Unsuitability ofTb157for light-neutrino rest-mass studies

We have measured accurately the intensity ratio of {ital L} and {ital K} x rays in the {sup 157}Tb{r arrow}{sup 157}Gd electron-capture decay as 0.932{plus minus}0.015. From this ratio, we have deduced a value of 4.0{plus minus}0.2 for the electron-capture ratio {ital P}{sub {ital L}}/{ital P}{sub {ital K}}, leading, in turn, to a decay energy of 60.0{plus minus}0.3 keV. This {ital Q} value is 2.9 keV lower than the currently accepted value. This decay energy rules out {ital L} capture to the daughter excited state at 54.54 keV; therefore, {sup 157}Tb is not a suitable candidate for light-neutrino rest-mass studies based on this {ital L}-capture branch.