Earth sciences

Ghosts from within

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The first detection of geoneutrinos from beneath our feet is a landmark result. It will allow better estimation of the abundances and distributions of radioactive elements in the Earth, and of the Earth's overall heat budget.

The decay of unstable isotopes of chemical elements within the Earth produces heat that contributes to its overall energy output — a fact recognized shortly after Henri Becquerel first discovered radioactivity in 1896. More than 100 years on, Araki et al. (page 499 of this issue)1 report the first measurement of antineutrinos produced by radioactive β-decay at the heart of the Earth. The results obtained from these so-called geoneutrinos are consistent with geochemical and geo-physical models of the planet, and provide a new way of determining where the unstable isotopes — radionuclides — are stored inside the Earth, and in what concentrations.

Figure 1
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Credit: KamLAND/LBNL

Antineutrinos, like their counterpart neutrinos, come in three varieties, each named after the charged particle with which they are paired: electron, muon and tau. Electron antineutrinos are produced in β-decays of an atomic nucleus that occur, for example, when potassium (40K) is transformed to the calcium isotope 40Ca, and also in the decay series of uranium (U) and thorium (Th). Neutrinos and antineutrinos are ghostly particles — they have no charge and almost no mass, and pass through most matter without interacting with it at all. Detecting them is thus extremely difficult.

The KamLAND (Kamioka Liquid-scintillator Anti-Neutrino Detector) apparatus was purpose-built to catch a glimpse of these elusive particles (see Fig. 5 on page 502). The detector is situated in the centre of the largest Japanese island, Honshu, in a mine one kilometre below the summit of Mt Ikenoyama, to reduce the effects of cos-mic rays formed from particles other than antineutrinos. Antineutrinos are occasionally captured by protons in KamLAND's 1-kilotonne, 13-metre-diameter scintillation detector (pictured above) in a process known as inverse β-decay. This produces a neutron, which combines with a proton to form a deuteron and produces a characteristic γ-ray ('scintillation light') with an energy of 2.2 MeV. The light that this reaction produces is detected as an electrical signal by an array of photomultiplier devices surrounding the detector.

In 2003, KamLAND played a fundamental role in documenting the phenomenon known as antineutrino oscillation2, in which the varieties of antineutrino are thought to change spontaneously into one another. That result demonstrated that antineutrinos have a mass (albeit a small one), and reinforced the discovery of oscillation among neutrino types from the Sun by the Sudbury Neutrino Observatory in Canada3. The oscillating antineutrinos detected by KamLAND were produced in nuclear reactors on average some 180 kilometres away. Now the detector has seen antineutrinos from an even more distant source — geoneutrinos, key to understanding where the energy output of the Earth comes from.

The total power dissipated by the Earth's interior is estimated to be between 30 and 44 terawatts (1 TW is 1012 watts)4,5. The spread in values stems from differences between global models of heat flow at the Earth's surface on the one hand, and estimates of heat dissipation at mid-ocean ridges, corrected for the effects of hydrothermal circulation in the oceans themselves, on the other. Several compositional models for the Earth find that the amount of K, U and Th in the planet contributes only around 19 TW of power6,7 to the total. These observations result in a Urey ratio (an assessment of the amount of heat produced by radioactive decay to total heat flow on the surface of a planet) of 0.4–0.6. The remaining heat must come from other potential contributors, such as core segregation, inner-core crystallization, accretion energy or extinct radionuclides — for example, the gravitational energy gained by metal accumulating at the centre of the Earth, which is converted to thermal energy, and the energy added by impacts during the Earth's initial growth.

Alternative models suggest that there is also K in the Earth's core8,9, and predict a higher Urey ratio. These models are, however, accompanied by geochemical consequences that limit their acceptability (see ref. 4 for a review). The heat flux across the core–mantle boundary and the nature of heat sources in the core are also subject to considerable speculation10. The KamLAND results1 show an upper limit (at the 99% confidence level) of radiogenic heat power from Th and U of 60 TW, and a central value of 16 TW that is consistent with model predictions.

The KamLAND results were not straightforward to obtain, and are not simple to interpret. Various 'pollutants' must be removed from the energy spectrum of the antineutrinos to achieve a pure signal: of a total of 152 events that were potentially from geoneutrinos, only 20–25 were considered true candidate geoneutrinos. The remaining, 'background' antineutrinos came from nearby nuclear power reactors (more than 50% of the total signal) and radioactive contamination in the detector (around 28%). Moreover, geoneutrinos produced from K decays are not — yet — detected at KamLAND, because their energies are below the threshold of 1.8 MeV required to trigger the existing detector system.

The data reported by Araki et al.1 are the results from their first experiment, which comprised just over two years of counting. Future observations at KamLAND, and at the Borexino detector under the Gran Sasso mountain in central Italy, which begins operation in 2006, will generate more data and provide greater sensitivity in testing the nature and sources of geoneutrinos. A crucial advance will be to confirm that the geoneutrino heat flux moving radially outwards from the Earth is directly proportional to the radiogenic heat flux. This will, however, require an exact knowledge of the abundance and distribution of K, Th and U in the Earth.

To this end, a first detailed assessment has been made11 of the predicted geoneutrino flux relative to the distribution of radioactive elements in the regional crust and underlying mantle near KamLAND, and throughout the Earth's interior. Further in the future, combining angle-integrated geoneutrino fluxes at different detector sites12 with element distribution maps will enable us to construct geoneutrino tomographic maps of the Earth that will tell us more about the planet-wide distribution of K, Th and U. Proposed sites for future (anti)neutrino detectors must therefore be sure to include areas beneath both continental regions rich in K, Th and U and oceanic regions where the three radionuclides are depleted.

The pioneering results from KamLAND presented by Araki et al.1, along with data from future work, will provide a fundamental constraint for the Earth's U and Th budget (and, it is to be hoped, shortly for that of K), and define the fractional contribution of radioactive heating to the total energy budget. Later this year, particle physicists and Earth scientists will gather to discuss these exciting and common areas of research at a meeting on Hawaii13.

References

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McDonough, W. Ghosts from within. Nature 436, 467–468 (2005) doi:10.1038/436467a

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