Pulsed-Decay and Extrapolation-Length Measurements in Graphite

Measurements of the shape of the fundamental mode of a thermalized neutron pulse in graphite for rectangular stacks has yielded a value of the extrapolation length, d = 1.825 ± 0.025 cm. The pulsed-neutron decay constants in graphite have been measured over a buckling range from 1.946 × 10⁻³ to 1.230 × 10⁻² cm ⁻² and fitted to three- and four-term power series in the buckling. To fit points at bucklings greater than 6.0 × 10⁻³ cm⁻², a cubic fit is necessary. An iterative method of fitting the decay constants results in an extrapolation length of 1.78 ± 0.01 cm. The diffusion constant was (2.0896 ± 0.0093) × 10⁵cm²/sec, and the diffusion cooling constant was (3.77 ± 0.35) × 106 cm4/sec at a density of 1.689 g/cm³. A technique of correcting the decay constants for the effect of spectral cooling has been developed. The decay constants corrected to room temperature can be fitted as a linear function of the buckling. This method interates on the heat-transfer coefficient, γ, and gives a value of γ = 1633 ± 89 sec⁻¹ for graphite. The heat-transfer coefficient relates the rate of energy transfer from a moderator to a cooled-neutron spectrum. With this approach the entire buckling range can be fitted with three parameters.