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Physics

At the heart of the atom

Experiments deepen knowledge about the structure of matter

eduardo cesarRibras: equipment that is unique in the Southern Hemisphere is helping to understand the reactions that occurred billions of years ago in the supernova starseduardo cesar

It was a high risk bet. The machine capable of producing exotic atomic nuclei – unstable particles, that last only 1 second and do not exist in nature – had not yet been tested. The researchers from the Physics Institute of the University of São Paulo (IF-USP) were not sure that it would work. Despite their apprehension, and with butterflies in their stomachs, they decided to maintain the date for the debut, February 2.

It was the beginning of the 13th Experimental Nuclear Physics Summer School and, in the audience, there were 50 postgraduate students from nine Brazilian states, and from Argentina, Colombia, and Cuba. “The students knew that it was the first time that the machine was going to work, and they were just as curious as we were”, says Alinka Lépine-Szily, one of those responsible for the project.

There were several hours of checking the components of the 7-meter long machine, whose main structures – two large horizontal cylinders – look like train coaches. And a few long seconds until the computers recorded the first information about the exotic nuclei generated inside it – it worked!, the researchers commemorated. The tension gave way to satisfaction. The results achieved should help to understand in more detail how the chemical elements arose, both at the beginning of the universe, minutes after the Big Bang, and in the explosions of supernova stars, 1 billion years later. Each explosion of a star generates thousands of exotic nuclei, which act like sparks and induce the formation of all known stable chemical elements.

The new equipment, called the Ribras Project, standing for Radioactive Ion Beams in Brazil, is the only one in the Southern Hemisphere – there is another, very similar, at the University of Notre Dame, in the United States, built about ten years ago. Installed in an enormous shielded shed, Ribras is coupled up to a particle accelerator installed at the institute some 30 years ago. In the experiment at the beginning of February, they accelerated a stable beam of lithium 7, a natural chemical element, which crashed with a fixed target of beryllium 9, also stable.

The nuclear reaction produced a series of particles, stable and unstable, which continued to propagate. The exception was the lithium 7 itself, as it is the primary particle beam, was blocked by a barrier, called a Faraday Cup, put in front of it. The other particles produced in the original collision, before the lithium stopped in the Faraday Cup, because of their directions which diverged from the main beam, escaped this blockage, and went in to the solenoids – 1-meter long coils, positioned inside cylinders and immersed in liquid helium. The solenoids produce a very intense magnetic field, using which it is possible to select the exotic nuclei, which are identified on the final detector, a silicon crystal 2 centimeters in diameter.

In the debut experiment, the researchers from USP’s Physics Institute produced approximately 10,000 particles per seconds of helium 6, an exotic nucleus, with two protons and four neutrons (normal helium has two protons and two neutrons). According to Alinka, the two extra neutrons stayed away from the nucleus, forming a halo, a sort of ring that determines a greater atomic radius than in common helium.

Exotic and rebellious
This peculiarity called attention: stable nuclei, even of different chemical elements, have the same density in the center, a not very diffused surface, and well defined shapes. Helium 6 showed an extensive region – comprising the area between the nucleus and the ring – in which the average density was very low, besides a not very well defined surface. “With the exotic nuclei, situations occur that do not manifest themselves in the stable ones. Our studies may confirm the idea that it is possible to find nuclear matter with different densities”, explains Rubens Lichtenthaler Filho, a member of the Ribras team. “We will be able to help to reformulate and to perfect old models about the atomic nucleus”, he adds.

In the laboratory, using the data obtained, the group of physicists is trying to detail the formation of chemical elements in the inside of stars. This was the moment when, starting with light gases – hydrogen, helium, and lithium – unstable and stable, heavier elements started to form, like carbon, oxygen, and nitrogen. It is as if the researchers were going up a ladder, the base of which is formed by the primordial chemical elements, and the top, by more complex byproducts.

Just one second
Each step forward taken makes a new combination of elements arise that, little by little, makes the complex scenario of the universe clearer. Oddly enough, the physicists have to be very quick to understand what happened billions of years ago: the lifetime of the exotic particles generated in the laboratory is very short – just one second. But they are not worried. “In this case, it is plenty of time, more than sufficient for all the information to reach the computer and to be analyzed”, guarantees Valdir Guimarães, a researcher on the project.

The idea of building Ribras was born in July 1995, when theoretical physicist Mahir Saleh Hussein returned from a spell of a year and a half at the Massachusetts Institute of Technology (MIT) and at Harvard University, in the United States. Convinced that Brazil could occupy a prominent place in the studies about exotic nuclei, Hussein organized an encounter at USP, which in February 1997 brought together some of the greatest scientific authorities from the area, like Richard Casten, from Yale University, James Kolata, from the University of Notre Dame, both in the United States, and Antonio Villari, a Brazilian physicist working at the Large National Heavy Ion Accelerator (Ganil), in France.

Carried out with the help of these specialists, the Brazilian project, then under the responsibility of Hussein, was approved before the end of 1997. The solenoids, which form the heart of the equipment, arrived only five years later, in April 2002, coming from the United States. As their assembly only ended in December 2003, there was no time to test the machine before the Summer School. “We decided to run the risk and to carry out the first scientific experiment as happens in real live, subject to hits and misses”, Lichtenthaler stresses.

Still during the Summer School, which lasted two weeks, from February 2 to 14, the team from USP caused a collision between lithium 8, exotic, and vanadium 51, stable, with the purpose of analyzing a phenomenon called elastic scattering. This is a kind of collision between particles without any loss of energy, already known with stable nuclei. With Ribras, in the same way that is being done on similar equipment in the United States and in France, the intention is to observe better details of the exotic nucleus – whether it is compact or nebulous, and whether it has a diffuse surface, or, on the contrary, a well defined one.

At this moment, the researchers from USP, with the comfort of counting on a machine that worked properly right from the first day, are getting ready to compare the collision of lithium 7, stable, with vanadium, to analyze whether different characteristics and manifestations occur, in relation to those that happen with exotic lithium.

The Projects
1.
The Brazilian Rib Facility Planned for the Pelletron-Linac Complex in São Paulo (nº 97/09956-5); Modality Regular Line of Grants for a Research Project; Coordinator Mahir Saleh Hussein – IF/USP; Investment R$ 1,082,150.75
2. Study of Nuclear Properties with Exotic Nuclei Beams; Modality Thematic Project; Coordinator Rubens Lichtenthaler Filho – IF/USP; Investment R$ 482,797.04

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