MEETINGS AND CONFERENCES

INTERNATIONAL CONFERENCE ON NUCLEAR REACTIONS

From July 2 to 7, 1956, an International Conference on Nuclear Reactions, organized by the Netherlands Physical Society, was held in Amsterdam (Netherlands). The conference was attended by more than 500 delegates from 27 countries, among them 28 scientists from the USSR.

The work of the conference opened with an introductory report by Bethe, who summarized the development of the physics of nuclear reactions over recent years. Especially important, in Bethe’s opinion, was the development of the quasi-optical theory of the interaction of neutrons with nuclei (the work of Weisskopf and others). The validity of this theory over a wide energy range is confirmed by the good agreement of calculated values of total and differential cross sections, as well as absorption cross sections, with experimental data. In the last two years the quasi-optical model has received, thanks to the work of Bruckner and others, a good theoretical foundation. In particular, this applies to the method for calculating the imaginary part of the neutron–nucleus interaction potential used by Lane and Wendell. Essential for understanding many nuclear processes was the development of the theory of surface phenomena (stripping) and the theory of collective states of nuclei (O. Bohr and Mottelson). It is interesting that many conclusions of the collective theory coincide with the results of the shell model of the nucleus. The appearance of all these “innovations” in the theory of nuclear reactions has led to less attention being paid to the statistical theory of nuclear reactions. It should be noted, however, that the statistical theory is still the only theory that qualitatively explains almost all nuclear reactions in the region from 1 to 50 MeV, and it need only be modified somewhat. Many valuable results have been obtained in recent years in the field of neutron physics. The development of neutron-selector techniques has made it possible to investigate in detail the density of nuclear levels, to find regularities in the distribution of levels by their widths, and so on. In problems of scattering of high-energy particles, primary attention is being paid to polarization phenomena, since only in this way can sufficiently complete information be obtained about the interaction of fast nucleons with nuclei. All these questions, touched upon in Bethe’s introductory report, were examined in detail at the individual sessions of the conference, each of which consisted of a survey report (25–45 min.) and several short presentations (5–10 min.).

On the first day, reports by Christy, Gugelot, and Hughes were heard. Christy reported on experimental and theoretical work on the scattering of low-energy protons by light nuclei, including Be^9, N^14, N^15, and others. Various methods of analyzing experimental data make it possible

to establish the properties of the lower excited states of nuclei. Scattering on narrow and broad resonances was investigated, and polarization in resonance scattering was also considered.

Gugelot gave a report on the scattering of medium-energy protons by transparent nuclei. An analysis of data on elastic proton scattering makes it possible to conclude that the potential interaction is small; at these energies it varies within the limits from 2 to 10 MeV. The mean free path of a proton in nuclear matter turns out to be of the order of the dimensions of the nucleus. In this connection the usual picture of the formation of a compound nucleus must be somewhat modified; namely, one must take into account not only the interaction on the nuclear surface but also the transparency of the nucleus. If the final state of the nucleus is quite unlike its initial state, as, for example, in the reaction \((p,\alpha)\), then one should expect a strong difference between the spectrum of \(\alpha\)-particles and the spectrum of inelastically scattered protons in the reaction \((pp')\). Indeed, experiments show that the emission of fast protons proves in many cases to be more probable than the emission of fast \(\alpha\)-particles. In the case of emission of low-energy protons or \(\alpha\)-particles, the emission probabilities turn out to be approximately the same.

Gus, in his report, gave a review of experiments on Coulomb excitation of nuclei. Theoretical data on angular distributions, transition probabilities, and excitation functions were presented and compared with experiment. The experimental data are interpreted from the point of view of the collective model of the nucleus.

The report by V. F. Weisskopf on the topic “Formation of the Compound Nucleus” was heard with interest. The problem of the compound nucleus, as is known, consists of two parts: the formation and the decay of the compound system. The speaker set forth the known material that makes it possible, from the point of view of the optical model of the nucleus, to explain the principal features of the first stage of the process—the formation of the compound nucleus. The report dealt, in particular, with the works of Soviet scientists (P. E. Nemirovskii), in which the process of formation of a compound nucleus in neutron scattering was studied with allowance for the properties of the diffuse surface of the nucleus. The speaker also considered the dependence of the formation of the compound nucleus and of the parameters of the optical model on the energy of the incident particle.

The second part of the problem—the decay of the compound nucleus—is much more complicated. Bohr’s hypothesis of the independence of the mode of decay of the compound nucleus from the mode of its formation was investigated. An analysis of this problem in the light of new data showed that the conditions for independence of decay from the mode of formation are fulfilled very rarely. Deviations from Bohr’s hypothesis are caused not only by processes of direct interaction of the incident particle with the emitted nucleon, but also by the lack of thermal equilibrium in the compound nucleus. In conclusion, the speaker stated that the modern theory gives a description of the processes of elastic scattering and of all inelastic-scattering processes in terms of the optical model, but cannot give a detailed picture of individual inelastic-scattering processes.

