High precision measurement of the $^{99}$Tc $\beta$ spectrum

Highly precise measurements of the $^{99}$Tc beta spectrum were performed in two laboratories using metallic magnetic calorimeters. Independent sample preparations, evaluation methods and analyses yield consistent results and the spectrum could be measured down to less than 1 keV. Consistent beta spectra were also obtained via cross-evaluations of the experimental data sets. An additional independent measurement with silicon detectors in a $4\pi$ configuration confirms the spectrum shape above 25 keV. Detailed theoretical calculations were performed including nuclear structure and atomic effects. The spectrum shape was found to be sensitive to the effective value of the axial-vector coupling constant. Combining measurements and predictions, we extracted $Q_{\beta}=$295.82(16) keV and $g_A^{\text{eff}} = 1.530 (83)$. Furthermore, we derived the mean energy of the beta spectrum $\overline{E}_{\beta}$=98.45(20) keV, $\log f = -0.47660 (22)$ and $\log ft = 12.3478 (23)$.