Precision Measurement of the 210Pb Β Spectrum Enables Threshold-free Detection from 0 keV, Probing Beyond the Standard Model

Beta decay, a cornerstone of nuclear physics, presents ongoing challenges for precise measurement and theoretical modelling, particularly at very low energies where subtle effects can significantly alter results. Shuo Zhang, Hao-Ran Liu, and Ke Han, working with colleagues including Xavier Mougeot, Paul-Antoine Hervieux, and Tao Sun, now report a groundbreaking, high-precision measurement of the complete decay spectrum of 210Pb. Their innovative approach, utilising a transition-edge sensor-based micro-calorimeter, captures particle energies from zero keV upwards, overcoming the limitations of previous methods and delivering unprecedented accuracy at the lowest energies. This first complete, high-precision spectrum not only resolves longstanding theoretical uncertainties surrounding the atomic exchange effect, but also provides a crucial benchmark for refining decay theory and establishing a firmer foundation for the search for new physics beyond our current understanding of the weak interaction, including investigations into dark matter and neutrino properties.

Accurate Beta Decay Spectra Improve Nuclear Data

This research details a complex project focused on precisely measuring beta decay spectra to improve the accuracy of nuclear data used in diverse fields, including neutrino physics, fundamental constant determination, and advanced detector development. The project combines experimental measurements with sophisticated theoretical calculations to address limitations in existing data and enhance the reliability of models used in these areas. A key emphasis lies in accurately modeling the shape of the beta spectrum, which is crucial for extracting key parameters. The primary goal is to obtain more accurate and reliable nuclear data specifically related to beta decay processes.

Accurate beta decay data is essential for understanding neutrino properties and searching for new physics beyond the Standard Model, such as neutrinoless double-beta decay. Furthermore, these measurements can be used to precisely determine fundamental constants and are crucial for establishing reliable radiometric standards. The experiment utilizes a sophisticated detector system to precisely measure the beta spectrum, representing the distribution of energies of emitted beta particles. Researchers employ complex theoretical calculations to model the beta decay process and predict the spectrum’s shape, carefully considering atomic effects like electron screening and radiative corrections. They also account for exchange currents and require accurate knowledge of the nuclear structure of the decaying isotope, employing computational methods like Relativistic Density Functional Theory and the Shell Model.

Low-Energy Beta Decay Measured with Micro-Calorimeter

Scientists pioneered a high-precision measurement of beta decay using a transition-edge sensor (TES)-based micro-calorimeter, enabling detection of particle energies from 0 keV upwards, a capability previously limited by reconstruction artifacts at low energies. They engineered a system where the energy deposition from both the beta particle and subsequent de-excitation events are measured simultaneously, effectively shifting the beta spectrum and eliminating the traditional low-energy threshold. The micro-calorimeter comprises a lead-tin spherical absorber containing a trace amount of 210Pb, directly coupled to the TES, and is maintained at a base temperature of 20 mK within a dilution refrigerator. To calibrate the system and ensure stability, researchers illuminated the micro-calorimeter with gamma rays from an americium-241 source, employing copper shielding to prevent interference from alpha particles. The 50-day measurement of the 210Pb beta decay spectrum was continuously monitored via the baseline fluctuations of a Superconducting Quantum Interference Device (SQUID), which remained remarkably stable. A multi-layer shield reduced ambient magnetic fields, further enhancing the stability of the TES over extended periods.

Low-Energy Beta Decay Spectrum Fully Measured

Scientists have achieved the first complete, high-precision measurement of a beta decay spectrum, starting from 0 keV, using a novel transition-edge sensor-based micro-calorimeter. This breakthrough focuses on the decay of 210Pb to excited states of 210Bi, enabling the detection of particle energies across the entire spectrum by simultaneously measuring the beta particle and subsequent de-excitation energy. This innovative approach circumvents traditional energy detection thresholds, providing unprecedented accuracy at low energies. Experiments reveal a statistically significant enhancement in beta emission probability near zero energy, confirming predictions from an accompanying theoretical framework that incorporates atomic, leptonic, and nuclear components.

This finding directly validates the importance of atomic quantum exchange effects, a phenomenon where a beta particle exchanges with an atomic electron, and contrasts with models that omit these crucial corrections. The data demonstrate that atomic quantum exchange increases emission probability by approximately 20% below 1 keV, a substantial effect previously difficult to verify experimentally. The team measured the complete beta decay spectrum, shifting the energy scale by the excitation energy of 46. 539 keV, allowing for direct measurement of atomic quantum exchange from 0 keV to the endpoint energy. This precise measurement provides a new benchmark for beta decay theory at low energies, deepening understanding of the weak interaction and establishing a critical foundation for searches for new physics beyond the Standard Model. The results are particularly relevant for dark matter detection experiments and precision studies of neutrinos.

Low-Energy Beta Decay Spectrum Precisely Measured

This research presents the first complete, high-precision measurement of the full decay spectrum of 210Pb, extending down to zero energy, achieved through a novel transition-edge sensor-based micro-calorimeter. The team successfully eliminated reconstruction artifacts that typically limit accuracy at low energies by employing a coincidence summing technique, detecting both the decay energy and subsequent de-excitation energy. Results demonstrate a statistically significant enhancement in emission probability near zero energy, confirming theoretical predictions incorporating atomic exchange effects and challenging models that omit these corrections. The findings establish a new benchmark for decay theory at low energies and deepen understanding of the weak interaction, a fundamental force in nuclear physics. By resolving uncertainties related to atomic exchange, this work provides a crucial foundation for ongoing searches for new physics beyond the Standard Model, including investigations into dark matter interactions and precision studies of neutrinos.