O. Bohr gave a report on the excitation of collective states of the nucleus. For the study of collective excited states of the nucleus, inelastic scattering is especially convenient, since it is accompanied by direct interaction without formation of a compound nucleus. At present the most complete quantitative information on collective states has been obtained in the study of Coulomb-excitation processes. The speaker gave a review of data concerning the lower excited levels of the nucleus. For even-even nuclei these levels can usually be ascribed to collective motions of the nucleons in the nucleus. In the case of nuclei with an odd number of nucleons, the degrees of freedom corresponding to the lower states of the nucleus also include the degrees of freedom of the last nucleon. The character of collective

... of motion in the nucleus depends essentially on the shell structure of the nucleus. For nuclei with a relatively small number of nucleons in unfilled shells, the collective motion is an oscillation about a spherical shape. If the number of nucleons in unfilled shells is sufficiently large, then the nucleus has the equilibrium shape of an ellipsoid. In this case the collective motion is a rotation and an oscillation about the equilibrium shape.

In the discussion of the report, A. S. Davydov communicated the results of work carried out jointly with G. F. Filippov on the moment of inertia of a system of interacting particles. V. M. Strutinskii spoke about excitation in the interaction of $\alpha$-particles with $\alpha$-decay of even-even nuclei. L. A. Sliv made a communication on the $\alpha$-decay of nuclei from the point of view of the shell model. The report by R. A. Vanetsian on experiments on elastic scattering of fast protons by light nuclei was listened to with interest, as was the communication by L. A. Sliv on the work of D. P. Alkhazov, D. S. Andreev, A. P. Grinberg, and I. N. Lemberg, in which experimental data on Coulomb excitation of nuclei by nitrogen ions are presented.

In his report “Neutrons and the Structure of the Nucleus,” Hughes set forth experimental material concerning resonances in the effective cross sections for the interaction of neutrons with nuclei. The parameters of these resonances provide significant information on the structure of the nucleus. Experimental data were considered on the following questions: 1) the radiation width of nuclear energy levels; 2) the distribution of the neutron width over levels; 3) the average spacing between levels for various nuclei, as well as the spacing between levels for individual nuclei; 4) the probability of formation of a compound nucleus; 5) the radius of the nucleus obtained from experiments with fast and slow neutrons.

On questions of nuclear reactions induced by neutrons, among other works by Soviet scientists, the report by I. I. Gurevich and M. I. Pevzner on regularities in the distribution of nuclear levels by energies was read, as well as the report by I. A. Radkevich, V. V. Vladimirskii, and V. V. Sokolovskii on measurements of total effective cross sections for the scattering of resonance neutrons for a number of elements.

Fremlin delivered a report on the topic “Disintegration of the Nucleus and Reactions Induced by Heavy Ions.” The disintegration of medium and heavy nuclei induced by deuterons and protons with an energy of about 100 MeV is accompanied by the emission of a comparatively small number of secondary charged particles, and for the heaviest nuclei fission becomes a competing process. The character of reactions induced by heavy ions changes noticeably from light nuclei to heavy ones. For light elements the probability of formation of a compound nucleus is comparatively small. In the case of medium nuclei the process of formation of a compound nucleus, accompanied by evaporation of particles, plays the principal role, while with increasing atomic weight the process of neutron evaporation becomes increasingly predominant.

Wheeler’s report covered the latest experimental and theoretical investigations on the question of nuclear fission, including: 1) predominance of symmetric fission for bismuth; 2) measurement of the angular asymmetry of fission as a function of the energy of the incident neutron or $\gamma$-quantum; 3) energy yield in symmetric and asymmetric fission; 4) nonmonotonic dependence of the effective fission cross sections in the neutron-energy region of the order of 1 MeV.

On questions of nuclear fission, among other works by Soviet scientists, the reports by L. E. Lazareva on the angular distribution of fission fragments in photofission of uranium and by V. G. Nosov on the energy dependence of the fission width were also presented.

Much attention at the conference was devoted to photonuclear reactions. Two survey reports were devoted to this topic (Endt, “Capture Reactions,” and Wilkinson, “Photodisintegration”) and many short...

