Where do the highest-energy atomic nuclei in the Universe come from? The Pierre Auger Project to investigate where the highest-energy atomic nuclei in the Universe come from was the subject of international basic agreements signed in Mendoza, Argentina, this month. Some 19 nations with 53 research institutions are taking part in the project, w... The Pierre Auger Project to investigate where the highest-energy atomic nuclei in the Universe come from was the subject of international basic agreements signed in Mendoza, Argentina, this month. Some 19 nations with 53 research institutions are taking part in the project, which is supported by UNESCO and named after the French physicist who first described the Earth's constant bombardment by atomic nuclei from the universe. The energy of these particles ranges from very small values up to energies that are a hundred million times higher than those available in our particle accelerators (for example at CERN in Geneva or DESY in Hamburg). Such particles should not even arrive on Earth, as they are rapidly decelerated to low energies in the universe. In the opinion of astronomers, there are no known objects within this "breaking distance" (black holes, quasars), which may generate atomic nuclei of such high energy. Major questions of modern astrophysics are therefore: "Why can these particles be measured on Earth?" and "Where are these particles generated?" The basic idea underlying the Pierre Auger project was developed at the beginning of the 1990s by the American Nobel Prize winner James Cronin and has been discussed extensively with the scientists and decision-makers since. After a long search, the scientists identified the sparsely populated Argentine pampas as a suitable site. The project is supported by the Argentine Government, which is providing investment in the long-term, world-wide scientific cooperation. This is where the Pierre Auger project starts. Over a total area of 3200 square km, about 9000 showers with energies larger than 10(power 19)eV and less than 100 showers with energies above 10(power 20)eV are expected within three years. The dimensions of the project obliged the researchers to select inexpensive and reliable detectors. About 1600 water tanks with a diameter of 3.4m and a height of 1.2m are deployed on a grid with a spacing of 1.5km in the pampas near Mendoza, Argentina. Each of these tanks is equipped with three photomultipliers measuring the so-called Cherenkov light that is generated by the incident particle showers. The showers consist of a total of 10 billion particles and will generate simultaneous signals in about ten tanks. So-called fluorescence detectors will be installed at three places around the edge and one in the centre of the detector field to observe the atmosphere above the measurement field and the "luminous trace" of the particle showers. The combined data will allow the researchers to determine the direction, energy and mass of the primary particles with good precision. The research involves international collaboration with Argentina, Brazil, France, Great Britain, Mexico and the USA operating the shower detector, and Germany and Italy who will mainly provide the fluorescence detectors.
