November of 2012 marks the 120th birth anniversary of Dmitry Vladimirovich Skobeltsyn (1892-1990), a great Russian physicist who has formed a major scientific school in nuclear physics, elementary particles and cosmic rays.

Dmitry Skobeltsyn. Early 1960s. Photo from the site of the Skobeltsyn Research Institute of Nuclear Physics.

Upon his graduation from the Department of Physics and Mathematics of Petrograd (St. Petersburg) University, Dmitry Skobeltsyn was selected as a candidate for professorship at the Physics Chair. Combining research with teaching, he started out as a research fellow at the Leningrad College of Physics and Technology (Ioffe Institute of Physics and Technology today). His first experiments dealt with the Kompton phenomenon.

In 1923 Arthur H. Compton, a US physicist (Nobel Prize in physics, 1927) discovered what became known as the Compton phenomenon. Thereby he laid a groundwork for the Compton scattering theory, i.e. microwave scattering on a free electron. Skobeltsyn showed a great interest for the interaction of gamma rays of radioactive substances, thought to be the hardest radiation at the time, with electrons. He found an apt methodic approach by using a Wilson cloud chamber placed in a static magnetic field for quantitative analysis of the interaction of relativistic particles with a substance. Experimenting, the young physicist concluded the Compton theory agreed, both in qualitative and quantitative terms, with the phenomenon of interaction of radiation quanta with free electrons. The Skobeltsyn method made it possible to discover a number of elementary particles and study many essential high-energy processes.

Subsequently Skobeltsyn applied the Compton phenomenon in spectroscopy of γ-rays. This paved the way to further research in β- and γ-rays spectra, and brought important theoretical and practical results.

By and large, Skobeltsyn concentrated on cosmic radiation. Examining shots taken in a Wilson cloud chamber in a sequence of experiments on the Compton phenomenon, he noted particle tracks not distorted by a magnetic field. As it turned out, among the tracts of β-particles and electrons knocked out by γ-rays and much distorted in a magnetic field, there were also straight tracks of particles with energies of ≥15 MeV. This fact demonstrated: the tracks thus detected could not be attributed to emissions of radioactive substances. More than that, the mean ionization generated by particles in the atmosphere agreed with air ionization by cosmic rays. And so, Skobeltsyn inferred: the straight tracks in the cloud chamber were left by the charged particles of cosmic rays. Even though this phenomenon was discovered as far back as 1912, the immediate cause of ionization was not clear yet--until Skobeltsyn found it in 1927. As a matter of fact, physicists showed but little interest in this phenomenon and approached it mainly from positions of geophysics.

Skobeltsyn's discovery was a turning point in this field of physics. Actually one turned to cosmic rays in good earnest only after he had demonstrated the presence of high-energy particles there.

Next came high-energy phenomena having no analogs. The point is that cosmic ray particles often occur in clusters, or showers, to use the present-day terminology. Skobeltsyn discovered them in 1929.

Two associated problems moved to the fore in the latter half of the 1930s: 1) life and characteristics of the new-discovered particles, mesons, and 2) the cascade theory of the electron/photon component. Dmitry Skobeltsyn made a substantial contribution in solving these two problems.

Skobeltsyn was likewise involved with other areas of nuclear physics. For one, he made a comprehensive study of how electron-positron pairs were formed in a gas within the Wilson cloud chamber under the action of γ-rays. He demonstrated that the phenomenon attending the interaction of photons with an electric field of atomic nuclei from different elements was well consistent with the experiment.

Studies of cosmic radiation at high altitudes were another facet of the Skobeltsyn-orchestrated research. As it turned out, the primary cosmic rays were in the form of electrically charged particles. Using the magnetic field of the earth by way of a measuring instrument, it became possible to grade this radiation according to energy. Cosmic rays were found to be absorbed fast in the atmosphere. Subsequent works of Skobeltsyn and his pupils indicated the cause of big energy losses--it lay in the interaction of charged particles with atomic nuclei in the atmosphere.

