The first confirmed sightings of antineutrinos produced by radioactive decay in the Earth’s mantle have been made by researchers at the Borexino detector in Italy. While such “geoneutrinos” have been detected before, it is the first time that physicists can say with confidence that about half of the antineutrinos they measured came from the Earth’s mantle, with the rest coming from the crust. The Borexino team has also been able to make a new calculation of how much heat is produced in the Earth by radioactive decay, finding it to be greater than previously thought. The researchers say that in the future, the experiment should be able to measure the quantities of radioactive elements in the mantle as well.
According to the bulk silicate Earth (BSE) model, most of the radioactive uranium, thorium and potassium in our planet’s interior lies in the crust and mantle. Accounting for about 84% of our planet’s total volume, the mantle is the large rocky layer sandwiched between the crust and the Earth’s core. Heat flows from the interior of the Earth into space at a rate of about 47 TW, but one of the big mysteries of geophysics is how much of this heat is left over from when the Earth formed, and how much comes from the radioactive decay chains of uranium-238, thorium-232 and potassium-40.
Peering deep underground
One way to settle the question is to measure the antineutrinos produced by these decay chains. These tiny particles travel easily through the Earth, which means that detectors located near the surface could give geophysicists a way of measuring the abundance of radioactive elements deep within the Earth – and thus the heat produced deep underground.
Back in 2005 physicists working on the KamLAND neutrino detector in Japan announced that they had detected 22 geoneutrinos, while Borexino, which has been running since 2007, reported in 2010 that it had seen 10 such particles. Both detectors have since spotted more geoneutrinos and, taken together, their measurements suggest that about one half of the heat flowing out of the Earth is generated by radioactive decay, although there is large uncertainty in this value.
The Borexino detector is made up of 300 tonnes of an organic liquid, and is located deep beneath a mountain at Italy’s Gran Sasso National Laboratory to shield the experiment from unwanted cosmic rays that would otherwise drown out the neutrino signal. Whenever electrons in the liquid are struck by an antineutrino, they recoil and create a flash of light. In the latest work, Borexino physicists have analysed a total of 77 detector events, with the team calculating – using data from the International Atomic Energy Agency – that about 53 of these antineutrinos were produced by nuclear reactors.
The remaining 24 geoneutrinos could have come from either the Earth’s crust or its core. However, scientists have a pretty good idea of how much uranium and thorium are in the crust, allowing the Borexino physicists to say that half of these geoneutrinos were produced in the mantle and the other half in the crust. Furthermore, the physicists can say with 98% confidence that they have detected mantle neutrinos – a much greater level of confidence than achieved in previous studies.
The team also calculated the heat generated by radioactive decay in the Earth and found it to be in the 23–36 TW range. This is larger than estimates based on assumptions about the amount of radioactive elements in the Earth, which are in the 12–30 TW range, and also larger than an estimate based on previous antineutrino measurements.
The Borexino team also tried to work out what proportion of the geoneutrinos came from the uranium decay chain and what proportion from the thorium chain. Potassium decays were not considered because they are not expected to make a significant contribution to the numbers detected. The data suggest that the currently accepted ratio of thorium to uranium in the Earth is correct, but that the uncertainty in the Borexino values is very large. More data, the Borexino physicists say, should let them make more precise measurements of the contributions of uranium and thorium to the heating of the Earth.
The study is reported in Physical Review D.