Measurement of theTb158electron-captureQvalue

An ultralow energy K-capture branch in the decay of 180-yr ${}_{}{}^{158}{}_{}{}^{}\mathrm{Tb}$ has recently been suggested to offer very favorable energetics for investigating the electron neutrino mass. The ${}_{}{}^{158}{}_{}{}^{}\mathrm{Tb}$ Q value relative to ${}_{}{}^{158}{}_{}{}^{}\mathrm{Gd}$ has been measured by means of a (p,d) reaction on a radioactive ${}_{}{}^{158}{}_{}{}^{}\mathrm{Tb}$ target. The ground state electron capture Q value of 1215.4(4.3) keV, inferred here, precludes ${}_{}{}^{158}{}_{}{}^{}\mathrm{Tb}$ K electron capture to the 1187 keV state of ${}_{}{}^{158}{}_{}{}^{}\mathrm{Gd}$.