During the Great Patriotic War of 1941-1945, together with the Physics Institute, Skobeltsyn was evacuated to the hinterland, to the city of Kazan east of Moscow, where he was in charge of defense and applied civilian projects. Yet he kept up research into extended air showers, discovered in 1938 by the French physicist Pierre Auger (Auger showers). On his initiative and under his guidance, detailed studies of Auger showers were launched in the Pamirs in 1944. Other field parties followed there in subsequent years. A total of data

thus obtained brought Skobeltsyn and his pupils to this conclusion. Such showers were not pure electron/photon avalanches, but were a product of a composite nuclear cascade process. Accordingly, Skobeltsyn and coworkers plotted a nuclear cascade pattern for Auger showers, one that gained universal recognition and proved to be of great use in further experiments.

To obtain more data on high-energy particles, Skobeltsyn advanced the idea of studying Auger showers at two levels simultaneously--on the terrestrial surface and underground.

Simultaneously, studies of cosmic rays at high altitudes were continued, and their composition analyzed. Such investigations were conducted in the stratosphere in different latitudes. In 1947 Skobeltsyn (elected meanwhile to the national Academy of Sciences in 1946) initiated experiments on rockets for studying cosmic rays beyond the atmosphere.

In his address at the plenary session of the USSR Academy of Sciences in February of 1950 Skobeltsyn outlined his vision of cosmic rays and related phenomena. The physics of cosmic rays, he reasoned, had become a chapter of high-energy nuclear physics. For their discovery of electronic-nuclear showers and the nuclear-cascade process as well as research in these fields Skobeltsyn and his fellow workers merited a top national award, the Stalin Prize, in 1951.

In his subsequent works Skobeltsyn furthered his concept of cosmic rays. Among other things, he attached great attention to the distribution of the energy of a primary nuclear-active particle among secondary particles generated by it. Encounters (collisions) of high-energy particles with light nuclei were found to be of great elasticity as a result. On the average a nucleon imparts only 30 to 40 percent of its energy to generated pimesons (pions) in the course of interaction, while retaining most of its energy. This feature is of major role in the scenario of absorption of cosmic rays energy in the atmosphere. Further indepth studies into extensive (Auger) showers brought research scientists to certain important conclusions. An encounter (collision) of two nucleons at 10¹¹-10¹² energies often produces a cluster of excited nuclear matter moving rather slowly and breaking up into pimesons. The data collected in these research works are now part and parcel of the science on cosmic rays and underlie the physics of high and superhigh energy processes.

Acad. Skobeltsyn has reared a large following expert in cosmic rays, atomic nucleus, and accelerators. He also excelled in teaching and managerial activities. In 1940 at Moscow University he founded an Atomic Nucleus Department and then a Nuclear Physics Department which he had been heading for many years. In 1946 he set up a Research Institute of Nuclear Physics at Moscow University and served as its director for 15 years. This Institute grew into a leading research center in nuclear and high-energy physics.

In 1951, after Acad. Sergei Vavilov's death, Skobeltsyn became director of the Lebedev Physics Institute that he had been heading for twenty years; he masterminded big changes in its research activities and opened up new avenues of research.

Acad. Skobeltsyn was an active public figure. For many years he had been heading the Committee on International Lenin Prizes "For the Promotion of Peace Among Nations". He was one of the initiators and leading figures of the Pugwash Movement of Scientists for Peace.

Acad. Skobeltsyn was in for many top national awards and titles. He was Hero of Socialist Labor and holder of Lenin, Stalin and State Prizes of the USSR, and merited six Lenin Orders together with other distinctions.

In 2004 the Russian Academy of Sciences instituted a Skobeltsyn Gold medal for signal accomplishments in elementary-particle physics and physics of cosmic rays, conferred every five years. Its first winner in 2005 was Acad. Georgi Zatsepin for his achievements in high-energy and elementary-particle physics.

In the following two articles we will tell you about some of the research works by Acad. Skobeltsyn.

Used in this article were material of the site of the Skobeltsyn Research Institute of Nuclear Physics (Moscow State University)