Glints from Inner Space: Sensing Earth’s hidden radioactivity

By recording faint blue-green twinklings in a huge subterranean detector, physicists have observed signatures of radioactivity deep within Earth. The new data enable the scientists to directly measure planet-wide quantities of the elements thorium and uranium, whose radioactive disintegrations generate about half of the planet’s heat, according to previous estimates.

The power from those nuclear decays—which exceeds that of 10,000 nuclear power plants—propels many dynamic features of the planet, including crustal motions that give rise to earthquakes and volcanoes and the convection of softened rock within the planet’s mantle.

Before the new measurements, “there were only guesses” about radioactivity’s contribution to Earth’s internal heat, says Giorgio Gratta of Stanford University, a leader of the experiment. He, coleader Atsuto Suzuki of Tohoku University in Sendai, Japan, and their colleagues spent more than 2 years looking for telltale flickers in the Kamioka Liquid Scintillator Antineutrino Detector (KamLAND)—a tough, transparent balloon filled with 1,000 tons of baby oil, benzene, and fluorescent chemicals. Suspended 1 kilometer underground near Toyama, Japan, the balloon is surrounded by more than 1,800 supersensitive light detectors.

When they show themselves, the elusive flickers signal arrivals of subatomic antimatter particles called antineutrinos. Droves of those particles and their ordinary-matter counterparts, neutrinos, are left over from the Big Bang. They’re also produced in nuclear reactions in stars and in commercial power plants and when cosmic rays hit atmospheric atoms.

Nuclear decays of thorium, uranium, and the isotopes into which those elements transform give off antineutrinos. The majority of these escape Earth unscathed. Occasionally, however, an antineutrino from an underground decay collides with a molecule in the KamLAND tank, resulting in a signature written in light: two flashes in rapid succession.

In the July 28 Nature, the KamLAND team reports approximately 25 of those rare, double-flash events. From that number, the researchers calculated terrestrial thorium and uranium quantities and power outputs that agree with earlier estimates by other scientists. Those prior values relied on indirect evidence, such as thorium and uranium concentrations in meteorites, whose compositions presumably reflect the makeup of the primordial material from which the planet formed.

Earth has a third major radioactivity source—an isotope of potassium—that KamLAND can’t detect. However, the isotope is probably responsible for less than 10 percent of the planet’s radiogenic heat.

More than merely confirming earlier thorium and uranium estimates, the KamLAND experiment “heralds the dawn of a new science—neutrino geophysics,” comments Ramaswamy S. Raghavan of the Virginia Polytechnic Institute and State University in Blacksburg. He, Suzuki, and other scientists proposed years ago that KamLAND could detect Earth-derived antineutrinos even though the detector was designed to observe properties of more energetic antineutrinos from nuclear-power reactors (SN: 12/14/02, p. 371: Identity Check: Elusive neutrinos morph on Earth, as in space; 1/24/98, p. 55:

Next year, a detector more sensitive than KamLAND to Earth-generated antineutrinos is slated to start operating under the Gran Sasso mountain in Italy. And geophysicists envision future generations of detectors that can discern the direction that the identified antineutrinos were traveling.

Such detectors could reveal unprecedented views of three-dimensional features of the planet’s interior, notes geochemist William F. McDonough of the University of Maryland, College Park in a commentary accompanying the new study.

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