Measurements of effective (resonance-shielded) neutron cross-sections in the keV region

Effective capture and transport cross-sections which are fundamental for Doppler coefficient calculations in fast reactor design have been measured on lead, uranium and tantalum in the low keV region. In this energy range the resonances of the cross-sections of fertile and fissile materials and of most structural materials are separated but are still experimentally unresolved. For the following measurements the pulsed method which has been proved in the thermal energy range for measuring capture and transport cross-sections is extended to the keV region. A short burst (1 or 10 ns) of nearly monoenergetic neutrons with energies below the threshold for inelastic scattering is injected into assemblies of lead, uranium and tantalum (parallelepipeds of 10 to 20 cm side length) and the decay of the neutron field in the block is measured. Moderation effects during the decay are small for materials as heavy as lead, uranium and tantalum and can be eliminated from each decay spectrum by a calculated moderation correction function F(t), which corrects the measured neutron density N(t) in the following way: N(t) / F(t) ∼e-α0t The corrected neutron density decays exponentially with a decay constant α0 = v0Σeffa + v0 / 3Σefftr B2 + CB4T. v0 is the neutron injection velocity, B2 the buckling of the assembly and CT a correction term due to transport theory. α0 is measured as a function of B2 and in this way Σeffa and Σefftr are obtained. The effective cross-sections are related to 'infinite dilution' average values <Σa> and <Σtr> by self-shielding factors fa and ft, which have been calculated for many nuclides by Abagjan and others. The measured effective cross-sections are compared with self-shielding factors and <Σ> values measured by other authors. Further dilution effects in mixtures of a resonance absorber (238U) and a potential scatterer (Pb) are investigated. (author)