Nuclear physics - the branch of physics devoted to the study of the structure of the atomic nucleus, the processes of radioactive decay and the mechanism of nuclear reactions. Elementary-particle physics is sometimes also included in nuclear physics. Lines of research that have become independent branches of engineering, such as accelerator technology and nuclear power, are sometimes still counted as parts of nuclear physics. Historically nuclear physics arose before the existence of the atomic nucleus had been established, and its age may be reckoned from the discovery of radioactivity.
A distinction is usually drawn between low-, intermediate- and high-energy nuclear physics. Low-energy nuclear physics covers problems of the structure of the nucleus, the study of the radioactive decay of nuclei, and also research on nuclear reactions induced by particles with energies up to 200 MeV. Energies from 200 MeV to 1 GeV are called intermediate, and those above 1 GeV high. This division is to a large extent arbitrary and took shape in accordance with the history of accelerator technology. In modern nuclear physics the structure of the nucleus is investigated with high-energy particles, while the fundamental properties of elementary particles are established from research on the radioactive decay of nuclei.
An extensive component of low-energy nuclear physics is neutron physics, which covers research on the interactions of slow neutrons with matter and on nuclear reactions induced by neutrons. A new field of nuclear physics is the study of nuclear reactions induced by multiply charged ions. These reactions are used both to search for new heavy nuclei and to study the mechanism by which complex nuclei interact with one another. A separate line of nuclear physics is the study of the interaction of nuclei with electrons and photons. All these branches of nuclear physics are closely interwoven with one another and are linked by common aims.
The arsenal of experimental means is varied and technically complex. Its basis is formed by accelerators of charged particles (from electrons to multiply charged ions), nuclear reactors, which serve as powerful sources of neutrons, and particle detectors. The modern nuclear experiment is marked by high intensities of the beams of accelerated charged particles or neutrons, which make it possible to study rare nuclear processes and phenomena, and by the simultaneous recording of several particles emitted in a single nuclear collision. The mass of data obtained in a single experiment requires the use of computers coupled directly to the recording apparatus.
The central problem of theoretical nuclear physics is the quantum problem of the motion of many bodies that interact strongly with one another. New lines of theoretical physics arose and developed out of the theory of the nucleus and of elementary particles; they later found application in other areas of physics and gave rise to new mathematical research (for example, the inverse problem of scattering theory and its applications to the solution of non-linear partial differential equations, and more). The applied importance of nuclear physics is great, and its practical applications are wide and varied — from nuclear weapons and nuclear power to diagnosis and therapy in medicine. At the same time nuclear physics remains a fundamental science, from whose progress one may expect the elucidation of deep properties of the structure of matter and the discovery of new laws of nature.