Function sommerfeld_eta 

pub fn sommerfeld_eta(za: f64, zb: f64, ma: f64, mb: f64, e_cm_ev: f64) -> f64

Sommerfeld parameter η for a Coulomb channel.

η = Z_a · Z_b · α · √(m_n·c² · μ̃ / (2·E_c))

where:

• Z_a, Z_b are the charge numbers stored in ParticlePair.za/.zb (ENDF LRF=7 stores the charge Z directly: neutron/photon = 0, proton = 1, alpha = 2; see SAMMY rml/mrml03.f Zeta formula)
• α = fine-structure constant ≈ 1/137.036
• μ̃ = MA·MB/(MA+MB) is the reduced mass in neutron mass units
• E_c is the CM kinetic energy in this channel (eV, must be > 0)
• m_n·c² in eV

SAMMY Reference

rml/mrml03.f — Zeta = Etac * Kza * Kzb * Redmas / Zke where Kza/Kzb are the charges read directly from the ENDF file.

Arguments

• za, zb — Charge numbers Z of the two particles (from ParticlePair.za/.zb). Neutron and photon: 0; proton: 1; alpha: 2; oxygen-16: 8.
• ma, mb — Particle masses in neutron mass units.
• e_cm_ev — CM kinetic energy in eV (must be > 0).