reports on the experiments carried out. In his report Endt noted the exceptional importance of capture reactions accompanied by the subsequent emission of $\gamma$-quanta for the detection of nuclear levels and their classification by momenta and parities. In particular, precisely the data on capture reactions on Al$^{25}$ and Mg$^{25}$ (the predominating role of E2 transitions as compared with M1) led to the supposition that these nuclei (and possibly others in this mass region as well) possess a sufficiently well-pronounced rotational structure. Wilkinson’s report was devoted to the analysis of experimental data on photodisintegration and to the discussion of the various mechanisms of this process. In particular, it was noted that calculations based on the shell and collective models are valid only in the region of comparatively small energies, since for large $\gamma$-quantum energies everything is determined by the detailed character of the wave functions of the nucleons in the nucleus. An analysis of radiation widths at low energies shows that for light nuclei the probabilities of dipole transitions are in good agreement with the prediction of the independent-particle model, although the widths of many transitions of higher multipolarity turn out to be unexpectedly large—as do the E2 widths—of transitions in heavy nuclei. Of great interest was that part of Wilkinson’s report in which he spoke about the mechanism of formation of the “giant resonance” in the photodisintegration cross section. It was shown that the “giant resonance” follows quite naturally from the nuclear shell model, according to which the process of interaction of a $\gamma$-quantum with a nucleus must be regarded as a process of direct interaction of the incident electromagnetic wave with one of the nucleons of the nucleus. At the same time it turns out that both the independent-particle model and the statistical model give very similar results, so that observation of a “Maxwellian distribution” of the disintegration products is not a sufficient criterion for choosing between these two models. Many reports by Soviet scientists were devoted to photonuclear reactions: O. V. Bogdankevich, L. E. Lazareva, and F. A. Nikolaev, “Inelastic Scattering of 115 MeV Photons,” B. I. Gavrilov and L. E. Lazareva, “Emission of Photoneutrons from Medium and Heavy Elements,” I. S. Shapiro, “Photonuclear Reactions and Scattering of Nucleons by Light Nuclei,” and others.

In a special report Bethe set forth the basic ideas of the work of Brueckner and others, in which an approximate solution is given to the many-body problem. The essence of the method consists in reducing the problem of a system of strongly interacting fermions to the problem of a self-consistent field in which the nucleons move, interacting with one another according to a law that depends on the magnitude and character of this common self-consistent field. Thus, the problem of the motion of nucleons in the nucleus is reduced: a) to the choice of a concrete form of the self-consistent potential, b) to the solution of the problem of the interaction of two nucleons in this potential, c) to the calculation, from what is obtained in point b), of the self-consistent potential of interaction between two nucleons, and d) to the variation of the potential chosen in point a) in such a way that it coincides with that calculated in point c). The first calculations of this kind were carried out in Brueckner’s work, and now a group of physicists in England headed by Bethe has joined in this work. It is significant that, despite the great mathematical difficulties of this method, important results have already been obtained which, to a considerable extent, substantiate the shell model of the nucleus and the quasi-optical theory of nuclear reactions of Weisskopf.

A special day was devoted to the discussion of stripping reactions and direct interaction. In addition to a large number of short communications, most of which reported on experiments performed, two large review reports were heard (Holt, “Experimental Data on Pick-up Reactions and Stripping Reactions,” and the theoretical report of Horowitz). Holt noted that significant progress in the technique

experiment, and the enormous quantity of experimental data now obtained on stripping and “pick-up” reactions is still not being used fully because of the imperfection of the theory. Nevertheless, analysis of angular distributions and total cross sections has in recent years yielded very valuable information on the moments and structure of many nuclear states. Much additional information can be obtained from experiments on angular correlations in \((d, p)\) and from measurements of the polarization of protons. In the discussion on stripping reactions, reports were heard by A. I. Akhiezer and A. G. Sitenko on diffraction breakup and the scattering of fast deuterons by nuclei; a report by G. F. Bogdanov, L. A. Vlasov, and others on the spectrum of neutrons in the bombardment of light nuclei by deuterons with an energy of 14 MeV; a report by I. A. Nemilov, K. P. Zherebtsov, and B. L. Vainshtein on the relation between the probabilities of stripping reactions and reactions with the formation of a compound nucleus; and other reports by Soviet scientists.

The scattering of high-energy particles was the subject of review reports by Segre (“Scattering and polarization at high energies”) and Van Hove (“Quasioptical model of elastic scattering of nucleons of high energies on nuclei”) and of several brief communications presenting experimental results on the scattering and polarization of protons with energies of 50–300 MeV on various nuclei. Segre’s report was mainly devoted to an analysis of \(pp\)- and \(np\)-scattering at 300 MeV. In this field there is now abundant experimental material (studied single, double, and triple scattering), which, however, has proved insufficient for an exact determination of the phases in \(pp\)-scattering. It turns out that there are four sets of phases satisfying, within the limits of experimental accuracy, all the measured quantities (differential cross section, polarization, depolarization, etc.), and the further task in this direction is to choose from these four sets the true set of phases. In the discussion Ya. A. Smorodinsky noted that such a choice can be made if the correlation of polarizations in \(pp\)-scattering is measured. In his report Van Hove presented data showing that the data on elastic scattering and polarization of high-energy protons on nuclei are reasonably well described by the model of a semitransparent nucleus with spin-orbit coupling taken into account.

During the conference a visit was organized to the largest physical laboratories and institutes of the Netherlands: the Kamerlingh Onnes Laboratory and the Institute for Theoretical Physics in Leiden, the laboratories of the University of Amsterdam, and others.

On the whole, the conference was well organized, was held in a friendly and businesslike atmosphere, and played an important role in establishing contacts among scientists of different countries.

A. B. and S. D.