The Kamioka liquid-scintillator antineutrino detector (KamLAND) has made the first observation of “geoneutrinos”. This comes just over 50 years since George Gamow, in a letter to Fred Reines in 1953, pointed out the possibility of detecting antineutrinos of terrestrial origin. KamLAND, which has already confirmed neutrino oscillations by detecting antineutrinos emitted from nuclear reactors, has opened up a new window of research, exploring the deep interior of the Earth by detecting geoneutrinos.
Geoneutrinos are created in the beta decays of radioactive isotopes in the Earth. The current geochemical and geophysical models suggest that the radiogenic power from the 238U and 232Th decay chains is 16 TW, approximately half the total measured heat-dissipation rate from the Earth. This heat helps to drive convective flows in the mantle and the outer core, resulting in plate tectonics, volcanism and terrestrial magnetism. Thus radiogenic heat is a key factor in understanding the Earth’s dynamics, formation and evolution. However, since geophysicists have never had a direct way to determine how uranium and thorium are distributed in the Earth’s interior, measuring their concentration inside the Earth sheds new light on geophysics.
KamLAND consists of about 1 kt of liquid scintillator, located in the Kamioka mine in Japan. It can detect geoneutrinos from the 238U and 232Th decay chains through an inverse beta-decay process with a threshold energy of 1.8 MeV. Using data collected between 9 March 2002 and 30 October 2004 with a detector live-time of 749 days, 25 geoneutrino events were obtained after subtracting the number of expected background events, mostly from reactor antineutrinos. Combining the event rate and energy spectrum of candidates yields between 4.5 and 54.2 geoneutrinos, with a central value of 28 at the 90% confidence interval (see figures). This assumes a Th:U mass ratio of 3.9, the value derived from chondritic meteorites and commonly understood to be the same for all materials in the solar system.
The result is consistent with the central value of 19 predicted by a geological model, and constrains the flux of geoneutrinos from uranium and thorium to less than 1.62 × 107 cm−2 s−-1 at 99% confidence limits. Although the present data have limited statistical power, they nevertheless directly provide an upper limit of 60 TW for the radiogenic power of uranium and thorium in the Earth.
These investigations should pave the way to more accurate measurements, which may develop into a new field of neutrino geophysics. There is a programme currently under way to reduce the radioactive content of the KamLAND detector, but further background reduction will require a new detector location, far away from nuclear reactors. In the future, a worldwide network of geoneutrino detectors would allow the production of a tomographic image of the radiogenic heat distribution.
Further reading
T Araki et al. 2005 Nature 436 467.
