Spectral Measurement of the $^{214}$Bi beta-decay to the $^{214}$Po Ground State with XENONnT

TL;DR

This work addresses precise beta-decay spectroscopy of $^{214}$Bi to the ground state of $^{214}$Po, a first-forbidden non-unique transition, using the XENONnT detector. The authors develop a dedicated BiPo tagging algorithm and an extended high-energy simulation pipeline to extract the ground-state spectrum up to the endpoint energy $E_0=3.27$ MeV, simultaneously calibrating electronic-recoil yields at MeV scales. By comparing to multiple nuclear-model predictions, they find the Conserved Vector Current (CVC) based spectrum provides the best description, while the allowed and sNME models are disfavored; systematic uncertainties are kept below 1%. The results demonstrate high-precision beta spectroscopy in a dual-phase xenon detector and offer valuable validation for nuclear-structure predictions and detector response modeling at MeV energies.

Abstract

We report the measurement of the $^{214}$Bi beta-decay spectrum to the ground state of $^{214}$Po using the XENONnT detector. This decay is classified as first-forbidden non-unique, for which theoretical predictions require detailed nuclear structure modeling. A dedicated identification algorithm isolates a high-purity sample of ground-state beta-decays, explicitly excluding events with associated gamma-rays emission. By comparing the measured spectrum, which covers energies up to 3.27 MeV, with several nuclear models, we find that the prediction based on the conserved vector current (CVC) hypothesis provides the best description of the data. Using this dataset, we additionally derive charge and light yield curves for electronic recoils, extending detector response modeling up to the MeV scale.

Figures (7)

• Figure 1: Decay scheme of [214]Bi to [214]Po and subsequently to [210]Pb. The diagram shows beta-decays from [214]Bi (left) either directly to the ground state (g.s.) or via excited states of [214]Po (center). Approximately 80 excited nuclear states are accessible during the decay (symbolically represented). All these excited states de-excite via gamma-decays emissions (red arrows), always including at least one transition greater than 609. The final step is the alpha-decay of [214]Po (black arrow) directly to the stable ground state of [210]Pb (right).
• Figure 2: Theoretical beta-decay energy spectra for ground-state to ground-state transitions of [214]Bi to [214]Po. The solid blue line shows the zeroth order Allowed spectrum, the dashed light blue line the CVC-based spectrum, the dashed red line the sNME1 fit, and the dot-dashed yellow line the sNME2 fit.
• Figure 3: Time evolution of the measured $^{222}$Rn activity in the XENONnT TPC fiducial volume during the calibration campaign. The activity is derived from alpha-decays, shown as a function of time since the source opening. The vertical dashed lines indicate the source opening and closing times.
• Figure 4: Waveform representation of a typical [214]BiPo event from measured data. The two green peaks on the left correspond to the S1 signals of [214]Bi and [214]Po, separated by a time interval $\Delta t$. The two dark blue peaks on the right, also separated by $\Delta t$, are identified as the corresponding S2 signals. The light blue peaks represent S2 signals from gamma emissions. Due to variations in electron drift distance, depending on whether the gamma travels upward or downward, its S2 signal appears earlier or later than the [214]Bi S2. The alpha-decay ([214]Po) exhibits significant recombination and quenching effects, resulting in a smaller S2 despite its higher energy compared to the preceding beta-decay . Conversely, these effects enhance the alpha’s S1 signal. Inset panels show the S1 waveforms on an expanded time scale, highlighting their much shorter duration compared to S2 signals.
• Figure 5: Charge yield (CY, top) and light yield (LY, bottom) as a function of reconstructed energy, extracted from beta-decays of [212]Pb (blue) and [214]Bi (black). The ER yield model (light blue) is obtained by fitting the charge yield, with the light yield derived as the complementary fraction of total quanta. The shaded band accounts for the systematic uncertainty from the energy-bias correction. A correction for residual excited-state leakage is applied to the data. Grayed-out [214]Bi points above 1.8MeV are excluded from the fit due to gamma contamination. For comparison, the default NEST model at 23V/cm (gray) and the XENONnT low-energy yield model (red) are shown. Statistical uncertainties are smaller than the marker size.