The Calculation of Effective Cutoff Energies in Cadmium, Samarium, and Gadolinium

A set of effective cutoff energies for 1/v detectors covered by cadmium, samarium, and gadolinium filters is presented. These cutoff energies have been calculated for both isotropic and beam fluxes which were made up of a Maxwellian thermal plus a 1/E tail. Calculations were performed as a function of r (the ratio of thermal flux to the resonance flux per unit lethargy), EM (the Maxwellian neutron energy), and the absorption coefficients of the 1/v foil and the resonance filter. The cutoff energies for samarium were found to be appreciably lower than those for cadmium, while those for gadolinium, especially for thicknesses of 30 to 40 mils, are similar to those of cadmium. It is felt that these rare earth filters might be able to supply valuable information in the region of low cutoff energies, especially in high temperature facilities where cadmium with its low melting point is difficult to use.