The problem of a self-sustaining detonation wave diffracting from confinement into an unconfined space through an abrupt area change is characterized by the geometric scale of the confinement and the reaction scale of the detonation. Previous investigations have shown that this expansion associated with a detonation transitioning from planar to spherical geometry can result in two possible outcomes depending upon the combustible mixture composition, initial thermodynamic state, and confining geometry. Competition between the energy release rate and expansion rate behind the diffracting wave is crucial. The sub-critical case is characterized by the rate of expansion exceeding the energy release rate. As the chemical reactions are quenched, the shock wave decouples from the reaction zone and rapidly decays. The energy release rate dominates the expansion rate in the super-critical case, maintaining the coupling between the shock and reaction zone which permits successful transition across the area change. A critical diffraction model has been developed in the present research effort from which the initial conditions separating the sub-critical and super-critical cases can be analytically determined. Chemical equilibrium calculations and detonation simulations with validated detailed reaction mechanisms provide the model input parameters. Experiments over a wide range of initial conditions with single- and multi-sequence shadowgraphy and digital chemiluminescence imaging support the model derivation and numerical calculations. Good agreement has been obtained between the critical diffraction model and experimental results.