Here 1.6022 × 10 -13 is conversion from MeV to J. The 0.74 is the estimated fraction overlap and 0.15 is the quantum uncertainty. The "fixing for maximum overlap" is done via potassium hydride crystals. Neutron reactivity at 2.45 MeV energy is relative low for 39K, [13, figure ]. Most of the neutrons will react with the protons in KH, provided the overlap > 0.