Experimental investigation of geologically produced antineutrinos with KamLAND

The detection of electron antineutrinos produced by natural radioactivity in the Earth could yield important geophysical information. The Kamioka liquid scintillator antineutrino detector (KamLAND) has the sensitivity to detect electron antineutrinos produced by the decay of 238 U and 232 Th within the Earth. Earth composition models suggest that the radiogenic power from these isotope decays is 16 TW, approximately half of the total measured heat dissipation rate from the Earth. Here we present results from a search for geoneutrinos with KamLAND. Assuming a Th/U mass concentration ratio of 3.9, the 90 per cent confidence interval for the total number of geoneutrinos detected is 4.5 to 54.2. This result is consistent with the central value of 19 predicted by geophysical models. Although our present data have limited statistical power, they nevertheless provide by direct means an upper limit (60 TW) for the radiogenic power of U and Th in the Earth, a quantity that is currently poorly constrained. Earthly powers The KamLAND experiment located in the Kamioka mine one kilometre beneath the Japanese Alps was primarily designed to detect antineutrinos produced by nuclear reactors. But radioactive elements in the Earth also release antineutrinos — known as geoneutrinos — and KamLAND should be sensitive enough to detect these too. And detect them it has. About 20 antineutrinos with characteristics typical of the products of uranium-238 and thorium-232 decay have so far been recorded. This opens up an exciting new era for geophysicists. Using geoneutrinos it should be possible to build up a three-dimensional image of the Earth's interior, and to establish how much geothermal heat is released by radioactive decay. On the cover, the half-globe to the left gives the neutrino rate at KamLAND from different locations on and beneath the Earth's surface.