The thermalization and diffusion of neutrons in polyphenyls

Abstract

The thermal neutron diffusion length in Dowtherm-A has been measured at 20°, 50°, 80°, 110°, 140°, 170°, 200° and 220°C. By varying the neutron absorption cross-section by the addition of an organic boron compound the neutron diffusion constant and diffusion heating constant were determined at 20°, 80°, 140° and 200°C. A theoretical model (analogous to the Nelkin model for water) for neutron scattering by protons bound in polyphenyls has been developed. The physical model comprises a free translator of mass 78, a hindered rotational oscillator of effective mass 21·3 at an energy of 0·02 eV and two vibrational oscillators at energies 0·146 and 0·38 eV with equal masses of 2·128. The predictions of this model are in good agreement with the present experimental results for Dowtherm-A, with experimental data on the diffusion parameters of diphenyl and with results of neutron scattering law measurements for Santowax-R. The sensitivity of the predictions of the model to the variation of mode parameters is investigated.