Signals from inside the Earth: Borexino Experiment releases new data on geoneutrinos

Exclusive insight into processes and conditions in the Earth's interior

30 January 2020

Scientists involved in the Borexino Collaboration, among them researches from the PRISMA+ Cluster of Excellence of Johannes Gutenberg University Mainz (JGU), have presented new results for the measurement of neutrinos originating from the interior of the Earth, so called geoneutrinos. These elusive "ghost particles" rarely interact with matter, making their detection difficult. With the newly presented analysis, the researchers have now been able to access 53 events in Borexino, which is almost twice as many as in the previous analysis of the data. The results provide an exclusive insight into processes and conditions in the Earth's interior that remain puzzling to this day.

The Earth is shining, even if it is not at all visible to the naked eye. The reason for this is geoneutrinos, which are produced in radioactive decay processes in the interior of the Earth. Every second, about one million of these elusive particles penetrate every square centimeter of our planet's surface.

Borexino has been collecting data on neutrinos for more than ten years

The Borexino detector, located in the world's largest underground laboratory, the Laboratori Nazionali del Gran Sasso in Italy, is one of the few neutrino experiments in the world capable of observing these ghostly particles. Researchers have been using it to collect data on neutrinos since 2007. By 2019, they were able to register twice as many events as at the time of the last analysis in 2015 and to thus reduce the uncertainty of the measurements from 27 to 18 percent, which is also due to new analysis methods.

"Geoneutrinos are the only direct traces of the radioactive decays that occur inside the Earth and which produce an as yet unknown portion of the energy driving all the dynamics of our planet," explained Professor Livia Ludhova, one of the two current scientific coordinators of Borexino and head of the neutrino group at the Nuclear Physics Institute at Forschungszentrum Jülich.

The interior of the Earth is built up in three shells: the solid Earth's crust down to a depth of about 50 kilometers, the viscous Earth's mantle down to a depth of 2,900 kilometers, and the molten outer and the solid inner core of the Earth below. Current models expect a large part of the geoneutrino signal from the Earth's crust, a smaller contribution from the mantle, and practically no geoneutrinos from the core region. For the first time the researchers in the Borexino Collaboration have extracted with an improved statistical significance the signal of geoneutrinos coming from the Earth's mantle by exploiting the well-known contribution from the Earth's uppermost mantle and crust, the so called lithosphere.

The intense magnetic field produced in the outer core, the unceasing volcanic activity, the movement of the tectonic plates, and mantle convection: The shell structure inside the Earth is in many ways unique in the entire solar system. Scientists have been discussing the question of where the Earth's internal heat comes from for over 200 years.

"The hypothesis that there is no longer any radioactivity at depth in the mantle can now be excluded at 99 percent confidence level for the first time. This makes it possible to establish lower limits for uranium and thorium abundances in the Earth's mantle," said Ludhova.

Interesting insights for Earth model calculations

These values are of interest for many different Earth model calculations. For example, it is highly probable, by about 85 percent, that radioactive decay processes inside the Earth generate more than half of the Earth's internal heat, while the rest is most probably largely derived from the original formation of the Earth. Radioactive processes in the Earth therefore provide a non-negligible portion of the energy that feeds volcanoes, earthquakes, and the Earth's magnetic field.

The Mainz scientists around Professor Michael Wurm, physicist at the Institute of Physics and the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz, have investigated how the underground generated by cosmic muons can be faded out as effectively as possible in data analysis. The problem: Cosmic muons are the only particles that have a sufficient range in the rock to reach the Borexino detector. Here, they sometimes produce signals delayed in time by radioactive decay, which at first glance cannot be distinguished from a real neutrino signal. "We have improved the analysis of muons and their subsequent events to the extent that less real signals from geoneutrinos have to be discarded," explained Wurm. "This allowed us to increase the usable data set by about ten percent. The detection of these cosmic underground sources is, so to speak, our specialty in the Borexino experiment."

The latest publication in Physical Review D not only presents the new results, but also explains the analysis in a comprehensive way from both the physics and geology perspectives, which will be helpful for next generation liquid scintillator detectors that will measure geoneutrinos. The next challenge for research with geoneutrinos is now to be able to measure geoneutrinos from the Earth's mantle with greater precision, perhaps with detectors distributed at different positions on our planet. One such detector will be the JUNO detector in China, where the Nuclear Physics Institute at Jülich and the Mainz neutrino group are also involved. The detector will be 70 times bigger than Borexino, which helps in achieving higher statistical significance in a short time span.