Nuclear physics is the branch of physics concerned with the nucleus of the atom. It has three main aspects: probing the fundamental particles (protons and neutrons) and their interactions, classifying and interpreting the properties of nuclei, and providing technological advances.

 

Nuclei do not lend themselves to exact theoretical understanding, because they are composed of many particles (protons and neutrons), but are not large enough to be accurately described as periodic, as done with crystals. So "nuclear models" that, singly or in combination, account for most nuclear behavior are used. Three of the four types of fundamental interaction play important roles in nuclei, the strong, electromagnetic and, on a longer time scale, weak.

 

Nuclei are held together by strong interactions (mostly exchanging pions), but electromagnetic repulsion of the positively charged protons tends to push them apart, according to Coulomb's law. The stable nuclei all have close to the lowest energy ratio of protons to neutron for their atomic weight. Nuclei near enough to this ratio to be bound but not close enough to be stable, give off electrons or positrons (beta decay) or take in electrons (and also give off neutrinos), to move closer to that ratio. This is the main place where the weak interactions come in. Nuclei that are too massive to be stable are pulled apart by the coulomb repulsion of their protons and either fission or give off alpha particles.

 

Though the number of energy levels is not infinite, as it is for the electron wave functions of atoms, most stable or nearly stable nuclei have many bound levels. These usually decay toward the ground state by emitting gamma ray photons.

 

Protons and neutrons are fermions, with different value of the isospin quantum number, so two protons and two neutrons can share the same space wave function. In the rare case of a hypernucleus, a third baryon called a hyperon, with a different value of the strangeness quantum number can also share the wave function.

 

The binding energies of the protons and neutrons are on the order of 1 % of their relativistic rest masses, so non-relativistic quantum mechanics can be used with errors usually smaller than those from other approximations. Once the chemists of the 18th century had elucidated the chemical elements, the rules governing their combinations in matter, and their systematic classification (Mendeleev's periodic table of elements) and John Dalton had, in 1803, applied Democritus's idea of atom to them, it was natural that the next step would be a study of the fundamental properties of individual atoms of the various elements, an activity that we would today classify as atomic physics. These studies led to the discovery in 1896 by Becquerel of the radioactivity of certain species of atoms and to the further identification of radioactive substances by the Curies in 1898. Ernest Rutherford next took up the study of radiation and its properties; once he had achieved an understanding of the nature of the radioactivity, he turned around and used radiated particles to probe the atoms themselves. In the process he proposed in 1911 the existence of the atomic nucleus, the confirmation of which (through the painstaking experiments of Geiger and Marsden) provided a new branch of science, nuclear physics.

 

Following Rutherford's work, physicists around the world began trying to "split" the atom. The first to achieve this were two of Rutherford's students, John Cockcroft and Ernest Walton, who divided an atom using a particle accelerator in 1932. In 1938, the German physicists Otto Hahn conducted the first successful experiment in nuclear fission.

 

In the 1940s and 1950s, it was discovered that there was yet another level of structure even more fundamental than the nucleus, which is itself composed of protons and neutrons. Thus nuclear physics can be regarded as the descendant of chemistry and atomic physics and in turn the progenitor of particle physics.

 

Experiments with nuclei continue to contribute to the understanding of basic interactions. Investigation of nuclear properties and the laws governing the structure of nuclei is an active and productive area of research, and practical applications, such as nuclear power, smoke detectors, cardiac pacemakers, and medical imaging devices, have become common.