Measurement of the $p$ to $s$ Wave Branching Ratio of ${}^{187}\mathrm{Re}$ $\beta$ Decay from Beta Environmental Fine Structure

The mixed occurrence of $s$-wave and $p$-wave contributions in a first forbidden unique Gamow-Teller $\beta$ decay has been investigated for the first time by measuring the beta environmental fine structure (BEFS) in a ${}^{187}\mathrm{Re}$ crystalline compound. The experiment has been carried out with an array of eight ${\mathrm{AgReO}}_4$ thermal detectors operating at a temperature of $\sim 100\mathrm{mK}$. A fit of the observed BEFS spectrum indicates the $p$-wave electron emission as the dominant channel. The complete understanding of the BEFS distortion of the ${}^{187}\mathrm{Re}$ $\beta$ decay spectrum is crucial for future experiments aiming at the precise calorimetric measurement of the antineutrino mass.

Figure 1

Kurie plot obtained from the sum of all 8 detectors, where $p$ is the electron momentum, $E$ is the electron kinetic energy, $F(Z,E)$ is the Coulomb factor, and $S(E)$ is the shape factor.

Figure 2

The filled circles are the experimental fit residuals derived as the difference between the theoretical ${}^{187}\mathrm{Re}$ $\beta$ spectrum for free rhenium atoms and the measured one, as explained in the text. The BEFS oscillations appearing in the experimental residuals are fitted by using (1) with the backscattering amplitudes and phase shifts calculated with the Filipponi et al. theory [16] (solid line).