Nuclear Physics is the branch of physics concerned with atomic nuclei, their structure, stability, interactions, reactions and transformations. It studies systems made from protons and neutrons, collectively called nucleons, as well as nuclei containing different numbers of these constituents. Nuclear physics examines how the strong interaction and electromagnetic interaction, together with quantum mechanics and nuclear many-body effects, determine nuclear properties. Its scope includes stable and radioactive nuclei, nuclear reactions, fission, fusion, nuclear astrophysics, nuclear forces and applications involving radiation and nuclear technologies.
Atomic nuclei are extremely compact systems with radii approximately described by a relationship of the form \(R=R_0A^{1/3}\), where \(A\) is the mass number and \(R_0\) is a characteristic nuclear length scale. Nuclear density is approximately constant across many nuclei because the nuclear radius grows roughly with the cube root of nucleon number. Protons carry positive electric charge and therefore repel one another electromagnetically, yet nuclei can remain bound because the strong interaction produces powerful attraction at nuclear distances. The balance between attractive nuclear forces, electromagnetic repulsion and quantum effects determines which nuclei are stable or unstable.
Nuclear structure is described using several complementary models because nuclei are complex quantum many-body systems. The liquid drop model explains broad trends such as binding energy, nuclear deformation and fission through collective properties. The shell model describes nucleons occupying quantised nuclear orbitals and explains the enhanced stability associated with magic numbers. Collective models describe rotational and vibrational excitations, while more microscopic approaches attempt to derive nuclear properties from nucleon interactions. No single model is universally sufficient, and nuclear physicists select approximations according to the observable, mass region and energy scale under investigation.
Binding energy is central to nuclear physics. The mass of a bound nucleus is less than the combined masses of its separated constituents by an amount corresponding to the nuclear binding energy through the mass energy relationship \(E=mc^2\). Binding energy per nucleon varies across the periodic table and is relatively high for nuclei around the iron and nickel region. This trend helps explain why fusion of very light nuclei and fission of very heavy nuclei can release energy: the products can have greater binding energy per nucleon than the initial systems. The energy difference appears as kinetic energy, radiation and excitation energy of the reaction products.
Nuclear reactions occur when nuclei interact with other nuclei, neutrons, protons, photons or other particles and transform into different states or products. Fission involves the splitting of a heavy nucleus into smaller nuclei, while fusion combines light nuclei into heavier ones. Neutron capture can produce heavier isotopes, sometimes followed by radioactive transformations. Nuclear reaction rates depend on energy, cross sections, target composition, particle flux and quantum effects such as tunnelling and resonances. Cross sections provide a quantitative measure of interaction probability and are essential for reactor physics, isotope production, astrophysics and radiation transport.
Nuclear physics has major practical applications. Nuclear reactors exploit controlled fission reactions to generate heat, which can then produce electricity. Medical nuclear physics supports diagnostic imaging using radioactive isotopes and therapeutic procedures using selected radiation types. Nuclear techniques are used in materials analysis, archaeological dating, industrial inspection, security screening and scientific research. Particle beams and nuclear reactions can also produce radioisotopes that are unavailable in useful quantities naturally. These applications depend on detailed understanding of radiation production, transport, interaction probabilities, shielding and biological effects.
Nuclear astrophysics connects nuclear physics with the origin and evolution of elements in stars and explosive astronomical environments. Stellar nucleosynthesis involves nuclear reactions that transform lighter nuclei into heavier ones, while neutron-rich environments can enable rapid neutron capture and the production of nuclei far from stability. Nuclear reaction rates and radioactive half-lives influence stellar energy generation, nucleosynthetic pathways and observable abundances. Laboratory measurements and theoretical nuclear models are therefore used together with astronomical observations to understand how the chemical elements were produced and distributed through cosmic history.
Nuclear physics must be distinguished from atomic physics. Atomic physics primarily concerns electrons and their interactions with the electromagnetic field around nuclei, whereas nuclear physics focuses on the nucleus itself and processes involving nucleons and nuclear states. Nuclear physics also differs from particle physics, although the fields overlap: particle physics studies elementary particles and fundamental interactions, while nuclear physics frequently studies composite nuclei and emergent collective behaviour. Modern nuclear research uses accelerators, detectors, theoretical calculations and large computational simulations to connect microscopic interactions with measurable nuclear properties and practical applications.
