Neutron absolute and total cross section difference measurements in the mass-140 region

Abstract

We have used a 100-MeV electron linac and neutron time-of-flight facility to measure precise neutron total cross sections in the mass-140 region for incident neutron energies of 3-60 MeV. We measured the absolute neutron total cross section of $^{140}\mathrm{Ce}$ and the total cross section differences of $^{139}\mathrm{La}$-$^{140}\mathrm{Ce}$, $^{141}\mathrm{Pr}$-$^{140}\mathrm{Ce}$, and $^{142}\mathrm{Ce}$-$^{140}\mathrm{Ce}$. These cross section differences oscillate with energy. Optical model calculations have been performed which fit the $^{140}\mathrm{Ce}$ total cross section well over the 3-60 MeV energy region. The $^{139}\mathrm{La}$-$^{140}\mathrm{Ce}$ and $^{141}\mathrm{Pr}$-$^{140}\mathrm{Ce}$ difference data were satisfactorily fit by small changes in the geometrical parameters of the potential. The $^{142}\mathrm{Ce}$-$^{140}\mathrm{Ce}$ data could not be fit by small changes in the geometry of the potential and we found that the changes required to achieve a satisfactory fit suggest that $^{142}\mathrm{Ce}$ is either nonspherical or more easily deformed than $^{140}\mathrm{Ce}$. Using our final optical model parameters we calculated $\ensuremath{\delta}({r}^{2})=^{A}〈{r}^{2}〉\ensuremath{-}^{140}〈{r}^{2}〉$ for the real part of the potential. The $^{139}\mathrm{La}$-$^{140}\mathrm{Ce}$ and $^{141}\mathrm{Pr}$-$^{140}\mathrm{Ce}$ optical model values of $\ensuremath{\delta}({r}^{2})$ were in agreement with corresponding $\ensuremath{\delta}{({r}^{2})}_{q}$ values calculated using muonic x-ray data. The value for $^{142}\mathrm{Ce}$-$^{140}\mathrm{Ce}$ was not.