Definition of Subcriticality Using the Importance Function for the Production of Fission Neutrons

Using an importance function describing the capability of a system for producing fission neutrons, a new definition of the subcriticality is proposed, which has the physical meaning of a multiplication factor in a real subcritical system with external sources. This multiplication factor k_s, which expresses the number of fission neutrons produced by a fission neutron in a steady state, is different from the usual criticality factor or the effective multiplication factor k_eff, since the former is calculated from the inhomogeneous equation with external source, whereas the latter is calculated from the homogeneous criticality equation without external source.