Chronicle

January Session of the Division of Physical and Mathematical Sciences of the USSR Academy of Sciences

The regular session of the Division of Physical and Mathematical Sciences of the USSR Academy of Sciences, held on January 23, 1947, was devoted entirely to new data obtained during 1946 concerning the composition of cosmic radiation.

At the morning meeting, reports were presented on the work of the Alagez high-mountain expedition of the Academy of Sciences of the Armenian SSR jointly with the Academy of Sciences of the USSR, in which the existence of a new elementary particle with a mass intermediate between the mass of the meson and that of the proton was undoubtedly established.

Corresponding Member of the Academy of Sciences of the USSR A. I. Alikhanyan, in his report, recalled that the preceding work of the Alagez expedition had shown the presence in cosmic rays, in addition to the so-called “hard” and “soft” components, of a certain “third” component, the nature of which still remains unknown. In particular, it was shown that this “third” component generates particles whose ionizing power is 2–3 times greater than that of relativistic mesotrons.

In 1946, at an altitude of 3200 m above sea level, Alikhanyan, Alikhanov, and Weissenberg carried out very thorough investigations of the deflection of the component of cosmic radiation in a magnetic field (4000 oersteds). At the same time, particles were sorted into those penetrating and those not penetrating through 5.4 cm of Pb, which made it possible to isolate the hard meson component.

It was found that, of the total flux of particles, about 10% consists of particles little deflected by the magnetic field and, at the same time, possessing low penetrating power. Special experiments established that these particles cannot be classified among the previously known ones. From the values of the momentum (found from the deflection in the magnetic field) and the energy (obtained from measurements of penetrating power), the mass of the particles was estimated; it lies in the range of 600–900 electron masses. These particles carry both negative and positive charges, the number of positively charged particles being somewhat greater than that of negatively charged ones. The newly discovered particle has been given the name “varitron.” The experimental data indicate the possibility of the existence of varitrons of different masses, up to a mass exceeding that of the proton.

Candidate of Physical and Mathematical Sciences S. Ya. Nikitin reported on the results of his investigation of the ionization spectra of cosmic radiation, carried out by him with the aid of an ionization telescope. A proportional counter made it possible to measure the magnitude of ionization pulses, while a Geiger-counter system made it possible to exercise simultaneous control over the number of ionizing particles. The speaker obtained ionization spectra of the “soft” and “hard” components and found a radical difference between these spectra. It was shown that about 15% of the particles (relative to the intensity of the hard component) entering into the composition of the soft component produce ionization 3 times greater than mesons.

The calculation of the mass of the particles on the basis of measurements of their penetrating and ionizing power gave values lying in the interval of 300–3500 electron masses, in agreement with the three maxima in the ionization spectrum. The study of the absorption of these particles in lead showed that the absorption curve has an unusual form.

It was also shown that the presence of these particles fully explains the discrepancies in determining the number of particles by the methods of ionization chambers and counters.

Thus one may regard as firmly established the presence in cosmic rays of a new kind of elementary particle, hitherto unknown—barytrons—with masses in the interval of 400–900 electron masses. Their low penetrating power compels the assumption that they do not penetrate the sensitive volume of the apparatus “third component,” which, possibly, is also responsible for the generation of “stars.”

At the evening session, Corresponding Member of the Academy of Sciences of the USSR V. I. Veksler reported some new data on the composition of cosmic rays, from among those obtained in 1946 by the Pamir Expedition of the Physics Institute of the Academy of Sciences of the USSR, which worked at altitudes up to 4800 m above sea level. The main aim of this cycle of investigations was to clarify the nature of the radiation that produces nuclear disintegrations.

In the work of Kurnosova and Lyubimov it was found that in thick layers of lead (10 cm and more) showers are generated that differ from electron-photon showers produced by ordinary mesons. At the same time a close correlation of these showers with narrow atmospheric showers was established. Careful measurements showed that the component generating showers in lead is absorbed in dense substances considerably more weakly than in an equivalent thickness of air. The latter may be explained either by the decay of the generating particles (but then, since these cannot be ordinary mesons, they must be some new particles), or, less probably, by the presence of a complex transition effect. There are grounds for considering that the occurrence of the observed special showers is connected with nuclear disintegrations.

In the work carried out by Shcherbakova and Bezotosny with the aid of proportional counters, a good correlation was established between nuclear disintegrations and narrow atmospheric showers, which indicates the presence in the latter of a component extremely effective with respect to the generation of nuclear disintegrations. It was shown, in agreement with the data of other authors (obtained by other methods), that a considerable fraction of the particles forming the generating component is non-ionizing.

A third method for studying nuclear disintegrations was obtained by Gorbunov, who used paired proportional counters of special design. He established that the principal part of the component generating nuclear disintegrations is penetrating. On the basis of measurements of absorption in dense substances and in the atmosphere, preliminary indications were obtained of the possibility of decay of the generating component.

The flux of the generating component at an altitude of 3860 m proved to be of the same order as the flux of mesons, whence it inevitably follows that the generating particles are neutral.

The totality of the experiments reported reveals a number of new phenomena closely connected with nuclear disintegrations in cosmic rays and, apparently, indicating the existence of a new unstable, possibly neutral, particle.

Academician A. I. Alikhanov, Academician V. A. Fok, Corresponding Member of the Academy of Sciences of the USSR B. M. Vul, Prof. V. L. Ginzburg, Doctor of Physical and Mathematical Sciences N. A. Dobrotin, Prof. D. D. Ivanenko, Doctor of Physical and Mathematical Sciences S. Z. Belenky, and others took part in the discussion of the reports.