Some Properties of the Width Fluctuation Factor

In this Note the subject of width fluctuation correction to average compound-nucleus cross sections is reviewed, with special emphasis on neutron capture and scattering cross sections. Recent statistical model theories on the calculation of cross sections offer a new approach to the calculation of this correction factor. For low energies, the classical integration method gives the best description. At higher energies, the approximation of Tepel et al. with a semi-empirical relation for the elastic enhancement parameter is recommended for practical calculations. In the discussion about properties of the width fluctuation factor (WFF), the concept of a lumped channel with an effective number of degrees of freedom, v_eff, has proven to be helpful. A new definition of v_eff is given in this Note. Under certain conditions, the WFF for nonelastic processes can become larger than unity. This effect can be important in neutron capture when strong nonelastic channel competition is present. An example of this effect is given for the reaction 100Mo(n,γ) at 0.9 MeV, where, due to width fluctuation effects, the capture cross section is enhanced by ≈30%. Some other examples of width fluctuation effects are given for neutron elastic and inelastic scattering.