Spectroscopic studies of two $\beta$ rays from ${}^{100}\mathrm{Mo}$ are shown to be of potential interest for investigating both the Majorana $\nu$ mass by neutrinoless double $\beta$ decay $\left(0\nu \beta \beta \right)$ and low energy solar $\nu$'s by inverse $\beta$ decay. With a multiton ${}^{100}\mathrm{Mo}$ detector, coincidence studies of correlated $\beta \beta$ from $0\nu \beta \beta$, together with the large $Q$ value ( $Q_{\beta \beta }$), permit identification of the $\nu$-mass term with a sensitivity of $\sim 0.03\mathrm{eV}$. Correlation studies of the inverse $\beta$ decay and the successive $\beta$ decay of ${}^{100}\mathrm{Tc}$, together with the large capture rates for low energy solar $\nu$'s, make it possible to detect, in real time, individual low energy solar $\nu$ in the same detector.

• Received 16 November 1999

DOI:https://doi.org/10.1103/PhysRevLett.85.2917