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The cosmic rays are arriving

Dozens of detectors come into operation at the Pierre Auger Observatory and collect the first pieces of information about the particles that travel 150 million light years before reaching the Earth

Known as a main region for the Argentinean wines production, the town of Mendoza has gained fame in another field, the scientific field. In a semi-desert area at the edge of the foothills of the Andes, with a dry climate and mild temperature, an international project is underway to study cosmic rays, the particles of highest energy found in nature, which could tell us, a little better than our knowledge of today, how the Universe came about and took on its current form.

After two years of work and a complex national and international effort (see magazine Pesquisa FAPESP No 56), the administrative buildings, the workshops and the infra-structure work on the Pierre Auger Cosmic Ray Laboratory – which brings together two hundred and fifty scientists from nineteen countries and has a budget of US$ 54 million – have been completed. FAPESP is participating in two ways: with close to US$ 1 million, approximately one third of the Brazilian participation, also financed through research support programs of the Ministry of Science and Technology (MCT), and in the administration of the project itself – since November of last year the Foundation’s Scientific Director, José Fernando Perez, has occupied the presidency of the Pierre Auger finance committee.

At this very moment, in a space which sometimes reminds one of the refinement of a spaceship or the robust workings of a hydroelectric plant, dozens of operators, technicians and researchers are working intensely setting up the cosmic rays measuring equipment. One by one the detectors are starting to function, and as soon as they register the first particles that fall from the sky, the researchers will sigh in relief because then they will be sure the largest project in the world built to study cosmic rays is running as expected.

In Pampa Amarilla, a desert in the vicinity of Malargüe, a town 450 km from Mendoza, the Provincial Capital, and at some 70 km from Las Leñas, a skiing complex, one can find forty working surface detectors, called Cerenkov tanks, each one containing 11,000 liters of pure water, which captures the blue radiation produced when a cosmic ray enters the water. Also, two fluorescence telescopes are in operation, the so called fly’s eyes, capable of registering the light produced by the cosmic radiation on colliding with the earth’s atmosphere. The telescopes are installed in a building constructed on the hill called Los Leones, which stands out on the landscape by being fifteen meters taller the rest of the plain and above all, because of the observation tower at a height of 51 meters. The second group of telescopes will be housed in another building, which is in the final phase of construction on the hill named Coihueco.

The researchers were delighted in December with the first twenty one hybrid events – when the same particles are registered by the telescopes and by the surface detectors. Another advance: the first golden event, as it was called, was registered on the 17th of January at 02:49. A golden event is a special hybrid, seen at the same time by a fluorescence telescope and by three or more tanks – important because, when one has more than one simultaneous registered point, it is much easier to calculate the speed, direction and energy of the cosmic rays.

“We are very excited because the results of the detectors of fluorescence and of the tanks are coinciding”, comments the Scottish physicist Alan Watson, of Leeds University in the United Kingdom, one of the project’s mentors. Exactly ten years ago that he and the North American James Cronin, (Nobel Prize in Physics in 1980), from Chicago University in the United States, launched the concept of the Pierre Auger Laboratory at a physicists meeting in Paris. “From the ceremony of the launching of the laboratory in March of 1999, we have made fantastic progress”, evaluates Watson.

Up until now, the detectors have registered only particles with energy three hundred times smaller than those that they really want to capture. However, as they install more tanks and telescopes, the probability grows of detecting particles of extremely high energy – comparable to a tennis ball of around 100 grams at the moment at which it leaves the serving racket of a professional tennis player with a speed of 200km/hr.

Evidently they have no possible idea of when the detectors and the telescopes are going to pick up the arrival of the first of these particles. It could be at any moment. Everything is ready: in the tanks there are sensors and processors that instantly transmit the information about the rays that fall into the water to the buildings with the fluorescence telescopes. And from there, the data from the tanks and the telescopes themselves go on through the Internet to the central building of the Observatory, in the urban zone of Malargüe.

However, the researchers will have to hold back their anxiety because they know: the cosmic rays that they intend to study are extremely rare. “One of the major problems of research in this area”, says the American Paul Mantsch, the project manager, “is that only one particle of extremely high energy arrives per square kilometer per century”. This is the reason the intention is to quickly widen the area occupied by the detectors – the larger the area, the greater the possibility of registering the arrival of theses space voyagers. The 1,600 surface detectors and the 30 fluorescence telescopes that will be installed by 2004 are going to be spread out over 3,000 km2, double the area occupied by the city of São Paulo.

The tanks – each one of them, 3.7 meters in diameter and 1.2 meters in height, and for some reason are identified by a woman’s name, such as Laura, Carmen, Fabiana and Flavia – are placed 1.5 km from each other in a low growth vegetation lined by vineyards and olive groves. In the middle of an immense plain, broken in the end by the mountains of the Andes, apparently close but in reality dozens of kilometers distance. There is also some grazing cattle, and when they are found at the side of the detectors, they attest to the possibility of harmony between the traditional cowboy culture and the science of the 21st century.

Communicating in English, the official language, but understanding also in Spanish, though one can also hear, more rarely, intriguing conversations (Is it Greek, Polish, Russian), those who work in this field don’t lose sight of the timetable goals: up until July one hundred tanks should be in operation, and the year should finish with three hundred capturing information about the particles that arrive from the sky. In 2004, the construction of the observatory in the Argentine having been completed, the frantic rhythm of the work is going to be transferred to the surroundings of Millard County, in the state of Utah in the United States, where the second half of the observatory will be built to capture the rays that also fall on the Northern hemisphere.

