by Vladimir VARLAMOV, Dr. Sc. (Phys. & Math.), laboratory head, Skobeltsyn Institute of Nuclear Physics, Moscow State University; Boris ISHKHANOV, Dr. Sc. (Phys. & Math.), chair head of the general nuclear physics, Department of Physics, Moscow State University; Vladimir NEDOREZOV, Dr. Sc. (Phys. & Math.), laboratory head, RAS Laboratory of Nuclear Research, Moscow

Nuclear physics and first of all, electromagnetic interaction of nuclei are associated with the name of Acad. Dmitry Skobeltsyn. Fundamentals of these research areas were laid down in 1940, when he organized an Experimental Chair of Nuclear Physics, the first one at Moscow State University and in the nation; these studies received further impetus in 1946 when Skobeltsyn came up with the initiative to set up an Institute of Nuclear Physics at Moscow University. In the 1950s, the Lebedev Physics Institute (FIAN) headed by Skobeltsyn explored new avenues in research of electromagnetic interaction of nuclei. In the early 1960s, the Nuclear Physics Institute and the Laboratory of Photonuclear Reactions set up at the RAS Institute of Nuclear Research became leaders among global photonuclear physics research centers.

GIANT DIPOLE RESONANCE

Atomic nuclei are studied by means of different research methods. One of the most effective methods involves bombardment by gamma quanta, i.e. by high-energy photons with a wavelength comparable to the nucleus in size. Its main advantage is that the properties of electromagnetic interaction of nuclei have been well studied; that is why it is not so hard to separate the two processes--the absorption of photons by the atomic nucleus and the subsequent dynamics of the excited nucleus.

Studies of electromagnetic interaction were begun in 1959, when a 35 MeV electron accelerator--betatron-was commissioned at the Nuclear Physics Institute. The research work was carried on under the guidance of Prof. Valerian Shevchenko. At FIAN research work was headed by Lyubov Lazareva, Dr. Sc. (Phys. & Math.).

In 1945 the theoretical physicist Arkady Migdal (elected to the Academy of Sciences in 1966) predicted that in the course of absorption of gamma-quanta by the nucleus under the effect of an electromagnetic field, coherent synchronous oscillations of a definite frequency (resonance) of all protons relative to all neutrons should take place. The atomic nucleus acts as if it were an electric dipole; that is why the resonance thus induced was called a giant dipole resonance (GDR). It was found to be typical of all atomic nuclei. Active scientific discussions that followed aimed at explaining this phenomenon. To describe GDR, an essentially new concept of the collective motion of the proton-neutron medium (as we know, the atomic nucleus consists of protons and neutrons, both called nucleons and that obey the laws of quantum mechanics) appeared in nuclear physics.

However, it became clear by the mid-1950s that this approach does not accord with new experimental data on the significant role of the independent motion of nucleons in the nucleus. A shell model of the nucleus, similar to the models of atomic shells, was developed, i.e. in the joint potential of attraction among all nucleons, independent protons and neutrons persist in definite energy states--within the corresponding nuclear shells. Low-energy shells, where particles are strongly, bound are called inner shells. As for outer shells, nucleons are loosely bound there.

According to Pauli's exclusion principle* applicable to all particles with a half-integral spin**, protons and neurons of the nucleus in their normal state are at low-energy levels. Since in the shell model these particles persist in different energy states, absorption of the gamma-quantum by the nucleus should produce an intermediate structure of GDR.

Discovery of the intermediate structure of GDR in medium and heavy nuclei made at the Nuclear Physics Institute played a major part in the development of new concepts of the atomic nucleus. The discovery was made on upgraded scientific equipment and tools by means of the "multichannel" method of measurement

* Pauli's exclusion principle--one of the basic principles in quantum physics; it explains the order of filling of atomic shells with electrons, and nuclear shells with protons and neutrons; the principle was formulated in 1925 by the Swiss physicist Wolfgang Pauli.--Auth.

