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Surround it in the air, capture it on the ground

Observatory will decipher high energy particles that streak through the atmosphere

In Pampa Amarilla, a semi-desert region in the south of the Argentinean Province of Mendoza, which extends westwards to the Andes foothills, is being born – with Brazilian participation as well as that of other two dozen countries – the Pierre Auger Observatory of Cosmic Rays. At the moment, only a communications tower, a building in the initial stages of construction and the first detector tank, announce the future enterprise in this low population density area, close to the town of Malargue – there are only a few cattle and goat breeders here and there on a landscape on which the plain is the rule, spreading itself out over an immense area of some 70 by 50 kilometers, broken up by some small elevations of at the maximum 60 meters.

However, in 2003 1,600 surface detectors of the observatory – the so called Cerenkov tanks will be spread out over a total area of 10,000 square kilometers (km2) at a regular distance of 1,5 kilometers between each one in the region. Four fluorescence detectors – the “eyes” telescopes – will also be installed, one of which will be spherical with twelve mirrors, in a small place called Los Leones and three will be semi-spherical, on peripheral points on the northern, southern and eastern limits of the immense southern site of the Pierre Auger Observatory. From that point on, the observatory will be completely equipped to detect, analyze and to interpret the rare particles of extremely high energy, which on hitting the atmosphere of the Earth at an altitude of 50,000 meters from the surface of the planet, trigger off a process of multiple production of new particles that cross a huge shower of more than one billion particles that swiftly cross through the atmosphere.

What do they want to know by hunting such particle? To know what they are and where they come from. And if possible, from this knowledge, to understand something more about the Big Bang – the powerful primordial explosion – which according to one of the most widely accepted theories of physics, gave origin to our Universe.

In concrete terms, the scientists involved in the Pierre Auger Project – which, as well as the southern site in Mendoza, will have in the future a northern site in Utah in the United States – are going to look into particles that arrive on the Earth with a frequency of only one per century per square kilometer. Therefore, they are the agents of a rare astrophysics phenomenon. To observe it directly , must have, besides clear skies, with low cloud cover and almost no interference from light coming from human activity (which would upset the work of the telescopes), a large extension of area and considerable numbers of detector equipment on the land.

To have a clearer idea of the rarity of the event, it is worth observing that on average only 30 particles per year hit the atmosphere over the area of 10,000 square kilometers (km2) reserved for the observatory in Mendoza. And once one of these particles has arrived at the atmosphere, in only 10 (-4) seconds or, what is the same as 100 micro seconds, the shower it gives origin reaches the surface of the Earth. Or that is to say, as well as being rare, the whole phenomenon is extremely quick, hence the necessity as well of a logical  system of transmission and analysis of information extremely precise and sophisticated such as that which was conceived for the Pierre Auger Project.

However, certainly the most essential characteristic of the particles in question is that, being sub-atomic, with a infinitesimal mass of  10(-27) kilograms, they have an energy that reaches 50 joules, something equivalent to the energy of a tennis ball of around 100 grams at the moment when it gets hit by the racket of a professional tennis player of the category of the Brazilian Gustavo Küerten or, even better, of the Swiss Marc Rosset, whose serving reaches around 200 kilometers per hour (kph). Thus, no wonder the fascination that this has over physicists: it is worth noting that the energy generated by the particle accelerator n the famous North American Fermi Laboratory, situated in Illinois, around 60 kilometers from Chicago – which happens to be nothing less than the most energetic phenomenon yet produced on the planet – is100 million times less than the energy of the particles that are the targets of the Pierre Auger Project.

Formally, the Brazilian participation in this project was announced on the 17th of last July at a ceremony in the Gleb Wataghin Physics Institute of the State University of Campinas (Unicamp), presided by its director (and president of FAPESP), Carlos Henrique de Brito Cruz. In financial terms, this will be translated up until 2003 into a total investment of US$ 3.5 million. For now, almost US$ 2 million has been granted. US$ 1.6 million are being invested through FAPESP and some US$ 340,000 by the Ministry of Science and Technology (MCT), through the Program of Centers of Excellence in Research (Pronex). On the part of the Foundation, as its scientific director, José Fernando Perez detailed, US$ 1 million has been destined to equipment (a good part manufactured in Brazil) and supplies, while US$ 600,000 are for doctorate and post doctorate scholarships for São Paulo researchers locked into the project.

Although the institutional entrance of Brazil into the Pierre Auger Project is recent, the effective participation of Brazilian researchers in the project has been going on since it started to be more seriously formulated. Indeed, the laboratory began to be thought out in 1992, by the American physicist James Cronin, winner of the Physics Nobel Prize in 1980 (see interview on page 36). Shortly afterwards, Cronin got the support of a Scottish colleague, Allan Watson. In 1994, it became clear that the observatory would have to be very large and would have to have the most advanced technology. This consequently demanded international cooperation – and under these terms he organized a working meeting in Paris. In July of 1995, there was another meeting with some 10 persons at the Fermi Lab. Here they found an enthusiastic Brazilian, Carlos Escobar, today a professor at the Cosmic Ray Department of the State University of Campinas (Unicamp). Originally – like Cronin – from the group of physicists working with particle accelerators, and at that time a professor in the Nuclear Physics Department of the Physics Institute of the University of São Paulo (USP), after having spent some years in the Physics/Mathematics Department, Escobar was invited a short time later to participate in a meeting of the Argentinean Physics Association, in Bariloche, where the question of the observatory would be discussed.

