Neutrinos to survey Earth's radioactivity

A significant fraction of the energy generated inside Earth comes from the decay of radioactive elements in its crust and mantle. The radioactive isotopes uranium-238 and thorium-232 are responsible for about 90 percent of this heat.

 Two groups of researchers now say it may be feasible to measure directly the global abundance of these two isotopes by detecting electron antineutrinos produced when the atoms decay. Neutrinos and their antimatter counterparts, antineutrinos, interact so little with ordinary matter that they zip readily through Earth's interior and fly off into space.

 Raju S. Raghavan of Bell Laboratories at Lucent Technologies in Murray Hill, N.J., and his collaborators describe their proposal in the Jan. 19 Physical Review Letters. Mark C. Chen and his coworkers at Princeton University detail their ideas in a report submitted to Geophysical Review Letters.

"We hope to get people in the geophysical community excited about this prospect," Chen says.

 Scientists have long been aware of the possibility of measuring the heat produced inside Earth by detecting antineutrinos. However, it took the recent development of special detectors, large-scale versions of liquid scintillators, to make such measurements feasible.

 Two massive liquid scintillation detectors are now under construction, one at the Borexino site in Italy and another at the Kamland experiment in Japan. Although both instruments are intended for other purposes, such as detecting neutrinos emanating from the sun, they would be sufficiently sensitive to pick up the low-energy antineutrinos generated by terrestrial radioactive decay.

"For the first time, we would have neutrino detectors that could look at Earth," Raghavan says. The Borexino experiment is slated to start up in 1999, the Kamland 2 years later.

 Nuclear power reactors also produce antineutrinos, which could potentially contaminate the results. However, the Princeton team shows that terrestrial antineutrinos can be distinguished from reactor antineutrinos and that the uranium and thorium contributions can be determined separately.

 Raghavan and his colleagues obtain similar conclusions and go on to suggest that, because one site is on continental crust and the other at the interface between continental and oceanic crust, it may even be possible to probe some aspects of the distribution of radioactive elements beneath Earth's surface.

"We could test geophysical models suggesting that most of the uranium and thorium is under the land masses rather than under the oceans," Raghavan says.

"The ratio of radioactive heat production to other sources, the distribution [of radioactive elements] between mantle and crust, and the distribution of the different nuclides are presently not known with any certainty," says geophysicist Raymond Jeanloz of the University of California, Berkeley.

 If such antineutrino data could be obtained, the resulting estimate of global radioactive heat production could shed light on what fraction of Earth's energy output is simply heat left over from the massive impact early in its history that created the moon, he remarks. "Such measurements could really change the textbooks."

 Chen says, however, that "it's unlikely that any one detector or combination of two detectors would have the precision to pinpoint local concentrations of uranium or thorium."

 Having a third antineutrino detector might enable researchers to map the distribution of uranium and thorium inside Earth, Raghavan notes. The Amanda detector at the South Pole, for example, is equipped to detect high-energy cosmic rays, but it could be adapted to detect antineutrinos too, he says.

"With these kinds of data, one could test the conceptual foundations of modern geophysics," Raghavan says.