Evaluation of the coefficient of B6 term in the decay constant of fundamental mode for BeO

The neutron flux in the case of pulsed finite moderator assemblies has been expanded in powers of B², following the method of Nelkin and Beckurts. Expressions for the diffusion cooling coefficient and the coefficient of B² term have been derived both on diffusion and transport theory. Using the Nelkin kernel in the incoherent approximation and transport cross sections as given by Singwi, these parameters have been evaluated for BeO moderator. Numerical calculations have been done with an energy mesh of 0·9 meV and cut-off energy of 0·3 eV.

The theoretical value of 3·4 × 105 cm⁵ sec⁻¹ for the diffusion cooling coefficient is on the lower side of the experimental value which lies between (3·5–6) × 105 cm⁴ sec⁻¹. In view of the large experimental errors on F-coefficient, no conclusion could be drawn.

On the basis of the theoretically calculated negative F-value, one concludes that the thermalization power for BeO decreases with a decrease in neutron temperature.