CONGRESSES AND CONFERENCES

II All-Union Conference on the Atomic Nucleus

N. Dobrotin, Moscow

On September 20–26 in Moscow, the II All-Union Conference on the Atomic Nucleus, convened by the Academy of Sciences of the USSR, took place. About 120 Soviet physicists working directly in this field took part in its work, as did a number of major foreign scholars: Pauli (Zurich), Auger (Paris), Williams (Manchester), and Feather (Cambridge). At the conference 28 reports were heard, of which 23 concerned the work of Soviet physicists. In addition to the reports at the conference itself, a series of survey lectures was organized for broader circles of scientific workers, students, and leading workers.

The program of the conference was devoted to five of the most pressing problems in the physics of the atomic nucleus: 1) the passage of $\gamma$ rays and fast electrons through matter; 2) cosmic rays; 3) $\beta$ decay; 4) the interaction of neutrons with nuclei; and 5) the theory of nuclear structure. In addition, at the first session of the conference reports were heard on high-voltage installations created in our Union in recent years, and on a new, recently discovered phenomenon: the luminescence of pure liquids under the action of fast electrons.

In the first report, K. D. Sinelnikov (Kharkov) described in considerable detail the Van de Graaff electrostatic machine built at the Ukrainian Physico-Technical Institute. Although the principle of this installation has been known for quite a long time and similar installations are already operating in America, constructing a large Van de Graaff machine is a very difficult matter.

As is known, the voltage that can be obtained by means of such an installation is limited by discharge in air, which occurs when there are any sharp edges in the installation. In this connection the radius of the spheres on which the charge accumulates must be very large, and their surface must be carefully finished. The spheres of the installation of the Ukrainian Physico-Technical Institute have a diameter of 10 m; the height of the columns on which they are mounted is also 10 m. To prevent discharge along the walls, the vacuum tube in which the beam of accelerated particles is obtained must likewise be made very large. In the installation described, its length reaches 15 m. To obtain a powerful beam of particles in the tube, a high vacuum must be maintained. In the installation of the Ukrainian Physico-Technical Institute, the gas pressure in the tube was brought to $4 \cdot 10^{-6}$ mm Hg. With this tube a beam of electrons with a current of 60–70 $\mu$A and a voltage of up to $2.5 \cdot 10^{6}$ V has already been obtained. The voltage on the tube can be increased still further. With this installation the laboratory intends to proceed directly to work on the atomic nucleus.

K. D. Sinelnikov’s report was supplemented by a communication by V. N. Rukavishnikov on the work of the Radium Institute in constructing an insta-

CONGRESSES AND CONFERENCES

new meson. Here, too, great technical difficulties were overcome, and recently it has already proved possible to obtain beams of accelerated protons and helium ions.

At the same session of the conference, a report by I. M. Frank was heard on a new type of radiation discovered at the Physical Institute of the Academy of Sciences by P. A. Cherenkov. As P. A. Cherenkov established in a series of his works, the essence of the phenomenon is as follows: if a beam of γ-rays or fast electrons falls on a liquid, then this liquid begins to emit a weak light, visible to the naked eye. Using a technique developed by Academician S. I. Vavilov for photometry of extremely weak intensities of visible light (the so-called “quenching method”), it proved possible to ascertain a whole series of characteristic properties of this radiation. Above all, it turned out that all liquids, and even solid bodies, emit light irrespective of their nature (in solid bodies, however, it is much more difficult to observe this radiation than in liquids, since in most cases in solid bodies there is a fairly strong ordinary fluorescence, and this masks the radiation found by Cherenkov). Experiments showed that the brightness of the radiation is proportional to the path length of the electrons; the radiation is not quenched by ordinary quenchers of fluorescence (for example KI); its light is partially polarized, with the predominant direction of oscillation of the electric vector parallel to the direction of the beam of exciting γ-rays. Finally, it turned out that the light is emitted not isotropically, but predominantly in the direction of motion of the exciting electrons. By acting on the exciting electrons with a magnetic field, one can change the direction of the maximum of the radiation. All these properties show that here we are dealing not with some kind of ordinary luminescence, but with a new, extraordinarily interesting phenomenon.

