LENINGRAD PHYSICOTECHNICAL INSTITUTE OF THE ACADEMY OF SCIENCES OF THE USSR

B. M. Gokhberg, Leningrad

The Leningrad Physicotechnical Institute is one of the first scientific institutes created by the Soviet government; it was organized already during the very first year of the October Revolution.

The organizer of the LPhTI, its permanent director and scientific leader, is Academician Abram Fedorovich Ioffe, whose sixtieth birthday and thirty-fifth anniversary of public and scientific activity are marked in this issue of the journal. Broad tasks were set before the new institute, both in the field of profound physical research and in the search for ways to establish a close connection between physics and technology.

There is no possibility, in a brief article, of dwelling on the history of the development of the Physicotechnical Institute. The subject matter of the institute continually expanded, embracing new areas of science and technology; a number of laboratories and groups grew into independent, already more specialized, physical and physicotechnical institutes.

The history of the development of the LPhTI is, to a considerable degree, the history of the development of Soviet physics. The purpose of the present survey is to give a description of the institute and of the principal directions of its work only in the most recent period.

Three problems encompass the entire subject matter of the Institute and determine its subdivision into three main groups: I. The group of atomic-nucleus physics; II. The group of electrophysics; and III. The group of molecular physics.

I. GROUP OF ATOMIC-NUCLEUS PHYSICS

At the present time the atomic nucleus is the extreme limit of our penetration into the depths of matter. The astonishing discoveries made in this field attract to this problem the great attention of physicists throughout the world. Physicists of the Leningrad Physicotechnical Institute have achieved significant successes in this field.

1. Nuclear isomerism. In 1935, I. V. Kurchatov, L. I. Rusinov, and others studied the transformations of artificially radioactive bromine, which served as the basis for establishing that there exist nuclei with identical charge and identical

mass, which, however, decay at different rates and, consequently, differ from one another in some way. By analogy with chemical compounds these nuclei were called isomeric. I. V. Kurchatov and his collaborators showed that the difference between the nuclei of isomers consists in their different energy state, when one of the isomers possesses greater energy than the other. The transition of an isomeric metastable nucleus to the ground state (a new kind of nuclear transformation—isomeric transformation) is accompanied by the emission of γ-rays, which, owing to internal conversion, give soft electron radiation. This phenomenon was studied in detail on bromine. The phenomenon of nuclear isomerism is of great interest for nuclear physics.

2. Uranium fission. The investigation of the interaction of neutrons with the nuclei of uranium and thorium is at present attracting much attention, since in this case the question is not only of interesting physical phenomena, but also of the possibility of the practical use of nuclear energy. In the laboratory of I. V. Kurchatov, the fission of uranium under the action of slow neutrons is being investigated, and the secondary neutrons arising in this fission are being studied. The investigations are aimed at clarifying the possibility of the formation of a chain reaction of uranium fission, which may in the future lead to the utilization of nuclear energy.

In the fission of uranium its nuclei break up into two approximately equal fragments, forming nuclei of the corresponding elements. A young collaborator of the institute, G. N. Flerov, together with a postgraduate student of the Radium Institute, K. A. Petrzhak, discovered a new remarkable phenomenon. It turned out that, in addition to the decay of uranium under the action of neutron bombardment, spontaneous decay is observed (also with the formation of approximately equal fragments, forming nuclei of new elements). The study of this phenomenon is of enormous importance for the theory of the stability of heavy atomic nuclei.

3. The problem of β-decay. The work of Alikhanov’s laboratory on the investigation of the β-spectra of radioactive elements has won worldwide recognition. Detailed studies of the spectra of electrons and positrons obtained in Alikhanov’s laboratory are a valuable contribution to the field of nuclear physics. From these investigations it follows that the neutrino (a particle hypothetically introduced by Pauli) must possess a finite definite mass. Experiments now being carried out on the precise measurement of electron decay and of the energy of recoil atoms arising both in β-decay and in the capture by the nucleus of orbital electrons make it possible to hope for a direct proof of the existence of the neutrino and for the determination of its properties.

