Thermal-Neutron Spectrum in Water with Temperature Discontinuity

The relaxation of neutron spectrum near a temperature discontinuity is studied both theoretically and experimentally. The P₁ approximation and Nelkin's model for light water are used. When the temperature difference is 45°C, theoretical values of 4.5 and 4.6 mm for the relaxation lengths on the warmer and colder side of the discontinuity are obtained. The experimental value is about 2.3 mm. Thus, there is large disagreement between the experimental and theoretical results. The reason for this cannot be pinpointed, but the following explanations are possible. 1) transport effects, 2) inapplicability of Nelkin's model, and 3) insufficient accuracy of the trial functions for flux and the treatment of its energy-dependence.