Measurements of Neutron Thermalization Time Constant of Light Water by Pulsed Neutron Method

The thermalization parameters of light water have been measured using a pulsed neutron source and non-l/v absorbers (Cd, Gd and Sm). The pulsed neutron source utilizing the D-D reaction was placed at the center of an effectively infinite system of water poisoned with non-1/v; absorbers. The time dependent capture γ-rays from these absorbers were detected by a Nal(Tl) scintillation counter. The neutron thermalization time constant of light water was determined to be 5.3 ± 0.3,μsec from the difference of the decaying reaction rates of the γ-rays, expressed as the sum of two asymptotical exponential components. The resulting value was larger than that of Möller's experiment (4.1 ± 0.4μsec), but agreed fairly well with the theoretical value of 5.6μsec, calculated by Hoshino based on the Nelkin model. A supplementary experiment was performed using thick neutronic filters of Cd and Ag.