4. Properties of fast electrons. With the aid of a magnetic spectrograph, in the laboratory of L. A. Artsimovich, absorption and scattering of fast electrons emitted by radioactive elements were investigated. The results obtained by Artsimovich established good agreement between experiment and theoretical conceptions—

Leningrad Physico-Technical Institute

the phenomena, whereas previously the conclusions of the theory had seemed highly controversial.

1. Cyclotron. The entire nuclear group of the LFTI sets as its central task the construction of a powerful cyclotron for obtaining particles with energies on the order of 10 MeV. This task is of first priority and the most important one, since the development of Soviet nuclear physics depends to the highest degree on the creation of a new technical base. By decree of the Council of People’s Commissars of the USSR, construction of the cyclotron at the LFTI is to be completed in 1940. The general study of nuclear reactions is of great practical interest; already now artificial radioelements have found wide application as indicators in chemistry, biology, and medicine. All these investigations have become possible on the basis of powerful technology for obtaining beams of charged particles of high energy. The most advanced means of obtaining artificial fast particles is the cyclotron.

The first cyclotron in the Soviet Union was built at the Radium Institute of the Academy of Sciences of the USSR. The cyclotron of the Leningrad Physico-Technical Institute will be considerably larger and more advanced than the cyclotron of the Radium Institute. It will make it possible to obtain beams of charged particles with energies up to 10 MeV; its magnet weighs 75 t, the high-frequency generator has a power of 100 kW, and the neutrons it produces will make the biologically dangerous radiation zone 50 m, which requires the installation of special shielding.

1. Electron accelerator. Simultaneously with the construction of the cyclotron, the nuclear group is carrying out the development of a new type of installation for obtaining fast electrons with energies on the order of tens of millions of electron-volts. The design of this installation—the electron accelerator (or quadrutron)—is based on the use of high-frequency voltage and multiple acceleration of electrons. In the electron accelerator the stream of electrons is repeatedly accelerated and its energy is brought up to millions of electron-volts, just as heavy particles—protons and nuclei of other elements—are accelerated in the cyclotron.

A new and original design of the electron accelerator was developed by Ya. L. Khurgin and G. Ya. Shchepkin. In 1940 they are to build and test a model of the accelerator, which should produce electrons with energies up to one million volts.

The creation of a powerful technical base, and first and foremost the creation of the cyclotron, will enable the nuclear group of the LFTI not only to pose the problem of studying the atomic nucleus more broadly and more deeply, but also to address questions of diverse applications of nuclear physics in chemistry, biology, and medicine.

II. GROUP OF ELECTROPHYSICS

The work of the electrophysics group of the Leningrad Physico-Technical Institute is devoted to: 1) the study of semiconductors and their technical application, and 2) questions of constant high voltages.

A. Semiconductors

As recently as ten years ago, electrical engineering used only metals that conducted current well, or insulators that did not conduct it at all. A vast class of materials with intermediate properties—semiconductors—remained completely forgotten and unused.

In the last decade semiconductors have been finding ever-increasing application in technology. In physics, interest in semiconductors is rapidly growing. It may be thought that the path to understanding the electrical properties of metals and insulators lies through the study of the intermediate region—the region of semiconductors. The diversity of semiconductors and of their properties is extremely great. In electrical respects they cover an interval of variation in electrical conductivity of 20 orders of magnitude.

In the work plan of the LPTI, the problem of semiconductors occupies a major place. Work in this field is directed immediately by A. F. Ioffe. Investigations in recent years have shown that even the still far from complete theoretical conceptions available make it possible to find new ways of solving practical problems and of making better use of semiconductors in technology. The principal directions of semiconductor work at the LPTI are the following:

1. Mechanism of electrical conductivity. One of the basic characteristics of a semiconductor is electrical conductivity and its dependence on temperature. The study of galvanomagnetic and thermoelectric phenomena makes it possible to determine the nature of the current carriers and the predominant mechanism of electrical conductivity (electron and “hole” conductivity). These investigations have been carried out for a large number of diverse semiconductors. The questions: 1) of the influence of impurities, whose content can radically change the properties of semiconductors, up to a change in the mechanism of electrical conductivity, 2) of the study of the energy levels of electrons in the crystal lattice and of the additional levels introduced by impurities and by heat treatment (the spectral distribution of the internal photoeffect—the absorption spectrum), 3) of the study of diffusion phenomena within the material and at its boundary surfaces—constitute another group of investigations devoted to elucidating the mechanism of electrical conductivity of semiconductors.

