Researchers Gustavo Alcalá and Alejandro Algora from the Gamma and Neutron Spectroscopy group at the Institute of Corpuscular Physics (IFIC) —a joint center of the Spanish National Research Council (CSIC) and the University of Valencia (UV)— have led a study published in the prestigious journal Physical Review Letters. The aim of the study is to understand the properties of antineutrinos emitted in nuclear fission reactors.

Neutrinos are fundamental subatomic particles with an extremely small mass, close to zero, and no electric charge, which makes them invisible to electromagnetic fields. They are the second most abundant particles in the universe, generated in stars and cosmic explosions, and they can pass through the Earth without being detected. For this reason, they are known as “the ghost particle”.

Every subatomic particle has its corresponding antiparticle (which has the same mass and spin as the particle, but with opposite electric charge and magnetic moment). Antineutrinos, the antiparticles of neutrinos, are emitted or produced during certain nuclear decays. Inside a nuclear reactor, unstable atomic nuclei are produced and decay in various ways. One of the most common forms is beta decay, in which the nucleus emits a beta particle (an electron) and an antineutrino.

Back in 2024, the Gamma and Neutron Spectroscopy group at IFIC, together with scientific teams from France, England, and Finland, announced the development of a new detection system intended to clarify some of the discrepancies between theoretical models and observed data in neutrino detection processes in nuclear reactors. The key to understanding these discrepancies, they stated, could lie in measuring the beta spectra of fission products. Now, this new detection system has borne fruit, and the group has published its results.

The study involved measurements of the shape of beta decay spectra that contribute most to the antineutrino spectrum of reactors, using high-purity radioactive beams produced at IGISOL, an internationally recognized Finnish experimental facility. Detecting emitted neutrinos to analyze their energy is extremely complex, but it is possible to infer it by measuring the energy of the beta particle, since both are linked by the principle of energy conservation. In other words, beta particles emitted in a reactor can be measured, and through them, antineutrinos can be studied.

With these results, Algora and Alcalá have succeeded in significantly correcting the calculated antineutrino spectrum of reactors, since the measurements focus on “a type of beta transition that represents a high percentage of the total and therefore directly affects the region of the spectrum that remains unexplained,” explains Alejandro Algora, principal investigator of the experimental proposal.

“These measurements,” adds Alcalá, “are the first of their kind performed with ultrapure radioactive beams, and they allow us to better assess the corrections used in the calculation of a reactor’s antineutrino spectrum.”

Thanks to the enormous number of antineutrinos emitted per second by a reactor (approx. 10²⁰), nuclear reactors have been highly relevant in the study of neutrino properties—such as their ability to transform from one type into another, a phenomenon known as neutrino oscillation—and, more importantly, they made it possible to directly demonstrate their existence for the first time.

It is now possible to monitor the power of a nuclear reactor using neutrino detectors. But these measurements could have major implications in the future: it may become possible to determine whether illegal manipulations of reactor fuel have occurred by measuring the antineutrino spectrum. Additionally, researchers believe it will eventually be possible to “see” what is happening inside a nuclear reactor through neutrinos.

The work, carried out within the framework of an international collaboration (IFIC–Subatech–Univ. Surrey–Univ. of Jyväskylä–Univ. Warsaw) and led by IFIC, is the first in a series of studies that will contribute to a better understanding of the anomalies that still persist in nuclear reactor neutrino physics.

Reference: https://journals.aps.org/prl/abstract/10.1103/hyj7-l22h