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Alpha decay is radioactive decay emitting an alpha particle, a bound group of two protons and two neutrons. The daughter atomic nucleus therefore has atomic number reduced by two and mass number reduced by four.

A released alpha particle must escape the Coulomb barrier outside the short-range nuclear attraction. Quantum tunnelling explains how emission can occur even when the alpha particle's measured kinetic energy is below the barrier's classical height. The strong sensitivity of tunnelling to energy helps explain widely differing decay lifetimes.

For a specified transition from a parent initially at rest, the alpha particle and daughter share the available Q value through two-body recoil. Different daughter nuclear energy levels can produce distinct alpha lines. Their line energies and populations provide information beyond simply counting the sample's total radioactive activity.

A gamma ray is a high-energy photon, conventionally associated with a transition of an atomic nucleus, radioactive decay or a particle reaction. X-rays and gamma rays are both electromagnetic radiation; their energy ranges overlap, so the production mechanism can be more informative than an arbitrary energy boundary.

A nuclear energy level transition emits a gamma ray whose energy is approximately the level difference, with a correction for nuclear recoil. Detectors infer the incoming energy from interactions such as photoelectric absorption, Compton scattering and, above threshold, pair production. A measured gamma-ray spectrum can therefore identify nuclear transitions even when a detector does not absorb the entire photon energy in each event.

Nuclear de-excitation is a transition from a higher to a lower nuclear energy level. It can transfer energy to an emitted gamma ray or to an atomic electron through internal conversion, among other channels when thresholds permit.

A nuclear energy level is an energy eigenvalue of an atomic nucleus. Differences between levels give nuclear excitation energies and the approximate gamma ray energies of transitions.

Nuclear physics studies atomic nuclei, their internal structure and the processes that change them. A nucleus contains protons and neutrons, whose short-range nuclear forces bind them while the Coulomb force repels the protons. The resulting nuclear energy levels and nuclear binding energies connect this subject to the low-energy consequences of quantum chromodynamics. Weak interactions additionally permit changes of nuclear composition through beta decay.

The mass defect makes the connection between structure and released energy quantitative. For a nucleus with $Z$ protons, $N$ neutrons and nuclear mass $M_{\mathrm{nuc}}$, its nuclear binding energy isA bound nucleus has less rest energy than its separated nucleons. In a nuclear reaction, the difference between the sums of initial and final rest energies supplies the Q value. The sign of that difference and the Coulomb barrier answer different questions: a reaction can release energy overall while still requiring a way to cross an entrance barrier.

Scattering experiments supplied an early structural clue. Rare large deflections of alpha particles by thin matter could be explained by close encounters with a highly concentrated central charge. Ernest Rutherford's 1911 analysis connected such angular measurements to a compact atomic centre, establishing a route from measured scattering cross-sections to internal structure.^[ref]

Nuclear structure concerns the states and internal organization of an atomic nucleus, including nuclear energy levels, nuclear spin, spatial distributions and the interactions among nucleons. It connects the nucleus's constituents to observable excitation and decay patterns.

A nuclear level scheme organizes measured transition energies and assignments. Gamma decay constrains differences between levels, while reaction and scattering measurements provide complementary information about allowed transitions and spatial distributions. Several models can describe different aspects of the same nucleus; matching one transition energy alone does not establish a unique microscopic arrangement.