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Mass excess

In nuclear physics, the mass excess (Δ) of a nuclide is defined as the difference between its atomic mass M in atomic mass units (u) and its mass number A, expressed as Δ = M − A, where A = Z + N with Z being the atomic number and N the neutron number; this quantity is often converted to energy units using the relation 1 u ≈ 931.494 MeV, making it Δ = [M (u) − A] × 931.494 MeV.^[1] The mass excess quantifies the deviation of a nuclide's mass from the integer mass number due to nuclear binding effects, with the carbon-12 isotope serving as the reference point where Δ(¹²C) = 0 by definition.^[2] This parameter is fundamental for organizing nuclear data in comprehensive evaluations, such as the Atomic Mass Evaluation (AME), which compiles experimentally measured and theoretically predicted mass excesses for thousands of nuclides to support research in nuclear structure and reactions.^[3] Mass excesses enable precise calculations of nuclear binding energies, given by B(A,Z) = Z \cdot m_H + (A - Z) \cdot m_n - M(A,Z) in mass units (converted to MeV), where m_H and m_n are the masses of the hydrogen atom and neutron, respectively; this reveals the energy required to disassemble a nucleus into its constituent nucleons.^[4] Beyond binding energies, mass excesses are essential for determining Q-values in nuclear reactions and decays, defined as the difference in mass excesses between reactants and products (Q = Σ Δ_reactants − Σ Δ_products in MeV), which indicates whether a process releases or absorbs energy and drives phenomena like stellar nucleosynthesis.^[2] For instance, in astrophysical processes such as the triple-alpha reaction forming ¹²C from three ⁴He nuclei, the Q-value derived from mass excesses (approximately 7.274 MeV total) underscores its role in powering stars.^[2] Accurate mass excess data, refined through experiments like Penning traps and theoretical models, remain crucial for predicting nuclear stability, fission barriers, and applications in energy production and medical isotopes.^[5]

Definition and Basics

Definition

The mass excess, denoted as Δ, of a nuclide is defined as the difference between its actual atomic mass m (in atomic mass units, u) and its mass number A, conventionally expressed in energy units as

\Delta = (m - A) \times 931.494 \, \mathrm{MeV},

where m is the atomic mass in u and 931.494 MeV/u is the conversion factor (1 u ≈ 931.494 MeV).^[1]^[6] This quantity is employed in nuclear physics to normalize nuclide masses relative to their mass number, facilitating straightforward comparisons of isotopic masses and emphasizing deviations arising from nuclear binding effects rather than the dominant contribution of nucleon masses.^[6]^[7] The semi-empirical mass formula, developed by Carl Friedrich von Weizsäcker in 1935, models nuclear binding energies and is used alongside mass excess data for theoretical predictions and experimental comparisons.^[8] For stable nuclides, mass excesses are positive for very light ones (e.g., ⁴He: +2.42 MeV, due to masses exceeding integer values on the carbon-12 scale), negative for medium-mass ones from ~¹³C to ~²⁰⁰ (e.g., ⁵⁶Fe near -65 MeV, ²⁰⁸Pb near -21 MeV), reflecting the increase in binding per nucleon to the iron peak and subsequent decrease, and positive again for very heavy ones near the line of stability (e.g., ²³⁸U: +47.3 MeV).^[9]

Notation and Units

The mass excess of a nuclide is standardly denoted by the symbol Δ, subscripted with the mass number A and atomic number Z as Δ(A, Z) to specify the isotope, or simply Δ when the context is clear.^[10] In nuclear data tables, values are organized by increasing Z and, for each Z, by increasing A, facilitating quick reference for specific nuclides.^[11] Mass excess is primarily expressed in energy units, most commonly mega-electronvolts (MeV), due to the mass-energy equivalence principle E = Δm c², where Δm is the mass difference and c is the speed of light; this convention aligns with nuclear reaction energetics.^[9] Although the underlying difference can be given in atomic mass units (u), where 1 u = 1.66053906660(50) × 10^{-27} kg, it is routinely converted to MeV for practical use in energy calculations.^[11] The precise conversion is achieved via the formula

\Delta \, (\text{MeV}) = 931.494 \times (m - A),

where m is the atomic mass in u and 931.494 MeV/u is the established factor linking mass and energy scales (with higher precision values like 931.49410372(29) MeV/u as of the 2022 CODATA adjustment available for advanced computations).^[11]^[12] In some detailed tabulations, such as those from the Atomic Mass Evaluation (AME), mass excess is reported in kiloelectronvolts (keV) to preserve precision in the least significant digits.^[9] Mass excesses are systematically compiled in authoritative databases like the AME, which aggregates experimental data from techniques such as Penning traps and time-of-flight mass spectrometry, providing recommended values for over 3,000 nuclides (AME2020 compiles data for 3167 ground-state nuclides and 659 isomers, the latest evaluation as of 2025).^[11]^[9] Each entry includes an uncertainty reflecting measurement precision and statistical analysis, typically ±0.01 MeV or better for stable nuclides, with values as low as ±1 eV for highly precise cases like ^{12}C.^[11] These uncertainties are derived from weighted least-squares adjustments of input data, ensuring reliability for nuclear structure studies.^[9]

