Electron spectral shape of the third-forbidden $$\beta $$-decay of $$^{87}$$Rb measured using a $$\hbox {Rb}_2\hbox {ZrCl}_6$$ crystal scintillator

In recent years, interest in experimental studies of β-decay electron spectra – often referred to as β spectra – has been growing. This is particularly true for β transitions where the electron spectra are sensitive to the effective value of the weak axial coupling, gA. Such measurements serve as important benchmarks for nuclear physics calculations and can also be used to characterize background in astroparticle physics experiments. In this work, a dedicated experiment has been carried out to investigate the spectral shape of the third-forbidden 87Rb β-decays, with the goal of estimating the effective gA value for this transition and of deriving the T1/2 value. This was done by comparing the experimental spectral shape with the estimates from various phenomenological models. The 87Rb source was embedded directly within the detector material of a new Rb2ZrCl6 crystal scintillator; the data taking was performed deep underground at Gran Sasso National Laboratory. The obtained experimental half-life value for the studied process is T1/2 = 5.08(13) × 10^10 yr; while a gA value in the range 0.4–0.6 is obtained when accounting for uncertainties and depending on the model adopted as discussed in detail in the text.