Appendix H — Glossary

This glossary defines the key terms used throughout this textbook. Each entry gives a concise definition (one to three sentences) and the chapter where the term is first introduced or most thoroughly discussed. Terms are listed alphabetically; cross-references to related terms are given in italics.

A

Absorbed dose. The energy deposited by ionizing radiation per unit mass of the absorbing material. The SI unit is the gray (Gy), equal to 1 J/kg. Ch. 16. See also equivalent dose, effective dose.

Abundance, isotopic. The fraction of a given element that exists as a particular isotope, either in a specific sample or as a terrestrial average. Expressed as a percentage or atom fraction. Ch. 1.

Accelerator. A device that uses electromagnetic fields to accelerate charged particles to high kinetic energies. Types include cyclotrons, synchrotrons, linear accelerators (linacs), and tandem Van de Graaff machines. Ch. 30.

Activity. The rate of radioactive disintegrations of a sample, defined as $A = \lambda N$ where $\lambda$ is the decay constant and $N$ is the number of atoms. The SI unit is the becquerel (Bq); the older unit is the curie (Ci). Ch. 12.

ALARA. Acronym for As Low As Reasonably Achievable. The guiding principle of radiation protection, which requires that radiation exposures be minimized below regulatory limits considering economic and practical constraints. Ch. 29.

Alpha decay. A radioactive decay mode in which a parent nucleus emits an alpha particle (${}^4_2$He), reducing its mass number by 4 and atomic number by 2. Alpha decay is the dominant decay mode for heavy nuclei with $Z \gtrsim 83$ and is explained by quantum mechanical tunneling through the Coulomb barrier (Gamow theory). Ch. 13.

Alpha particle. A nucleus of helium-4, consisting of two protons and two neutrons in a tightly bound configuration. Because of its high binding energy (28.3 MeV), the alpha particle is emitted as an intact unit in nuclear decay and reactions. Ch. 1, 13.

AME (Atomic Mass Evaluation). A comprehensive, critically evaluated compilation of experimentally measured atomic masses and derived quantities (binding energies, separation energies, Q-values). The current edition is AME2020. Ch. 2, Appendix E.

Anapole moment. A parity-violating electromagnetic moment arising from weak interactions inside the nucleus. The nuclear anapole moment has been measured in cesium-133 and provides information on hadronic weak interactions. Ch. 32.

Angular momentum coupling. The quantum-mechanical procedure for combining two angular momenta $\mathbf{j}_1$ and $\mathbf{j}_2$ into a resultant $\mathbf{J}$ with $|j_1 - j_2| \le J \le j_1 + j_2$. The expansion coefficients connecting uncoupled and coupled bases are the Clebsch-Gordan coefficients. In nuclear physics, angular momentum coupling is central to the shell model, electromagnetic transition rates, and reaction theory. Ch. 5, 9.

Angular correlation. The correlation in the emission directions of successive radiations (typically two gamma rays in a cascade). Angular correlations provide information about the spins and parities of nuclear states and the multipolarities of the transitions connecting them. Ch. 9.

Asymmetry term. The term in the semi-empirical mass formula that penalizes nuclei with unequal numbers of protons and neutrons, reflecting the Pauli exclusion principle applied to the proton and neutron Fermi gases. Proportional to $(N-Z)^2/A$. Ch. 4.

Asymptotic freedom. The property of quantum chromodynamics (QCD) whereby the strong coupling constant $\alpha_s$ decreases at short distances (high energies), causing quarks to behave as nearly free particles inside hadrons at very high momentum transfers. Discovered by Gross, Wilczek, and Politzer (Nobel Prize 2004). Ch. 31.

Atomic mass unit (u). A unit of mass defined as one-twelfth the mass of a carbon-12 atom. Equivalent to 931.494 MeV/$c^2$ or $1.66054 \times 10^{-27}$ kg. Also written "amu" in older literature. Ch. 1.

Auger electron. An electron emitted when the energy released by an electron filling an inner-shell vacancy is transferred directly to another electron, ejecting it. Competes with characteristic X-ray emission. Ch. 15.

B

Backbending. A sudden increase in the moment of inertia of a rotating nucleus at a critical angular momentum, observed as a characteristic S-shape (or backbend) in a plot of moment of inertia versus rotational frequency. Caused by the breaking of a nucleon pair, which aligns its angular momentum with the rotation axis. Ch. 8.

Background radiation. Ionizing radiation from natural sources (cosmic rays, terrestrial radioactivity, radon, internal radionuclides) and ubiquitous man-made sources. The global average annual effective dose from background radiation is approximately 2.4 mSv. Ch. 29.

Barn (b). A unit of area used for nuclear cross sections, equal to $10^{-24}$ cm$^2$ or 100 fm$^2$. The name reportedly originated as slang for a cross section as "big as a barn" (by nuclear standards). Ch. 1, 17.

Bateman equations. A set of coupled first-order differential equations describing the time evolution of activities in a radioactive decay chain $A \to B \to C \to \cdots$. The general solution gives the number of atoms of each member of the chain as a function of time. Ch. 12.

Becquerel (Bq). The SI unit of radioactivity, defined as one disintegration per second. Named after Henri Becquerel, who discovered radioactivity in 1896. Ch. 12.

Beta decay. A radioactive decay process mediated by the weak interaction in which a neutron converts to a proton (beta-minus, $\beta^-$), a proton converts to a neutron (beta-plus, $\beta^+$), or an orbital electron is captured by the nucleus (electron capture, EC). Each process changes $Z$ by $\pm 1$ while keeping $A$ constant. Ch. 14.

Beta spectrum. The continuous energy distribution of electrons (or positrons) emitted in beta decay, ranging from zero to the endpoint energy $Q_\beta$. The continuous spectrum, unexplained by two-body kinematics, led Pauli to postulate the neutrino (1930). Ch. 14.

Bethe-Bloch formula. The formula describing the mean energy loss per unit path length ($-dE/dx$, the stopping power) of a charged particle passing through matter, as a function of the particle's velocity and the target material's properties. Ch. 16.

Bethe-Weizsacker formula. See semi-empirical mass formula. Ch. 4.

Binding energy. The energy required to completely disassemble a nucleus into its constituent free protons and neutrons. Equivalently, the difference in mass-energy between the separated nucleons and the bound nucleus: $B = [Z m_p + N m_n - M(Z,N)]c^2$. A more tightly bound nucleus has a smaller mass, not more energy stored inside. Ch. 1, 4.

Binding energy per nucleon ($B/A$). The average binding energy contributed by each nucleon. The $B/A$ curve, peaking near $A \approx 56$--62 (iron-nickel region), explains why fission of heavy nuclei and fusion of light nuclei both release energy. Ch. 1, 4.

Bohr compound nucleus model. A model of nuclear reactions proposed by Niels Bohr (1936) in which the projectile is absorbed by the target nucleus, forming a highly excited compound nucleus that subsequently decays independently of how it was formed (the Bohr independence hypothesis). Ch. 18.

Borromean nucleus. A three-body nuclear system in which the complete system is bound but any two-body subsystem is unbound. Named after the Borromean rings (three interlocked rings, none of which are linked pairwise). Examples include ${}^{6}$He (alpha + $n$ + $n$) and ${}^{11}$Li (${}^{9}$Li + $n$ + $n$). Ch. 10.

Brachytherapy. A form of radiation therapy in which a sealed radioactive source is placed inside or next to the area requiring treatment. Commonly uses ${}^{192}$Ir, ${}^{137}$Cs, or ${}^{125}$I sources. Ch. 27.

Bragg peak. The sharp maximum in the energy deposition profile (dose vs. depth) of a charged particle beam, occurring near the end of the particle's range where it slows down and its stopping power increases. Exploited in proton and heavy-ion cancer therapy. Ch. 16, 27.

Branching ratio. The fraction of decays of a radioactive nuclide that proceed through a particular decay mode or to a particular final state, when more than one mode is energetically allowed. Ch. 12.

Breit-Wigner formula. The formula describing the energy dependence of a nuclear reaction cross section near an isolated resonance. For a single level with total width $\Gamma$ and partial widths $\Gamma_a$, $\Gamma_b$: $\sigma(E) \propto \Gamma_a \Gamma_b / [(E - E_r)^2 + (\Gamma/2)^2]$, producing a Lorentzian peak centered at the resonance energy $E_r$. Ch. 18.

Bremsstrahlung. Electromagnetic radiation (photons) emitted when a charged particle is decelerated by the electric field of a nucleus. An important energy-loss mechanism for electrons in matter and a source of continuous X-ray spectra. Ch. 16.