Physical Interpretation

Relation to Nuclear Binding Energy

The nuclear binding energy B(A, Z) of a nuclide with mass number A and atomic number Z represents the energy required to disassemble it into its constituent protons and neutrons, arising from the mass deficit due to strong nuclear interactions. This energy is given by B(A, Z) = [Z m_H + (A - Z) m_n - m(A, Z)] c^2, where m_H is the mass of the hydrogen atom, m_n is the neutron mass, m(A, Z) is the atomic mass of the nuclide, and c is the speed of light.^[9]^[13] Mass excess \Delta(A, Z) provides a convenient way to express this relation, defined as \Delta(A, Z) = [m(A, Z) - A u] c^2, where u is the atomic mass unit (u \approx 931.494 MeV/c^2). Substituting the mass excesses of the constituents, the binding energy simplifies to B(A, Z) = [Z \Delta_H + (A - Z) \Delta_n - \Delta(A, Z)], with \Delta_H \approx 7.289 MeV for ^1H and \Delta_n \approx 8.071 MeV for the neutron. To derive this, start with the atomic masses: m_H = u + \Delta_H / c^2, m_n = u + \Delta_n / c^2, and m(A, Z) = A u + \Delta(A, Z) / c^2. Inserting into the binding energy expression yields B(A, Z) / c^2 = Z (u + \Delta_H / c^2) + (A - Z) (u + \Delta_n / c^2) - (A u + \Delta(A, Z) / c^2), which reduces to the summed mass excesses form after canceling the A u terms. This formulation accounts for the use of atomic masses in tables, implicitly including electron bindings, which are negligible compared to nuclear scales.^[9]^[13]^[14] Mass excess data thus directly quantify nuclear stability, as greater binding energy corresponds to smaller (more negative) mass excess relative to unbound nucleons. Plots of mass excess per nucleon versus A reveal valleys of stability, aligning with the semi-empirical mass formula (SEMF), which approximates B(A, Z) / A through volume, surface, Coulomb, asymmetry, and pairing terms; minima in these curves indicate nuclei with maximal binding per nucleon around iron isotopes.^[15]^[16] In infinite nuclear matter at saturation density, the average binding energy per nucleon reaches approximately 8 MeV, a value reflected in typical mass excesses for stable heavy nuclei, underscoring the near-constant binding scale across most elements.^[15]^[9]

Atomic vs. Nuclear Mass Excess

The atomic mass excess is defined as the difference between the mass of a neutral atom and the mass number A times the atomic mass unit u, expressed in energy units via multiplication by c^2: \Delta_\text{atomic} = [m_\text{atom} - A u] c^2. This quantity is the standard in comprehensive atomic mass tables, such as the Atomic Mass Evaluation (AME), which tabulate masses for neutral atoms including the Z electrons.^[11] In contrast, the nuclear mass excess refers to the same difference but applied to the bare nucleus, excluding electrons: \Delta_\text{nuclear} = [m_\text{nucleus} - A u] c^2. It is rarely tabulated directly and is instead derived from atomic data by accounting for electron contributions, approximately \Delta_\text{nuclear} \approx \Delta_\text{atomic} - Z \times 0.511 MeV, where 0.511 MeV is the rest energy of the electron.^[17] Electron binding energy corrections are added, typically on the order of 13.6 eV per electron for hydrogen-like atoms but totaling up to a few hundred keV for heavy elements; these are negligible relative to nuclear energy scales of MeV.^[18]^[2] The explicit relation between nuclear and atomic masses is m_\text{nucleus} = m_\text{atom} - Z m_e + B_e / c^2, where m_e is the electron rest mass (0.511 MeV/c^2) and B_e is the total atomic electron binding energy. This adjustment highlights that atomic masses simplify data handling in nuclear physics, as electron masses nearly cancel in calculations involving binding energies or conserved charges, avoiding the need for separate electron corrections in most cases.^[18]^[4] Atomic mass excesses are particularly preferred in contexts like beta decay, where the process involves changes in electron number; using neutral atomic masses inherently accounts for the emitted or created electrons without additional adjustments. The discrepancy between atomic and nuclear mass excesses is tiny, on the order of 0.0005% of the nuclear mass, but becomes relevant for high-precision studies of exotic nuclei far from stability.^[19]^[11]

