Function urr_cross_sections 

pub fn urr_cross_sections(
    urr: &UrrData,
    e_ev: f64,
    ap_fm: f64,
) -> (f64, f64, f64, f64)

Compute Hauser-Feshbach average cross-sections in the Unresolved Resonance Region.

Returns (total, elastic, capture, fission) in barns. All four are zero when e_ev falls outside the URR energy band [urr.e_low, urr.e_high].

elastic = compound-nuclear elastic + smooth potential scattering. total = elastic + capture + fission + competitive (GX). Potential scattering σ_pot = 4π·AP²/100 (AP in fm, result in barns) is folded in here rather than at the call site. SAMMY ref: unr/munr03.f90 includes the hard-sphere background.

SAMMY Reference

unr/munr03.f90 Csig3 — Hauser-Feshbach cross-section kernel.

Arguments

• urr — Parsed URR parameters (LRF=1 or LRF=2).
• e_ev — Neutron lab-frame energy in eV.
• ap_fm — Scattering radius in fm at this energy. The caller is responsible for evaluating any AP(E) table (NRO≠0) or falling back to the constant urr.ap.