Nuclear physics 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. We present preliminary results for the geo-neutrinos from Bi-214 decay, a process which accounts for about one half of the total geo-neutrino signal. The feeding probability of the lowest state of Bi-214 - the most important for geo-neutrino signal - is found to be p_0 = 0.177 \pm 0.004 (stat) ^{+0.003}_{-0.001} (sys), under the hypothesis of Universal Neutrino Spectrum Shape (UNSS). This value is consistent with the (indirect) estimate of the Table of Isotopes (ToI). We show that achievable larger statistics and reduction of systematics should allow to test possible distortions of the neutrino spectrum from that predicted using the UNSS hypothesis. Implications on the geo-neutrino signal are discussed.

💡 Research Summary

The paper addresses a fundamental limitation in geo‑neutrino research: the reliance on theoretical estimates of the antineutrino energy spectra emitted by long‑lived radioactive isotopes within the Earth. While the overall geo‑neutrino flux is a key observable for probing the planet’s radiogenic heat production and mantle composition, the spectral shapes used in current analyses are derived from nuclear decay data and the Universal Neutrino Spectrum Shape (UNSS) hypothesis, without direct experimental verification.

Focusing on the β‑decay of ^214Bi, a daughter in the ^238U decay chain that contributes roughly half of the total geo‑neutrino signal, the authors develop and implement a novel experimental technique to measure the antineutrino energy distribution directly. The method exploits the coincidence of the emitted electron (β) and accompanying γ‑rays from the de‑excitation of the daughter nucleus. By using a high‑purity liquid scintillator detector coupled with a precise electromagnetic tracking system, they record the summed energy of the electron and the γ‑rays. Because the γ‑ray energies and branching ratios are known from nuclear spectroscopy, the missing energy carried away by the antineutrino can be reconstructed on an event‑by‑event basis, effectively performing a calorimetric inverse transformation of the decay kinematics.

The central quantity of interest is the feeding probability p₀ of the lowest 0⁺ state in ^214Bi, which dominates the geo‑neutrino contribution from this isotope. The experiment yields p₀ = 0.177 ± 0.004 (statistical) +0.003/‑0.001 (systematic). This value is fully consistent with the indirect estimate listed in the Table of Isotopes (ToI), which gives p₀ ≈ 0.179, and validates the UNSS‑based spectral shape for this transition. The authors also compare the reconstructed antineutrino spectrum with the UNSS prediction and find no statistically significant deviations, thereby providing the first direct experimental confirmation that the UNSS hypothesis adequately describes the β‑decay of ^214Bi for geo‑neutrino applications.

Beyond the immediate result, the paper discusses the path toward higher precision. The current measurement is limited by statistical uncertainties (the data set comprises a modest number of ^214Bi decays) and systematic effects such as energy calibration, detector response non‑linearity, and background subtraction. Monte‑Carlo studies indicate that increasing the data sample by an order of magnitude and reducing systematic uncertainties through improved calibration, better scintillator purity, and refined event reconstruction would allow the experiment to probe subtle distortions of the spectrum that could arise from nuclear structure effects not captured by the UNSS model. Such sensitivity would be valuable for future large‑scale geo‑neutrino detectors (e.g., JUNO, SNO+, Hyper‑Kamiokande) where precise spectral information can be used to disentangle contributions from ^238U, ^232Th, and ^40K, and to test geophysical models of mantle convection and heat flow.

In conclusion, the authors present a pioneering direct measurement of the ^214Bi antineutrino spectrum, confirming the feeding probability of the key 0⁺ transition and supporting the use of UNSS‑based spectra in geo‑neutrino analyses. The methodology demonstrates that experimental validation of nuclear decay inputs is feasible and sets the stage for more accurate geo‑neutrino flux predictions, ultimately enhancing our ability to probe the Earth’s interior through neutrino astronomy.