Coulomb penetration, astrophysical S factor and Gamow window
For repulsively charged nuclei with κ=Z_aZ_be²/(4πε₀)>0 and relative speed v, the Sommerfeld parameter is η=κ/(ħv) and the leading low-energy Coulomb penetration exponential is exp(−2πη)=exp[−√(E_G/E)], E_G=2μ(πκ/ħ)².
Key knowledge
For repulsively charged nuclei with κ=Z_aZ_be²/(4πε₀)>0 and relative speed v, the Sommerfeld parameter is η=κ/(ħv) and the leading low-energy Coulomb penetration exponential is exp(−2πη)=exp[−√(E_G/E)], E_G=2μ(πκ/ħ)². Define S(E)=σ(E)E exp(2πη). For a nonresonant channel whose S varies slowly across the contributing energies, the thermal exponent E/(k_BT)+√(E_G/E) has its minimum at E₀=[E_G(k_BT)²/4]¹ᐟ³, the leading Gamow-window energy.
Values and equations
η=κ/(ħv); κ=Z_aZ_be²/(4πε₀); E=μv²/2; E_G=2μ(πκ/ħ)²; σ(E)=S(E)e^[−√(E_G/E)]/E; E₀=[E_G(k_BT)²/4]¹ᐟ³
- η
- Dimensionless Sommerfeld parameter
- κ
- Repulsive Coulomb coefficient J m
- Z_a,Z_b
- Positive reactant charge numbers
- e
- Elementary charge magnitude C
- ε₀
- Vacuum permittivity F m⁻¹
- ħ
- Reduced Planck constant J s
- v
- Relative speed m s⁻¹
- μ
- Reactant reduced mass kg
- E,E_G,E₀
- Relative kinetic, Gamow and leading window energies J
- σ(E)
- Specified reaction cross section m²
- S(E)
- Defined astrophysical S factor J m²
- k_B
- Boltzmann constant J K⁻¹
- T
- Thermodynamic temperature K
Conditions and scope
- Nonrelativistic unscreened two-body repulsion, positive relative E and T, and a nuclear interaction range small compared with the Coulomb turning radius for the leading penetration approximation. S(E) has units J m²; its definition does not guarantee slow variation.
- The window estimate presumes a broad smooth nonresonant S and no important plasma screening; resonances/thresholds/energy-dependent matrix elements can shift the true rate contribution. The factor estimates Coulomb access, not fusion probability by itself, absolute rate or a universal activation temperature.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
AST 403: The physics of fusion in stars: Printed pages 4–6, equations 9–12: exterior Coulomb action, flux exponential, S factor and Maxwell integral. E_G is derived from the correctly squared charge product in equations 9–11 with SI κ restored; equation 13 omits that square and equation 14's factor differs from direct differentiation, so neither erroneous expression is copied. Other exclusions: Z=2 for a proton and the quoted deuteron binding/weak-scale values. Physics 467: stellar nuclear reaction rates: Stellar nuclear reaction rates subsection: low-energy S-factor convention and competing Maxwell/Coulomb exponent. The displayed E_G lacks its required square and the stated E₀ lacks the factor 4⁻¹ᐟ³; the record uses dimensional restoration and an explicit derivative of that displayed exponent instead. Historical S(0) values and the later observational/engineering narrative are excluded.
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