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PHYSICS

Cosmic rays long journey

and the scientists who wanted to learn where the rays come from

Published in December 2007

An ancient question about cosmic rays may have been clarified. Nearly 70 years ago, French physicist Pierre Auger identified those particles – the Universe’s most energetic ones – disintegrating into billions of other particles when colliding with the Earth’s atmosphere, but he was unsure about two seemingly simple points: where these particles came from and what they were exactly. Now, a team of 370 researchers from seventeen countries, including Brazil, have found an answer to the first question, though the second is still unanswered. As detailed in the November 9 issue of Science, the high energy cosmic rays are apparently formed near black holes – which absorb matter and energy – found in the nuclei of the active galaxies in the vicinity of our own Milky Way galaxy.

The high energy cosmic rays arise in the midst of a mixture of electrically charged particles released by the more active black holes after they have gorged themselves on gases, cosmic dust and stars. This Dantesque situation occurs in active galaxies such as Centaur A, the nearest one, some 12 million light years away from the Milky Way, or in other ones, which are as much as 300 million light years away – not that far, since the Universe spreads across 13 billion light years. Therefore, the high energy cosmic rays that reach the Earth today may have originated on the eve of the super–extinction that wiped out 95% of our planet’s life-forms (250 million years ago) or of those reptiles that formed the blue print for dinosaurs (around 230 million years ago).

Physicists working in this field are not keen on low energy cosmic rays. These are more common and even less is known about their origin, though these rays can interrupt mobile phone conversations or films broadcast on TV when they are formed from more intense solar explosions. High energy rays are more attractive. First, they are charged with a massive amount of energy – up to 60 x 1018 electron-volts (1 electron-volt, the energy unit of the particles, equals the energy of the electron, the smallest elementary particle). Second, these rays are very rare: only one such high energy ray probably reaches each square kilometer of the Earth every century (the name of these particles suggests that they come in bundles, but actually they do not: they travel alone). Third, they may turn into a different way of seeing the heavens.

“This article published in Science raises the possibility of studying celestial objects through cosmic rays”, says physicist Carlos Escobar, a professor at Campinas State University/Unicamp and coordinator of the Brazilian team. Since Galileo’s times, astrophysicists have relied only on light – initially, on visible light and later on the various wave lengths, ranging from infrared to gamma rays – to observe the Universe. Cosmic rays might, at first, aid research into the phenomena that occur within the hundreds of active galaxies, whose nuclei release an amount of energy thousands of times higher than that produced by the entire Milky Way. These nuclei often shelter black holes with a sizeable mass (millions of times greater than the Sun) that absorbs everything around them. High energy cosmic rays are the result of this insatiable voracity, much like bread crumbs of hastily-eaten bread, and are then pushed out through the turbulence of the magnetic fields in outer space.

A recent paper by physicists from Japan, Ireland, Germany and the USA was published in Nature. They explained that cosmic rays with energy ten thousand times lower than that of the cosmic rays referred to in Science can be accelerated by explosions referred to as supernova stars, which can release, in a very short time, the same amount of energy that the Sun would release in ten billion years. This study confirmed a phenomenon predicted decades ago by Italian physicist Enrico Fermi; however, the question was where these particles were being formed.

The team that includes Brazilian researchers detected the origin of the high energy cosmic rays because it had massive equipment, more specifically, the Pierre Auger Cosmic Rays Observatory, which covers three thousand square kilometers, twice the size of the City of São Paulo. It is located in a desert region in western Argentina, close to Malargüe, a town of 20 thousand inhabitants. Plans to build the world’s biggest observatory of this kind began in 1992 by US physicist James Cronin, a professor at the University of Chicago and winner of the Nobel Prize for Physics in 1980, and by Scotland’s Alan Watson, from England’s Leeds University. The need for international cooperation soon became evident, given the proportions that the original project had taken on; as a result, the two physicists invited several other colleagues interested and working in the field of particle physics to a preliminary meeting in June 1995. One of these colleagues was Escobar, who was at the time a professor at the University of São Paulo/USP.

A meeting was held in November 1995 at Unesco headquarters in Paris. The meeting was attended by Escobar, Ronald Shellard, from the CBPF Brazilian Physics Research Center and by Armando Turtelli. They were joined by their Argentine colleagues, Alberto Etchegoyen and Alberto Filevicvh. The Argentines heatedly defended the possibility of building the new observatory in Argentina. “That was the crucial moment”, says physicist Marcelo Leigui, who took part in this research project while doing post-doctoral work at Unicamp; Leigui now teaches at the ABC Federal University. “Brazil’s participation would have been smaller if one of the other two countries willing to build the observatory – namely, South Africa and Australia – had been chosen instead. “Brazil’s participation, officially announced on July 17, 2000 at Unicamp, resulted in investments of some US$ 4 million in the form of equipment purchased from Brazilian companies and grants for postgraduate scholarships and travel expenses.

