The Princeton University scientists and others in the Borexino Collaboration have detected geoneutrinos at the Gran Sasso National Laboratory of the Italian Institute of Nuclear Physics. The discovery could explain how reactions taking place in the planet’s deep interior affect events on the surface. This stainless steel sphere is part of the neutrino detector used in the project, located nearly a mile below the surface of the Gran Sasso mountain about 60 miles outside of Rome.
A neutrino is an elementary particle that usually travels close to the speed of light, is electrically neutral, and is able to pass through ordinary matter almost undisturbed. This makes neutrinos extremely difficult to detect. Neutrinos have a very small, but nonzero mass. They are denoted by the Greek letter ν (nu).
Neutrinos are created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms. There are three types, or “flavors”, of neutrinos: electron neutrinos, muon neutrinos and tauon neutrinos (or tau neutrino); each type also has a corresponding antiparticle, called antineutrinos. Electron neutrinos (or antineutrinos) are generated whenever protons change into neutrons (or vice versa), the two forms of beta decay. Interactions involving neutrinos are mediated by the weak interaction. Most neutrinos passing through the Earth emanate from the Sun, and more than 50 trillion solar neutrinos pass through the human body every second
Neutrinos are produced as a result of natural background radiation. In particular, the decay chains of 238 92U and 232 90Th isotopes, as well as 4019K, include beta decays which emit anti-neutrinos. These so-called geoneutrinos can provide valuable information on the Earth’s interior. A first indication for geoneutrinos was found by the KamLAND experiment in 2005. KamLAND’s main background in the geoneutrino measurement are the anti-neutrinos coming from reactors. Several future experiments aim at improving the geoneutrino measurement and these will necessarily have to be far away from reactors.
Scientists expect that geoneutrinos will aid them in better identifying what constitutes matter deep within the Earth. “It’s a very significant discovery and holds much promise for better understanding the composition of the Earth and how the Earth operates,” said Thomas Duffy, a professor of geosciences at Princeton, who was not involved in the research.
Earth scientists would like to know more about the crucial role that decaying elements such as uranium and thorium play in heating up the Earth and causing convection in its mantle — the slow, steady flow of hot rock in the interior carrying heat from great depths to the Earth’s surface. Convection, in turn, drives plate tectonics and all the accompanying dynamics of geology seen from the surface — continents moving, seafloor spreading, volcanoes erupting and earthquakes occurring. No one knows whether radioactive decay dominates the heating action or is just a player among many factors.
The origin of the power produced within the Earth is one of the fundamental questions of geology, according to Calaprice. The definite detection of geoneutrinos by the Borexino experiment confirms that radioactivity contributes a significant fraction — possibly most — of the power, he said.
The detector is composed of a nylon sphere containing 1,000 tons of a hydrocarbon liquid. An array of ultrasensitive photodetectors is aimed at the sphere that is encased within a stainless steel sphere. All of this is surrounded by 2,400 tons of highly purified water held within another steel sphere measuring 59 feet.
Scientists can envision a day when a series of geoneutrino-detecting facilities, located at strategic spots around the globe, can sense particles to get a detailed understanding of the Earth’s interior and the source of its internal heat. This data could provide enough information to predict the occurrence of events such as volcano eruptions and earthquakes.