Absorption of 6.42 MeV photons

Precise measurements of the total attenuation coefficients of 6.418 MeV photons in 23 elements between Be and U were carried out with uncertainties which are generally less than 0.3%. The method utilizes resonance scattering of monochromatic photons obtained from thermal neutron capture in titanium. The resonant scatterer is ¹³⁹La; it serves as an analyser of the γ-beam energies passing through the absorber. The effective energy definition in this method is of the order of 20 eV. The measured coefficients are generally in close agreement (≈ 0.6%) with calculated values. A significant deviation was observed only in the case of Ta.