**Spin--intrinsic angular momentum of a microparticle, an analogue of the classical rotation of a body around its inner axis; it can have integral and half-integral values (in the units of Planck's constant).--Ed.

of a photonuclear cross-section* based on a quick (with a frequency of 50Hz) cyclic change of gamma-radiation energy. This method allowed to carry out such measurements simultaneously at 512 values of photon energy and actually excluded recording equipment errors. The better measurement accuracy of photonuclear reactions to a record value of 0.1 percent had never been achieved before. In addition, a highly efficient neutron sensor was designed to register particles by 80 counters. The efficiency of this apparatus reached 45 percent, or it was better than all available analogues used in gamma-quantum experiments. In the 1960s-1970s, special photonuclear data management methods were developed under the supervision of the mathematician Acad. Andrei Tikhonov, now actively used in world laboratories. Upgraded methods in data management for a large number of nuclei (about 40) in a wide range of mass numbers** (A= 9-208) made it possible to detect the intermediate structure.

Research work in photonuclear reactions conducted in 1955 to 1980s at the Nuclear Physics Institute laid a groundwork for the nuclear physics. At that time Lyubov Lazareva suggested a complete photoabsorp-tion cross-section measurement method now widely used in leading research centers of the world. To conduct these measurements, a 9-channel magnetic pain gamma quantum spectrometer with a unique system of digital data coding was built. For the first time it became possible to demonstrate the impact of shell effects on the form and disintegration of GDR for a

* Cross-section of a nuclear reaction--value characterizing the probability of its occurrence, measured in cm² or barns (1 bam = 10²⁴ cm²).--Auth.

** Mass number (A), expressed by the sum of protons (Z) and neutrons in the nucleus.--Ed.

numerous group of light nuclei. In particular, data on the cross-section of complete photoabsorption for the oxygen isotope nucleus ¹⁶O, measured at the Nuclear Physics Institute are cited in many Russian and foreign monographs and textbooks.

Absorption of gamma-quanta by the nucleus results in the excitation in it of disintegrating GDR; it gives off neutrons and protons in light nuclei and several neutrons in heavy nuclei. To explain the nature of GDR, in 1970-1975 a novel research method was suggested by the Nuclear Physics Institute: it was proposed to study this phenomenon proceeding from excited nuclei formed in photonuclear reactions. For the first time scientists measured cross-sections of as many as 300 (A=11-60). Analysis of the new experimental data enabled a qualitative leap in the understanding of this phenomenon. It became known how the energy brought to the nucleus by a gamma-quantum is redistributed among nucleons. One determined the probability of a direct mechanism responsible for GDR decay, when the energy of an incident gamma-quantum is transmitted directly to one of the outgoing nucleons: in the gamma-quantum-proton reaction this mechanism accounts for about 50 percent of the cross-section, and in the gamma-quantum-neutron reaction, as much as 60 percent. It has been proved that the increase in the excitation energy and mass number of the nucleus A reduces the probability of direct GDR decay from 100 percent at A=12 to 50 percent at A=40, while nuclei at A>100 are characterized by decay via a complex nucleus formed when the energy of an incident gamma-quantum is redistributed among many nucleons of the nucleus.

It was also established that the intermediate structure of GDR correlates with configurational disinte-

gration, or energy breakdown of nucleons traveling in between the outer and the inner shells of the nucleus. Experiments conducted in the 1980s to 2000s by physicists of Australia, Belgium, USA, Switzerland and Japan, with nuclei being excited by incident particles, showed: the regularity discovered in the pioneering studies at the Nuclear Physics Institute and FIAN was of universal nature. It was observed in reactions triggered not only by gamma-quanta, but by other particles as well. The concept of configurational disintegration helped to understand the "response" of light nuclei to the action of different external fields.

The newly discovered regularities needed new theoretical approaches, which was always in the center of attention of Dmitry Skobeltsyn. Theoretical works at the Nuclear Physics Institute and FIAN on a microscopic description of GDR contributed to the progress of experimental studies. The shell structure of the nucleus was found to be important for a description of the GDR width; determined likewise was the role of single-particle states (associated with one nucleon traveling in between shells) in the formation of collective excitation in nuclei.