I. M. Frank and I. E. Tamm succeeded in explaining this phenomenon, and not only qualitatively but also quantitatively. The basic idea of this explanation is as follows. If an electron moves uniformly in a medium, it produces an electromagnetic disturbance in this medium.

Disturbances from all points of the electron’s trajectory will arrive at a certain point in space. It can be shown that if the velocity of the electron is less than the velocity of light in the given medium, then the disturbances arriving at the given point will interfere in such a way that the resultant disturbance will be equal to 0. But if the electron moves with a velocity greater than the phase velocity of light, then complete cancellation of the disturbances will no longer occur. This means that, in its motion, the electron will radiate and, consequently, will experience additional braking. A phenomenon occurs analogous to the appearance of additional braking when an artillery shell moves with a velocity greater than the velocity of sound (“singing shell”). Developing these considerations, I. E. Tamm and I. M. Frank succeeded in constructing a mathematical theory that well explains the properties of the new radiation observed by P. A. Cherenkov.

The morning session of September 21 was devoted to the passage of γ-rays through matter, more precisely, to pair production. On this question reports were heard by A. I. Alikhanov, B. S. Dzhelepov, and I. M. Frank. In these reports a large amount of experimental material was summarized, obtained recently at the Leningrad Physico-Technical Institute and at the Physical Institute of the Academy of Sciences. It is valuable that the phenomenon of pair production was studied by different methods. In Alikhanov’s laboratory it was studied with the aid of a magnetic spectrograph, while I. M. Frank, together with L. V. Groshev, worked with a Wilson chamber. These reports showed above all that the theory of pair production developed chiefly by Bethe and Heitler on the basis of Dirac’s theory is in good-

quantitative agreement with experiment. The experiments confirmed not only the general conclusion of the theory concerning the energy relation:

\[ h\nu - 2mc^2 = W_{e^-} + W_{e^+}, \]

which expresses the law of conservation of energy for the formation of pairs, but also the conclusions of the theory concerning the magnitude of the effective cross section, its dependence on the atomic number of the nucleus near which this process occurs, the angular distribution of the components of the pair, the distribution of energy between the particles, etc.

A. I. Alikhanov showed in his report that the study of the phenomenon of pair formation makes it possible to investigate the γ-rays themselves that produce these pairs. Thus, by studying the spectrum of pairs formed in this way, he succeeded in finding new γ-lines in the active deposit of thorium. This method may prove especially convenient in the study of γ-rays emitted by elements with relatively small atomic number, since in this case the method of ordinary internal conversion is almost inapplicable because of the sharp decrease in the coefficient of internal conversion with decreasing atomic number.

Matters stand quite differently, as regards agreement with theory, in the questions that were considered in the reports of D. V. Skobeltsyn and E. G. Stepanova.

In these reports the anomalous phenomena discovered by D. V. Skobeltsyn and his collaborators, occurring during the absorption of electrons with energies of the order of several \(mc^2\), were examined in detail. The first anomaly is connected with the phenomenon of electron scattering. As is known, Mott considered this phenomenon theoretically from the point of view of wave mechanics and obtained a formula giving the effective cross section for the scattering of electrons by nuclei. Since wave mechanics should be fully applicable to phenomena in this energy region and, in particular, since its conclusions agree excellently with experiments on pair formation, it might have been expected that Mott’s formula for electron scattering would also be confirmed experimentally. However, it turned out that in reality this is not so. The number of cases of scattering through large angles in lead is many times greater than what can be expected on the basis of the theory. Moreover, a number of tracks of electrons were found which show that an electron can lose a large part of its energy in a single act. The electron track breaks off in the gas, ending in a dense dot or a characteristic curl corresponding to an electron that has lost almost all its energy. The possibility of such sudden losses of energy is not provided for at all by the modern theory, so that here we are already dealing with a qualitative divergence between experiment and wave mechanics. It is especially interesting that this divergence is observed precisely in the region in which wave mechanics should be fully applicable.

