AbstractMonte Carlo calculations have been performed for neutrons reflecting between two Type 04 concrete plane-parallel interfaces. A new 23-group albedo data base (SAIL) was used in conjunction with the MORSE code to calculate the neutron spectrum at each reflection. It was found that, after thermalization, for the same order of reflection, the number of neutrons in the cavity is dependent on the initial energy of the neutrons. Typical results are presented along with a brief history of the motivations for these calculations.