Prof. Rudolf Eramzhyan, who worked at FIAN in the 1980s to 1990s, has made a significant contribution to theoretical research into photonuclear reactions. He participated in conceptualizing the fundamental principles for GDR configurational disintegration. These principles were then confirmed in joint research at the Nuclear Physics Institute and FIAN on photodisinte-gration of^6,7 Li isotopes. In addition to these problems Eramzhyan, together with his Russian and foreign colleagues took an active part in further research into the physics of the nucleus and elementary particles.

In 1987 the USSR State Committee for Inventions and Discoveries made a No. 342 entry in the list of discoveries: "Regularity of Configurational Disintegration of Giant Dipole Resonance in Light Atomic Nuclei" (authors--research fellows of the Nuclear Physics Institute and FIAN Prof. Boris Ishkhanov, Igor Kapitonov, Vladimir Neudachin, Valerian Shevchenko, Nikolai Yudin, Rudolf Eramzhyan).

So far we have considered GDR only in spherical nuclei, i.e. nuclei close to a sphere in shape. However, research into reactions acted upon by gamma-quanta gave an unexpected result: most of the nuclei were deformed and looked like oblong or oblate ellipsoids. It was proved that the spherical symmetry of the nuclear shape was reflected in GDR in the form of one maximum, and the most spectacular effect was in non-spherical ellipsoid-like nuclei, with GDR breaking into two maxima corresponding to the oscillation of the nuclear substance under the effect of an electromagnetic field along the major and minor axes of the ellipsoid nucleus. In the nuclei of ²³²Th, ²³⁵U, ²³⁸U, ²³⁹Pu, with the number of protons ~90, a transition from a spherical to a heavily deformed nuclear shape was discovered. This unique information was obtained in the pioneering studies at FIAN into uranium and thorium fission reactions.

Research of photonuclear reactions in heavy nuclei brought forth yet another puzzle. It turned out that

Internediate structure of GDR of the ⁹⁰Zr nucleus.

Configurational disintegration of GDR of the ²⁸Si nucleus.

cross-section maxima in reactions implicating outgoing protons are shifted to higher energies relative to cross-section maxima in reactions involving outgoing neutrons, and the exit of protons from heavy nuclei at A>100 turned out to be 10² or 10³ more vigorous, than predicted in theory. Comparison of the exit of protons and neutrons was the first step toward the idea of isotopic invariance--independence of nuclear interactions from the electric charge of particles.

STRONG AND WEAK INTERACTIONS OF NUCLEONS IN THE NUCLEUS

Recall that before the discovery of the atomic nucleus only two types of interaction were known: the

Change in GDR shape and width depending on the deformation of the nucleus.

gravitational, describing the mutual attraction of heavy bodies--the motion of planets in the solar system, the fall of objects dropping to the terrestrial surface under the effect of gravitation, and the electromagnetic interaction that describes the motion of charged bodies and magnets. Unlike the gravitational interaction resulting in the mutual attraction of bodies, the electromagnetic interaction leads to both attraction and repulsion.

After the discovery of the atomic nucleus composed of positively charged protons and neutrons with a zero electric charge, it became clear: none of the interactions can bind them into the atomic nucleus: the gravitational interaction, due to a small mass of protons and neutrons, the other one, the electromagnetic interaction, due to a zero electric charge of neutrons. These particles are bound by an unknown type of interaction that is hundredfold stronger than electromagnetic interaction. This strong interaction is manifest at a distance of ~10^-13 cm, but this is enough to bind protons and neutrons, as typical scales of atomic nuclei are of the same level.

Protons and neutrons have many similar characteristics, including equal spins and almost equal masses. However protons, unlike neutrons, carry a positive electric charge. That is why, in terms of nuclear physics with its predominating electric forces, the difference between these two particles is immense. An extra neutron added to the atom converts into another isotope of the same element with almost equal chemical characteristics, while an extra proton increases the atomic number of this element by one unit that results in the formation of a new chemical element with essentially different characteristics.

Note that electric forces in nuclear physics are not the main thing, they are second to short-lived, but far more intensive nuclear forces. Now, in terms of nuclear interactions protons and neutrons behave identically. This enables scientists to treat them not as different particles, but as two charge states of one particle, the nucleon. These two states are described by projections in the charge medium of the quantum number* of the isotopic spin** characterizing the charge symmetry of strong interactions. The discovery of GDR isotopic disintegration made it possible to achieve significant progress in interpreting the large resonance

*In quantum physics the state of a particle is described by a set of quantum numbers that determine all possible values of a physical quantity characterizing different coupled quantum systems, the atoms and atomic nuclei.--Auth.

