Nuclear physics is the branch of physics concerned with the structure, properties, interactions, and transformations of the atomic nucleus. Its scope includes stable and radioactive nuclei, the internal structure of their constituent particles, and nuclear matter under extreme conditions. It overlaps with particle physics, which investigates fundamental particles and forces, but places particular emphasis on how interacting constituents form nuclei and other nuclear systems. Experimental measurements and theoretical models together explain nuclear behavior and provide knowledge used in energy production, isotope applications, and studies of the universe. (energy.gov)
Historical development
The field emerged from investigations of radioactivity and atomic structure. In 1911, Ernest Rutherford interpreted measurements of alpha-particle scattering as evidence that an atom contains a small, massive, positively charged nucleus. His interpretation replaced models in which positive charge was distributed throughout the atom and established scattering as a central method for investigating subatomic structure. James Chadwick’s discovery of the neutron in 1932 subsequently supported the description of nuclei as systems of protons and neutrons, collectively called nucleons. (nobelprize.org)
During the following decades, quantum mechanics supplied the framework for explaining nuclear energy levels and transitions. The nuclear shell model, developed by Maria Goeppert Mayer and J. Hans D. Jensen in the late 1940s, explained important patterns of stability through the filling of nucleon energy levels. Alongside models of collective motion, it established that nuclear behavior requires both descriptions of individual particles and descriptions of the nucleus as an interacting whole. (nobelprize.org)
Nuclear composition and binding
A nucleus contains protons, which carry positive electric charge, and usually neutrons, which are electrically neutral. The proton number, (Z), identifies the chemical element; the neutron number is (N), and the mass number is (A=Z+N). Isotopes of an element have the same proton number but different neutron numbers. Nuclear dimensions are typically measured in femtometres, or (10^{-15}) metres, much smaller than atomic dimensions. For many nuclei, the radius approximately follows (R=r_0A^{1/3}), reflecting their roughly constant interior density. (energy.gov)
The attraction between nucleons derives from the strong interaction, while electrical repulsion opposes binding between protons. At a deeper level, nucleons contain quarks and gluons, whose interactions are described by quantum chromodynamics. Nuclear theory therefore connects descriptions in terms of nucleons with the underlying theory of their constituents. (energy.gov)
Nuclear binding energy is the energy required to separate a nucleus completely into free protons and neutrons. A bound nucleus has less mass than the sum of those free constituents. Through mass–energy equivalence, this mass difference gives the binding energy:
[ E_b=\left[Zm_p+Nm_n-M_{\mathrm{nucleus}}\right]c^2, ]
where (m_p) and (m_n) are the free nucleon masses and (c) is the speed of light. Binding energy per nucleon broadly increases toward the iron–nickel region and decreases for heavier nuclei. This pattern explains why combining certain light nuclei or splitting certain heavy nuclei can release energy. (openstax.org)
Structure and theoretical models
Nuclei possess discrete energy levels and can exhibit changes of shape, rotation, and vibration. The nuclear shell model treats protons and neutrons as occupying quantum states within an effective nuclear potential. Closed shells produce particularly strong binding at certain “magic” proton or neutron numbers. Strong coupling between a nucleon’s spin and orbital motion is essential to reproducing the observed shell structure. (nobelprize.org)
Collective descriptions emphasize coordinated motion of many nucleons rather than individual configurations alone. Modern theoretical work also calculates nuclear properties from interactions among nucleons and, where feasible, from quarks and gluons. Comparing predicted masses, shapes, energy levels, and reaction behavior with observations tests the assumptions and limits of these approaches. (energy.gov)
Radioactive decay and nuclear reactions
Unstable nuclei can transform spontaneously through radioactive decay. In alpha decay, a nucleus emits a helium-4 nucleus. In beta decay, the proton–neutron composition changes through processes involving electron or positron emission, or electron capture. Gamma decay releases excitation energy as electromagnetic radiation without changing the proton or neutron number. (energy.gov)
For an ensemble of identical radioactive nuclei with a constant decay probability per unit time,
[ N(t)=N_0e^{-\lambda t}, \qquad T_{1/2}=\frac{\ln 2}{\lambda}. ]
Here, (\lambda) is the decay constant and half-life (T_{1/2}) is the time required for half the initial nuclei to decay. The equation describes an ensemble statistically rather than specifying when an individual nucleus will transform. (openstax.org)
Nuclear reactions can also be induced by incoming particles. In nuclear fission, a heavy nucleus splits into smaller nuclei, often releasing neutrons that can initiate further reactions. In nuclear fusion, light nuclei combine into heavier products. Reaction probabilities depend on the incident energy and the nuclei involved; cross sections quantify this dependence. (energy.gov)
Experimental methods and applications
Experiments use particle accelerators, nuclear targets, and radiation detectors to investigate nuclear structure and reactions. Electron and ion beams probe nuclei through scattering, while radioactive beams provide access to short-lived isotopes. Measurements of emitted particles and radiation reveal nuclear properties and test theoretical predictions. High-energy collisions of heavy nuclei produce quark–gluon plasma, allowing investigation of matter resembling that present shortly after the Big Bang. (energy.gov)
Nuclear data underpin nuclear power, isotope production, and applications in medicine and industry. Nuclear astrophysics investigates nucleosynthesis: the formation of nuclei in the early universe, stars, and explosive environments. Measurements of nuclear reactions and decay properties help explain stellar energy generation and the production of elements, including heavy nuclei formed in neutron-rich environments such as neutron-star mergers. (energy.gov)