Borexino gets a first look inside the Earth

The Borexino Collaboration has announced the observation of geoneutrinos at the underground Gran Sasso National Laboratory of the Italian Institute for Nuclear Physics (INFN). The data reveal, for the first time, an antineutrino signal well above background with the energy spectrum expected for radioactive decays of uranium and thorium in the Earth.

The Borexino Collaboration, comprising institutes from Italy, the US, Germany, Russia, Poland and France, operates a 300-tonne liquid-scintillator detector designed to observe and study low-energy solar neutrinos. Technologies developed by the collaboration have enabled them to achieve very low background levels in the detector, which were crucial in making the first measurements of solar neutrinos below 1 MeV. The central core of Borexino now has the lowest background available for such observations and this has been key to the detection of geoneutrinos.

Geoneutrinos are antineutrinos produced in the radioactive decays of naturally occurring uranium, thorium, potassium and rubidium. Decays from these radioactive elements are believed to contribute a significant but unknown fraction of the heat generated inside the Earth. This heat produces convective movements in the mantle, which influence volcanic activity and the tectonic-plate movements that induce seismic activity, as well as the geo-dynamo that creates the Earth’s magnetic field.

The importance of geoneutrinos was pointed out by Gernot Eder and George Marx in the 1960s and in 1984 a seminal study by Laurence Krauss, Sheldon Glashow and David Schramm laid the foundation for the field. In 2005, the KamLAND Collaboration reported an excess of low-energy antineutrinos above background in their detector in the Kamioka mine in Japan. Owing to a high background from internal radioactivity and antineutrinos emitted from nearby nuclear power plants, the KamLAND Collaboration reported that the excess events were an “indication” of geoneutrinos.

With 100 times lower background than KamLAND, the Borexino data reveal a clear low-background signal for antineutrinos, which matches the energy spectrum of uranium and thorium geoneutrinos. The lower background is a consequence both of the scintillator purification and the construction methods developed by the Borexino Collaboration to optimize radio-purity, and of the absence of nearby nuclear-reactor plants.

The origin of the known 40 TW of power produced within the Earth is one of the fundamental questions of geology. The definite detection of geoneutrinos by Borexino confirms that radioactivity contributes a significant fraction, possibly most, of this power. Other sources of power are possible, the main one being cooling from the primordial condensation of the hot Earth. A powerful natural geo-nuclear reactor at the centre of the Earth has been suggested, but is ruled out as a significant energy source by the absence of the high rate of antineutrinos associated with such a geo-reactor that should have been observed in the Borexino data.

Although radioactivity can account for a significant part of the Earth’s internal heat, measurements with a global array of geoneutrino detectors above continental and oceanic crust will be needed for a detailed understanding. By exploiting the unique features of the geoneutrino probe, future data from Borexino, KamLAND and the upcoming SNO+ detector in Canada should provide a more complete understanding of the Earth’s interior and the source of its internal heat.