Buildup factor. A multiplicative correction factor $B$ that accounts for the contribution of scattered radiation (Compton-scattered photons, secondary particles) to the transmitted intensity through a shield. Always $B \geq 1$. Ch. 16.

C

Chain reaction. A self-sustaining sequence of nuclear fission events in which neutrons released by one fission induce further fissions. A chain reaction is sustained when the effective neutron multiplication factor $k_\text{eff} = 1$ (critical). Ch. 20.

Chart of nuclides. A two-dimensional plot of all known nuclides arranged with neutron number $N$ on the horizontal axis and proton number $Z$ on the vertical axis. Each cell represents a nuclide and is color-coded by decay mode. The chart reveals the valley of stability, drip lines, and magic numbers. Ch. 1.

Chiral effective field theory (chiral EFT). A systematic framework for constructing the nuclear force from QCD symmetries (chiral symmetry) and the relevant low-energy degrees of freedom (pions and nucleons). Provides a hierarchy of two-nucleon, three-nucleon, and higher-body forces with controlled uncertainties. Ch. 3, 31.

CKM matrix. The Cabibbo-Kobayashi-Maskawa matrix, a unitary matrix describing the mixing of quark flavors in weak interactions. In nuclear physics, the element $V_{ud}$ is extracted with high precision from superallowed $0^+ \to 0^+$ beta decays and tests the unitarity of the Standard Model. Ch. 14, 31.

Clebsch-Gordan coefficient. A numerical coefficient that arises in the coupling of two angular momenta $\mathbf{j}_1$ and $\mathbf{j}_2$ to form a resultant $\mathbf{J}$. Written $(j_1 m_1 j_2 m_2 | J M)$, it gives the amplitude for finding the coupled state $|JM\rangle$ in the uncoupled product state $|j_1 m_1\rangle |j_2 m_2\rangle$. Ch. 5.

Cluster radioactivity. A rare radioactive decay mode in which a nucleus emits a light cluster heavier than an alpha particle but lighter than a typical fission fragment (e.g., ${}^{14}$C, ${}^{24}$Ne, ${}^{28}$Mg). Observed primarily in the actinide region. Ch. 13.

Coherent elastic neutrino-nucleus scattering (CE$\nu$NS). A process in which a neutrino scatters coherently off all nucleons in a nucleus via the neutral weak current, with a cross section approximately proportional to $N^2$ (the square of the neutron number). First observed by the COHERENT experiment at Oak Ridge (2017). Now used to probe nuclear neutron form factors, neutrino properties, and physics beyond the Standard Model. Ch. 32.

CNO cycle. A catalytic cycle of hydrogen burning in stars in which carbon, nitrogen, and oxygen isotopes serve as catalysts for the net conversion of four protons into one alpha particle. Dominates over the pp chain for stellar core temperatures above approximately $1.7 \times 10^7$ K (stars with $M \gtrsim 1.3 M_\odot$). Ch. 21, 22.

Collective motion. Coherent motion of many nucleons acting together, as opposed to single-particle excitations. Includes vibrational modes (surface oscillations) and rotational modes (rotation of a deformed nucleus). Ch. 8.

Compound nucleus. The intermediate, highly excited nuclear system formed when a projectile is fully absorbed by a target nucleus. The compound nucleus exists long enough ($\sim 10^{-15}$--$10^{-18}$ s) for the excitation energy to be shared statistically among all nucleons before it decays. Ch. 18.

Compton scattering. The inelastic scattering of a photon by a free or loosely bound electron, in which the photon transfers part of its energy to the electron and is deflected at a reduced energy. The dominant photon interaction mechanism in the 0.5--5 MeV range for most materials. Ch. 16.

Confinement (color confinement). The property of QCD that prevents isolated quarks or gluons from existing as free particles. As quarks are separated, the strong force between them increases rather than decreasing, confining them permanently inside hadrons. Ch. 31.

Conversion coefficient. See internal conversion coefficient. Ch. 15.

Core-collapse supernova. The catastrophic death of a massive star ($M \gtrsim 8 M_\odot$) that occurs when the iron core exceeds the Chandrasekhar mass and collapses to a neutron star or black hole. The gravitational energy released ($\sim 3 \times 10^{53}$ erg) powers the explosion and drives explosive nucleosynthesis. Ch. 23.

Coulomb barrier. The electrostatic potential energy barrier that must be overcome (or tunneled through) for two positively charged nuclei to come close enough for the short-range nuclear force to act. The barrier height is approximately $V_C = z_1 z_2 e^2 / (4\pi\epsilon_0 R)$, where $R$ is the sum of the nuclear radii. Ch. 13, 17, 21.

Coulomb excitation. The excitation of nuclear states by the electromagnetic (Coulomb) interaction between a projectile and target nucleus during a close passage. Coulomb excitation is a purely electromagnetic process (no nuclear contact) and provides a clean measurement of electromagnetic transition strengths, particularly $B(E2)$ values. Ch. 9.

Coulomb term. The term in the semi-empirical mass formula that accounts for the electrostatic repulsion energy among all protons in the nucleus. Proportional to $Z(Z-1)/A^{1/3}$, it penalizes proton-rich nuclei and favors a neutron excess for heavy nuclei. Ch. 4.

Critical mass. The minimum mass of fissile material required to sustain a self-propagating chain reaction for a given geometry and composition. Depends on enrichment, shape, density, and the presence of reflectors or moderators. Ch. 20, 28.

Cross section ($\sigma$). A measure of the probability that a specific nuclear reaction or scattering process will occur, expressed as an effective target area. Defined operationally by $R = \sigma \cdot I \cdot n$, where $R$ is the reaction rate, $I$ is the beam intensity, and $n$ is the areal density of target nuclei. The unit is the barn. Ch. 1, 17.

Curie (Ci). A historical unit of radioactivity, defined as $3.7 \times 10^{10}$ disintegrations per second (the approximate activity of 1 gram of ${}^{226}$Ra). Replaced by the becquerel in SI but still widely used in medical and industrial contexts. Ch. 12.

D

Decay chain. A sequence of radioactive decays in which a parent nuclide decays to a daughter, which itself is radioactive and decays further, continuing until a stable nuclide is reached. The four natural decay chains originate from ${}^{232}$Th, ${}^{235}$U, ${}^{238}$U, and (extinct) ${}^{237}$Np. Ch. 12.

Decay constant ($\lambda$). The probability per unit time that a radioactive nucleus will decay. Related to half-life by $\lambda = \ln 2 / t_{1/2}$ and to mean lifetime by $\lambda = 1/\tau$. Ch. 12.

Deformation parameter ($\beta$). A dimensionless parameter characterizing the deviation of the nuclear shape from a sphere. The quadrupole deformation $\beta_2$ is the most common: $\beta_2 > 0$ indicates a prolate (elongated) shape, $\beta_2 < 0$ an oblate (flattened) shape, and $\beta_2 = 0$ a sphere. Typical values for well-deformed nuclei are $|\beta_2| \approx 0.2$--$0.4$. Ch. 8.

Delayed neutron. A neutron emitted by a fission product (a delayed neutron precursor) following beta decay, on a timescale of seconds to minutes after fission. Though comprising only $\sim 0.65\%$ of fission neutrons for ${}^{235}$U, delayed neutrons are essential for the safe control of nuclear reactors. Ch. 20, 26.

Density of states (level density). The number of nuclear energy levels per unit energy interval at a given excitation energy. Level density increases roughly exponentially with excitation energy and is a key input to statistical model (Hauser-Feshbach) reaction calculations. Ch. 18.

Density saturation. The observation that the central density of nuclei is approximately constant ($\rho_0 \approx 0.16$ nucleons/fm$^3$, or $\approx 2.3 \times 10^{17}$ kg/m$^3$) independent of mass number $A$. This reflects the short-range, saturating nature of the nuclear force and leads to the radius formula $R \approx r_0 A^{1/3}$. Ch. 1, 2.

Deuteron. The bound state of one proton and one neutron, the nucleus of deuterium (${}^2$H). The deuteron is the simplest composite nuclear system and provides fundamental constraints on the nucleon-nucleon interaction. It has $J^\pi = 1^+$, a binding energy of 2.224 MeV, and a small electric quadrupole moment indicating a slight D-state admixture. Ch. 3.

Differential cross section ($d\sigma/d\Omega$). The cross section per unit solid angle, giving the probability of scattering into a particular direction. Units: barn/steradian. Ch. 1, 17.

Direct reaction. A nuclear reaction in which the projectile interacts with one or a few nucleons on the nuclear surface, transferring energy or nucleons in a single step without forming a compound nucleus. Occurs on a timescale comparable to the nuclear transit time ($\sim 10^{-22}$ s). Examples include stripping, pickup, and knockout reactions. Ch. 19.

