On Curaçao (yellow dot), the first receiving station of the EARTH project is to be built. The island is about 60 kilometers off the coast of Venezuela. The two nuclear power plants (red dots) on Cuba and Puerto Rico have been shut down. The next operational nuclear power plant in Florida is about 1600 kilometers away. (Source: International Nuclear Safety Center, US Department of Energy)
Reading aloud Scientists are planning to build an "antineutrino camera" that will deliver images from the Earth's interior. It will consist of ten stations distributed throughout the globe. If you want to take pictures, you need light. More precisely, one needs light that either reflects off the selected object or that is emitted by itself and then hits the camera. Of course, photos of the Earth's interior are not possible this way. Because the earth is not translucent - at least not for light. But she is transparent to anti neutrinos. The EARTH project, initiated by Dutch scientists, aims to capture antineutrinos that are produced by radioactive decay in the Earth's interior, with ten receiving stations distributed all over the Earth's surface, and to make pictures of them from within the Earth.

The EARTH project is extremely ambitious. For only in 2005 was it possible for Japanese researchers from the so-called Kamland project to capture antineutrinos from the Earth's interior for the first time. "The Japanese colleagues were barely able to distinguish the antineutrinos coming from the Earth's interior from those coming from nuclear power plants on Earth, " said 66-year-old physics professor Rob de Meijer until his retirement at the Universities of Groningen and Eindhoven, where he and his colleagues also launched the EARTH project. Today de Meijer is the director of the EARTH project and teaches at the South African University of the Western Cape in Belleville, a suburb of Cape Town.

The antineutrino detectors used in the Kamland project were not sensitive to direction, that is, they were unable to determine which direction the registered antineutrinos had reached them. It would be like putting a photo film on the table, leaning over it, grinning at it, hoping to find a picture of its face on the film. In fact, the "image" will be black, proof that light fell on the film. But to represent his face, one would have to make sure that only the light hits the film, which was reflected from the face.

Accordingly, the EARTH project must ensure that the detectors only register antineutrinos from within the Earth that have been christened "geoneutrinos" by researchers, leaving no doubt as to their origins. It is not enough to know only the rough direction: If you really want to reconstruct something like a picture from the registered geoneutrinos, then the direction must be known as precisely as possible. "And if we also want to get a three-dimensional picture, then we have to capture the geoneutrinos from different directions, " adds de Meijer. "That's why we distribute the ten receiving stations all over the globe." Furthermore, the detectors can measure the energy of the antineutrinos. This makes it possible to distinguish whether they were emitted, for example, in the decay of uranium or thorium. display

In addition, to get a clear picture from the Earth's interior, there should be no disturbing "light effects" in the form of other antineutrino sources - nuclear power plants - nearby. "With sufficiently sensitive directionality of our detectors, we could filter out the interference, " explains de Meijer. But these detectors are currently being developed by the EARTH team. "Since we do not yet know how good they will be, we have decided to use our receiving stations to keep a minimum distance of 1, 000 kilometers to the nearest nuclear power plant. Such a location is not easy to find in Europe and the US, but it should not be that difficult in the Caribbean, Hawaii, Australia, New Zealand, South America and South Africa. "

The choice of researchers for the first of the ten sites fell on the Caribbean, more specifically on the island of Curacao, which lies 60 kilometers off the coast of Venezuela. "Although there are nuclear power plants in Puerto Rico and Cuba, but they are shut down. The next operational nuclear power plants are in Florida. We therefore expect no disturbances, "says de Meijer and adds mischievously:" Unless a nuclear-powered submarine would come too close to Curaçao. "Another criterion for the choice of Curaçao was that there are no large granite formations. Because these usually contain radioactive elements such as Thorium-232 or uranium-238, which also radiate antineutrinos. Last but not least, the fact that the island belongs to the Netherlands Antilles and thus to the territory of the Netherlands played a role in the decision for Cura ao.

But the main venue for the activities of the EARTH team is currently in South Africa. This is where physicists from the University of Cape Town and the laboratory iThemba LABS are working on the development of the direction-sensitive antineutrino detectors. De Meijer anticipates the completion of the first prototypes in two years. The detectors will then first be tested at the Koeberg nuclear power plant, 25 kilometers north of Cape Town. If the test is positive, the researchers could begin building the first receiving station on Cura ao in three to four years. If Geoneutrinos are successfully received there from the Earth's interior, then the researchers want to begin in about 10 years with the construction of the other receiving stations.

The questions that de Meijer hopes will be answered by the EARTH project are manifold. Recent research suggests that there are high concentrations of the radioactive elements uranium and thorium at the boundary between the Earth's core and the Earth's mantle approximately 2900 kilometers below the Earth's surface . "If that's true, then the question is whether these radioactive elements are evenly distributed across the entire core-cladding boundary or are they concentrated in small areas, " says de Meijer. Based on the antineutrinos emitted by these elements, their distribution could be determined.

If the radioactive elements were concentrated in a small area, they could form a natural nuclear reactor, similar to the one that started two billion years ago in Oklo in the West African state of Gabon. "That could explain the mysterious occurrence of the helium isotope He-3 in the Earth's interior, " says de Meijer. This helium isotope can not be produced by ordinary radioactive decay, but arises only as a decay product of tritium, which in turn can only be found in nuclear reactors with a certain minimum mass and minimum amount of uranium. The only alternative explanation would be that this helium isotope was already present at the formation of the Earth and was previously formed inside a later exploded star.

Areas with a high concentration of radioactive elements at the core-mantle boundary would, in addition, have the fluid movements of the liquid iron in the outer due to the large heat production Earth's core influences and would have an impact on the Earth's magnetic field. In conclusion, de Meijer puts it in a nutshell: "The ideas that we have today about the Earth's interior could drastically change with the help of the Geoneutrinos."

Rob de Meijer et al .: "Towards Earth AntineutRino Tomography (EARTH)", pre-release in arXiv.org (physics / 0607049), will be published in Earth, Moon and Planets Axel Tillemans

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