**Isotopic spin (isospin)--a quantum characteristic that determines a charged state of strongly interacting particles.--Auth.

width, which was impossible before in the available theoretical models.

Further studies of atomic nuclei characteristics resulted in the discovery of another type interaction--the weak interaction occurring at a distance of ~10^-16 cm. A free proton not bound by intranuclear forces is a stable particle. A free neutron is unstable (its half-life period, the time needed for decay of half of the initial number of particles, is 10.23 min). But protons and neutrons change their characteristics as a result of strong and weak interactions within the nucleus. Protons may act as stable particles or decay into neutrons, positrons and neutrinos. The same applies to intranuclear neutrons.

Weak and strong interactions of nucleons are revealed in a phenomenon not covered by classical physics, and this is radioactivity, as one nucleus is transformed into another.

New experiments proceeded alongside the steady improvement of the experimental routine and the development of new particle acceleration methods. At present the Nuclear Physics Institute is running

Isospin disintegration of GDR of the ⁹⁰Zr nucleus (straight line, the reaction: gamma-quantum, neutron; dots, the reaction: gamma-quantum, proton).

3 split-design 35, 55 and 75 MeV microtrons* designed in cooperation with the National Research Nuclear University (MIFI), FIAN, the Alikhanov Institute of Theoretical and Experimental Physics, the Moscow Institute of Physics and Technology, and Saratov State University. These setups opened up essentially new experimental opportunities.

The mechanism of proton-nucleus interaction changes substantially depending on photon energy. In the energy range of up to ~20 MeV the photon interacts with the nucleus as an integer object, but at high energies it interacts with separate systems consisting of few nucleons. As a result, the proton imparts high energy and a weak impulse to the nucleus. Therefore, when its energy is ~20 MeV, the part of this energy transferred to a single nucleon is compensated through the interaction of this nucleon with other nucleons of the nucleus.

In the course of interaction between 50-70 MeV photons and nuclei, the absorbed energy is released through emission of several particles giving rise to radioactive nuclei. A registration method was developed to evaluate radioactivity induced in such photon-bombarded target

*Split-design microtron--a modern electron accelerator combining many characteristics of the classical cyclic microtron and linear accelerator.--Auth.

nuclei. Finally, isotopes were obtained after the exit of 6 or 7 neutrons from the parent nucleus. These experiments showed how the characteristics of nuclei change depending on the correlation of the number of protons and neutrons.

The photon energy increase proportionate to the attenuation of the photon wavelength results in that the pairs of closely located neutrons and protons come to be involved in such interaction. As it was found at the Nuclear Physics Institute, a quasi-deutron mechanism associated with this phenomenon plays an important role for 30 to 70 MeV photons.

NUCLEUS: A GAS, LIQUID, PLASMA?

Studies of the gamma-quantum--atomic nucleus interactions showed that the nucleus is a unique physical object exhibiting mutually exclusive properties. The atomic nucleus--what is it?

One of the principal properties of the atomic nucleus consists in the motion of independent (weakly interacting) nucleons. This means that such motion is similar to the degenerate Fermi gas, i.e. a solid perfect gas composed of particles obeying the Pauli exclusion principle. But this analogy is a restricted: the nucleus has a finite size and well-nigh constant density, and it does not tend to expand as much as possible like a gas does. In other

words, it keeps within the bounds of a constant volume. Constant density of the nuclear substance makes the nucleus similar to a drop of liquid that, since it shows quantum properties, is known as the Fermi liquid. Like an ordinary liquid, the nucleus should have a spherical shape. That is why the existence of such spherical nuclei in a state of equilibrium is a weighty argument for the atomic nucleus--drop-of-liquid analogy. At the same time, the natural occurrence of deformed atomic nuclei makes them similar to a solid body. But this is also a

restricted similarity due to the independent motion of single protons and neutrons, which is typical of a gas, not of a solid body.

