July 20, 2011 – JAPAN – Radioactive decay of uranium, thorium, and potassium in Earth’s crust and mantle is a principal source, and in 2005 scientists in the KamLAND collaboration, based in Japan, first showed that there was a way to measure the contribution directly. The trick was to catch what Kamioka Liquid-scintillator Antineutrino Detector (KamLAND) dubbed geoneutrinos – more precisely, geo-antineutrinos – emitted when radioactive isotopes decay. KamLAND scientists have now published new figures for heat energy from radioactive decay in the journal Nature Geoscience. Based on the improved sensitivity of the KamLAND detector, plus several years’ worth of additional data, the new estimate is not merely “consistent” with the predictions of accepted geophysical models but is precise enough to aid in refining those models. One thing that’s at least 97-percent certain is that radioactive decay supplies only about half the Earth’s heat. Other sources – primordial heat left over from the planet’s formation, and possibly others as well – must account for the rest. The first signal comes when the antineutrino converts the proton to a neutron plus a positron (an anti-electron), which quickly annihilates when it hits an ordinary electron – a process called inverse beta decay. The faint flash of light from the ionizing positron and the annihilation process is picked up by the more than 1,800 photomultiplier tubes within the KamLAND vessel. A couple of hundred millionths of a second later the neutron from the decay is captured by a proton in the hydrogen-rich fluid and emits a gamma ray, the second signal. This “delayed coincidence” allows antineutrino interactions to be distinguished from background events such as hits from cosmic rays penetrating the kilometer of rock that overlies the detector. This is more heat energy than the most popular BSE model suggests, but still far less than Earth’s total. Says Freedman, “One thing we can say with near certainty is that radioactive decay alone is not enough to account for Earth’s heat energy. Whether the rest is primordial heat or comes from some other source is an unanswered question.” Better models are likely to result when many more geoneutrino detectors are located in different places around the globe, including midocean islands where the crust is thin and local concentrations of radioactivity (not to mention nuclear reactors) are at a minimum. –Physics.org
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