Distance of closest approach. The minimum distance between two charged particles in a head-on Coulomb collision. For an alpha particle with kinetic energy $T$ approaching a nucleus of charge $Z$: $d = 2kZe^2/T$. Ch. 1.

Distorted Wave Born Approximation (DWBA). A theoretical framework for calculating direct reaction cross sections in which the incoming and outgoing waves are distorted by optical potentials (rather than being plane waves as in the simple Born approximation). The standard tool for extracting spectroscopic factors from transfer reaction angular distributions. Ch. 19.

Double beta decay ($2\nu\beta\beta$). The simultaneous emission of two electrons and two antineutrinos, converting two neutrons in a nucleus to two protons. This second-order weak process occurs in even-even nuclei where single beta decay is energetically forbidden. Half-lives are extremely long ($\sim 10^{18}$--$10^{24}$ years). Ch. 14, 32.

Drip line. The boundary on the chart of nuclides beyond which the addition of one more proton (proton drip line) or neutron (neutron drip line) would result in an unbound system (negative separation energy). Nuclei beyond the drip lines undergo prompt nucleon emission. Ch. 4, 10.

E

Effective field theory (EFT). A theoretical framework for describing physics at a given energy scale using only the relevant degrees of freedom at that scale, with higher-energy effects systematically parameterized by contact interactions and organized by a power-counting scheme. In nuclear physics, the principal implementations are chiral EFT (pions and nucleons as degrees of freedom) and pionless EFT (nucleons only, valid below the pion mass scale). Ch. 3, 31.

Effective dose ($E$). The sum over all irradiated organs of the products of equivalent dose $H_T$ and tissue weighting factor $w_T$: $E = \sum_T w_T H_T$. It provides a single number representing the overall risk from a non-uniform irradiation. The unit is the sievert (Sv). Ch. 16, 29, Appendix F.

Electric dipole (E1) transition. An electromagnetic transition in which a nucleus emits (or absorbs) a photon carrying one unit of angular momentum with a parity change. E1 transitions are typically the fastest electric transitions, with Weisskopf estimates giving lifetimes on the order of $10^{-15}$ s for MeV gamma rays in medium-mass nuclei. Ch. 9.

Electric dipole moment (EDM). A permanent electric dipole moment of a particle, atom, or nucleus would violate both parity (P) and time-reversal (T) symmetry. Searches for EDMs in the neutron, diamagnetic atoms (${}^{199}$Hg, ${}^{225}$Ra), and polar molecules provide some of the most sensitive probes of CP violation beyond the Standard Model. Current upper limits constrain new physics at energy scales of $10$-$100$ TeV. Ch. 32.

Electric quadrupole moment ($Q$). A measure of the deviation of the nuclear charge distribution from spherical symmetry. A prolate nucleus has $Q > 0$; an oblate nucleus has $Q < 0$; a spherical nucleus has $Q = 0$. Ch. 2, 8.

Electron capture (EC). A beta-decay process in which the nucleus captures an inner-shell atomic electron, converting a proton to a neutron and emitting a neutrino. Competes with positron emission when the $Q$-value exceeds $2m_e c^2$; is the only available beta-plus process when $Q < 2m_e c^2$. Ch. 14.

Electron scattering. A technique for measuring nuclear charge distributions by scattering electrons off nuclei. Because the electron interacts electromagnetically (not via the strong force), the analysis is clean. Electron scattering provided the first precise measurements of nuclear radii and revealed the Fermi-type charge distribution. Ch. 1, 2.

ENDF (Evaluated Nuclear Data File). The primary U.S. evaluated nuclear reaction data library, containing neutron, charged-particle, and photonuclear cross sections, decay data, and fission yields. The current release is ENDF/B-VIII.0. Ch. 35, Appendix E.

ENSDF (Evaluated Nuclear Structure Data File). A comprehensive evaluated database of nuclear structure information including level schemes, transition data, and decay data for all known nuclides. Maintained by the NNDC at Brookhaven. Ch. 35, Appendix E.

Equation of state (EOS). A thermodynamic relation between pressure, density, temperature, and composition of matter. In nuclear physics, the nuclear EOS describes the behavior of nuclear matter at densities ranging from sub-saturation to several times nuclear saturation density, and is a key input for neutron star structure calculations. Ch. 25.

Equilibrium (radioactive). A condition in a decay chain in which the ratio of daughter activity to parent activity becomes constant over time. Secular equilibrium occurs when $t_{1/2}(\text{parent}) \gg t_{1/2}(\text{daughter})$; transient equilibrium when $t_{1/2}(\text{parent}) > t_{1/2}(\text{daughter})$ but not vastly so. Ch. 12.

Equivalent dose ($H_T$). The absorbed dose in a tissue or organ $T$, weighted by the radiation weighting factor $w_R$ for the type and energy of the radiation: $H_T = \sum_R w_R D_{T,R}$. The unit is the sievert (Sv). Ch. 16, 29, Appendix F.

EXFOR. The international database of experimental nuclear reaction data, maintained jointly by the IAEA, NNDC, and other data centers. Contains over 24,000 datasets of measured cross sections, angular distributions, and related quantities. Ch. 35, Appendix E.

F

Fermi decay (Fermi transition). A beta-decay transition in which the emitted lepton pair (electron + antineutrino, or positron + neutrino) carries zero total angular momentum ($\Delta J = 0$, no parity change). The nuclear operator is the isospin ladder operator. Superallowed $0^+ \to 0^+$ Fermi transitions provide the most precise determination of $V_{ud}$ and test CKM unitarity. Ch. 14.

Fermi distribution. The charge density profile commonly used to parametrize nuclear charge distributions: $\rho(r) = \rho_0 / [1 + \exp((r-c)/a)]$, where $c$ is the half-density radius and $a$ is the diffuseness (skin thickness) parameter. Ch. 2.

Fermi energy. In the nuclear Fermi gas model, the kinetic energy of the highest occupied nucleon state at zero temperature. For symmetric nuclear matter at saturation density, the Fermi energy is approximately 37 MeV. Ch. 4, 5.

Fermi's golden rule. The formula giving the transition rate from an initial state to a set of final states under a weak perturbation: $\Gamma = (2\pi/\hbar)|\langle f | \hat{V} | i \rangle|^2 \rho(E_f)$, where $\rho(E_f)$ is the density of final states. The starting point for deriving decay rates and reaction cross sections in nuclear physics. Ch. 5.

Fertile material. A nuclide that is not itself fissile but can be converted to a fissile nuclide by neutron capture and subsequent beta decays. The principal fertile materials are ${}^{238}$U (converts to ${}^{239}$Pu) and ${}^{232}$Th (converts to ${}^{233}$U). Breeding reactors exploit fertile-to-fissile conversion. Ch. 26.

Fissile nuclide. A nuclide that can sustain a chain reaction with thermal (low-energy) neutrons. The three most important are ${}^{233}$U, ${}^{235}$U, and ${}^{239}$Pu. Contrast with fissionable, which includes any nuclide that can undergo fission at some neutron energy. Ch. 20.

Fission. The splitting of a heavy nucleus into two (or rarely three) medium-mass fragments, typically accompanied by the emission of several neutrons and the release of approximately 200 MeV of energy. May be spontaneous (quantum tunneling through the fission barrier) or induced (by neutron capture or other reactions). Ch. 20.

Fission barrier. The potential energy barrier that a nucleus must overcome (or tunnel through) to undergo fission. In the liquid drop model, the barrier arises from the competition between the surface energy (which resists deformation) and the Coulomb energy (which favors it). Shell effects create a double-humped barrier in the actinide region. Ch. 20.

Fissility parameter ($x$). The ratio $x = E_C / 2E_S$ of the Coulomb energy to twice the surface energy in the liquid drop model. A nucleus with $x \ge 1$ has no fission barrier and is unstable against spontaneous fission. For the actinides, $x \approx 0.7$-$0.8$, meaning a finite but penetrable barrier exists. The fissility parameter determines the height of the liquid-drop fission barrier. Ch. 4, 20.

Fission product. One of the two (or more) nuclei produced by nuclear fission. Fission products are typically neutron-rich and undergo a series of beta decays toward the valley of stability. The mass distribution of fission products is typically asymmetric for thermal fission of actinides. Ch. 20.

Form factor. The Fourier transform of the nuclear charge or matter distribution, obtained from elastic electron scattering. The form factor $F(q)$ as a function of momentum transfer $q$ encodes the spatial structure of the nucleus. Ch. 2, 31.

