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New Scientist has today published a nice little article on the use of antineutrino detectors to measure the radioactivity of the Earth

…These particles, called antineutrinos, suggest that about half of Earth’s heat comes from the radioactive decay of uranium and thorium – and give clues to the location of geological stashes of these elements.

Heat is needed to drive the convection currents in Earth’s outer core that create its magnetic field. But exactly how much of this heat comes from radioactive decay wasn’t known until now…

…The researchers also had enough neutrinos to confirm that some must be coming from places other than the crust, something that wasn’t possible before. “The uncertainty is small enough that some contribution must be from the mantle,” says Giorgio Gratta, a physicist at Stanford University in California who is part of the KamLAND collaboration.

The ability to determine the location of the radioactive elements could permit better models of the Earth’s interior, says Gratta. Seismic waves tell us about elasticity of the crust and mantle: now we have a small window into their chemistry, which should allow their behaviour to be better modelled. The presence of radioactive elements in the mantle, for example, could affect its flow…

This work was done with a detector in Japan, but my understanding is that there are two such detectors looking out for antineutrinos in order to learn more about the geophysics of the Earth.

An interesting aspect to this is the use of antineutrino detectors to measure the geophysical distribution of uranium and thorium

Terrestrial antineutrino flux measurements are the only identified, feasible method to experimentally determine the distribution of uranium and thorium in the interior of the Earth. Measurements from two geologically distinct detection sites remote from nuclear reactors provide model-independent estimates of uranium and thorium concentrations in continental crust and mantle. This information is vital for understanding the Earth’s geophysical structure and dynamics. If, as expected, these elements are much less concentrated in the mantle than in the continental crust, the oceanic detector needs to be several times larger than the continental detector…

This is a point also made previously at Science News Online

“The ratio of radioactive heat production to other sources, the distribution [of radioactive elements] between mantle and crust, and the distribution of the different nuclides are presently not known with any certainty,” says geophysicist Raymond Jeanloz of the University of California, Berkeley…

…Chen says, however, that “it’s unlikely that any one detector or combination of two detectors would have the precision to pinpoint local concentrations of uranium or thorium.”

Having a third antineutrino detector might enable researchers to map the distribution of uranium and thorium inside Earth, Raghavan notes…

I do not have sufficient knowledge to discern whether the distribution of uranium and thorium refers to the distribution of these elements within the deep interior of the Earth only or whether it would also include more precise mapping of uranium and thorium within the continental crust. Should the latter be also the case then one can readily appreciate the potential implications for both uranium exploration and non-proliferation policy.

Antineutrino detectors have indeed been spoken of in the context of improving nuclear reactor safeguards, as the following Lawrence Livermore analysis outlines

Reactor fuel rods contain the isotopes uranium-238 and uranium-235 . Inside a reactor core, these isotopes absorb neutrons and undergo fission, producing antineutrinos with each decay. Some 238
U isotopes capture neutrons and decay into isotopes of plutonium-239, which also fission and emit antineutrinos. However, the decay of 239Pu produces substantially fewer antineutrinos than does the decay of 235U within the energy range required for detection. Over the course of a reactor’s fuel cycle, the antineutrino count rate drops as uranium content decreases and plutonium increases. In addition, the antineutrino count rate is proportional to the fission rate of the isotopes and thus is approximately proportional to the reactor’s power. By monitoring this count rate, scientists can track both the thermal power and the fissile inventory of the reactor over time. Any deviation from what is considered “normal” would identify a potential problem…

…Antineutrino detectors could provide a more precise method to confirm that reactors are operating according to IAEA standards and that fissile materials are not being diverted for use in an undeclared nuclear weapons program. The Livermore–Sandia team has developed autonomous detectors that continuously and accurately monitor antineutrinos in real time throughout the one- to two-year fuel cycle of a standard pressurized water reactor…

Although according to the following 2003 paper available on the ArXiv

Antineutrino monitoring of reactors can provide unique information on the burn-up in the core from outside the containment vessel. If accurate knowledge of the reactor power is known through an independent measurement, the variation of the number of detected antineutrino events reflects the build-up of plutonium. Thus, antineutrino monitoring could be used to detect gross deviations from the declared operational mode of a reactor. A measurement of the average anti-neutrino energy or of the shape of the spectrum would provide valuable additional information and would greatly reduce uncertainties in relating the antineutrino spectrum to core burn-up. However, the magnitude of the expected change in the antineutrino count rate of less than 1% in the case of the diversion of a critical mass of plutonium suggests that antineutrino monitoring is unlikely to be sensitive to this class of safeguards…

I find this aspect, related in the Science News Online article linked above, to be kinda cool

If such antineutrino data could be obtained, the resulting estimate of global radioactive heat production could shed light on what fraction of Earth’s energy output is simply heat left over from the massive impact early in its history that created the moon…

 

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