The probabilities of single-particle excitations that preserve completely paired configurations of the nuclear proton and neutron subsystems are formulated as a function of excitation energy in the framework of the superfluid nuclear model. These calculations are used to analyse measured data on even-odd structure in fission-fragment yields. Excitation energies acquired at scission are deduced. The strongly differing even-odd structures in proton and neutron numbers are explained within the assumption of thermal equilibrium of intrinsic excitations at scission.