In each of the laboratories the physicists hope to register, per year, thirty cosmic rays of extremely high energy, in the order of 1020 (the number 1 followed by 21 zeros) electron Volts (eV, the unit of energy measurement for atomic particles). Until today, there have been close to a dozen events with energy above this level, observed at the Akeno Giant Air Shower Array (Agasa) in Akeno, Japan, and at the Fly’s Eye Group in Utah, in the United States.

“Since we are in the southern hemisphere, we have the privilege of observing the cosmic rays that come from the center of the Milky Way”, observes Carlos Ourívio Escobar, the professor at the Cosmic Ray Department of the State University of Campinas (Unicamp) and the Brazilian representative in the project. However, it is not because they come from the most dense region of our galaxy, the center, that the cosmic rays have been formed there.

Particles that arrive with such high energy, the physicists suppose, must have been produced in a location relatively close, around 150 million light years – in the surroundings of the Milky Way. One of the models being studied today to explain cosmic rays of extremely high energy”, comments Escobar, “attributes this radiation to the disintegration of super heavy particles, relics of the Big-Bang, imprisoned in neighboring galaxies and in our very own that are only now disintegrating”. Therefore the mother particles must be 15 billion years old, as old as the Universe.

Two basic questions haunt the physicists. One of them refers to the origin of cosmic rays and the other, regarding of their being, in a more informal language, moving packages of intense energy. An electron, which normally has less than 10 eV, can be accelerated up to 20 million times its energy (107) of eV in order to irradiate and destroy cancer tissue – even so, an intensity one hundred and twenty times smaller than that of the particles awaited in the Argentine.

It is already taken as certain that cosmic rays are protons, one of the basic particles of the atomic nucleus, almost two thousand times heavier than an electron. What is not known is how it is that they can exhibit an energy 100 million times higher to the particles of the same type produced at the Tevatron, the most powerful particle accelerator in the world, located at the Fermilab in the United States. “Theoretical physics doesn’t forecast the mechanisms of production of such huge energy”, emphasizes Mantsch.

What is reasonably well understood is the scandal that they provoke when they hit the earth, where they arrive literally breathing fire. On colliding with the atmosphere, at a speed close to that of light (300,000 km per second), the cosmic rays begin a cascade of particles, that become greater and denser as they get closer and closer to the surface. The successive collisions with the hydrogen molecules in the air spawn other types of particles such as electrons, photons, pions and muons, and produce light that can be observed at a distance – the telescopic mirrors and the detectors capture this luminosity at 20km from the point at which it was generated. A cascade of cosmic rays with energy of 1020 eV originate hundreds of billions of particles, cover close to 50 km2 and lasts around ten millionths of a second.

One might have the impression that their has been little advance since the French physicist Pierre Victor Auger (1899-1993) reported, in 1938, the first shower or cascade of high energy cosmic rays – of 1015 eV, ten million times greater than any other energy form of that era. It so happens that research into cosmic rays is living through a dramatic problem: it is not the speed of the phenomenon (in particle accelerators the collisions have duration close to that reported by Auger), but the shortage of raw material of high quality: the greater the energy the rarer the particles. If the energy increases by a factor of ten, then the number of particles falls by one hundred. Mantsch, in one of the articles that he wrote about Pierre Auger, did the calculation: the most energetic cosmic rays have more than 1019 eV – and only one of them arrives on the earth per square kilometer per year. For the particles with 1020 eV, the number drops to one per square kilometer per century.

From now onwards, besides the preliminary results announced at the International Cosmic Ray Conference held in October of last year in Hamburg, Germany, there is another form of evaluating the impact of the observatory in Argentina – and on the very land on which it is taking shape. The infrastructure work, and afterwards, the setting up of the tanks absorb dozens of operators laid off by the company YPF, one of the major companies of the region, which has given up prospecting there for oil and it going to concentrate only on the exploration of areas already surveyed.

As well as this, English courses are spreading in Malargüe. The town acquires na international air, with so many visitors with different languages, and its inhabitants, once their initial inhibition overcome, are today expressing an interest in the project – one of the weekend programs is to visit the observatory’s headquarters, with tanks and its other works. With time, it is probable that not only do the high energy particle fall from the sky, but as well that the production of scientific knowledge about the cosmic rays becomes a reason for pride among the inhabitants of Mendoza, just like their wines.

Equipment from the four corners of the earth
The Pierre Auger Observatory has equipment from nineteen different countries: Argentina, Australia, Bolivia, Brazil, China, Czech Republic, France, Germany Greece, Italy, Japan, Mexico, Poland, Russia, Slovenia, Spain, the United Kingdom, the United State and Vietnam. “In a general manner, 80% of the contributions of the countries is with components”, comments he Argentinean physicist Carlos Hojvat, the project’s assistant manager.

The Brazilian products have been there for some time. Right from the start of last year, the company Alpina, a company from São Paulo, has been sending the Cerenkov tanks, on journeys that don’t take less than two weeks. The Schwantz company in the town of Indaiatuba, manufactures the correcting lenses, whilst the Equatorial company, from São José dos Campos, is going to set up a 2.5 meter device which will allow for the automatic regulation of the telescope’s lenses and the shutters that allow the telescope to make night observations.

Argentina has contributed with the infrastructure and with the water purification equipment. It is also going to manufacture part of the tanks and the batteries for the solar panels that will electrically feed the surface detectors, in a division of tasks with the Mexicans and the Americans. From Australia the cloud detectors have arrived, and from France the electronic devices for surface detection. The Czechs have sent the telescope mirrors and the Spanish the tanks’ solar panels. The fluorescence light detectors of the telescopes are going to be hook up Italian cameras and the electronic command devices are part English and part German.

The Project
Pierre Auger observatory; Modality Thematic Project; Coordinator
Carlos Ourívio Escobar – Unicamp; Investment R$ 1,884,287.12