Williams’s report was closely connected with these reports. Williams presented the results of his own experiments, as well as some as-yet unpublished results of other authors, on anomalous phenomena occurring in the absorption of fast electrons. It also turned out that there is a fairly large number of cases of sharp losses of energy by electrons and of electron scattering through large angles. The results of D. V. Skobeltsyn’s experiments were thus fully confirmed. No theoretical explanation of these phenomena, however, could be found.

The discussion that followed these reports showed that, although there are discrepancies in the results obtained by different authors, it can nevertheless be considered that the modern theory does not in fact provide for any phenomenon giving these sharp losses of energy and scattering through large angles.

The most interesting day of the conference was the day devoted to cosmic rays (September 22 and partly September 23). The special interest in this day is explained by the fact that several months before the confer-

Neddermeyer and Anderson published an article in which, on the basis of their experiments, they came to the conclusion that in cosmic rays there is a new particle whose charge is equal to the electron charge, while its mass is greater than the electron mass by several tens of times and, consequently, much less than the proton mass. It is therefore quite natural that the conference devoted considerable attention to the question of the existence of these particles.

In a number of works of recent years, largely thanks to the work of P. Auger, it has become clear that cosmic rays may be divided into two components: a “hard” and a “soft” one. The “soft” component is characterized by an absorption coefficient of approximately \(30 \cdot 10^{-3}\frac{\mathrm{cm}^2}{\mathrm{g}}\), the “hard” one by \(1 \cdot 10^{-3}\frac{\mathrm{cm}^2}{\mathrm{g}}\). The study of the properties of the “soft” component, and in particular the determination of the position of the maximum of the Rossi curve (i.e. the curve giving the dependence of the number of showers on the thickness of the material from which they are knocked out) for showers consisting of different numbers of particles, speaks in favor of the cascade theory of showers. According to this theory, the mechanism of shower formation consists in the successive formation of pairs when a particle passes through matter. The mathematical development of this theory, carried out and reported by L. D. Landau and Yu. B. Rumer, makes possible a detailed comparison of it with experiment and gives hope that in the near future considerable progress will be made in understanding the phenomenon of showers. But even the material already available makes it possible to conclude that the “soft” component consists of electrons of both signs and photons arising when cosmic rays pass through the Earth’s atmosphere.

Fig. 1.

The situation with the “hard,” penetrating component is considerably less clear. The assumption that it too consists of electrons or positrons with very high energy encounters very serious difficulties. The energy of these particles can be determined directly with the aid of a Wilson chamber placed in a strong magnetic field. According to these experiments it is equal to \(10^8\) eV and higher. On the basis of the modern theory one can calculate the radiation losses for electrons with such an energy. It then turns out that the losses to radiation are so large that such electrons would have to possess a range many times smaller than what is observed experimentally. In Fig. 1 are shown theoretical curves depicting the dependence of the magnitude of the losses on the particle energy, and experimental points according to Blackett’s data. It follows from this that either the theory of radiative losses is fundamentally wrong, or the particles are not electrons. The supposition was made that these are protons. It is known that, according to the theory, radiation losses are inversely proportional to the square of the particle mass; therefore for protons there are essentially only ionization losses. However, this supposition encounters the following difficulty: in that case there would be many slow protons in cosmic rays, which could easily be detected by their large ionization. But, as this with especial

with great clarity from the report of V. I. Veksler; such a conclusion contradicts observations. There are also other objections to the hypothesis of the proton nature of this component.

The supposition that the contemporary theory is incorrect for these energy regions is unlikely. In the question of the scattering of particles with high energy, the theory is in good agreement with experiment, and it would be extremely strange if it gave a completely wrong value for the braking of particles. On the contrary, a whole series of difficulties in the field of cosmic rays finds a simple resolution on the basis of the assumption that the “hard” component of cosmic rays is formed by particles with the charge of the electron, but with a larger mass.