2. Contact phenomena. In studying transition layers semiconductor–metal, A. V. Ioffe established that the contact of a metal with a semiconductor possessing an electron mechanism of conductivity leads to the formation of a semiconductor layer of increased resistance in the case where the contact potential of the metal is higher than the contact potential of the semiconductor. If the semiconductor has a “hole” mechanism of conductivity, then a layer of increased resistance is formed upon contact with a metal whose contact potential is lower than that of the semiconductor. This phenomenon has great fundamental significance for understanding the insulating properties of dielectrics and the difficulty of entry of electrons from metallic electrodes into a dielectric.

Theories of contact phenomena and the rectifying effect are the subject of works by B. I. Davydov.

1. Solid photoelements. By the time the Institute began work on this problem, research and production of solid selenium photoelements, which had found wide practical application, were being carried out in many places. On the basis of extensive experimental and theoretical study of semiconductors, it was possible to hope to find other solid photoelements with higher characteristics. At the suggestion of A. F. Ioffe, members of the Institute—Yu. P. Maslakovets and B. T. Kolomiets—investigated the possibility of producing various photoelements. Especially interesting results were obtained with sulfur-thallium photoelements. It turned out that good results were obtained only by those photoelements made of thallium sulfide in which the metal is charged positively. Yu. P. Maslakovets found that light, tearing an electron out in the semiconductor, frees a place for another electron passing from the adjacent metal. This new “positive” photoeffect finds its explanation in the quantum theory of semiconductors.

Sulfur-thallium photoelements with the positive photoeffect possess a broad region of spectral sensitivity (from 0.4 to 1.3 μ), with the maximum sensitivity lying at 1.0 μ. Their integral sensitivity exceeds the sensitivity of selenium photoelements by a factor of 10–20.

B. T. Kolomiets, together with the Lenkinap plant, implemented sound cinema using sulfur-thallium photoelements. This installation has already been working faultlessly for more than a year in one of the cinemas of Leningrad and possesses a number of important advantages (absence of extraneous noises). In a short time sulfur-thallium photoelements are beginning to find application in various kinds of automatic devices.

1. Solid rectifiers. In 1939, at the LFTI, in the laboratory of P. V. Sharavsky, a technological process was developed for the production of powerful rectifiers from copper oxide for hundreds and thousands of amperes. In setting before industry the task of mass production of copper-oxide rectifiers, it was necessary to study the influence of the quality of the copper (of various impurities) on the properties of the rectifiers. This work is being carried out on a broad scale by P. V. Sharavsky.

In parallel with copper-oxide rectifiers, much attention is being devoted to improving the quality of selenium rectifiers and to developing new types of them.

The work of B. V. Kurchatov led to new rectifiers of the copper sulfide—magnesium system. This rectifier makes it possible to rectify currents up to 100 A with an operating area of only about 4 cm². As a result of prolonged and persistent work, B. V. Kurchatov’s laboratory succeeded in eliminating one of the main shortcomings—aging; technical models of rectifiers of this type already exist. At present the task of successive—

of connecting individual elements (which previously presented a number of difficulties).

In comparison with cuprous-oxide rectifiers, the current density in rectifiers made of sulfurous copper is increased by approximately 200 times. The extremely small size and great mechanical strength promise this rectifier a wide range of practical applications. It should be noted that during the past year analogous rectifiers have also appeared in the USA; however, they are 10 times worse than the rectifiers of B. V. Kurchatov.

1. Thermoelements. Semiconductors possess thermoelectromotive forces several times greater than the thermoelectric emf of semiconductors. Thermoelements already exist which, when heated, yield 25 times more electrical energy than analogous thermoelements in certain experiments abroad. The study of the thermoelectric and thermal properties of semiconductors opens up the possibility of creating much more sensitive receivers of radiant energy, reducing their inertia, and also producing powerful thermoelements.

As we see, alongside serious theoretical achievements, the semiconductor group has a number of valuable practical results, partly already introduced, and partly being introduced into our industry.