In was in Bariloche that, along with Argentinean and Brazilian colleagues, Escobar prepared himself to lead the effort for the implantation of the southern site of the observatory in Argentina. On his return to Brazil, he got in touch with Perez, the then scientific director of FAPESP, and Lourival Carmo Mônaco, the president of the Financier of Studies and Projects (Finep) at that time, to sound them out on the possibilities of institutional assistance for the project. “We’d imagined at that moment that the Brazilian participation in the project would have to be in the order of US$ 10 million, since Menem (Carlos Menem, at that time the president of Argentina) hinted that Argentina would come up with US$ 15 million to implant the observatory in his country”, recalls Escobar. The sum sounded a little far-fetched, and it did turn out to be overblown. The remaining Menem’s allusions proved to be beyond the real possibilities of the Argentineans, but Escobar received support from the two Brazilian development agencies to take the project a step further. He made a point of emphasizing the support that he had also received from the physicist Oscar Sala, the ex-president of FAPESP. “People understood the scope that the project could have for research, and even for Brazilian industry since the resources needed would be mostly spent in the country”, he comments.

Escobar explained that a clause in the observatory project says that at the maximum 20% of the investments from each country should be destined to the common fund of the enterprise.

In November of 1995, “with their homework well prepared” a group of 20 Argentineans and Brazilians, among them Escobar, Ronald Shellard from the Brazilian Center of Physics Research (CBPF) in Rio de Janeiro and Armando Turtelli Jr., from Unicamp, went to a meeting at the headquarters of Unesco in Paris, where – among other things – the location of the southern site of the Pierre Augar Observatory was to be decided. The Argentineans put forward three different locations, while Australia and South Africa each proposed a location, all of them having previously been visited by a search team made up by a Frenchman, an Englishman and an American. To shorten the story, with better conditions and a well organized fans group (at the meeting there were only two Australian scientists and two from South Africa), the Argentineans ended up winning the race with the area of Mendoza. The sad fact about the Argentinean choice was Japan temporarily left the project complaining about the large distance between the two countries. Japan might take part in the northern site that is likely to go to Utah, according to the decision taken in 1997 and whose implementation is conditioned to the confirmation that the general technicalof the observatory is good.

Another decision made at the meeting in Paris was the choice of Escobar as the chairman of the Collaboration Board of the project for a period of two years, to which he was re-elected – since September of last year, the position has been occupied by the French physicist Murat Boratav.

Since that meeting of 1995, the project has taken many steps forward. Today, two tanks are being installed in the area of the observatory, and another eighteen should be following them by the end of September, and if everything goes according to the schedule in November, there will be forty. They are all being produced by Alpina Termoplástica Ltda., a São Paulo company located in the district of Jabaquara, in the capital, which can be defined as a small to medium-sized company (it has 100 employees), and belongs to a family group that was founded in 1953 from Alpina Equipamentos Industriais Ltda. “We’re very proud of the work of this project’, says the company’s general manager, Estéban Peres, a Spaniard from Madrid who has been working with the group for some 45 years. “We were picked to carry out the work by scientists who visited several other companies in other countries. If they concluded that we had the skill to do it, it is because we’re, in a certain manner, in the technological vanguard in this particular area”, he completes.

The tanks, made of a special resin, are 3.60 meters in diameter and 1.20 meters tall, in the reservoir part that can hold 12,000 liters of water. Considering as well the reinforced structure of the upper part, the total height reaches 1.60 meters. What happens in these tanks when the particles hit the water is first the production of a bluish radiation, which is captured by photo sensors immersed in the water. We are talking about a radiation that is different from the one produced in the atmosphere, which sensitive enough to be captured by telescopes, because in this case the particles excite the molecules of nitrogen, and then they suffer a “de-excitation” and this what gives off a light that goes out in all directions with the intensity equivalent to a 5 Watt bulb. This light passes through the atmosphere at the speed of light, 30 kilometers in distance. Consequently, we are dealing with an atomic process. However in water, explains Escobar, “the particle changes the dielectric properties of the liquid, and it is collectively that its molecules are going to emit radiation”. And here we are dealing with a much more intense radiation, with a much greater quantity of photons and in a conical direction, like the wave of a ship’s bow. “Since the speed of the particle that enters the water is greater than the speed of light in the water, the capture of the radiation by this medium is much more efficient”, explains Escobar.

However, it is the combination between the two detection processes that is the beauty of the Pierre Auger project. Because if the particles of a shower enter, in a determined instant into the tank, there is a single piece of highly precise information, while in one of the telescopes that watches the observatory area is going to produce a more continuous measurement of the shower  radiation, reflecting the phenomenon accompaniment at various points along its 30-kilometer path. And more than this, there is a “dialogue” between the two systems carried out by a logical system. How?  Firstly, the tank photo sensors send signals to the logical system of the tank itself and, if the radiation points surpass a determined limit, this system alerts the central system of the computer, which distributes the information to the telescope and to other tanks. Then in the telescope, if its photo sensors are set off and three of them form a reasonable geometric standard, the sending of information to the central system and from there to the tanks also takes place. Afterwards, in the area of the data processing, coordinated by Shellard, the event will be analyzed and, if it is the case, stored. “However, in a general fashion our database will be more occupied with calibration events than with physics events”, forecasts Escobar.

However, the big question that remains for laymen is why the physicists believe that this observation can lead to their understanding of what these highly energetic particles are, and where they have come from. Well, they believe that the particles in question are not energy photons, but are matter: they have mass and electrical charge. Therefore, in principle they can be deviated by electromagnetic fields that permeate the universe. However, these fields are weak and the energy of the particles, on the contrary, is immense. Thus, such a deviation seems difficult, and the particles probably maintain their trajectory, and perhaps we might be able to investigate it as far as 300 million light years away. In the end of this trajectory perhaps we might find collision of galaxies, active nuclei of galaxies or, more probably, dark matter, relics of the big-bang imprisoned in the halo of the galaxy, a dark mass hidden in the universe. For now, nobody knows. The physicists of the Pierre Auger Observatory want to find out.

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