Methods and Cross Sections for Calculating the Fast Effect

The Spinrad three-group theory of the fast effect is formulated. Recipes are presented for determining the necessary cross sections directly from fundamental data and three-group cross sections for various materials are tabulated. A comparison is made between the three-group theory and one-group theory with respect to the calculation of R, the ratio of fast-to-thermal fissions, and it is found that the three-group theory appears to give better agreement with experiments. The three-group values of Є — 1 are about 20% higher than those calculated from the one-group formula.