A NEW THERMAL NEUTRON FLUX CONVENTION

A new thermal neutron flux convention is proposed where the neutron slowing down spectrum per unit energy varies as 1/E1+β (| β ln E | << 1). The neutron capture rate per atom for a detector may be written as nν0 σ0 [Gthg(T) + f C(β, T) Grh(β, Gr)I'/σ0] where n is the neutron density, ν0 is the conventional 2200 m/s velocity, σ0 is the 2200 m/s neutron capture cross-section, C(β, T) is a renormalisation correction to the epithermal flux fraction f at temperature T, h(β) is the correction to the reduced resonance integral I' defined for a 1/E flux, g(T) is the Westcott function and Gth and Gr are the thermal and resonance self-shielding factors. For the case β = 0 this expression reduces to the Westcott form. Tables of h(β) are presented for a number of commonly used isotopes, and C(β, T) has also been tabulated. A simple experimental method for measuring β is given which only involves the ratio of the cadmium-covered activities of standard manganese and gold detector foils, and the method has been applied to several different standard neutron flux assemblies.