Gamow factor ($e^{-2\pi \eta }$)

The semiclassical penetration factor for two repelling point charges is approximately $\exp (-2\pi \eta )$, where $\eta =Z_1{Z}_2{\alpha }_{\mathrm{fs}}\sqrt{\mu c^2/(2E)}$. It represents the long-range Coulomb suppression, not the nuclear reaction probability after penetration.