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Tomographic beta-gamma spectroscopy of nuclear beta decay

PandaX Collaboration, Zhe Yuan, Zihao Bo, Wei Chen, Xun Chen, Yunhua Chen, Chen Cheng, Xiangyi Cui, Manna Deng, Yingjie Fan, Deqing Fang, Xuanye Fu, Zhixing Gao, Yujie Ge, Lisheng Geng, Karl Giboni, Xunan Guo, Xuyuan Guo, Zichao Guo, Chencheng Han, Ke Han, Changda He, Jinrong He, Houqi Huang, Junting Huang, Yule Huang, Ruquan Hou, Xiangdong Ji, Yonglin Ju, Xiaorun Lan, Chenxiang Li, Jiafu Li, Mingchuan Li, Peiyuan Li, Shuaijie Li, Tao Li, Yangdong Li, Zhiyuan Li, Qing Lin, Jianglai Liu, Yuanchun Liu, Congcong Lu, Xiaoying Lu, Lingyin Luo, Yunyang Luo, Yugang Ma, Yajun Mao, Yue Meng, Binyu Pang, Ningchun Qi, Zhicheng Qian, Xiangxiang Ren, Dong Shan, Xiaofeng Shang, Xiyuan Shao, Guofang Shen, Manbin Shen, Wenliang Sun, Xuyan Sun, Yi Tao, Yueqiang Tian, Yuxin Tian, Anqing Wang, Guanbo Wang, Hao Wang, Haoyu Wang, Jiamin Wang, Lei Wang, Meng Wang, Qiuhong Wang, Shaobo Wang, Shibo Wang, Siguang Wang, Wei Wang, Xu Wang, Zhou Wang, Yuehuan Wei, Weihao Wu, Yuan Wu, Mengjiao Xiao, Xiang Xiao, Kaizhi Xiong, Jianqin Xu, Yifan Xu, Shunyu Yao, Binbin Yan, Xiyu Yan, Yong Yang, Peihua Ye, Chunxu Yu, Ying Yuan, Youhui Yun, Xinning Zeng, Minzhen Zhang, Peng Zhang, Shibo Zhang, Siyuan Zhang, Shu Zhang, Tao Zhang, Wei Zhang, Yang Zhang, Yingxin Zhang, Yuanyuan Zhang, Li Zhao, Kangkang Zhao, Jifang Zhou, Jiaxu Zhou, Jiayi Zhou, Ning Zhou, Xiaopeng Zhou, Zhizhen Zhou, Chenhui Zhu, Marlom Ramalho, Jouni Suhonen

TL;DR

This work introduces tomographic β-γ spectroscopy (TBGS), a method that detects energies of $β$, $γ$, and internal conversion electrons while reconstructing their energy-deposition vertices, enabling direct, vertex-resolved measurements of beta-decay schemes. Demonstrated with the PandaX-4T liquid xenon TPC, TBGS yields precise branching ratios for the $^{214}$Pb decay to ground and excited states of $^{214}$Bi in a single dataset, revealing significant deviations from the NDS2021 values. By coupling TBGS with state-of-the-art nuclear-shell-model-based β-spectra calculations, the study provides updated $eta$-spectral shapes and BRs that improve background modelling for xenon-based dark matter and rare-event searches. The results establish TBGS as a versatile tool for fundamental nuclear physics and offer a pathway to extend tomographic spectroscopy to additional isotopes and decay channels, with broad implications for background control and precision tests of weak interactions.

Abstract

Nuclear $β$ 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 $β$-$γ$ spectroscopy (TBGS) of nuclear $β$ decay, a method that detects the energies of $β$, $γ$, 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$σ$ 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.

Tomographic beta-gamma spectroscopy of nuclear beta decay

TL;DR

This work introduces tomographic β-γ spectroscopy (TBGS), a method that detects energies of , , and internal conversion electrons while reconstructing their energy-deposition vertices, enabling direct, vertex-resolved measurements of beta-decay schemes. Demonstrated with the PandaX-4T liquid xenon TPC, TBGS yields precise branching ratios for the Pb decay to ground and excited states of Bi in a single dataset, revealing significant deviations from the NDS2021 values. By coupling TBGS with state-of-the-art nuclear-shell-model-based β-spectra calculations, the study provides updated -spectral shapes and BRs that improve background modelling for xenon-based dark matter and rare-event searches. The results establish TBGS as a versatile tool for fundamental nuclear physics and offer a pathway to extend tomographic spectroscopy to additional isotopes and decay channels, with broad implications for background control and precision tests of weak interactions.

Abstract

Nuclear 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 - spectroscopy (TBGS) of nuclear decay, a method that detects the energies of , , 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 Pb, a key background isotope in searches for dark matter and Majorana neutrinos. For the first time, transitions of Pb to both the ground and excited states of Bi are measured concurrently, revealing discrepancies in branching ratios of up to 4.7 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.

Paper Structure

This paper contains 9 sections, 2 equations, 5 figures, 2 tables.

Table of Contents

  1. Introduction
  2. The challenge of $\beta$ decay spectroscopy
  3. PandaX-4T TPC for TBGS
  4. $^{214}$Pb data and fitted BRs
  5. Summary and outlook
  6. Methods
  7. Signal model of the $\beta$ spectra
  8. $^{214}$Pb data in PandaX-4T
  9. Fit and results

Figures (5)

  • Figure 1: a. The $\beta$ decay scheme of $^{214}$Pb and the corresponding branching ratios in different databases. Uncertainties follow the NDS2021 values in parentheses. No uncertainties are quoted in the other two databases. b.c.d. Illustration of $\beta$ decay spectroscopic techniques. The spectra do not include detector effects.
  • Figure 2: Schematic diagrams of tomographic signatures of GS and ES decays. a. a $\beta$ from GS decay registers one single S2 and is identified as a SS event. b. $\beta$ and $\gamma$ from an ES decay register overlapping S2s and are identified as a SS event. c. $\beta$ and $\gamma$ from an ES decay register separated S2s and are identified as an MS event. d. a $\gamma$ from an ES decay may experience one or more scattering before been absorbed in LXe. The $\beta$ and $\gamma$ vertices register separated S2s and are identified as an MS event.
  • Figure 3: The $^{214}$Pb data and the final best-fit are shown for SS (a) and MS (b), with a bin size of 2 keV. The horizontal axis represents the reconstructed energy in the data. The shaded green area in (a) represents the excluded region. The lower panel in each figure shows the fit residuals together with $\pm 1 \sigma$, $\pm 2 \sigma$, and $\pm 4 \sigma$ bands.
  • Figure 4: Fitted branching ratios of $^{214}$Pb $\beta$ decay for each iteration. The solid squares represent the best-fit results with $1\sigma$ error bars. The NDS2021 values are shown as the green dashed line with error bands.
  • Figure 5: Comparison of the energy response spectra of the detector due to the decay branches of $\mathrm{GS}$, $\mathrm{ES_{295}}$, and $\mathrm{ES_{534}}$ using Geant4 default $\beta$ energy spectra and using theoretical $\beta$ energy spectra. The top graph is the $\beta$ energy spectra, and the bottom is the detector response energy spectra.