Applications in Nuclear Reactions

Role in Q-value Calculations

The Q-value of a nuclear reaction represents the energy released or absorbed, defined as Q = (m_\text{initial} - m_\text{final}) c^2, where m_\text{initial} and m_\text{final} are the total rest masses of the initial and final particles, respectively.^[2] When using mass excesses \Delta, which are the deviations of atomic masses from integer multiples of the atomic mass unit, the Q-value simplifies to Q = (\sum \Delta_\text{initial} - \sum \Delta_\text{final}) \times 931.494 MeV/u, as the mass number A is conserved and the large A u terms cancel out.^[2] This approach leverages tabulated atomic mass excesses for direct computation without needing full mass values.^[2] For specific decay processes, mass excesses enable precise Q-value determination. In alpha decay, Q_\alpha = \Delta_\text{parent} - \Delta_\text{daughter} - \Delta_\alpha, using atomic mass excesses where the electron binding energies are negligible compared to nuclear scales.^[20] For beta-minus decay, Q_{\beta^-} = \Delta_\text{parent} - \Delta_\text{daughter}, again with atomic mass excesses; the emitted electron and antineutrino account for the charge change, with electron masses balancing automatically in the atomic formulation.^[20] In reactions like the (p, n) process, the Q-value is Q = \Delta_\text{target} + \Delta_p - \Delta_\text{residual} - \Delta_n, facilitating assessments of neutron production in accelerators or stars. Mass excesses offer advantages over raw atomic masses in Q-value calculations by minimizing numerical cancellations; the dominant A u contributions (around 931 MeV per nucleon) subtract nearly exactly, leaving only the small excess differences (typically a few MeV) for high-precision arithmetic in tables and computations.^[2] A negative Q-value signifies an endothermic reaction, requiring a minimum threshold kinetic energy in the center-of-mass frame to proceed, which is critical in nuclear astrophysics for evaluating stellar nucleosynthesis rates where such reactions influence reaction chains at finite temperatures.^[2]

Energy Scales and Thresholds

Mass excesses establish the fundamental energy scales for nuclear processes, distinguishing them sharply from atomic phenomena. For stable nuclei, values typically span from approximately +7 MeV for proton-rich light isotopes like hydrogen-1 to -60 MeV for tightly bound medium-mass nuclei such as iron-56, before rising again to +47 MeV for neutron-rich heavy isotopes like uranium-238. Unstable light nuclei often exhibit higher positive mass excesses, reaching around +18 MeV in cases like helium-6. These variations in mass excess directly determine the Q-values of nuclear reactions—differences in mass excess multiplied by 931.494 MeV/u—which range from keV for weak decays to several MeV for reactions like capture or scattering. In contrast, atomic energy scales, governed by electron transitions, operate at eV levels, underscoring the immense energy density of nuclear interactions.^[21]^[22]^[23]^[24] Threshold energies for reactions are critically influenced by mass excess differences, particularly for endothermic processes where Q < 0. The laboratory-frame threshold energy required for the projectile to initiate such a reaction is E_{th} = -Q \left(1 + \frac{m_p}{m_t}\right), derived from ensuring sufficient center-of-mass energy to cover the -Q deficit while accounting for kinematic constraints. Here, m_p and m_t are the projectile and target masses, respectively. This threshold often exceeds the absolute value of Q due to the mass ratio factor, which approaches 1 for equal masses but grows for light projectiles on heavy targets. Additionally, charged-particle reactions face the Coulomb barrier, an electrostatic repulsion with heights typically spanning 1-10 MeV, depending on nuclear charges; for instance, proton-nucleus barriers are ~1-5 MeV, while alpha-heavy ion barriers reach ~5-10 MeV. These barriers, combined with thresholds, dictate minimum beam energies for observable reaction rates in accelerators.^[25] In broader applications, mass excess asymmetries underpin the energetics of fission and fusion. For fission of heavy nuclei like uranium-235, the transition to medium-mass fragments with more negative mass excesses per nucleon (higher binding) releases ~200 MeV total energy per event, primarily as kinetic energy of fragments, with ~6% as prompt neutrons and gamma rays. This release stems from the curve of binding energy peaking around A=56, making fission exothermically favorable despite the initial endothermic barrier penetration. In fusion, light nuclei converge toward this stability peak, yielding small positive Q-values; for example, deuterium-tritium fusion liberates 17.6 MeV, enabling stellar burning and thermonuclear devices despite high Coulomb thresholds.^[26] Mass excess trends across the nuclear chart uniquely reveal shell effects, with abrupt changes signaling magic numbers (2, 8, 20, 28, 50, 82, 126) where filled subshells enhance stability and favor reactions like beta decay or capture toward those configurations. These kinks in mass excess plots, observed in systematic measurements, predict regions of enhanced reaction cross-sections near magic shells. Unbound nuclides, incapable of stable existence, have mass excesses exceeding the particle separation energy—averaging ~8 MeV for neutron emission in light nuclei—leading to immediate dissociation. Historically, precise mass excess tabulations were instrumental in Manhattan Project computations, enabling Q-value assessments for uranium-235 fission chains and confirming the viability of self-sustaining reactions.^[27]^[28]

