Neutron strength functions and average total cross sections II. The behavior of the average cross sections and the S-wave scattering lengths

Abstract

Average neutron total cross sections have been measured in the keV region for natural samples of KCl, Sc, V, Mn, Co, Cu, Zn, As, Se, RbBr, CsI, Ce, Nd, Gd, Tb, Dy, Ho, and Hg, and for the separated isotopes 144,146,148Nd and 149,152Sm. The measurements were combined with those made earlier here (TUNL) and elsewhere to give total data set which is comprised of the total corss sections for most of the natural elements and many separated isotopes. The cross sections were averaged over hundreds of keV and compared to various optical model calculations. The data are in fair agreement with spherical optical model calculations below A = 140 and in poor agreement for heavier nuclei. The calculations using deformed potentials agree much better with the data, especially above A = 140. The s-, p-, and d-wave strength functions, the s-(or R′) and p-wave phase shifts were extracted from the average cross sections of the nuclei mentioned above by the Duke average cross section method, but in this paper only < σ T > and R′ are discussed (see Part III for strength functions). The values of R′, supplemented by those from other measurements, made both here and elsewhere, were compared to available optical model calculations which use deformed optical potentials. The calculations agree fairly well with the data. Measurements of R′ indicate that it rises much more rapidly at A = 140 than would be predicted by the spherical optical model and confirm the rapid rise predicted by the deformed model. Similarly, a rise in R′ is evident around mass number A = 50, however, enough experimental points are not available for a detailed comparison with theory.