Neddermeyer and Anderson also adduce direct arguments in favor of the existence of such “semi-heavy” particles. They measured the relative loss of energy of particles as they passed through a platinum plate in a Wilson chamber. It turned out that the curve giving the distribution of particles according to the magnitude of the relative energy loss has two distinct maxima. One of them Neddermeyer and Anderson ascribe to electrons, the other to “semi-heavy” particles. As E. Williams reported in his paper, Blackett, in a recent and still unpublished work, did not obtain such a distinct separation of particles into two categories.

Nevertheless, there exist other arguments in favor of the existence of new particles. Thus Anderson and Williams obtained photographs of several particle tracks which, by their curvature, specific ionization, and range, cannot be assigned either to protons or to electrons. However, one still cannot be entirely certain that these tracks have not experienced any distortions due to some irregularities in the operation of the Wilson chamber. Therefore, despite a number of arguments in favor of the new particle, the question still cannot be regarded as finally solved. Reiter, who, not being able to come to the conference, was forced to confine himself to sending in a written report, arrives at similar conclusions on the basis of analogous reasoning.

In addition to the reports listed above on cosmic rays, the Conference heard three further communications: D. V. Skobeltsyn on the work of S. N. Vernov, who is now on an expedition; A. B. Verigo on the results of measuring the intensity of cosmic rays during the ascent of the USSR-1-bis stratospheric balloon; and V. I. Veksler on the measurement of ionization shocks by the coincidence method on Elbrus. In the first report D. V. Skobeltsyn reported on the successful use by S. N. Vernov of Prof. Molchanov’s radiosondes for the investigation of cosmic rays in the stratosphere. This method is of especially great importance when working at high and low latitudes, where it is almost impossible to find the instruments after the flight, and therefore the use of ordinary sondes is extremely difficult. This method had already been successfully applied by S. N. Vernov in the summer of last year in an expedition to Yerevan, where he obtained substantial data on the geomagnetic effect at these latitudes.

A. B. Verigo described measurements of the intensity of cosmic rays during his ascent in a stratospheric balloon. During this flight the measurements were made with the aid of five electrometers of various types, some of which were protected by lead shielding. It turned out that all the electrometers give fairly well concordant readings, and this made it possible to determine rather accurately the ionization produced by cosmic rays at the ceiling altitude of the stratospheric balloon. In addition, A. B. Verigo observed a noticeable decrease in ionization when the lead ballast located beneath the gondola of the stratospheric balloon and, consequently, beneath the electrometers was dropped. This very interesting observation apparently indicates that at great heights there is a fairly large number of particles traveling from below upward.

Finally, V. I. Veksler reported on the application to the study of cosmic rays of an original method developed by him. It consists in using, for counting particles, gas proportional amplifiers operating according to a coincidence scheme. This makes it possible to determine not only the number of particles that have passed through these counters, but also to measure the ionization they produce. V. I. Veksler worked with such an apparatus in the summer of this year during an expedition to Elbrus. It turned out that at an altitude of 4200 m above sea level there are readily absorbed and strongly ionizing particles. At sea level the number of such particles is considerably smaller than at the height of Elbrus. Their number is so small that these observations cannot be reconciled with the assumption that cosmic rays contain an intense proton component. In addition, V. I. Veksler obtained more direct indications of the secondary character of these particles. The existence of such particles in cosmic rays had been indicated earlier by a number of authors. But with such distinctness they were discovered for the first time. Thus these first experiments with proportional gas amplifiers already yielded very valuable results. And there can be no doubt that further application of this method will make it possible to achieve very substantial successes both in the study of heavy particles and in the investigation of showers.