B. Constant High Voltages

1. Electrostatic generators. From the point of view of nuclear physics and high-voltage engineering, high-voltage electrostatic generators are acquiring great interest. High-voltage generators of the Van de Graaff type are striking in their gigantic dimensions. A. F. Ioffe proposed a new type of electrostatic generator which can produce not only high voltages but also considerable currents, while occupying a comparatively small volume. A. F. Ioffe, together with B. M. Gokhberg, developed and built the corresponding models, which fully confirmed the calculated data. It may be expected that these generators will find application not only in physical laboratories but also for high-voltage tests, for X-ray technology, and for creating powerful electron beams.

For better use of insulation under constant high voltage, studies were carried out on the forced distribution of potential over the surface of the insulation by means of semiconducting layers. The studies gave positive results, and semiconducting layers are already beginning to be used in generators of this type.

1. Electric strength of gases. In connection with work on electrostatic generators, the laboratory of B. M. Gokhberg is conducting research on the electric strength of various gases. It was known that some gases possess a strength two to three times greater than the strength of air. The majority of these

gases, because of unfavorable other physical and chemical properties, could not find significant practical application. At present, the gas “elegas” has been investigated at LPTI; it has increased strength (twice that of air) and possesses physical and chemical properties favorable for its use in high-voltage engineering, and first of all in the cable industry. These works are being carried out by LPTI jointly with the Sevkabel plant and the State Institute of Applied Chemistry in Leningrad. Together with the Sevkabel plant, tests have been conducted on sections of cable filled with elegas, which gave positive results. By decision of the Council of People’s Commissars of the USSR, a pilot semi-industrial installation for the production of elegas is being built, and cables filled with it will be tested.

Alongside questions of the practical investigation of elegas, the laboratory has as one of its tasks the study of the reasons for the increased electrical strength of certain gases and the comparison of electrical-strength data with the molecular constants of gases.

III. GROUP OF MOLECULAR PHYSICS

The work of the molecular-physics group of LPTI has recently been devoted to the study of the mechanical properties of both amorphous and crystalline bodies. For the first group of bodies, the main attention has been directed toward the study of polymeric substances used as plastics and rubbers. For the second group of bodies, the basic question is the investigation of mechanical properties and the testing of metals.

A. Amorphous Bodies

Studies of previous years, carried out in the laboratory of P. P. Kobeko, showed an analogous course of change in the electrical and mechanical properties of simple amorphous bodies. These studies made it possible, to a considerable extent, to form clearer ideas about the structure of amorphous bodies. The chemistry of polymeric substances, i.e., substances consisting of large aggregates of molecules and used as plastics and rubbers, has developed considerably in recent years and at present makes it possible to obtain polymeric substances of the most diverse chemical nature. However, the physics of polymeric substances is at an initial stage of development; the question of the connection between the physical properties of polymers and their chemical constitution and structure has as yet been little clarified. Therefore, the technology of polymeric materials does not have a guiding theory that would make it possible to find a way to control the properties of these bodies. Recently, the central problem of LPTI work on the amorphous body has been the study of polymers.

1. General Regularities of the Properties of Polymers

The studies carried out in the laboratory by P. P. Kobeko and A. P. Aleksandrov showed that the regularities determining the properties of polymeric materials are very similar to the regularities occurring in simple amorphous bodies, not complicated by high-

molecular structure. At the same time, the conditions of bonding of individual molecules into a complex aggregate, and of these aggregates with one another, determine all the properties of polymers. The study of copolymerization and the preparation of samples of copolymers have yielded a number of interesting and important results in the direction of obtaining polymers with improved qualities. The heat resistance of a number of polymers has been investigated, and methods of increasing it are being clarified.

1. Frost resistance of rubber. On the basis of the results of their research on polymers, P. P. Kobeko and A. P. Aleksandrov, working together with the Lebedev plant, achieved an increase in the frost resistance of rubber made from synthetic caoutchouc. This is a very important practical result.

2. Plasticization of hard caoutchoucs. An interesting study was carried out by a member of the Institute, S. N. Zhurkov; he succeeded in finding a method for plasticizing hard sodium-divinyl caoutchoucs. The kinetics of the mechanism of oxidative processes leading to degradation, which accounts for the plasticization of caoutchouc while at the same time not diminishing its mechanical strength, has been traced. Work is being conducted jointly with the plant on developing the technology for mass production of such rubbers. The experiments are giving positive results.

