The confusing issue of the neutron capture cross-section to use in thermal neutron self-shielding computations

In the absence of scattering, the thermal neutron self-shielding as occurs in neutron activation experiments can be corrected for if the macroscopic thermal absorption cross-section of the material is known. It is generally accepted that the average cross-section 〈 Σ 〉 to be used for this purpose differs from the cross-section Σ0 for neutrons with a velocity of 2200 m/s by a factor of φ2. In this paper, it is shown that the correct factor to use in the case of thin samples is not φ2, but 2φ. An empirical formula for the computation of self-shielding factors for samples of intermediate thickness is also presented.