Chronicle

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

On March 18, a regular session of the Division of Physical and Mathematical Sciences of the Academy of Sciences of the USSR was held in Moscow.

At the morning meeting, Doctor of Physical and Mathematical Sciences I. Ya. Pomeranchuk delivered a report, “On the Ambiguity of the Elimination of Infinities in the Quantum Theory of Fields.” As is known, modern quantum field theory leads to divergent expressions of three types: the so-called “even” ones, which have a classical nature and are connected with the representation of a point particle; “odd” ones, which have a purely quantum character and are due in their origin to the peculiarities of field quantization and the existence of zero-point oscillator energy; and, finally, “logarithmic” ones, to which the Dirac theory of the vacuum leads. All attempts made thus far to get rid of these divergences by introducing into the theory certain “form factors” that determine the “structure” of an elementary particle have either run up against the impossibility of satisfying the basic requirements of the theory of relativity, or, as for example in the case of Dirac’s limiting \(\lambda\)-process, have rid the theory of only one type of divergence while fully preserving the others.

The speaker succeeded in finding a whole class of such form factors which, together with the limiting \(\lambda\)-process, both satisfy the relativistic requirements and free the theory from all divergences simultaneously. However, by varying the form of the form factor, one can turn the divergent expressions not only into zero but also into any finite number. Thus, the situation with quantum field theory has changed radically. In place of the previously assumed impossibility of getting rid of divergent expressions without a fundamental breakdown of the theory, an ambiguity of such elimination has arisen. In the speaker’s opinion, in this connection an alternative arises: either, relying, for example, on experimental measurements of the masses of elementary particles, one can find certain ways of selecting particular form factors from among the possible ones, and then it will be possible to construct a rigorous theory free of divergences but incapable of explaining the observed mass spectrum. Such a theory would be a significant step forward, but, remaining limited, would be only a temporary “working” theory—a “transitional” theory, in the speaker’s words—or the ambiguity of the form factor is fundamental and irremovable, and then the only path of development is a radical transformation of the entire theory.

Academician S. I. Vavilov, Corresponding Member of the Academy of Sciences of the USSR B. M. Vul, and Corresponding Member of the Ukrainian Academy of Sciences N. N. Bogolyubov spoke in the discussion of the report.

Corresponding Member of the Ukrainian Academy of Sciences N. D. Morgulis reported the results of a comprehensive study, carried out by him jointly with P. G. Borzyak and B. I. Dyatlovitskaya, of the optical and photoelectric properties of a new modern antimony–cesium cathode. A layer of antimony was deposited by evaporation on the walls of the tube (partly coated with platinum), and was subjected to...

then etched with cesium. The thickness of the cathode obtained in this way could be varied monotonically along the tube, gradually decreasing to zero. With the aid of a monochromatic light probe, interference patterns (in transmitted and reflected light) were obtained for various wavelengths of the visible spectrum. Comparison of the experimentally observed interference patterns with the results of their theoretical calculation (carried out for various combinations of values of the refractive index and the absorption coefficient) made it possible for the first time to determine, with a high degree of reliability, the optical constants of a surmone-cesium cathode. It proved that the absorption coefficient increases continuously with decreasing wavelength, while the refractive index undergoes changes characteristic of the region of anomalous dispersion. At the same time it proved possible to determine accurately the thickness of the cathode in various sections of the tube.

With the aid of the same probe, photoelectric emission was studied in various sections of the cathode both when the cathode was illuminated from the side of the emitting surface and when it was illuminated from the opposite side (through the layer). It became clear that in both cases the distribution of the photocurrent along the tube has an unusual character, forming a series of maxima and minima. The latter exactly follow the distribution of the energy of the light wave in the immediate vicinity of the emitting surface of the cathode, as obtained from analysis of the interference pattern. Hence, as also from analysis of the theoretical equations for the photocurrent, it follows that the effective depth of the exit zone of photoelectrons is of the order of only about 100 Å, as well as a number of other conclusions essential for the problem of the photoeffect.

The evening session was devoted to new results of studies of atmospheric showers of cosmic rays. A report on the so-called “narrow” showers was given by Corresponding Member of the Academy of Sciences of the USSR A. I. Alikhanyan.

A group directed by him and by Academician A. I. Alikhanov, working on Mount Aragats, has since 1943 been studying atmospheric showers. As a result of this work, alongside “wide” atmospheric showers there were also found to be showers that covered an area not exceeding a few square meters and had a density of several particles per square meter. It was established that narrow showers are not genetically connected with Auger showers. The number of narrow showers is greater than the number of Auger showers and increases from sea level to an altitude of 3200 m by approximately 2.5–3 times. Measurements show that the penetrating power of particles at the center of the shower is greater than at the periphery, and that in 5.5 cm of lead about 50% of the shower particles are absorbed.

The entire complex of data on the penetrating power of shower particles, including the absence of multiplication in lead, indicates that they are neither electrons nor protons.

Observations on the deflection of shower particles by a magnetic field, carried out with the large magnet of the Aragats expedition, showed that the hard component consists mainly of particles with energy $\sim 1$ MeV and, apparently, is mesotronic. The soft component undergoes considerably greater deflections in the magnetic field and is not mesotronic. The possibility is not excluded that it consists of particles with a mass intermediate between the masses of the mesotron and the proton—varitrons. The small diameter of the showers compels one to suppose that they are formed at a low altitude.

The report of Candidate of Physico-Mathematical Sciences N. A. Dobrotin was devoted to work on the study of wide Auger atmospheric showers, carried out in 1946 in the Pamirs by FIAN graduate student G. T. Zatsepin and diploma students V. V. Miller, A. L. Rozental, and L. Kh. Eidus under the direction of Academician D. V. Skobeltsyn. The energy of the particles generating such showers is, as is known, $10^{14}$–$10^{17}$ electron-volts, which makes it possible to assume, alongside the cascade mechanism of multiplication, the presence of certain nuclear processes.

One of the objects of study was the “root-mean-square radius” of showers, which depends little on the energy of the particles. The method developed by the authors, using fourfold coincidences, reduced the number of random coincidences to fractions of a percent and made it possible to study the distribution of particles in a shower at distances between registering instruments up to 1000 meters (instead of the previously available 300 meters). It turned out that at large distances the number of coincidences considerably exceeds that predicted by cascade theory. Similar results were also obtained when selecting a hard component penetrating 12 cm of lead.

Measurement of the penetrating power of shower particles for two- and threefold coincidences showed that showers of high density have a greater penetrating power than showers of low density. Comparison of absorption in aluminum and lead showed that the main role in the absorption of shower particles penetrating 12–16 cm of lead is played by radiation losses, i.e., the particles are electrons or photons, and not mesons, as had previously been assumed. However, showers also contain mesons, which constitute an appreciable fraction of the particles in showers of low density.

Further, it was found that high-energy particles scattered in a shower through a considerable (hundreds of times) angle larger than that predicted by cascade theory. The variation of shower density with altitude, from sea level to 3800 m, also proved inconsistent with cascade theory. Studies of shower density at altitudes of 3800–4800 m above sea level showed that the energy spectrum of the primary particles producing showers has the form of a power function \(E^{-\gamma}\), where \(\gamma \simeq 1.8\).

The results presented indicate that cascade theory, which takes account only of electromagnetic interaction, is not capable of giving a complete description of the processes of shower formation.

Taking part in the discussion were Academician A. I. Alikhanov, Corresponding Member of the Academy of Sciences A. I. Alikhanyan, G. T. Zatsepin, and others.