Unsuitability of 157Tb for light-neutrino rest-mass studies.

Abstract

We have measured accurately the intensity ratio of L and K x rays in the $^{157}\ensuremath{\rightarrow}^{157}$Gd electron-capture decay as 0.932\ifmmode\pm\else\textpm\fi{}0.015. From this ratio, we have deduced a value of 4.0\ifmmode\pm\else\textpm\fi{}0.2 for the electron-capture ratio ${\mathit{P}}_{\mathit{L}}$/${\mathit{P}}_{\mathit{K}}$, leading, in turn, to a decay energy of 60.0\ifmmode\pm\else\textpm\fi{}0.3 keV. This Q value is 2.9 keV lower than the currently accepted value. This decay energy rules out L capture to the daughter excited state at 54.54 keV; therefore, $^{157}\mathrm{Tb}$ is not a suitable candidate for light-neutrino rest-mass studies based on this L-capture branch.