Function rutherford_cross_section 

pub fn rutherford_cross_section(
    z1: f64,
    z2: f64,
    energy: f64,
    angle: f64,
) -> f64

Rutherford scattering differential cross section. dσ/dΩ = (Z1 Z2 k_e e² / (4E))² / sin⁴(θ/2)