Four-factor formula. An expression for the infinite neutron multiplication factor in a reactor: $k_\infty = \eta f p \varepsilon$, where $\eta$ is the reproduction factor, $f$ is the thermal utilization, $p$ is the resonance escape probability, and $\varepsilon$ is the fast fission factor. Ch. 20, 26.

ft-value. The comparative half-life for a beta-decay transition, defined as $ft = K / (G_F^2 |M_{fi}|^2)$, where $f$ is the Fermi function (statistical rate function), $t$ is the partial half-life, $K$ is a constant, and $M_{fi}$ is the nuclear matrix element. The $\log ft$ value classifies transitions as superallowed ($\log ft \approx 3.0$--3.7), allowed ($\approx 4$--6), or forbidden (progressively higher). Ch. 14.

Fusion. The combining of two light nuclei to form a heavier nucleus, releasing energy when the product has a higher binding energy per nucleon than the reactants. Fusion powers stars (pp chain, CNO cycle) and is the basis for thermonuclear weapons and experimental fusion reactors. Ch. 21.

G

Gamma decay. The emission of a high-energy photon (gamma ray) by a nucleus transitioning from an excited state to a lower-energy state. Gamma decay does not change $Z$ or $A$; it carries away energy, angular momentum, and (through its multipolarity) information about the structure of the nuclear states involved. Ch. 15.

Gamow factor. The exponential tunneling probability for a charged particle to penetrate the Coulomb barrier: $P \propto e^{-2\pi\eta}$, where $\eta = z_1 z_2 e^2 / (\hbar v)$ is the Sommerfeld parameter. The Gamow factor controls the energy dependence of charged-particle reaction cross sections at sub-barrier energies and is the key ingredient in the Gamow peak calculation. Ch. 13, 21.

Gamow peak. The energy window in which thermonuclear reactions in stellar interiors are most probable, determined by the overlap between the Maxwell-Boltzmann energy distribution of the ions (which decreases exponentially with energy) and the tunneling probability through the Coulomb barrier (which increases exponentially with energy). Ch. 21.

Gamow theory (of alpha decay). The quantum mechanical theory of alpha decay developed independently by Gamow and by Condon and Gurney in 1928. The alpha particle, preformed inside the nucleus, tunnels through the Coulomb barrier with a probability that depends exponentially on the barrier height and width, naturally explaining the Geiger-Nuttall law. Ch. 13.

Gamow-Teller transition. A beta-decay transition in which the emitted lepton pair carries one unit of angular momentum ($\Delta J = 0, \pm 1$, with $0 \to 0$ forbidden, no parity change). The nuclear operator involves both spin and isospin. Ch. 14.

Geiger-Nuttall law. The empirical relation between the alpha-decay half-life and the alpha-particle kinetic energy: $\log t_{1/2} = a / \sqrt{Q_\alpha} + b$, where $a$ and $b$ are constants for a given element. Explained by Gamow theory. Ch. 13.

Giant resonance. A collective excitation of the nucleus in which many nucleons participate coherently. The most prominent is the Giant Dipole Resonance (GDR), a collective oscillation of protons against neutrons that appears as a broad peak in the photonuclear cross section at $E \approx 80 A^{-1/3}$ MeV. Ch. 9.

Gluon. The gauge boson mediating the strong force between quarks in QCD. Gluons carry color charge and interact with each other, giving rise to confinement. There are eight gluon types. Ch. 31.

Gray (Gy). The SI unit of absorbed radiation dose, equal to 1 joule of energy deposited per kilogram of matter. Replaces the older unit rad (1 Gy = 100 rad). Ch. 16.

Gravitational wave. A ripple in spacetime produced by accelerating masses, predicted by general relativity and first directly detected by LIGO in 2015. In nuclear physics, the landmark event was GW170817 (2017) -- gravitational waves from a neutron star merger accompanied by a kilonova, confirming that neutron star mergers are a major r-process site and constraining the nuclear equation of state through tidal deformability measurements. Ch. 23, 25.

H

Half-life ($t_{1/2}$). The time required for half of the atoms in a radioactive sample to decay. Related to the decay constant by $t_{1/2} = \ln 2 / \lambda \approx 0.693 / \lambda$. Ch. 12.

Half-value layer (HVL). The thickness of a shielding material required to reduce the intensity of a radiation beam by half. Related to the linear attenuation coefficient by HVL $= \ln 2 / \mu$. Ch. 16, Appendix F.

Halo nucleus. A weakly bound nucleus with an extremely extended spatial distribution, in which one or two valence nucleons (typically neutrons) tunnel far beyond the range of the nuclear potential, forming a dilute "halo" around a compact core. Classic examples are ${}^{11}$Li (two-neutron halo) and ${}^{11}$Be (one-neutron halo). Ch. 10.

Hauser-Feshbach theory. A statistical model for compound nuclear reactions that calculates the cross section by averaging over overlapping compound nucleus resonances. The cross section for $a + A \to B + b$ is proportional to $T_a T_b / \sum_c T_c$, where $T$ denotes transmission coefficients. Ch. 18.

Hyperon. A baryon containing one or more strange quarks ($\Lambda$, $\Sigma$, $\Xi$, $\Omega$). Hyperons may appear in the dense cores of neutron stars if the neutron chemical potential exceeds the hyperon rest mass, a possibility known as the "hyperon puzzle" that softens the equation of state. Ch. 25.

I

Impact parameter ($b$). The perpendicular distance between the initial trajectory of a projectile and a parallel line through the center of the target. Smaller impact parameters correspond to larger scattering angles in Coulomb scattering. Ch. 1.

Interacting boson model (IBM/IBA). A model of collective nuclear structure in which correlated pairs of valence nucleons (proton pairs and neutron pairs) are treated as bosons ($s$-bosons with $L=0$ and $d$-bosons with $L=2$). The IBM reproduces vibrational, rotational, and transitional nuclear spectra as limiting symmetries of the U(6) group. Ch. 8.

Internal conversion. A process competing with gamma-ray emission in which the electromagnetic energy of a nuclear transition is transferred directly to an atomic electron, which is then ejected from the atom. The ratio of conversion electrons to gamma rays is the internal conversion coefficient $\alpha$, which increases strongly with multipolarity and $Z$ and decreases with transition energy. Ch. 15.

Iron peak. The group of elements near iron ($Z = 26$), cobalt, and nickel that have the highest binding energies per nucleon and are produced in nuclear statistical equilibrium during the silicon-burning phase of massive stars and in Type Ia supernovae. Ch. 22.

Island of inversion. A region of the nuclear chart (centered near $N = 20$, $Z \approx 10$--12) where nuclides exhibit deformed ground states rather than the spherical shapes expected from traditional magic numbers. The shell closure at $N = 20$ is broken by the residual interaction, demonstrating shell evolution far from stability. Ch. 10.

Island of stability. A predicted region of relatively long-lived superheavy nuclei near closed proton and neutron shells (predicted at $Z \approx 114$ or 120, $N = 184$). Shell effects are predicted to create a local maximum in binding energy and significantly enhance stability against fission. Ch. 11.

Isobar. One of a set of nuclides sharing the same mass number $A$ but different $Z$ and $N$. Isobars connected by beta decay form an isobaric chain. Ch. 1.

Isomer. A nuclear state with a measurably long half-life (typically $\gtrsim$nanoseconds), generally arising because the available decay transitions are highly forbidden (high multipolarity or low transition energy). Examples include ${}^{99m}$Tc ($t_{1/2} = 6.01$ h) and ${}^{178m2}$Hf ($t_{1/2} = 31$ y). Ch. 7, 15.

Isospin. A quantum number treating the proton and neutron as two states of a single particle (the nucleon), analogous to spin-1/2. Total isospin $T$ and its projection $T_3 = (N-Z)/2$ are approximate quantum numbers reflecting the approximate charge independence of the nuclear force. Ch. 2, 3.

Isotone. One of a set of nuclides sharing the same neutron number $N$ but different $Z$. Ch. 1.

Isotope. One of a set of nuclides sharing the same atomic number $Z$ (same element) but different neutron number $N$. Ch. 1.

ITER. The international experimental tokamak fusion reactor under construction at Cadarache, France, designed to demonstrate net energy production from magnetically confined deuterium-tritium fusion. Ch. 21.

K

K-quantum number. In a deformed (axially symmetric) nucleus, the projection of the total angular momentum $J$ on the nuclear symmetry axis. Rotational bands are built on states with a given $K$ value, with $J$ taking values $K, K+1, K+2, \ldots$ for $K > 0$ and $0, 2, 4, \ldots$ for $K = 0$ (ground-state band of even-even nuclei). Ch. 8.

