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Adapted from New scientist article
EARTH'S natural radioactivity has been measured for the first time using the Kamioka Liquid Scintillator Anti-Neutrino Detector (KamLAND). The measurement will help geologists find out to what extent nuclear decay is responsible for the immense quantity of heat generated by Earth.

The earth consists of several layers. The three main layers are the core, the mantle and the crust. The core is the inner part of the earth, the crust is the outer part and between them is the mantle.

The Earths heat output drives the convection currents that churn liquid iron in the outer core, giving rise to Earth's magnetic field. Just where this heat comes from is a big question. Measurements of the temperature gradients across rocks in mines and boreholes have led geologists to estimate that the planet is internally generating between 30 - 44 terawatts of heat.

The radioactive isotopes inside the Earth generate heat. In particular, decays of the daughter nuclei in the decay chains of 238U and 232Th, and 40K generate most of the radiogenic heat produced. According to the estimated concentrations of these isotopes, the radiogenic heat production rates are 8.0, 8.3, and 3TW for 238U series, 232Th series, and 40K decays, respectively. The sum of the estimated radiogenic heat production rate, about 19TW is only about the half of the total heat flow measured using the borehole measurements. According to some of the mantle convection models, these two numbers, 44TW (or 31TW) for the total heat dissipation rate from the Earth, and 19TW for radiogenic heat production rate should be similar.

Some of this heat comes from the decay of radioactive elements. Based on studies of primitive meteorites known as carbonaceous chondrites, geologists have estimated Earth's uranium and thorium content and calculated that about 19 terawatts can be attributed to radioactivity.
But until now there has been nothing definitive about exactly how much uranium there is in the planet, says geologist Bill McDonough of the University of Maryland in College Park.

"There are fundamental uncertainties"

KamLAND began taking data at the end of January, 2002. KamLAND has since observed anti-neutrino deficit as well as energy spectral distortion confirming neutrino oscillations. The existence of neutrino oscillations means that neutrinos have mass. KamLAND has also conducted a geoneutrino investigation. The KamLAND estimation of the radiogenic heat produced inside the Earth agrees with the current Earth models.

There is one way to lessen this uncertainty, and that is to look for antineutrinos. These particles are the antimatter equivalent of the uncharged, almost massless particles called neutrinos and are released when uranium and thorium decay to form lead. If antineutrinos are being created deep within the planet they should be detectable, because they can pass through almost all matter.

Now, the KamLAND antineutrino detector in Kamioka, Japan, has counted such antineutrinos. An international team of scientists analysed the data and found about 16.2 million antineutrinos per square centimetre per second streaming out from Earth's core. They calculate that the nuclear reactions creating these particles could be generating as much as 60 terawatts, but are most likely putting out about 24 terawatts.

"We have made the first measurements of the radioactivity of the whole of Earth" - John Learned, heads of the KamLAND group at the University of Hawaii in Manoa.

The KamLAND group's finding is like unwrapping a birthday present, says McDonough.
With time, as more antineutrinos are detected, KamLAND may be able to determine once and for all whether radioactivity is entirely responsible for heating Earth or whether other sources, such as the crystallisation of liquid iron and nickel in the outer core, also play a significant role.

"(Detecting anti-neutrinos) is the way of the future in terms of hard numbers about the system" - Bill McDonough.

Antineutrinos could also reveal the radioactive composition of the crust and mantle, which will give geologists clues as to when and how they formed. But to do that, they will have to be able to pin down exactly where the antineutrinos are coming from, and this will require a whole network of detectors.

"We are heading towards doing neutrino tomography of the whole Earth. This is just the first step" - John Learned.

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