Tomographic beta-gamma spectroscopy of nuclear beta decay

TL;DR

This paper introduces tomographic beta-gamma spectroscopy (TBGS), a novel technique that reconstructs decay schemes of nuclear beta decay with high precision, providing new insights into nuclear structure and background modeling for rare-event physics.

Contribution

The paper presents the first implementation of TBGS using the PandaX-4T detector, enabling concurrent measurement of transitions in $^{214}$Pb and revealing significant discrepancies in branching ratios.

Findings

Precise decay scheme of $^{214}$Pb obtained.

Concurrent measurement of transitions to ground and excited states.

Discrepancies in branching ratios up to 4.7$\sigma$ compared to previous data.

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, driven by recent advances in spectrometric techniques. Here we introduce tomographic $\beta$-$\gamma$ spectroscopy (TBGS) of nuclear $\beta$ decay, a method that detects the energies of $\beta$, $\gamma$, and internal conversion electrons while simultaneously reconstructing the energy deposition vertices. Using the PandaX-4T detector operated as a TBGS, 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 modelling in rare-event searches and highlight the potential of TBGS as a versatile tool for fundamental physics and nuclear applications.