Observation ofNandMorbital-electron capture in the decay ofHo163

The ${}_{}{}^{163}{}_{}{}^{}\mathrm{Ho}$ nucleus decays by electron capture with the lowest $Q$ value of any known $\beta$-decay process and is sensitive to the effects of a finite electron neutrino mass. Low energy $N$ and $M$ orbital-electron capture events have been observed from a gaseous form of ${}_{}{}^{163}{}_{}{}^{}\mathrm{Ho}$ internally dispersed in a heated gas counter. The ratio $\frac{{\lambda }_N}{{\lambda }_M}=3.86\mathrm{}\pm 0.12$ provides a constraint on $Q_{\mathrm{ec}}$ and $m_{\nu }$. For $m_{\nu }=0\mathrm{}$, we find $Q_{\mathrm{ec}}=2600\mathrm{}\pm 30$ eV. Using independent $Q_{\mathrm{ec}}$ values, the range for $m_{\nu }$ is discussed.