3. Principles of tire design. M. O. Kornfeld developed a method for testing rubber for fatigue; the corresponding instruments have already been installed at several plants and in institutes of the rubber industry. As a result of a review of the literature on tires over the last twenty years and of the laboratory’s own investigations, M. O. Kornfeld obtained data that help improve methods for calculating tires.

In addition to studies of polymers, the group of amorphous bodies is continuing investigations of simple amorphous bodies and, in particular, studies of elastic properties over the range from the liquid to the solid state.

Interesting results may be expected from the study of the properties of amorphous bodies under high pressure. In E. V. Kuvshinskii’s laboratory, a methodology for these tests at pressures up to 10,000 atm. is being developed and mastered.

The work on the study of amorphous bodies carried out at the LPTI is characterized by the fact that it has been performed with the aid of new methods developed by this group. This has made it possible to construct new designs of instruments, which are being successfully used at a number of industrial enterprises.

B. Strength and plasticity of metals

Metal is the principal structural material in technology, and therefore the study of its mechanical properties is of great interest. At present, among these properties the LPTI is focusing its attention on the fundamental questions of strength and plasticity that have great industrial significance; these are questions of the cold brittleness of steel, i.e., the especially dangerous destruction of it without plastic deformation, under a single action.

loads—impact—and problems of the plastic deformation of nonferrous metals at high temperatures, at which the technical processing of metals is usually carried out.

These works are a direct development of A. F. Ioffe’s work on the study of the strength and plastic properties of crystals.

1. Brittleness. Questions of brittleness have been studied in N. N. Davidenkov’s laboratory for many years. Whereas abroad they have limited themselves to establishing the fact of the existence of a critical temperature of brittleness, the Leningrad Physico-Technical Institute has undertaken a detailed analysis of the process and the elucidation of the laws of brittle fracture under various conditions.

A staff member of the Institute, F. F. Vitman, established a connection between the critical temperature of brittleness and the velocity of impact. This work is especially important, since recently erroneous notions have appeared abroad, based on experiments conducted there incorrectly. The work is continuing on a specially built pendulum impact-testing machine, with a particularly high impact velocity (up to 100 m/sec). The established connection will be checked for still higher deformation rates on a large number of materials.

F. F. Vitman and E. M. Shevandina are studying the influence of the scale factor on impact strength. This question is of enormous importance in testing materials and is of great interest to factory designers. The causes of the decrease in impact strength with increasing dimensions are being investigated; apparently, they are connected with an increase in the probability of the presence of defective cracks.

At the same time E. M. Shevandina is investigating the influence of the geometry (shape) and the stressed state of specimens on impact strength. This work supplements the study of the influence of the scale factor.

1. Plasticity of metals. Modern technological methods (for example, continuous rolling) are closely connected with questions of plastic deformation at high temperatures. Therefore, in developing its earlier work on plastic deformation, the LPTI sets as its task the study of the laws of plastic deformation and fracture of metals at high temperatures, especially at temperatures close to the melting point. Investigations of these questions are being carried out by A. V. Stepanov in two directions:

2. Study of the mechanical properties of nonferrous alloys in the single-crystal and polycrystalline states over a wide temperature interval. The aim of the work is to elucidate the causes governing the dependence of the mechanical properties and various types of fracture on the service conditions of alloys. At present the work is being carried out on brasses of various compositions.

3. New methods are being developed for studying the stressed state during plastic deformation with the aid of models made of transparent crystalline material (silver chloride).

From the above (far from complete) brief description of the work being carried out at the Leningrad Physicotechnical Institute, it is evident that the aspiration of its head and director—Academician Abram Fedorovich Ioffe—to establish a continuous and close connection between physics and technology has now been achieved to a considerable degree. Practical results have been achieved not by lowering the theoretical level of research but, on the contrary, thanks to a rigorously scientific approach to phenomena of importance for technology.

There is no doubt that, in the future, under the leadership of A. F. Ioffe, the Institute will achieve still greater successes in this direction.