Ilya Mikhailovich Frank (born 23.10.1908) is a Soviet physicist and an academician (1968; corresponding member 1946). He was born in St Petersburg. He graduated from Moscow University (1930). In 1930 — 34 he worked at the State Optical Institute, and in 1934-70 at the Physical Institute of the USSR Academy of Sciences (from 1941 as head of a department, from 1947 as head of a laboratory). From 1957 he has been director of the Laboratory of Neutron Physics of the Joint Institute for Nuclear Research in Dubna, and from 1971 head of a laboratory at the Institute for Nuclear Research of the USSR Academy of Sciences. From 1940 he has also been professor at Moscow University.
His work was in the field of physical optics and of neutron and low-energy nuclear physics. In 1937, with I. E. Tamm, he developed on the basis of classical electrodynamics the theory of Vavilov — Cherenkov radiation, showing that the source of this radiation is electrons moving in a medium at a speed greater than the phase velocity of light (USSR State Prize, 1946; Nobel Prize, 1958). He later studied many features of this radiation. He constructed the theory of the so-called complex Doppler effect — the Doppler effect in a medium with account of its refracting properties and dispersion (1942) — and, together with V. L. Ginzburg, of the anomalous Doppler effect (for a source moving at a superluminal speed) (1947). In 1946, with V. L. Ginzburg, he predicted transition radiation, which arises when a moving charge crosses a plane interface between two media.
In 1938, together with L. V. Troshev, he studied pair production by gamma quanta in krypton and nitrogen, obtaining the fullest and most correct comparison of the theory of the phenomenon with experiment.
In the mid-1940s he carried out extensive theoretical and experimental studies of the multiplication of neutrons in heterogeneous uranium-graphite systems, which helped to understand and refine the basic laws of neutron transport in nuclear reactors; in particular they made it possible to determine with high accuracy the critical size and the neutron multiplication factor in an infinite system and to study their dependence on the properties of the uranium-graphite lattice. He proposed and developed a pulsed method of studying the diffusion of thermal neutrons and detected (1954) the dependence of the mean diffusion coefficient on a geometrical parameter (the diffusion cooling effect). He also developed a new method of neutron spectrometry — by the time of their slowing down in lead.
He carried out a cycle of experimental studies of reactions on light nuclei in which neutrons are emitted, of the interaction of fast neutrons with the nuclei of tritium, lithium and uranium, and of the fission process. He began the study of short-lived quasi-stationary states and of the fission of nuclei under the action of mesons and high-energy particles. He took part in the creation and start-up of the pulsed fast-neutron reactors IBR-1 (1960) and IBR-2 (1981).
He founded a school of physicists. USSR State Prizes (1954, 1971). The S. I. Vavilov Gold Medal (1980).