Attenuation coefficients of γ-rays at 9.00 and 11.39 MeV

Precise measurements of the total attenuation coefficients at 9.00 and 11.39 MeV in 10 elements between Be and U are presented. The γ beam was obtained from the (n,γ) reaction on disks of metallic nickel, placed along a tangential beam tube of the IRR-2 research reactor. To reduce the effects of small angle scattering, the γ beam, after passing the absorbers, was collimated to 0.3deg angular aperture. The intensity of the γ beam was measured directly, as a function of absorber thickness, using a 12.7x12.7 cm2 NaI crystal. Six absorbers were used for each element and the total thickness was about one free path. The running time for each thickness was 10 min. and about 6 runs were made for each element. The attenuation coefficients were obtained from the average of these measurements. The results for several elements are listed and compared with theoretical calculations. The comparison leads to the conclusion that the measured cross sections are in good agrement with the calculated ones for the light elements at 9.00 MeV. However at 11.39 MeV the experimental cross sections are usually higher than the theoretical. This difference seems to increase with Z and E. From the systematic behaviour of the differences it can be assumed that the cross section for pair production is under-estimated. (B.G.)