Neutron-resonance capture gamma rays inBa136

Abstract

Earlier experiments have reported an anomalous $M1$ strength for the ground-state radiative transition in $^{136}\mathrm{Ba}$ following neutron capture. Barium has been restudied in discrete resonance and average resonance capture. The $\ensuremath{\gamma}$-ray widths are measured and tabulated. We find a $\ensuremath{\gamma}$-ray strength function for $M1$ transitions of (20 \ifmmode\pm\else\textpm\fi{} 7) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}9}$ Me${\mathrm{V}}^{\ensuremath{-}3}$ from a study of capture in discrete resonances below 500 eV. From a study of neutron capture in an energy band 2 keV wide at 24.5 keV we find comparable values. These values are near the mean value (18 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}9}$) Me${\mathrm{V}}^{\ensuremath{-}3}$ observed for all nuclei; hence we conclude that the average $M1$ strength in $^{136}\mathrm{Ba}$ is quite normal. Additional widths for primary $E1$ and $M1$ transitions are also reported. A detailed multilevel analysis of the partial ($n,\ensuremath{\gamma}$) cross sections below the first $^{135}\mathrm{Ba}$ resonance at 24 eV shows a negligible direct $M1$ component.[NUCLEAR REACTIONS $^{135}\mathrm{Ba}(n,\ensuremath{\gamma})^{136}\mathrm{Ba}$; measured spectra, ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}\ensuremath{\lambda}f}$, ${J}^{\ensuremath{\pi}}$, resonances up to 500 eV; average resonance capture at ${E}_{n}=24$ keV; $M1$ photon strength functions.]