The existence of collective excited states increases the similarity between the nucleus and gas (sound waves), solid body (quasiparticles--phonons), and plasma (qua-siparticles--plasmons). But the similarity of the nucleus to plasma characterized by the long-distance Coulomb (also known as the electrostatic) force is also restricted, as the short-time nuclear interaction between nucleons plays the main role in the atomic nucleus. Thus overall, we may conclude that the nuclear substance is a new hitherto unknown state of matter with unique properties in the "macroscopic" world around us. Their diversity gave rise to models reflecting different characteristics of atomic nuclei symmetry.

With the increase in the photon energy (E>100 MeV), the photon wavelength becomes comparable in--size to the nucleon (10-13 cm), which opens up new research opportunities in studying the inner structure of the proton and the neutron. Absorption of gamma-quanta by neutrons gives rise to excited states of the nucleon manifested in the form of resonances in the photon absorption cross-section in the energy range >300 MeV (nucleonic resonances). This phenomenon can be accompanied by realignment of quarks* within the nucleon and changes in their orbital moments. In particular, a change in the spin direction of one quark in the proton results in delta resonance disintegrating with emission of pions (pi-mesons), or light particles with a mass of ~140 MeV/s².

The inner proton and neutron structure is studied with the use of special-purpose high energy accelerators. Since such experiments are expensive and laborious, they usually involve joint groups of scientists from different countries. Physicists of the Skobeltsyn Institute and FIAN participate in many such projects, in particular, in experiments carried out at the JLAB (USA), MAMI (Germany), ESRF (France) accelerators. The data thus obtained show that both the proton and the neutron have a complex inner structure. As the radii of these particles are equal to ~0,8x10^-13 cm, modern accelerators in the energy range of ~5 to 10 GeV make it possible to observe separate components of nucleons. First of all, these are three valent quarks surrounded by gluons that provide for a strong interaction of quarks. Gluons engender new quark-antiquark pairs turning again into gluons.

* Quarks, basic particles composed of strongly interacting particles; never observed in a free state, within the proton quarks are bound by gluons, the particles transferring strong interaction.--Auth.

NEW LINES OF RESEARCH

Experimental and theoretical studies of photo disintegration of atomic nuclei carried out at the Skobeltsyn Nuclear Physics Institute, FIAN and other research centers need were in intensive computer-based data processing. In this context, the Skobeltsyn Institute and the Moscow University Department of Computing Mathematics and Cybernetics developed unique software systems. These works, initiated in the mid-1970s with active support from Rem Khokhlov and Anatoly Logunov, Moscow University rectors, are also backed by the incumbent rector Acad. Viktor Sadovnichy. In the late 1970s, the Photonuclear Experiment Data Center was established at the Nuclear Physics Institute so as to collect and process data, and inform physicists on the properties of atomic nuclei and characteristics of nuclear reactions. The center is taking part in the work of the international IAEA Network of Nuclear Data Centers, and it has made an important contribution to the dissemination of relevant data obtained by the global scientific community. These data, available in the Internet (http://cdfe.sinp.msu.ru), are widely used in scientific research and education.

In recent years research in the physics of electromagnetic interaction of nuclei has expanded, and new trends have appeared. For one, the research of spin structural functions of the lightest nuclei, nucleons and newly discovered elementary particles. Also studied are the nonlinear effects of quantum electrodynamics in the interaction of intense electromagnetic fields with a substance on the beams of relativistic ions, electrons and femtosecond TW-lasers. These works are coordinated by the RAS Council for Electromagnetic Interactions of Nuclei. Russian scientists, the active participants in different international programs, are always welcome to the international Workshops for Electromagnetic Interactions held every three years by FIAN, the Skobeltsyn Institute and Moscow State University.

Now, to summarize. The above research works have served as a basis for further studies in the field of higher energies. The focus was on the characteristic features of the nucleon structure and on elementary particles--the fundamental questions of nuclear physics. The results obtained in the long-time research into electromagnetic interactions of nuclei brought physicists to reconsider many ideas concerning the atomic nucleus structure and in many ways shaped the present physical picture of the microworld.