Kilonova. The electromagnetic transient powered by the radioactive decay of heavy elements synthesized in the rapid neutron capture process (r-process) following a neutron star merger. The kilonova associated with GW170817 (August 2017) provided the first direct evidence that neutron star mergers produce r-process elements. Ch. 23.

Kurie plot. A linearized representation of the beta-decay energy spectrum, plotting $\sqrt{N(E)/[p^2 F(Z,E)]}$ versus the electron kinetic energy $E$, where $N(E)$ is the measured spectrum, $p$ is the electron momentum, and $F(Z,E)$ is the Fermi function. For an allowed transition, the Kurie plot is a straight line intersecting the energy axis at the endpoint $Q_\beta$. Ch. 14.

L

Lattice QCD. A computational approach to solving quantum chromodynamics by discretizing spacetime on a four-dimensional grid (lattice) and evaluating the path integral numerically. Lattice QCD provides first-principles calculations of hadron masses, nucleon structure, and (increasingly) nuclear interactions. Ch. 31.

Lawson criterion. The condition for a fusion plasma to produce net energy: the product of ion density $n$, confinement time $\tau_E$, and temperature $T$ must exceed a critical value. For deuterium-tritium fusion: $n \tau_E T \gtrsim 5 \times 10^{21}$ keV$\cdot$s/m$^3$. Ch. 21.

Level density. See density of states. Ch. 18.

Level scheme. A diagram showing the energy levels of a nucleus, together with the gamma-ray transitions connecting them, their multipolarities, and intensities. The experimental level scheme is the fundamental structural data for a nucleus. Ch. 6, 9.

Linear energy transfer (LET). The energy deposited per unit path length by a charged particle in matter. High-LET radiation (alpha particles, heavy ions) produces dense ionization tracks and is more biologically damaging per unit dose than low-LET radiation (electrons, photons). Ch. 16, 27.

Linear no-threshold model (LNT). The hypothesis that the risk of stochastic radiation effects (cancer) is proportional to dose with no threshold below which the risk is zero. The LNT model is the basis for current radiation protection regulations, though its validity at very low doses remains debated. Ch. 29.

Liquid drop model. A model of the nucleus that treats it as an incompressible drop of quantum liquid, with properties analogous to a classical liquid: constant density, short-range interactions, surface tension. The semi-empirical mass formula and the Bohr-Wheeler fission theory are based on the liquid drop model. Ch. 4, 20.

M

Magic numbers. The nucleon numbers (2, 8, 20, 28, 50, 82, and 126 for neutrons; 2, 8, 20, 28, 50, 82 for protons) at which nuclei exhibit exceptional stability due to the complete filling of nuclear shells. Evidence includes discontinuities in separation energies, high-energy first excited states, and high abundances of neighboring nuclei in the cosmos. Ch. 1, 6.

Magnetic dipole moment. The nuclear magnetic moment, arising from the orbital motion and intrinsic spins of the constituent nucleons. Measured in units of nuclear magnetons ($\mu_N = e\hbar/2m_p$). For a single nucleon in a state with angular momentum $j$, the Schmidt lines give the expected values. Ch. 2.

Mass defect. The difference between the mass of the separated nucleons and the mass of the assembled nucleus: $\Delta M = Z m_p + N m_n - M(Z,N)$. The corresponding energy $B = \Delta M c^2$ is the binding energy. Ch. 1.

Mass excess ($\Delta$). The difference between the actual atomic mass $M$ and the mass number times the atomic mass unit: $\Delta = M - A \cdot u$, typically expressed in keV. Mass excess is the standard way nuclear masses are tabulated (AME). Ch. 2, 4.

Mean field. An effective single-particle potential experienced by each nucleon, generated self-consistently by averaging over the interactions with all other nucleons in the nucleus. The mean-field concept underlies the nuclear shell model: nucleons move approximately independently in a common potential (typically modeled as a Woods-Saxon potential). Mean-field theory is the starting point for Hartree-Fock and density functional theory calculations. Ch. 6, 7.

Mean lifetime ($\tau$). The average time a radioactive atom survives before decaying: $\tau = 1/\lambda = t_{1/2} / \ln 2 \approx 1.443 \, t_{1/2}$. Ch. 12.

Mirror nuclei. A pair of nuclei in which the proton and neutron numbers are interchanged: $(Z, N)$ and $(N, Z)$. Mirror nuclei have nearly identical level schemes due to the approximate charge symmetry of the nuclear force, with energy differences attributable primarily to the Coulomb interaction. Ch. 2, 3.

Moderator. A material used to slow down (thermalize) fast neutrons, typically through elastic scattering off light nuclei. Effective moderators include water ($^1$H), heavy water ($^2$H), and graphite ($^{12}$C). The moderator slowing-down properties are critical for thermal reactor design. Ch. 20, 26.

Mossbauer effect. The recoil-free emission and resonant absorption of gamma rays by nuclei embedded in a crystal lattice. Because the recoil momentum is absorbed by the entire crystal (rather than a single atom), the gamma-ray energy is not Doppler-broadened, allowing extremely precise energy measurements. Applications include testing general relativity, studying hyperfine interactions, and materials science. Ch. 15.

Multipole radiation. Electromagnetic radiation classified by the angular momentum $\lambda$ and parity it carries. Electric multipole radiation of order $\lambda$ ($E\lambda$) has parity $(-1)^\lambda$; magnetic multipole radiation ($M\lambda$) has parity $(-1)^{\lambda+1}$. Ch. 9.

N

Neutrino. A neutral, weakly interacting lepton emitted in beta decay and other weak processes. Nuclear physics uses neutrinos primarily in beta-decay kinematics, double beta decay searches, and astrophysical nucleosynthesis (supernova neutrino processes). The question of whether neutrinos are Dirac or Majorana particles is probed by neutrinoless double beta decay experiments. Ch. 14, 32.

Neutrinoless double beta decay ($0\nu\beta\beta$). A hypothetical lepton-number-violating process in which two neutrons convert to two protons with the emission of two electrons but no neutrinos. Observation would prove that neutrinos are Majorana particles (their own antiparticles) and would measure the effective Majorana mass. One of the highest-priority experiments in nuclear and particle physics. Ch. 14, 32.

Neutron. An electrically neutral baryon (composed of one up quark and two down quarks) with mass 939.565 MeV/$c^2$ and spin-1/2. Free neutrons are unstable, decaying by beta-minus emission with a half-life of approximately 10.2 minutes. Ch. 1.

Neutron capture. A nuclear reaction in which a neutron is absorbed by a nucleus, forming a heavier isotope and typically emitting a gamma ray: $(n,\gamma)$. The dominant neutron reaction channel at low energies for most nuclides and the mechanism underlying the $s$-process and $r$-process. Ch. 18, 22, 23.

Neutron star. A compact stellar remnant consisting primarily of neutrons, formed in core-collapse supernovae. Neutron star properties (mass, radius, composition, cooling) are determined by the nuclear equation of state at extreme densities. Typical parameters: $M \approx 1.4 M_\odot$, $R \approx 10$--13 km, central density $\rho \approx 5$--$10 \rho_0$. Ch. 25.

Nilsson model. A single-particle model for deformed nuclei in which nucleons move in an axially deformed harmonic oscillator potential (plus spin-orbit and $l^2$ corrections). Nilsson diagrams show single-particle energies as functions of deformation. Each level is labeled by the asymptotic quantum numbers $K^\pi[N n_z \Lambda]$. Ch. 7, 8.

Nuclear force. The strong interaction between nucleons, characterized by short range ($\sim 1$--2 fm), strong attraction at intermediate distances, a repulsive core at very short distances ($\lesssim 0.5$ fm), spin dependence, approximate charge independence, and saturation. The nuclear force is ultimately a residual effect of the QCD interaction between quarks. Ch. 3.

Nuclear magneton ($\mu_N$). The natural unit for nuclear magnetic moments: $\mu_N = e\hbar / 2m_p = 3.152 \times 10^{-8}$ eV/T = $5.051 \times 10^{-27}$ J/T. Smaller than the Bohr magneton by a factor of $m_e/m_p \approx 1/1836$. Ch. 2.

Nuclear pasta. Exotic configurations of nucleonic matter predicted to exist in neutron star crusts at densities just below nuclear saturation density ($\rho \sim 10^{14}$ g/cm$^3$), where the competition between nuclear attraction and Coulomb repulsion produces non-spherical shapes. These phases are named by culinary analogy: "gnocchi" (spheres), "spaghetti" (rods), "lasagna" (slabs), "anti-spaghetti" (tubes), and "anti-gnocchi" (bubbles). Nuclear pasta may affect neutron star cooling, neutrino transport, and gravitational wave emission. Ch. 25.

