Function statistical_weight 

pub fn statistical_weight(j_total: f64, target_spin: f64) -> f64

Spin statistical weight factor g_J.

g_J = (2|J| + 1) / ((2I + 1) · (2s + 1))

where I = target spin, s = neutron spin (1/2), J = total angular momentum.

Uses |J| so that the SAMMY sign convention (negative J to distinguish spin groups with the same |J|) produces the correct weight. ENDF data always has J ≥ 0, so the abs() is a no-op for standard ENDF inputs.

Arguments

• j_total — Total angular momentum J (may be negative per SAMMY convention).
• target_spin — Target nucleus spin I.