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Site work for what will be the world's largest cosmic-ray observatory is about to begin in Argentina, after a consortium of 53 institutes in 19 countries signed an agreement last month.

The Pierre Auger Southern Observatory, in Mendoza province, will study ultra-high-energy cosmic particles whose origin is unknown and which are so rare that less than a dozen have so far been detected. They generate showers of secondary particles when they hit the Earth's atmosphere.

The US$50 million observatory will comprise 1,600 surface detectors placed at 1.5-kilometre intervals over an area of 3,000 square kilometres, complemented by four optical telescopes. The detectors — water tanks equipped with photomultipliers — will measure the so-called Cherenkov radiation generated in the water when the secondary particles hit at Earth level. The telescopes will detect the luminous traces in the sky created by particle showers.

The combined data will allow the direction, energy and mass of the primary particles to be determined much more accurately than at smaller cosmic-ray observatories, such as those in Akeno, Japan, and Utah, USA.

The largest single contributor to the Pierre Auger project is Argentina. The central and Mendoza governments have between them put up US$15 million, which will be used primarily to prepare the infrastructure of the site, an elevated desert plain just east of the Andes.

Roads and a research station will be built in the province's capital, Malargüe. Data from components scattered around the region will be sent to the research station using mobile-telephone technology.

Brazil, France, Mexico, Britain and the United States will work with Argentina on the water-tank array, while Germany and Italy will provide the telescope technologies.

The observatory should be complete by 2003. According to a spokesman, Alberto Etchegoyen, of the National Atomic Energy Commission's Tandor Laboratory in Buenos Aires, data may be collected as soon as 2001, when many of the detectors will be in place.

Although low-energy cosmic-ray particles with energies of a few million electron volts are plentiful — one particle strikes every square metre of the earth every second — ultra-high-energy particles of over 1020 electron volts, which interest scientists in the Pierre Auger collaboration, strike one square kilometre every century. “It is therefore obviously important to cover as much of the sky as possible,” says Hans Blümer, a spokesman and head of the Institute for Nuclear Physics at the Forschungszentrum Karlsruhe.

The collaboration decided in 1996 to build a ‘twin’ Northern Observatory in Utah, of the same size, as soon as funding is found for the project.

For Blümer, the project is a triumph of “grass-roots science”. As a large-scale project organized by a consortium of small institutes, he says, “no single very large institute is in charge”. He praises the organizational skills of the project's pioneers, Nobel laureate James Cronin, head of the Enrico Fermi Institute at the University of Chicago, and Alan Watson, professor of astrophysics at the University of Leeds in Britain, who first proposed the project eight years ago.

Etchegoyen says the observatory will provide a tremendous boost for young Argentinian scientists. He believes it will encourage funding agencies to create more fellowships for young astrophysicists, which is important “because the physics community here is growing old”.