Detectors zero in on Earth’s heat : Nature News & Comment

Detectors zero in on Earth’s heat : Nature News & Comment.

– extracts from the article –

A window on the deep Earth opened unexpectedly in 2011, when Japan’s nuclear reactors were shut down after the Fukushima disaster. Before the closure, an underground particle detector called KamLAND based in Kamioka, Japan, was monitoring a torrent of neutrinos streaming from dozens of nearby nuclear reactors, seeking clues to the nature of these hard-to-catch subatomic particles. After those plants fell silent, KamLAND scientists could see more clearly a signal that had largely been obscured: a faint trickle of neutrinos produced inside the planet.

These detections are not just curiosities. Geoneutrinos offer the only way to measure one of Earth’s internal heat sources. The total heat flow, measured with sensors in deep mines and amounting to 47 terawatts (TW) of power, drives everything from plate tectonics to Earth’s magnetic field. Some of it comes from the decay of radioactive elements, the rest is primordial heat left over from when Earth was formed by the violent collision of planetary building blocks.

Enter KamLAND and Borexino, which spot geoneutrinos as a sideline to their other neutrino studies. Both experiments use liquid scintillator detectors, in which huge vats of fluid capture the occasional sparkle of light when a passing neutrino interacts with atomic nuclei in the liquid.

The team at Borexino, a vat containing 300 tonnes of liquid buried under the Italian Alps, captured 14 candidate geo­neutrinos between December 2007 and August 2012 (ref. 2). Scientists at KamLAND, with 1,000 tonnes of liquid, say that they detected 116 probable geoneutrinos between March 2002 and November 2012 (ref. 3).

Assuming that uranium and thorium are spread uniformly in the mantle, the KamLAND findings suggest that about 11 of the 47 TW come from the radioactive decay of those elements. A similar calculation for Borexino yields about 18 TW.

One challenge is that emissions from uranium and thorium much nearer the surface in the continental crust can mask the geoneutrino signal coming from deeper in the planet (see ‘Under the sea’). Next year, for example, the retro­fitted Sudbury Neutrino Observatory (SNO) in Ontario, Canada, will start taking data with a 780-tonne detector that is sensitive to geoneutrinos. But SNO+, as the upgrade is called, sits smack in the middle of continental crust. Separating crustal from mantle geo­neutrinos is crucial, says Steve Dye, a physicist at Hawaii Pacific University in Honolulu, as “the mantle is really what contributes to the rate of cooling of the planet”.

Meanwhile, China is working on its Daya Bay II experiment, a 20,000-tonne detector on land that could be ready to hunt for geoneutrinos in 2019. Borexino has funds to run for at least another four years. And KamLAND plans to keep going for at least five more years, says team member Hiroko Watanabe of Tohoku University in Sendai, Japan. Even after Japan’s nuclear reactors restart, the detector will still be able to find geoneutrinos — just not as easily.

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