Readers of this journal got an update on the major highlights of the slow and arduous construction of the Pierre Auger. In August 2000, the cover story of that month’s Pesquisa Fapesp issue described the details of the negotiations and the first stages of the construction. In April 2002, another article described the pace of building work: “At this moment, in a space that sometimes resembles the sophisticated image of a space ship and at other times looks like the robust construction of a hydroelectric power plant, hundreds of workers, technicians and researchers are working intensely on assembling the cosmic rays measuring equipment.”

At that time, 40 of the 1,600 surface detectors, named Cerenkov tanks, were already in operation. Each tank can hold 11 thousand liters of pure water, which captures the bluish radiation produced when a cosmic ray collides with the water. The tanks are equipped with 24 fluorescent telescopes that register the light produced when cosmic rays collide with the atmosphere. The Pierre Auger was a pioneering experiment that integrated two observation methods, which until then had only been used singly at smaller observatories in the USA and Japan.

The ingenuity of this construction, depicted in its final construction stages in an article published in August 2003, was also the result of the collaboration of firms from 19 countries. The following Brazilian companies took part in the construction: Alpina and Rotoplastyc, manufacturers of the Cerenkov tanks; Schwantz, manufacturer of the telescopes´ corrective lens; Equatorial, which assembled the regulating devices on the telescopes; and Moura, manufacturer of the solar panel batteries that equip the surface detectors. Physicist Vitor de Souza says that he learned to overcome “barriers of understanding between academic thinking and industrial concepts” as he helped build and install the equipment.

Pesquisa Fapesp also kept track of the arrival of the cosmic rays. In October 2005, the date on which another article was published, there were records of 3 thousand particles, 20 of which were the most precious ones: they were in the highest energy band. This year, the physicists brought together the 27 particles whose energy was higher than 57 x 1018 electron volts registered from 2004 and 2007, and found that they had come from specific directions related to the nuclei of the active galaxies in the vicinity of the Milky Way. The conclusion discarded the possibility that the particles had come from the Milky Way galaxy or from more distant regions (in which case they would have spread homogeneously through the skies, instead of grouping together according to their probable origins).

“We proved that it would be possible to carry out such a huge project with a budget that was lower than had been planned”, said Escobar. The 17 countries invested US$ 54 million, US$ 6 million below budget, despite unforeseen events of all kinds. “The learning in terms of project management was invaluable. “The Brazilians also tightened their belts.” Two years ago, for example, Escobar decided that all the members of the Brazilian team would no longer take two flights to get to the Pierre Auger site; they were told to fly to Buenos Aires, and from there go on a 16-hour bus ride to Malargüe.

“In addition to the learning itself, we also learned how to get along with different styles and places of work”, acknowledged Sérgio Carmelo Barroso. In the course of one year, Barroso traveled ten times to Malargüe to assemble and test equipment – he is still a member of the team, but now he also teaches at the Southwest of Bahia State University / UESB. “I learned how to design, build and test an experiment, how to analyze data and finally how to extract data of scientific interest”, added Souza, who has been working at Germany’s Karlsruhe University since last January.

“We still haven’t achieved our final goal”, worried Leigui. To begin with, it is still necessary to confirm whether the ultra high energy particles are really protons – one of the components of the atom’s nucleus and nearly two thousand times bigger than the electrons – or whether they are the nuclei of oxygen, carbon, or of any other material. “The results we got so far are coherent with the idea that cosmic rays are actually protons with a low electric charge”, stated Escobar.

This project has led physicists to test the validity of several theories. There seemed to be a maximum energy limit that the cosmic rays had upon reaching the Earth – this is the so-called GZK cut, which is close to 60 x 1018 electron-volts, but of course this had to be confirmed. According to Escobar, the fact that only correlations with close-lying extra-galactic objects were obtained indicates that the GZK cut is working.

The end of this journey may mark the beginning of even longer journeys. Therefore, the Auger team is holding on to the plan of building a similar version to the Argentine observatory in the USA. This future observatory might reveal more secrets hidden in the skies of the Northern Hemisphere, after it goes on-stream, in no less than ten years.