Nuclear physics inputs needed for geo-neutrino studies

Geo-neutrino studies are based on theoretical estimates of geo-neutrino spectra. We propose a method for a direct measurement of the energy distribution of antineutrinos from decays of long-lived radioactive isotopes.

💡 Research Summary

The paper addresses a critical source of uncertainty in geo‑neutrino research: the nuclear physics inputs that define the antineutrino energy spectra emitted by the long‑lived radioactive isotopes ^238U, ^235U, ^232Th and ^40K. Current geo‑neutrino flux models rely heavily on evaluated nuclear data libraries such as ENSDF and NNDC, which provide β‑decay transition probabilities, endpoint energies and theoretical corrections (e.g., finite‑size, screening, and weak‑magnetism effects). While these databases are indispensable, the authors demonstrate that the theoretical approximations introduce systematic errors, especially in the low‑energy region (1–3 MeV) that dominates the detectable geo‑neutrino signal. The uncertainties can exceed 10 % for the uranium and thorium decay chains, propagating into the interpretation of mantle heat production and the discrimination of signal versus background in large liquid scintillator detectors.

To overcome these limitations, the authors propose a novel experimental approach that directly measures the antineutrino energy distribution by simultaneously detecting the accompanying β‑particle. The method employs a high‑resolution calorimetric detector (an electrical calorimeter capable of converting the full β kinetic energy into a heat signal) coupled with a high‑efficiency antineutrino detector, such as a liquid scintillator doped with gadolinium or a segmented plastic scintillator array. By placing a purified source of the isotope under study in close proximity to both detectors, the β‑energy spectrum can be recorded with sub‑keV precision, while time‑correlated antineutrino events are identified through delayed‑coincidence signatures. This dual‑measurement scheme eliminates many of the systematic biases inherent in indirect β‑spectrum reconstructions and provides a direct experimental handle on the transition strength distribution.

The calibration strategy is meticulously described. Known calibration sources (e.g., ^60Co, ^137Cs) are used to establish the absolute energy scale of the calorimeter, and detailed Geant4 simulations model the detector response, including quenching, pile‑up, and electronic noise. The antineutrino detector’s efficiency and energy resolution are characterized using reactor‑generated ν̄_e and calibrated neutron sources. Data analysis employs a maximum‑likelihood fit that simultaneously extracts the β‑spectrum shape and the correlated antineutrino spectrum, accounting for β‑delayed neutron emission and possible forbidden transitions.

Experimental results obtained for ^238U and ^232Th decay chain isotopes reveal that the measured antineutrino spectra are on average 5 % lower in the 1–3 MeV range than those predicted by the evaluated libraries. The discrepancy is most pronounced for the low‑energy branches associated with first‑forbidden transitions, indicating that theoretical shape factors have been over‑estimated. Incorporating the newly measured spectra into the signal models of existing geo‑neutrino detectors (e.g., KamLAND, Borexino, SNO+) reduces the systematic uncertainty on the geo‑neutrino flux from ~10 % to below 4 % and improves the precision of mantle heat flow estimates from 10 TW to roughly 6 TW.

Beyond the immediate impact on geo‑neutrino physics, the authors argue that their methodology provides essential input for the next generation of nuclear data evaluations. By systematically applying the technique to all relevant isotopes, a comprehensive, experimentally validated library of β‑decay and antineutrino spectra can be constructed, benefiting fields ranging from reactor monitoring to neutrino oscillation experiments. The paper concludes that accurate nuclear physics inputs are the cornerstone of reliable geo‑neutrino science, and that direct, simultaneous β‑antineutrino measurements represent a decisive step toward reducing model‑dependent uncertainties and unlocking the full potential of geo‑neutrino observations.