Nuclear reaction. A process in which two nuclei (or a nucleon and a nucleus) interact, producing different nuclei and/or particles. Nuclear reactions conserve charge, baryon number, energy, momentum, and angular momentum. Standard notation: $a(b,c)d$ or $a + b \to c + d$, where $a$ is the target, $b$ the projectile, $c$ the ejectile, and $d$ the residual nucleus. Ch. 17.

Nuclear matter. An idealized infinite system of nucleons interacting via the nuclear force, without the Coulomb interaction and without surface effects. Nuclear matter at saturation density ($\rho_0 \approx 0.16$ fm$^{-3}$, binding energy $\approx 16$ MeV/nucleon) serves as a reference for the nuclear equation of state. Ch. 3, 4, 25.

Nuclear statistical equilibrium (NSE). A condition in which nuclear reactions proceed rapidly enough in both forward and reverse directions that the abundance distribution is determined entirely by nuclear binding energies, temperature, and density (through Saha-like equations), independent of individual reaction rates. Achieved at $T \gtrsim 5 \times 10^9$ K and produces the iron-peak abundance pattern. Ch. 22.

Nucleon. A generic term for a proton or neutron — the constituents of the atomic nucleus. Ch. 1.

Nucleosynthesis. The creation of atomic nuclei from pre-existing nucleons and nuclei. The principal nucleosynthesis processes are Big Bang nucleosynthesis (H, He, Li; Ch. 24), stellar burning (up to Fe; Ch. 22), and explosive/capture processes (s-process, r-process, p-process, rp-process, $\nu$-process) for elements heavier than iron (Ch. 23). Ch. 22, 23, 24.

Nuclide. A specific nuclear species characterized by its proton number $Z$ and neutron number $N$ (equivalently, $Z$ and $A$). Each distinct combination of $Z$ and $N$ defines a different nuclide. Ch. 1.

O

Optical model. A model of nuclear scattering and reactions in which the interaction between the projectile and target is described by a complex potential: the real part accounts for elastic scattering (refraction) and the imaginary part accounts for absorption into non-elastic channels. Ch. 17.

P

Pair production. The creation of an electron-positron pair by a high-energy photon interacting with the electromagnetic field of a nucleus. Requires a photon energy exceeding $2m_e c^2 = 1.022$ MeV. Dominant photon interaction mechanism at energies above approximately 5--10 MeV (material-dependent). Ch. 16.

Pairing energy (pairing term). The term in the semi-empirical mass formula that accounts for the tendency of nucleons to form spin-zero pairs. Even-even nuclei (all nucleons paired) have extra stability; odd-odd nuclei have reduced stability; odd-$A$ nuclei are intermediate. The pairing energy is $\delta \approx \pm 12/\sqrt{A}$ MeV. Ch. 4, 7.

Pairing interaction. A short-range attractive interaction between nucleons in time-reversed orbits, analogous to the Cooper pairing in superconductors. Pairing produces an energy gap $\Delta \approx 12/\sqrt{A}$ MeV in even-even nuclei, explains the systematic odd-even staggering in binding energies, and is responsible for nuclear superfluidity and reduced moments of inertia relative to the rigid-body value. Ch. 7.

Parity ($\pi$). A quantum number describing the behavior of the nuclear wave function under spatial inversion ($\mathbf{r} \to -\mathbf{r}$). Even parity ($\pi = +$) means the wave function is unchanged; odd parity ($\pi = -$) means it changes sign. Nuclear states are labeled by $J^\pi$. Ch. 2, 5.

Parity violation. The non-conservation of parity in weak interactions, discovered by Wu et al. (1957) in the beta decay of polarized ${}^{60}$Co. Nuclear physics experiments continue to test parity violation through measurements of anapole moments, parity-violating electron scattering, and neutron spin rotation. Ch. 14, 32.

Partial wave. A component of a scattering wave function with definite orbital angular momentum $l$. In partial-wave analysis, the scattering amplitude is expanded as $f(\theta) = \sum_l (2l+1) a_l P_l(\cos\theta)$, and the cross section is expressed in terms of partial-wave amplitudes or phase shifts. At low energies, only the lowest partial waves contribute significantly. Ch. 5, 17.

Partial width ($\Gamma_i$). The contribution to the total decay width $\Gamma$ of a resonance from a specific decay channel $i$. The branching ratio for channel $i$ is $\Gamma_i / \Gamma$. Related to the partial half-life by $\Gamma_i = \hbar / \tau_i$. Ch. 18.

Pauli exclusion principle. The requirement that no two identical fermions (including protons or neutrons) occupy the same quantum state. In nuclear physics, the Pauli principle determines the shell structure, produces the asymmetry energy, and constrains allowed transitions. Ch. 4, 5, 6.

Penning trap. A device that confines a single charged particle (ion) using a combination of static electric and magnetic fields. In nuclear physics, Penning traps achieve atomic mass measurements with precisions of $\delta m/m \sim 10^{-9}$ or better, enabling precise determination of $Q$-values, separation energies, binding energies of exotic nuclei, and tests of CKM unitarity via superallowed beta-decay $Q$-values. Ch. 2, 30.

PET (Positron Emission Tomography). A medical imaging technique that detects the coincident 511 keV annihilation photon pairs produced when positrons from a radioactive tracer (typically ${}^{18}$F-FDG) annihilate with electrons in tissue. Ch. 27.

Photoelectric effect (nuclear context). The absorption of a photon by an atom, ejecting an inner-shell electron. Dominates the photon attenuation cross section at low energies (below $\sim$100 keV to 1 MeV, depending on $Z$). Cross section scales approximately as $Z^{4\text{--}5}/E^{3.5}$. Ch. 16.

Photodisintegration. The breakup of a nucleus by absorption of a high-energy photon, e.g., $\gamma + d \to p + n$ or ${}^{56}$Fe($\gamma$,$\alpha$)${}^{52}$Cr. In astrophysics, photodisintegration of iron-group nuclei in stellar cores triggers core collapse. Ch. 22, 23.

pp chain. The proton-proton chain, a sequence of nuclear reactions that converts four protons into one alpha particle, two positrons, and two neutrinos, releasing 26.73 MeV. The dominant energy source in stars with core temperature below approximately $1.7 \times 10^7$ K (including the Sun). Ch. 21, 22.

Proton drip line. The boundary on the neutron-deficient side of the chart of nuclides beyond which the proton separation energy becomes negative and the nucleus is unbound to proton emission. The proton drip line has been experimentally established up to approximately $Z \approx 90$. Ch. 4, 10.

Proton radioactivity. The emission of a proton from a nuclear ground state or long-lived isomer, observed in very neutron-deficient nuclei beyond the proton drip line. The proton must tunnel through the combined Coulomb and centrifugal barrier. Ch. 10.

Proton therapy. A form of external-beam radiation therapy that uses proton beams to treat cancer. The Bragg peak of protons allows precise dose delivery to the tumor volume with sharp dose falloff beyond the target, sparing downstream healthy tissue. Carbon-ion therapy extends this concept with an even sharper Bragg peak and higher relative biological effectiveness. Ch. 27.

Q

Q-value. The net energy released or absorbed in a nuclear reaction or decay, equal to the difference in total rest-mass energy between the initial and final states: $Q = (M_\text{initial} - M_\text{final})c^2$. Positive $Q$ indicates an exothermic process; negative $Q$ indicates an endothermic process requiring an energy input. Ch. 12, 13, 17.

Quadrupole moment. See electric quadrupole moment. Ch. 2, 8.

Quark. A fundamental fermion carrying color charge, fractional electric charge, and flavor. Quarks come in six flavors (up, down, strange, charm, bottom, top). Protons ($uud$) and neutrons ($udd$) are each composed of three quarks bound by the strong force (QCD). Ch. 31.

Quark-gluon plasma (QGP). A state of matter in which quarks and gluons are deconfined and move freely over distances larger than a nucleon. Created in ultrarelativistic heavy-ion collisions at RHIC and the LHC. Ch. 31.

R

r-process (rapid neutron capture process). A nucleosynthesis mechanism in which seed nuclei capture neutrons on a timescale faster than beta decay, building up very neutron-rich isotopes that subsequently beta-decay to stability. Responsible for producing roughly half of all elements heavier than iron. Confirmed to occur in neutron star mergers (GW170817/kilonova). Ch. 23.

Radiation weighting factor ($w_R$). A dimensionless factor used to convert absorbed dose (Gy) to equivalent dose (Sv), reflecting the biological effectiveness of different radiation types. See Appendix F for values. Ch. 16, 29, Appendix F.

