Precise tracking spectroscopy of beta-gamma cascade in nuclear decay
Abstract
Nuclear $\beta$ decay, a sensitive probe of nuclear structure and weak interactions, has become a precision test bed for physics beyond the Standard Model (BSM), driven by recent advances in spectroscopic techniques. Here we introduce tracking spectroscopy of $\beta$-$\gamma$ cascades, a method that reconstructs decay vertices while simultaneously detecting $\beta$ particles and all associated de-excitation energies. Using the PandaX-4T detector operated as a tracking spectrometer, we obtain a precise and unbiased decay scheme of $^{214}$Pb, a key background isotope in searches for dark matter and Majorana neutrinos. For the first time, transitions of $^{214}$Pb to both the ground and excited states of $^{214}$Bi are measured concurrently, revealing discrepancies in branching ratios of up to 4.7$\sigma$ relative to previous evaluations. Combined with state-of-the-art theoretical spectral shape calculations, these results establish a new benchmark for background modeling in rare-event searches and highlight the potential of tracking spectroscopy as a versatile tool for fundamental physics and nuclear applications.