On the Variation in Scattering and Absorption Cross Sections with Resonance Neutron Energy. I

Abstract

(1) The scattering, and absorption cross sections were separately measured for C, Fe, Ni, Pb, Zn, Al, Hg, Cl, Co, and Mn with various resonance neutrons by the method of back scattering. The investigated resonance neutrons are as follows: “thermal” (In 54'), “D₁” (Rh 44''), “D2” (In 54'), “A” (Ag 23''), “Mn” (Mn 150'), “Br” (Br 18'), “I₁” (I 25'), and “I₂” (I 25', with 0.32g/cm² iodine filter). Absolute determinations of the scattering cross sections were performed only for carton scatterer, and for the other elements they were measured relative to it. The scattering cross-sections of carbon were found to bd independent of the neutron energy ranging from thermal to eV in the order of hundreds, though a tendency to decrease slightly with the increase of neutron energy may be seen. The results are summarized in Fig. III. It can be said that: (I) the scattering cross-section of the so-called good scatterers such as Ni and Fe is independent of neutron energy with a tendency toward a slight decrease with higher neutron energy, (II) for Hg, Cl, Co, Zn, and Mn the scattering shows sharp changes with some parallelism to absorption excluding, the 1/v factor in the region of thermal energies.
(2) The scattering cross-section of Hg were calculated according to the dispersion formulae based on the theory of compound nucleus. The Γ_N and Γ of Hg were assumed to be equal to those of Cd. The suggestions of Bethe on the existense of a resonance level at a negative energy was confirmed. The scattering shows best agreement with the experimental results if we assume the position of the level to be at -0•8eV. For the other scatterers, it is difficult to explain the behavior of scattering cross sections without resorting to very artificial assumptions for the resonance levels