Measurements on the Thermalization Process in Beryllium Oxide Using Spectral Indicators and Comparison with Theory

The reaction rate of 235Pu as a fraction of time and space in a large block of BeO following the injection of a burst of high-energy neutrons has been measured. Fission counters provided the reaction rates and the block was a cube of side 60 cm using BeO of density 2.87 g/cm3. The time range investigated was from immediately after, to 1 ms after the input pulse. The value 17.5 ± 3.0 μs found for the slowing-down time to the 239Pu resonance is in good agreement with that of Zhezherun. The pulsed source was provided by a 3-MeV Van de Graaff accelerator with beam deflection in the top terminal backed up by post-acceleration deflection. The beam could be externally triggered to produce beam pulses ranging from 2 to 500 μs. A multigroup code is described which approximates the leakage by a DB2 term and which solves the time dependence by a time stepping technique. The time-dependent reaction rates for indium, cadmium, plutonium and samarium have been calculated using the code and the slowing-down times to those resonances compared with experimental values. It is proposed to extend the present work to cover the range 0.1-1.0 eV. A brief description is given of a method of doing this using suitable (n, γ) resonances and a large sodium iodide crystal. (author)