On the question of β-decay, the conference heard an experimental report by A. I. Alikhanian and two theoretical ones: by R. Peierls and W. Pauli. The main experimental task in this field is to determine the exact form of the β-spectra of various elements, and in particular the upper limits of these spectra. A detailed report by A. I. Alikhanian was devoted to the great successes achieved in this direction by the collaborators of the Leningrad Physico-Technical Institute. On extensive material he showed that the forms of the β-spectra are in good agreement with Fermi’s theory, developed and supplemented in recent years by Uhlenbeck and Konopinski. Only at the maximum energies of the emitted electrons are deviations observed from the predictions of the theory made under the assumption that the neutrino mass is zero. Thus one might have thought that this circumstance indicates that the neutrino mass is finite.

However, the reports by Peierls and especially by Pauli showed that the modern theory of β-decay itself is still very far from perfection. In its mathematical formulation there is arbitrariness; and the further development of the theory leads to still greater difficulties, since divergences are obtained in it as well. Therefore the agreement between the experimental results and the consequences of the theory, about which A. I. Alikhanian reported, cannot yet be regarded as confirmation of the validity of this theory. This circumstance emphasizes still more the importance of the further accumulation of accurate experimental data on β-decay, necessary for the construction and verification of the correct theory.

On the question of neutrons, one should first of all note the major introductory report by I. V. Kurchatov. In this report the large amount of experimental material obtained recently on the absorption of slow neutrons was analyzed in considerable detail. I. V. Kurchatov succeeded in showing that these data are well accounted for within the framework of the theory developed very recently by N. Bohr. According to these ideas, the process of capture of a neutron by a nucleus should be regarded as a many-body problem. The selective absorption of neutrons having a definite velocity is explained by a resonance effect, observed in the case when the energy of the incident neutron coincides with one of the virtual levels of the complex nucleus. Therefore each nucleus has its own characteristic groups of especially readily absorbed neutrons. Further

These views were further developed in the works of Breit, Wigner, and Bethe–Placzek. The results obtained in these works for resonance-level widths are in good agreement with experimental data. It should be noted, however, that the experimental data themselves are not yet sufficiently reliable. This is explained chiefly by the imperfection of the methods themselves for determining the widths and positions of resonance levels.

To determine the positions of the resonance levels of nuclei, the absorption of neutrons of the corresponding group in boron is measured. In processing the results of these experiments one has to assume that the given nucleus has only one absorption band, whereas this assumption is not justified in a number of cases. It is possible, however, as V. Kurchatov pointed out, that a thorough study of the phenomenon of the scattering of slow neutrons will make it possible to overcome this difficulty.

The interaction of fast neutrons with nuclei has been studied even less than the absorption and scattering of slow neutrons. Nevertheless, it may be stated that the values of the effective cross sections for capture and inelastic scattering, and the general characteristics of the spectra of \(\gamma\)-radiation emitted by nuclei upon the capture of fast neutrons, are likewise in good agreement with the general propositions of Bohr’s new theory of the nucleus.

Thus, this theory gives a reliable theoretical interpretation of a whole series of experimental facts and may serve as a guiding thread for further work.

Two experimental reports from the laboratory of the Ukrainian Physico-Technical Institute dealt with questions of the slowing down of neutrons at low temperatures and the scattering and absorption of photoneutrons. The first report, delivered by F. Goutermans, was devoted to experiments carried out chiefly in order to test the law

\[ \frac{1}{v}, \]

which follows from theory for the dependence of neutron absorption on their velocity in boron and in certain other elements. Experiments in which the effective cross section for the absorption of slow neutrons was determined as a function of the temperature of the substance slowing down these neutrons in general confirmed the conclusions of the theory for boron and silver. In cadmium, however, there apparently exist some deviations from this law. In addition, it follows from these experiments that at room and higher temperatures the distribution of neutrons of group C corresponds to a Maxwellian one. At the lowest temperatures this correspondence is violated. This circumstance must be connected with the fact that the protons, in collisions with which the neutrons are slowed down, can no longer be regarded as free.

