Nuclear Physics

The spontaneous transformation of an unstable atomic nucleus into a more stable configuration by emitting particles or electromagnetic radiation. The three primary modes are $\alpha$ decay (emission of a $^{4}_{2}\text{He}$ nucleus), $\beta$ decay (conversion of a neutron to a proton or vice versa with emission of an electron or positron), and $\gamma$ decay (emission of high-energy photons).

The splitting of a heavy atomic nucleus into two or more lighter nuclei, accompanied by the release of a large amount of energy, free neutrons, and $\gamma$ radiation. Fission can be spontaneous or induced by bombarding a nucleus with neutrons.

The process by which two light atomic nuclei combine to form a heavier nucleus, releasing enormous energy due to the mass difference between reactants and products. Fusion requires extremely high temperatures and pressures to overcome the electrostatic repulsion between positively charged nuclei.

The fundamental force that binds protons and neutrons together inside the nucleus. It is the residual effect of the strong interaction between quarks mediated by gluons, and it is approximately 100 times stronger than the electromagnetic force at nuclear distances but acts only over ranges of about 1 to 3 femtometers.

The energy required to disassemble a nucleus into its individual protons and neutrons. It arises from the mass defect -- the difference between the mass of the assembled nucleus and the sum of the masses of its individual nucleons -- and is calculated using Einstein's equation $E = mc^2$.

The time required for half of the atoms in a sample of a radioactive isotope to undergo decay. Half-life is a statistical property of a large ensemble of nuclei and is constant for a given isotope regardless of external conditions such as temperature or pressure.

A theoretical model that describes the structure of the nucleus in terms of energy levels (shells) occupied by protons and neutrons, analogous to electron shells in atomic physics. Nuclei with completely filled shells (magic numbers: 2, 8, 20, 28, 50, 82, 126) exhibit exceptional stability.

A self-sustaining sequence of nuclear fission reactions in which the neutrons released from each fission event trigger additional fissions. A chain reaction is controlled when exactly one neutron per fission goes on to cause another fission (critical state) and uncontrolled when more than one does (supercritical state).

Einstein's principle that mass and energy are interchangeable, expressed by $E = mc^2$. In nuclear reactions, the small amount of mass converted into energy accounts for the enormous energy yields because $c^2$ is an immense conversion factor.

The process of slowing down fast neutrons produced in fission to thermal energies (around 0.025 eV) so they are more likely to be captured by fissile nuclei and sustain a chain reaction. Moderators are materials with low atomic mass that efficiently absorb neutron kinetic energy through elastic collisions.