Thermal two-body nuclear reaction rate
In a dilute nondegenerate gas of independently distributed reacting nuclei at common temperature T, reaction events per volume/time are R_ab=n_an_b⟨σv⟩/(1+δ_ab), where δ_ab=1 for the same species and 0 otherwise.
Key knowledge
In a dilute nondegenerate gas of independently distributed reacting nuclei at common temperature T, reaction events per volume/time are R_ab=n_an_b⟨σv⟩/(1+δ_ab), where δ_ab=1 for the same species and 0 otherwise. The nonrelativistic Maxwell relative distribution gives ⟨σv⟩=√[8/(πμ)](k_BT)⁻³ᐟ²∫₀∞σ(E)Eexp[−E/(k_BT)]dE, using relative energy E=μv²/2. The identical-species factor counts each incoming pair once.
Values and equations
R_ab=n_an_b⟨σv⟩/(1+δ_ab); ⟨σv⟩=√(8/(πμ))(k_BT)⁻³ᐟ²∫₀∞σ(E)E e^[−E/(k_BT)]dE; μ=m_am_b/(m_a+m_b)
- R_ab
- Expected channel events per volume/time m⁻³ s⁻¹
- n_a,n_b
- Reactant number densities m⁻³
- δ_ab
- One for identical reactant species, zero otherwise
- ⟨σv⟩
- Thermal two-body rate coefficient m³ s⁻¹
- σ(E)
- Specified reaction cross section m²
- E
- Relative center-of-momentum kinetic energy J
- v
- Relative speed m s⁻¹
- μ,m_a,m_b
- Reduced and reactant masses kg
- T
- Common thermodynamic temperature K
- k_B
- Boltzmann constant J K⁻¹
Conditions and scope
- Specified two-body channel/cross-section convention, positive masses and T, independently thermalized classical velocity distributions with common bulk rest frame and sufficiently large populations; μ=m_am_b/(m_a+m_b). The thermal integral must converge.
- This is not a three-body/triple-alpha rate, degenerate distribution or arbitrary nonthermal beam model. Excited target populations, plasma screening and resonant/inelastic channel structure must be represented in the actual σ or a justified extended rate model; Q alone never determines ⟨σv⟩.
Sources · 2
References
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Evidence and review
Physics 467: stellar nuclear reaction rates: Stellar nuclear reaction rates subsection: displayed per-target n_bvσ, volumetric pair rate K n_an_b⟨vσ⟩ with identical-pair K=1/2, and relative Maxwell speed/energy distributions. No historical reaction table, stellar-neutrino/device narrative, or erroneous displayed Gamow-energy/peak prefactor is used. AST 403: The physics of fusion in stars: Printed page 5, equation 12 first line: Maxwell-averaged arbitrary cross-section integral with the reduced mass. Source M is renamed μ; no old solar values, unsourced author identity or approximate pp-rate number is asserted.
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Entry details
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