Theoretically, this latter question was considered by I. Pomeranchuk. But, unfortunately, the conclusions of his theory cannot be compared with the results of the existing experiments. With its further development, however, this theory will to a considerable extent help clarify the picture of neutron slowing down in the crystal lattice, including in those cases that are of direct interest to the experimenter.

The second report, delivered by a collaborator of the Ukrainian Physico-Technical Institute, L. Timoshuk, concerned the scattering and absorption of photoneutrons. As is known, when Be is split by \(\gamma\)-rays, and by the products of its decay, neutrons with energies in the interval \(100\text{—}250\ \mathrm{eV}\) are obtained. Neutrons with such intermediate energy have so far scarcely been studied. Therefore, a systematic study of their absorption in various elements was of interest. The experiments carried out showed that for these neutrons the principal role is played not by the absorption of neutrons, but by their scattering. The magnitude of the effective cross section for scattering on heavy nuclei changes smoothly with change of atomic number. In light elements, however, it changes by jumps. This show-

shows that light nuclei have resonance levels and, for these too, regions of energy.

The report by N. A. Dobrotin was devoted to the question of the angular distribution of particles in collisions of fast neutrons with protons. The great fundamental importance of this question is determined by the fact that the curve of the angular distribution of particles obtained in such experiments makes it possible to draw certain conclusions about the character of the forces acting between a neutron and a proton. A number of works on this question lead to a uniform distribution of particles in the coordinate system connected with the center of gravity of the neutron and proton, which indicates the presence of short-range forces between the neutron and proton. However, Harkins’s recent experiments and their collaborator contradict this. Unfortunately, in all the experiments carried out on the angular distribution of particles, the role of scattered neutrons was not taken into account. Nevertheless, analysis of these works, as well as Harkins’s work, shows that Harkins’s experiments apparently are not convincing, and the forces acting between the proton and neutron are, after all, short-range.

The last report on neutrons was given by N. N. Dmitriev. It was devoted to the formation of light artificial radioelements under bombardment by neutrons. It is known that in some cases neutron capture and the formation of radioactive nuclei are accompanied by the emission of $\alpha$-particles or protons. Until now this phenomenon had been studied very little, partly because the ranges of these particles are very small. N. N. Dmitriev used, for investigating this phenomenon, a Wilson chamber operating at reduced pressure. This enabled him to observe rather short ranges. The greatest effect proved to be in sulfur. In this case (per 1000 chamber expansions) about 25 protons were obtained. Unfortunately, Dmitriev’s observations were made visually, and this, of course, affected their accuracy.

The last session of the conference was devoted to reports by theoreticians on the forces of interaction between particles in the nucleus and on the theory of nuclear structure. A large survey report on these questions was given by I. E. Tamm. This report showed very clearly how little we still know about the nucleus. Our information about nuclear forces essentially comes down to the following, very general propositions: these forces cannot be reduced to ordinary Coulomb forces and have some special feature unknown to us. Their radius of action is equal to $10^{-13}$ cm; thus nuclear forces are very short-range. At these small distances their magnitude is very great. From the experimental fact that the effective cross section for the capture of a neutron by a proton increases sharply as the neutron velocity decreases, it follows, apparently, that these forces depend on the relative orientation of the spins of the particles. Until recently it was usually considered that in the nucleus only the forces between neutrons and protons play an essential role. The interaction between two identical particles, i.e. between two neutrons or two protons, was regarded as Coulomb forces, small in comparison with the interaction between two identical particles. But experiments published last year by Tuve, Heydenburg, and Hafstad on the scattering of protons by protons showed that this conclusion was wrong. It turned out that between two protons, and consequently probably also between two neutrons, forces of the same order act as between a proton and a neutron. This, in essence, is all that we know about these forces.

