Neutron wave propagation in beryllium

The propagation of thermal neutron waves has been studied in four beryllium parallelepipeds. The transverse bucklings of these parallepipeds ranged from 0·0497 to 0·2572 cm⁻², values which are greater than the critical buckling (0·00116 cm⁻²). From the space-time profiles, the dispersion law for beryllium was deduced over the frequency range 0 < ω < 1000 Hz. For the larger systems good agreement was found with calculations based on thermalization theory at frequencies less than 600 Hz. The effect of continuum contamination was apparent in all the systems but in the larger blocks this contamination had about the same magnitude and attenuation as the discrete modes. For the smaller systems, the continuum contamination was associated with neutrons having energies below the Bragg cut-off and was manifested by oscillations in the complex inverse relaxation lengths at the higher frequencies.