Radioactive dating. The determination of the age of a sample by measuring the ratio of a radioactive parent to its stable daughter or the amount of a cosmogenic nuclide. Common systems include ${}^{14}$C (archaeology, up to $\sim$50,000 years), ${}^{40}$K-${}^{40}$Ar (geology), and ${}^{238}$U-${}^{206}$Pb (geology, up to $\sim$4.5 Gy). Ch. 12.

Radioactive ion beam (RIB). A beam of short-lived radioactive nuclei produced at accelerator facilities (via ISOL or fragmentation methods) and used to study nuclear reactions and structure far from stability. Major RIB facilities include FRIB, ISOLDE, RIKEN-RIBF, and FAIR. Ch. 10, 30.

Radon. A radioactive noble gas (${}^{222}$Rn, $t_{1/2} = 3.82$ d, from the ${}^{238}$U decay chain) that seeps from soil and rock into buildings. Radon and its short-lived progeny are the largest single contributor to natural background radiation exposure, delivering dose primarily to the lungs. Ch. 29.

Reduced transition probability ($B(E\lambda)$, $B(M\lambda)$). The nuclear structure quantity that encodes the intrinsic strength of an electromagnetic transition, obtained by stripping the kinematic and angular momentum factors from the transition rate. Measured in units of $e^2$fm$^{2\lambda}$ (electric) or $\mu_N^2$fm$^{2\lambda-2}$ (magnetic). Large $B(E2)$ values indicate collective (deformed or vibrational) character. Ch. 9.

Resonance. A sharp peak in a nuclear reaction cross section at a specific energy, corresponding to a quasi-bound state of the compound system. Characterized by resonance energy $E_r$, total width $\Gamma$, and partial widths $\Gamma_i$. Described by the Breit-Wigner formula. Ch. 18.

Rotational band. A sequence of nuclear states built on a common intrinsic state (bandhead) in a deformed nucleus, with energies following approximately $E(J) = (\hbar^2/2\mathcal{I})J(J+1)$, where $\mathcal{I}$ is the moment of inertia. Even-even nuclei have ground-state bands with $J^\pi = 0^+, 2^+, 4^+, \ldots$ Ch. 8.

Rutherford scattering. The elastic scattering of a charged particle by the Coulomb potential of a nucleus. The Rutherford cross section is $d\sigma/d\Omega = (a/4)^2 \csc^4(\theta/2)$, where $a = z_1 z_2 e^2/(4E_\text{CM})$ is the half-distance of closest approach. Ch. 1, 17.

S

s-process (slow neutron capture process). A nucleosynthesis mechanism in which neutron captures proceed more slowly than beta decay, so the nucleosynthesis path follows the valley of stability. Occurs primarily in asymptotic giant branch (AGB) stars. Produces roughly half of all elements heavier than iron. Ch. 23.

Saturation (nuclear). The property of the nuclear force by which each nucleon interacts effectively only with a limited number of nearest neighbors, not with all other nucleons. This leads to constant nuclear density and the approximate proportionality of binding energy to mass number $A$ (rather than $A^2$). Ch. 2, 3, 4.

Schmidt lines. The predicted values of the nuclear magnetic moment for a single odd nucleon in a state of definite $j$, calculated from the orbital and spin gyromagnetic ratios. The "Schmidt values" systematically overestimate the spread of measured moments, indicating the importance of configuration mixing and meson exchange currents. Ch. 2.

Secular equilibrium. A condition in a radioactive decay chain where the half-life of the parent is much greater than that of any descendant, so that the activity of each daughter becomes equal to the activity of the parent. Ch. 12.

Selection rules. Rules governing which transitions between nuclear states are allowed and which are forbidden, based on conservation of angular momentum and parity. For electromagnetic transitions of multipolarity $\lambda$: $|J_i - J_f| \leq \lambda \leq J_i + J_f$, with parity change $(-1)^\lambda$ for $E\lambda$ and $(-1)^{\lambda+1}$ for $M\lambda$. Ch. 5, 9, 14.

Semi-empirical mass formula (SEMF). The Bethe-Weizsacker formula for nuclear binding energy: $B(Z,N) = a_V A - a_S A^{2/3} - a_C Z(Z-1)A^{-1/3} - a_A(N-Z)^2/(4A) + \delta(A,Z)$, where the five terms represent volume, surface, Coulomb, asymmetry, and pairing energies. Reproduces measured binding energies to $\sim$1% accuracy for medium and heavy nuclei. Ch. 4.

Seniority. A quantum number $\nu$ counting the number of unpaired nucleons in a shell-model configuration. States with low seniority (few unpaired nucleons) dominate the low-energy spectrum due to the pairing interaction. Ch. 7.

Separation energy. The minimum energy required to remove one nucleon (or a pair of nucleons) from a nucleus. $S_n$ is the one-neutron separation energy; $S_p$ is the one-proton separation energy. Discontinuities in separation energies at magic numbers are key evidence for nuclear shells. Ch. 1, 4, 6.

Shell model. The model of nuclear structure in which nucleons move independently in a mean-field potential (usually Woods-Saxon with spin-orbit coupling), filling discrete energy levels (shells). The filling order of shells accounts for the magic numbers and predicts ground-state spins, parities, and electromagnetic moments. Ch. 6.

Sievert (Sv). The SI unit of equivalent dose and effective dose, equal to 1 J/kg when weighted by radiation quality and tissue sensitivity factors. Replaces the older unit rem (1 Sv = 100 rem). Ch. 16, Appendix F.

Sommerfeld parameter ($\eta$). A dimensionless parameter characterizing the strength of the Coulomb interaction relative to the kinetic energy in a nuclear collision: $\eta = z_1 z_2 e^2 / (\hbar v)$, where $v$ is the relative velocity. Large $\eta$ indicates strong Coulomb effects (sub-barrier regime). Ch. 13, 17, 21.

Spectroscopic factor. The overlap between the actual nuclear wave function and the single-particle wave function assumed in a direct reaction calculation. Extracted from (d,p) or (e,e'p) data, it measures the purity of the single-particle nature of a state. A value near 1 indicates a nearly pure single-particle state; fragmented strength gives values $\ll 1$. Ch. 19.

Spin-orbit interaction. The interaction between a nucleon's orbital angular momentum $\mathbf{l}$ and its intrinsic spin $\mathbf{s}$, proportional to $\mathbf{l} \cdot \mathbf{s}$. In nuclei, the spin-orbit interaction is large and has the opposite sign from the atomic spin-orbit interaction. It splits levels with $j = l \pm 1/2$, producing the correct sequence of magic numbers. Proposed by Mayer, and independently by Haxel, Jensen, and Suess (1949). Ch. 6.

Statistical model. See Hauser-Feshbach theory. A model for compound nuclear reactions in which the cross section is calculated by averaging over many overlapping resonances, valid when the level density of the compound nucleus is high. The statistical model is the standard tool for calculating reaction cross sections at excitation energies above a few MeV. Ch. 18.

Stopping power ($-dE/dx$). The mean energy loss per unit path length of a charged particle in matter. The electronic stopping power (energy loss to atomic electrons) dominates at high energies; the nuclear stopping power (elastic collisions with atoms) dominates at very low energies. Ch. 16.

Stripping reaction. A direct nuclear reaction in which one or more nucleons are transferred from the projectile to the target. The classic example is the (d,p) reaction, in which a deuteron deposits a neutron into a target nucleus, producing a proton. Ch. 19.

Strong force. The fundamental force between quarks, mediated by gluons, described by quantum chromodynamics (QCD). At the hadronic scale, the nuclear force between nucleons is a residual effect of the strong force, analogous to the van der Waals force between neutral atoms. Ch. 3, 31.

Superallowed beta decay. An allowed Fermi beta-decay transition between $0^+$ states in mirror or isospin-multiplet nuclei, proceeding at the maximum possible rate ($\log ft \approx 3.0$--3.1). The $\mathcal{F}t$-values of superallowed decays provide the most precise determination of $V_{ud}$, the up-down element of the CKM quark mixing matrix. Ch. 14, 32.

Superdeformation. A nuclear shape with an axis ratio of approximately 2:1 (corresponding to $\beta_2 \approx 0.6$), observed as rotational bands with anomalously large moments of inertia and uniform gamma-ray spacing ($\sim$50 keV intervals). First discovered in ${}^{152}$Dy (1986). Ch. 8.

Symmetry energy. The coefficient $a_\text{sym}$ in the semi-empirical mass formula that accounts for the energy cost of having unequal numbers of protons and neutrons: $E_\text{sym} \propto (N-Z)^2/A$. The symmetry energy and its density dependence are critical for the nuclear equation of state, neutron star structure, and the location of the neutron drip line. At saturation density, $a_\text{sym} \approx 30$-$33$ MeV. Ch. 4, 25.