Unfortunately, we cannot find the law according to which nuclear forces depend on distance. The point is that the wavelength of those neutrons with which we can experiment is large compared with the radius of action of the forces. In order to probe in detail

...the course of the potential curve, one must work with neutrons having an energy of at least \(20\text{–}25 \cdot 10^6\) eV, and this is still beyond our capabilities. True, recently Heller and Schwinger have pointed out a certain indirect route. The wavelength of neutrons slowed down by collisions with protons at the temperature of liquid hydrogen is close to the distance between the nuclei in the hydrogen molecule. Therefore the pattern obtained in the interference of neutrons scattered by hydrogen molecules proves to be very sensitive to a number of properties of nuclear forces. One may therefore hope that a careful study of this scattering will somewhat broaden our information about nuclear forces. As for the very numerous attempts to calculate the structure of light nuclei on the basis of their binding energy, I. E. Tamm showed very thoroughly that their value is extremely doubtful.

In exactly the same way, now—especially after the appearance of Bohr’s fundamental work on the nucleus—the inapplicability to calculations of heavy nuclei of the approximate statistical methods of Fermi, Thomas, Hartree, and Fock has become clear.

However, it is precisely the properties of heavy nuclei that make it possible to draw some additional qualitative conclusions about the properties of nuclear forces. It is known that the binding energy of the individual particles in the nucleus, and the volume occupied by them, are almost independent of the atomic weight of heavy nuclei. This indicates that nuclear forces possess, like molecular forces, the property of saturation. In turn, this apparently speaks for the exchange character of nuclear forces.

Summing up, I. E. Tamm pointed out that the only conclusion about the nature of nuclear forces that we can draw at present is that these forces are forces of an entirely special kind and are not electromagnetic in character.

However, Acad. A. F. Ioffe, in his remarks on I. E. Tamm’s report, noted that even this assertion may be disputed. In particular, the circumstance that the neutron possesses a magnetic moment speaks rather convincingly in favor of the electrical nature of the neutron.

L. D. Landau, in his report on the statistical theory of nuclei, showed that, proceeding from Bohr’s ideas about the nucleus and treating the nucleus as a droplet of liquid, one can draw a number of very substantial conclusions. In particular, one succeeds in obtaining the distribution of levels in the nucleus and in showing that the ratio of the width of levels to the distance between them depends only on the speed of the neutrons. These conclusions are in agreement with the experimental facts. Finally, the broadening of levels observed for neutrons of high energy can be explained by inelastic scattering.

According to Bohr’s ideas, there can be no ready-made \(\alpha\)-particles in the nucleus, since the binding energy of the \(\alpha\)-particle is small in comparison with the energy of the particles in the nucleus. Therefore, to explain the phenomena of \(\alpha\)-decay one must assume that \(\alpha\)-particles are formed in the process of their emission. Thus the question of \(\alpha\)-decay must be revised to some extent. Ya. I. Frenkel’s report was devoted to such a revision, carried out from the statistical point of view.

P. P. Pavinsky reported on his calculations of the scattering of protons by protons, made under the assumption that the interaction energy between protons can be represented in the form of a rectangular potential well. Unfortunately, the validity of this assumption remains unclarified, and this limits the value of these calculations.

Finally, the last report at the Conference was Achieser’s report on joint work with P. L. Landau on the coherent scattering of \(\gamma\)-rays by nuclei. As the calculation shows, the dependence of the effective cross section for this process on the photon frequency has a maximum, which...

which lies in the region of the Compton wavelength. The maximum effective cross section reaches a value of the order of \(10^{-27}\) cm.

In summing up the results of the Conference, Academician A. F. Ioffe noted that it had given a clear picture of the broad development of work on the atomic nucleus that has taken place in recent years here in the USSR. True, our technical base in this field still lags behind the level of techniques used in work on the atomic nucleus abroad, and especially in America; but we have every reason to believe that, thanks to the tremendous assistance rendered by the Soviet government to all Soviet science, and to the physics of the atomic nucleus in particular, we shall in the near future be able to overcome this lag as well. There is no doubt that the results of the Conference will help to expand still further the work on the study of the atomic nucleus.