Examples and Calculations

Illustrative Example

A concrete illustration of mass excess in nuclear processes is the beta decay of carbon-14 to nitrogen-14, a key reaction in radiocarbon dating. The atomic mass excess of ^{14}C is 3019.89 \pm 0.004 keV, while that of ^{14}N is 2863.42 \pm 0.001 keV, both sourced from the 2020 Atomic Mass Evaluation (AME2020).^[29] To compute the Q-value for this beta-minus decay, retrieve these values from the AME table and apply the formula Q_\beta = \Delta(^{14}\mathrm{C}) - \Delta(^{14}\mathrm{N}), yielding Q_\beta = 3019.89 - 2863.42 = 156.47 keV. This Q-value represents the total energy released, available as kinetic energy shared between the emitted electron (with maximum kinetic energy equal to the endpoint energy of approximately 156 keV) and the antineutrino, neglecting minor atomic binding corrections. For alpha decay, consider the process ^{238}\mathrm{U} \to ^{234}\mathrm{Th} + ^4\mathrm{He}. The relevant mass excesses from AME2020 are \Delta(^{238}\mathrm{U}) = 47307.3 \pm 1.2 keV, \Delta(^{234}\mathrm{Th}) = 40608.9 \pm 3.5 keV, and \Delta(^4\mathrm{He}) = 2424.92 \pm 0.0002 keV.^[29] The Q-value is calculated as Q_\alpha = \Delta(^{238}\mathrm{U}) - \Delta(^{234}\mathrm{Th}) - \Delta(^4\mathrm{He}) = 47307.3 - 40608.9 - 2424.92 = 4273.5 keV (or 4.274 MeV), which is the total kinetic energy imparted to the alpha particle and thorium recoil nucleus (AME2020 values; note that ^{238}U has been refined post-2020 to 47308.367 \pm 0.014 keV).^[30] This example demonstrates how mass excesses enable precise energy release predictions for spontaneous decays. The 2020 AME refinement of these values, often by ~0.01 MeV or better in uncertainties for well-measured nuclides, directly impacts decay rate calculations and half-life predictions in astrophysical and geophysical models.

Comparison with Mass Defect

The mass defect, denoted as δ, is defined as the difference between the total mass of the constituent protons and neutrons in a nucleus and the actual mass of the nucleus itself, expressed as δ = Z m_H + (A - Z) m_n - m, where Z is the atomic number, A is the mass number, m_H is the mass of a hydrogen atom, m_n is the mass of a neutron, and m is the atomic mass of the nuclide.^[31] This quantity directly corresponds to the mass equivalent of the nuclear binding energy, representing the "missing" mass due to the strong nuclear force that holds the nucleus together. In contrast, the mass excess, denoted as Δ, is the deviation of the nuclide's atomic mass from its mass number times the atomic mass unit, given by Δ = m - A u, where u is the unified atomic mass constant.^[32] A key distinction lies in their formulation and utility: the mass defect is an absolute measure tied intrinsically to the binding energy of a single nuclide, while the mass excess is a relative quantity normalized to the integer mass number, facilitating additive calculations in nuclear reactions and decay processes.^[33] The two concepts are related through the mass excesses of the hydrogen atom and neutron, approximately as δ c² ≈ Z Δ_H + (A - Z) Δ_n - Δ, where c is the speed of light and the Δ terms are in energy units; this relation underscores how the mass defect can be derived from tabulated mass excesses for practical computations.^[34] Historically, the mass defect concept emerged in the early 1920s from Francis Aston's mass spectrometry work, which revealed non-integer atomic masses and their implications for nuclear stability.^[35] The mass excess, however, was formalized later in the mid-20th century for efficient tabulation in nuclear data compilations, such as those by the Atomic Mass Evaluation project.^[32] For bound nuclei, the mass defect is invariably positive, reflecting the mass reduction from binding, whereas the mass excess can be negative (e.g., for stable medium-mass nuclides like oxygen-16), zero (by definition for carbon-12), or positive, highlighting deviations from the reference scale.^[33] In the liquid drop model of the nucleus, both quantities arise from similar semi-empirical terms—volume, surface, Coulomb, asymmetry, and pairing—but the mass excess particularly emphasizes systematic trends across isotopic chains due to its normalized form.^[34] Consequently, the mass defect is preferentially used in educational contexts to illustrate binding energy for individual nuclides, while the mass excess dominates in database-driven applications like reaction energy predictions.^[33]

References

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