Superheavy element (SHE). An element with atomic number $Z \geq 104$ (beyond the actinides), whose existence depends on the stabilizing effect of nuclear shell structure against the destabilizing Coulomb repulsion. Elements up to $Z = 118$ (oganesson) have been synthesized. Ch. 11.

Surface term. The term in the semi-empirical mass formula that accounts for the reduced binding of nucleons at the nuclear surface compared to those in the interior. Proportional to $A^{2/3}$ (the surface area of the nucleus). Ch. 4.

T

Tensor force. A component of the nucleon-nucleon interaction with the operator structure $V_T \propto S_{12} = 3(\boldsymbol{\sigma}_1\cdot\hat{r})(\boldsymbol{\sigma}_2\cdot\hat{r}) - \boldsymbol{\sigma}_1\cdot\boldsymbol{\sigma}_2$, analogous to the magnetic dipole-dipole interaction. The tensor force mixes the $^3S_1$ and $^3D_1$ partial waves in the deuteron, producing the deuteron's nonzero electric quadrupole moment, and is a key driver of shell evolution in exotic nuclei (especially the quenching of the $N = 20$ and $N = 28$ gaps far from stability). Ch. 3, 10.

TENDL. TALYS-based Evaluated Nuclear Data Library. A comprehensive nuclear data library produced by systematic nuclear model calculations with the TALYS code, providing neutron and proton reaction cross sections for all nuclides. Ch. 35, Appendix E.

Theranostics. A treatment paradigm in nuclear medicine combining diagnostic imaging and targeted radionuclide therapy using the same molecular targeting agent labeled with different isotopes (or the same isotope with both imaging and therapeutic emissions). Example: ${}^{68}$Ga-PSMA for PET imaging paired with ${}^{177}$Lu-PSMA for therapy. Ch. 27.

Thermal neutron. A neutron in thermal equilibrium with its surroundings, with kinetic energy $E \approx kT \approx 0.025$ eV at room temperature (293 K). The most probable velocity is approximately 2,200 m/s. Cross sections for thermal neutron interactions are conventionally tabulated at this energy. Ch. 18, 20.

Three-nucleon force. An interaction among three nucleons that cannot be decomposed into a sum of pairwise (two-body) interactions. Three-nucleon forces are essential for reproducing nuclear binding energies, saturation properties, and the positions of drip lines. They arise naturally in chiral EFT at next-to-next-to-leading order. Ch. 3.

Threshold energy. The minimum kinetic energy (in the laboratory frame) that a projectile must have to make an endothermic ($Q < 0$) nuclear reaction energetically possible. For a projectile of mass $m$ on a stationary target of mass $M$: $T_\text{th} = |Q|(1 + m/M + Q/2Mc^2)$ approximately. Ch. 17.

Tissue weighting factor ($w_T$). A dimensionless factor representing the relative radiation sensitivity of different organs and tissues for stochastic effects. Used to compute effective dose. See Appendix F for values. Ch. 16, 29, Appendix F.

Tokamak. A toroidal magnetic confinement device for fusion plasmas, using a combination of toroidal and poloidal magnetic fields to confine a hot, dense plasma. The most developed approach to controlled thermonuclear fusion. Ch. 21.

Tolman-Oppenheimer-Volkoff (TOV) equation. The equation of hydrostatic equilibrium for a self-gravitating, spherically symmetric body in general relativity. Given a nuclear equation of state relating pressure to density, the TOV equation determines the mass-radius relationship for neutron stars. Ch. 25.

Transient equilibrium. A condition in a radioactive decay chain where the parent half-life is longer than, but comparable to, the daughter half-life ($t_{1/2,P} > t_{1/2,D}$ but not $\gg$). In transient equilibrium, the daughter activity exceeds the parent activity by a constant ratio, and both decay with the parent's half-life. The ${}^{99}$Mo/${}^{99m}$Tc generator used in nuclear medicine operates in transient equilibrium. Ch. 12, 27.

Transfer reaction. A direct nuclear reaction in which one or more nucleons are transferred between the projectile and target. Includes stripping (nucleon transferred to target) and pickup (nucleon transferred from target). Used to measure single-particle structure via spectroscopic factors. Ch. 19.

Transmission coefficient. In quantum scattering, the probability that a particle incident on a potential barrier will penetrate through it. In nuclear reactions, transmission coefficients determine the probability of forming or decaying through a particular channel. Ch. 17, 18.

Triple-alpha process. The nuclear reaction sequence $3 {}^4$He $\to {}^{12}$C that produces carbon in helium-burning stars. Proceeds through the Hoyle state, a $0^+$ resonance in ${}^{12}$C at 7.654 MeV that dramatically enhances the reaction rate. One of the most important reactions in nuclear astrophysics. Ch. 22.

Tunneling (quantum mechanical). The quantum mechanical phenomenon in which a particle penetrates through a potential energy barrier that it cannot classically surmount. In nuclear physics, tunneling explains alpha decay (Gamow theory), sub-barrier fusion in stars, and spontaneous fission. Ch. 5, 13, 21.

U

Uranium. Element 92, the heaviest element found in significant quantities in nature. ${}^{235}$U (0.72% natural abundance) is fissile; ${}^{238}$U (99.27%) is fertile (convertible to ${}^{239}$Pu by neutron capture). Natural uranium is the starting point for both the nuclear fuel cycle and weapons-grade enrichment. Ch. 20, 26, 28.

V

Valley of stability. The region on the chart of nuclides occupied by stable and long-lived nuclei, following a curve in the $(N, Z)$ plane that starts along $N = Z$ for light nuclei and bends toward neutron-rich configurations ($N > Z$) for heavier nuclei. The shape is determined by the competition between the Coulomb and asymmetry terms in the SEMF. Ch. 1, 4.

Volume term. The dominant term in the semi-empirical mass formula, proportional to $A$, reflecting the fact that each nucleon interacts primarily with its nearest neighbors (saturation). The volume term represents the bulk binding energy of nuclear matter. Ch. 4.

W

Weak interaction. One of the four fundamental forces, responsible for beta decay, electron capture, and neutrino interactions with nuclei. Mediated by the $W^\pm$ and $Z^0$ bosons. The weak interaction violates parity, changes quark and lepton flavors, and operates on a timescale much slower than the strong or electromagnetic interactions. Ch. 14, 31.

Weisskopf estimate. A single-particle estimate of the rate of an electromagnetic transition of multipolarity $\lambda$, assuming a single nucleon transitions between shell-model states. Weisskopf estimates provide a benchmark: $B(E\lambda)$ or $B(M\lambda)$ values significantly exceeding the Weisskopf unit indicate collective enhancement; values much below it indicate hindrance. Ch. 9.

WKB approximation. The Wentzel-Kramers-Brillouin semiclassical approximation, used to estimate tunneling probabilities through potential barriers. In nuclear physics, the WKB method is applied to alpha decay (Gamow theory), fission barriers, and sub-barrier fusion. The tunneling probability is $P \sim \exp(-2 \int_{r_1}^{r_2} \kappa(r) \, dr)$, where $\kappa(r)$ is the local wave number in the classically forbidden region. Ch. 5, 13.

Woods-Saxon potential. A widely used model for the nuclear mean-field potential: $V(r) = -V_0 / [1 + \exp((r - R)/a)]$, with depth $V_0 \approx 50$ MeV, radius $R = r_0 A^{1/3}$, and diffuseness $a \approx 0.65$ fm. More realistic than the harmonic oscillator for nuclear shell-model calculations because it has the correct asymptotic behavior and finite depth. Ch. 6.

X

Xenon poisoning. The buildup of the fission product ${}^{135}$Xe ($\sigma_a = 2.65 \times 10^6$ b), a powerful neutron absorber, in an operating nuclear reactor. Xenon poisoning reduces reactivity and can prevent restart of a reactor that has been shut down. Ch. 26.

Y

Yukawa potential. The potential $V(r) = -g^2 e^{-\mu r}/r$, proposed by Hideki Yukawa (1935) to describe the nuclear force as mediated by the exchange of a massive particle (the pion). The range of the potential is $\hbar / m_\pi c \approx 1.4$ fm. The Yukawa model was the first successful meson-exchange description of the nuclear force. Ch. 3.

Yrast line. On a plot of excitation energy versus angular momentum $J$, the yrast line connects the lowest-energy state at each value of $J$. States on or near the yrast line are preferentially populated in heavy-ion fusion reactions and emit gamma rays in cascades that follow the yrast line downward. "Yrast" is from the Swedish superlative of "yr" (dizzy). Ch. 8, 9.

Z

Z (atomic number). The number of protons in a nucleus, which determines the chemical identity of the element. Ch. 1.