Function photon_survival_probability

pub fn photon_survival_probability(x: f64, z: f64) -> f64

Photon survival probability P_s(x, z).

P_s(x, z) = exp(−x_c(z)/x)

Probability that an injected photon at frequency x survives absorption by DC/BR processes. At high x (x >> x_c), P_s → 1 (photon survives). At low x (x << x_c), P_s → 0 (photon is absorbed).

Reference: Chluba (2015), arXiv:1506.06582, Eq. 24