High precision measurement of the Tc 99 β spectrum

Highly precise measurements of the Tc99β spectrum were performed in two laboratories using metallic magnetic calorimeters. Independent sample preparations, evaluation methods, and analyses yielded consistent results and the spectrum could be measured down to less than 1keV. Consistent β spectra were also obtained via cross evaluations of the experimental data sets. An additional independent measurement with silicon detectors in a 4π configuration confirms the spectrum shape above 25keV. Detailed theoretical calculations were performed using the nuclear shell model and including 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 Qβ=295.82(16)keV, gVeff=0.376(5), and gAeff=0.574(36), which seems to solve an inconsistency of the quenched coupling constants between first and higher forbidden nonunique transitions. Furthermore, we derived the mean energy of the β spectrum, Ēβ=98.51(23)keV, logf=-0.47660(22), and logft=12.3478(23).