protons and n and IC mesons could not be used to determine the structure of particles, because when they come close to each other fresh particles are generated by the strong interaction; electrons, on the other hand, can smoothly "burrow" into the proton and undergo deflections by the electric forces stemming from the charge distribution within the particle. An accumulator for electrons was recently built in Novosibirsk.
Let us now pause to consider a very interesting phenomenon detected in circular accelerators, which involves the emission of electromagnetic waves by relativistic electrons moving at nearly the speed of light.
This special kind of radiation, of "radiating electrons", also known as "synchrotron" radiation, is the only instance of light emission that does not involve the collision of electrons with matter, i. e. , with other particles. In this case the relativistic electrons may be pictured as moving at such high accelerations that the electromagnetic field cannot "keep up" with them and detaches itself, i. e. , is emitted. In the energy range of tens to hundreds of Mev the maximum of this radiation lies in the visible region. This unusual type of radiation, visible to the naked eye, was discovered by Pollack in 1944, thus bearing out the theoretical predictions of Soviet physicists Ivanenko, Pomeranchuk, Artsimovich, and Sokolov. The electrons lose a considerable part of their energy by intense synchrotron radiation, and that sets a practical limit to the energies at which circular electron accelerators, for instance, the betatrons (Kerst) and their variations, the synchrotrons (V.I. Veksler, McMillan), can be operated.
Photographs have been taken of beams of radiating electrons. At F.A. Korolev's laboratory a detailed study was made, on the synchrotron of the Academy of Sciences of the USSR, of the spreading of an electron beam with the increase of energy, owing to quantum oscillations of the orbits. As has been predicted by Sokolov and Ternov, at very high energies account has to be taken of the recoil experienced by the electrons when they emit photons.
At Frascati the emission of a single electron has been recorded photogra phically, i. e. , the electron itself was made visible, rather than its tracks, which are seen in cloud chambers or photographic emulsions. We may note that the same kind of "synchrotron" radiation is emitted by fast relativistic electrons in interstellar and stellar magnetic fields and under other astronomical conditions.
In the latter case the radiation is of much longer wavelength and is received as radio emission from stars, galaxies, etc. Synchrotron radia tion also plays a part in plasma, where there are similarly relativistic electrons moving in a magnetic field (Trubnikov).
Improvements in detection techniques (such as protracted irradiation of plates at high altitudes by means of planes and rockets, the launching of artificial satellites, the use of stacks of photographic emulsion measuring some cubic meters, etc.) will certainly provide new means of using cosmic rays for discovering particles and studying their properties. The cosmic rays involve processes of stupendous energies of the order of to electron-volts, which are not likely to be attained by terrestrial accelerators in the near future, even though there is some promise in the process of accelerating particles by means of self-regulation (Mints, Petukhov in Moscow), by means of intense laser radiation, and by accelerating plas moids (V.I. Veksler), etc.
It is an extremely complicated matter to discover a new particle, as was once the case with the discovery of a new chemical element, or, at a different time, of a new radioactive isotope. Thus, it has happened that after some particles had been "discovered" they had to be "undiscovered" later. This was the case with the now defunct varitrons and some heavy mesons, thought to exist in the mass range of 1000-1400 m, which later proved to be none other than the K meson with a basic mass of 966 m. Preliminary data on D-meson tracks (Van Ganchan in Dubna, 1959) prompted a reexamination of the old material on particles with masses of 1200-1500 m in the hope of discovering perhaps some rare D-meson tracks. They were, in fact, never found.
A. I. Alikhanyan and his coworkers reported a flux of particles with a mass of approximately 550 m (about 1% of the number of mesons) observable in cosmic rays, but no such evidence has been uncovered in the experiments performed by Conversi (Rome) and in some other laboratories.