Earthly whispers of geoneutrinos

Neutrinos originating from inside the Earth have at last been detected — a landmark discovery that will lead to a deeper understanding of the radioactive make-up of our planet, and of its overall heat budget.

The decay of unstable isotopes, or radionuclides, within the Earth’s interior gives off heat that contributes to the total energy output of the planet (known as radiogenic power). In particular, isotopes of uranium and thorium, ²³⁸U and ²³²Th, decay to produce electron antineutrinos, which propagate undisturbed through the Earth, thanks to their extremely weak interactions with matter. Given a sufficiently large detector and enough time, however, some of these neutrinos can be detected at the surface, enabling us to probe the reactions that go on inside the Earth. That is exactly what T. Araki and colleagues¹, of the KamLAND experiment, set out to do and their findings are reported in this week’s issue of Nature.

The total power dissipated from the Earth has already been measured using thermal techniques, but alternative interpretations of the data (which adopt different heat-flow models near mid-ocean ridges) have led to a rather broad power estimate: between 30 and 44 terawatts. The Kamioka Liquid-Scintillator Anti-Neutrino Detector (KamLAND), which was custom-built to detect the elusive signatures of neutrinos, has now shed new light on this debate. Located a kilometre below ground in the Kamioka mine in Japan, KamLAND consists of one thousand tonnes of liquid scintillator shielded from the effects of cosmic rays. Occasionally, protons in the liquid capture electron antineutrinos through inverse beta-decay, emitting a characteristic 2.2-MeV gamma-ray that is subsequently detected by an array of photomultipliers.

The major challenge is to fight down the background processes that mimic the signatures of genuine geoneutrinos. In this instance, the main background antineutrinos come from nearby nuclear power reactors and radioactive contamination within the detector. Out of a total of 152 geoneutrino candidates, the researchers were left with 20–25 signal events after background subtraction. This constrains the antineutrino emission from thorium and uranium in the planet, establishing an upper limit to the radiogenic power of 60 terawatts, and a central value of 16 terawatts — consistent with the prediction of 19 terawatts from current geophysical models.

Further data from KamLAND, and from other detectors around the world, will provide greater sensitivity in testing the nature and source of geoneutrinos. The hope is to be able to construct geoneutrino tomographic maps of the Earth, and ultimately to gain a detailed understanding of the world beneath our feet.