One of the great mysteries in geophysics is the nature of the interior of the Earth. We have a reasonable idea of the planet’s general internal structure, various techniques for imaging structures beneath the surface and a rough idea of how much heat is generated from the Earth’s interior.
But geophysicists want much more. One question, in particular, is exactly where heat is generated inside the Earth.
Physicists know that almost all of this heat is generated by the decay of radioactive elements such as potassium-40, thorium-232 and uranium-238. But how are these elements distributed and how much heat does each contribute?
In the next few years, geophysicists hope to get some detailed answers to this question thanks to the emerging science of neutrino geophysics. The radioactive decay inside the Earth produces subatomic particles known as antineutrinos. So an experiment that measures the antineutrinos coming out of the Earth should provide a detailed picture of the distribution of these elements within it.
That’s the theory. In practice, this is a tricky task for two reasons. The first is that antineutrinos are famously hard to detect because they interact so weakly with ordinary matter.
In recent years, however, physicists have made great strides in building detectors that can spot them, particularly those generated beyond our shores by the Sun, for example. And in the last couple of years, two of these experiments have spotted the first geophysical antineutrinos from inside the Earth.
That’s triggered huge interest from physicists who want to repeat the experiments in other parts of the world. That brings us to the second problem.
This is that nuclear reactors also produce antineutrinos and so generate a strong background signal that can swamp the signal from inside the planet. What’s needed, of course, is a global map showing where the background antineutrino signal is strongest. And today, we get it thanks to the work of Barbara Ricci at the University of Ferrara in Italy and a few pals who have made the most detailed map of reactor antineutrinos ever produced.
Their approach is straightforward. These guys take data from the International Agency of Atomic Energy giving the thermal power of every reactor on the planet. That gives Ricci and co a good idea of how many antineutrinos each reactor core produces and allowed them to calculate the antineutrino flux all over the world. Next, they considered an antineutrino detector containing 10^32 protons and calculated how many antineutrinos from reactor cores this device would spot over the course of a year. Finally, they plotted their results for the entire planet showing the areas where the background signal for any antineutrino detector would be highest. You can see the map here ( 1 TNU= 1 event/yr/10^32 detector protons ).