The dynamics of liquid H2O and the thermalization of neutrons

Abstract

A study has been made of the asymptotic as well as time-dependent decay of neutron pulses in water assemblies at various temperatures in the range 05-60 °C. A new scattering kernel for neutrons has been proposed which takes account of low-energy collective oscillations of H₂O molecules in the Debye approximation and which also includes contributions from rotational and intramolecular vibrational modes. The calculated values of the total neutron scattering cross section for water at 20 °C are found to agree rather well with the experimental results. The values of the asymptotic decay constant calculated for different B² at various temperatures also agree well with the corresponding experimental results. The temperature variation of the diffusion coefficient D₀ and the diffusion cooling coefficient C is also in agreement with most of the experimental results. Transient spectra in assemblies with different B² and at various temperatures are successfully compared with those reported by Ishmaev et al (1965) and Menzel et al (1970). The values of thermalization times t_th obtained by the authors are consistent with the waiting times quoted by Ishmaev et al (1965) The importance of introducing rotational and intramolecular vibrational modes, apart from the collective elastic modes, is demonstrated by neglecting the contribution of these other modes. Two cases have been considered: one corresponding to θ_D=200 K, and these other to θ_D=250 K. It is shown that under certain conditions the exclusion of rotational and intramolecular vibrational modes markedly affects various neutron thermalization parameters.