The two-photon luminosity, L??, is calculated as the product of the equivalent (Weizsäcker-Williams) photon-fluxes, n(?,b), from the two sources, integrated over impact parameter. For nuclei, nuclear impact parameters for which the nuclei overlap, bɚR, have to be excluded.
L?? = ?? n(?1,b1) n(?2,b2) ?(?b1-b2?-2R) d2b1 d2b2
G Baur and L.G. Ferreira Filho: Nucl. Phys A 518 (1990) 786.
R.N. Cahn and J.D. Jackson: Phys. Rev. D 42 (1990) 3690.