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Particle shock

Experiments in USP's accelerator reveal behavior of exotic nucleuses

The first original scientific results have come out of the experiments done in São Paulo with a machine that is revealing a bit more of the behavior of atomic particles called exotic nucleuses, endowed with more protons or neutrons than the stable nucleuses of the same chemical elements. In the equipment known as Ribras, which stands for Radioactive Ion Beams in Brazil, installed two years ago in the Physics Institute of the University of São Paulo (USP), exotic nucleuses of the chemical element helium – helium 6 – collided with a fixed target, made up of a film of pure aluminum.

The physicists found that the probability of this exotic nucleus of helium breaking up, after colliding with the nucleuses of aluminum, is only between 10% and 20% greater than with other particles that do not show such a not very dense cloud made up of two neutrons that spin around the center – the halo, typical of helium 6.

On these collisions, which last less than a billionth of a second, information is emerging that helps to understand in more depth the reactions that originated the chemical elements about 14 billion years ago, when the Universe was formed, and those that even today are occurring in the interior of stars like the Sun, which result in light and heat for the Earth. It is also possible to get to know better the limits of the forces that act between the elementary particles of matter.

Some species of exotic nucleuses are much larger than nucleuses with the same number of particles. This is the case of helium 6, made up of two protons (particles with a positive electric charge) and four neutrons (without an electric charge) – two neutrons more than helium 4. It s these two extra neutrons that form the halo, a sort of ring with a diameter equal to that of the nucleus of lead 208, with 82 protons e 126 neutrons.

In the last few years, in particle accelerators in France, Belgium or the United States, the physicists are studying how the neutrons of the halo can influence the collision with other nucleuses. In these experiments, the helium 6 collides with nucleuses endowed with a much greater mass than the mass of aluminum 27, like uranium 238 and lead 208. In these cases, according to Alinka Lépine-Szily, a researcher from USP’s Physics Institute, the intense electric field of the heavier nucleuses repels the helium 6, since the two nucleuses have a positive charge, and the helium 6 breaks up even before the nuclear collision.

In 2001 and 2002, Alinka was a member of the team that prepared and analyzed some of these experiments, carried out in the accelerator of the Centre de Recherches du Cyclotron in Louvain-la-Neuve, in Belgium. This work showed that the exotic nucleuses, in spite of housing extra particles and of breaking up easily during the collision, merge with other nucleuses in the same way as normal nucleuses. Detailed in an article published in October 2004 in the Nature magazine, this conclusion ran counter not only to intuition, but also to the theoretical models, according to which the exotic nucleuses would be natural donors of protons or neutrons.

Back in Brazil, Alinka planned another kind of experiment, with the other two researchers from Ribras, Rubens Lichtenthaler Filho and Valdir Guimarães, and with an experimental nuclear physicist, from the Fluminense Federal University (UFF). Choosing as a target for the helium 6 a much lighter atomic nucleus, aluminum 27, the nucleus of which is made up of 13 protons and 14 neutrons, they managed to bring down the Coulomb barrier, defined as the repulsive electric potential between the colliding nucleuses, which makes the nucleuses break up before the nuclear collision.

“These were the first experiments with collisions of exotic nucleuses with light targets at low energies, close to the Coulomb barrier”, Alinka says. “We wanted to discover what the probability is of the helium 6 breaking up when colliding with a target with a far more tenuous electromagnetic field.” It was a way of making the exotic nucleus arrive intact close to the target, to the point of being attracted by one of the elementary forces, strong nuclear interaction, which maintains the particles close to each other.

Throughout one week, in December 2004, the physicists from USP, in collaboration with Gomes’s group, worked day and night on these experiments. They created beams of ions (electrically charged particles) on the eighth floor of the tower that houses USP’s particle accelerator, the Pelletron, inaugurated in 1972. The beams are accelerated, go down to the surface, and are diverted to several pieces of equipment – one of them is the Ribras, 7 meters in length.

Of each million nucleuses of helium 6, roughly only one nucleus would go on precisely in the direction of the target, break the Coulomb barrier, and collide with the aluminum nucleus. As a consequence of the shock, it could break up into fragments, sometimes losing the two neutrons furthest from the heart of the nucleus, which could – or not – be incorporated into the target. Another possibility could be that, after the collision, it would be diverted as if nothing had happened, like a billiard ball hitting another.

The physicists then found that the probability of the helium 6 breaking up is greater than with other normal particles, whose behavior had already been characterized by means of experiments done by other research groups in the last few years. These results will be announced in March at an international congress on nuclear fusion and constitute the raw material for the doctoral thesis of one of  Alinka’s pupils, Elisangela Benjamin, presented at the end of January.

It was theoretical physicist Mahir Saleh Hussein, also from USP’s Physics Institute, who concluded that the helium 6, because of the two neutrons of the halo, which do not break up so easily, shows a chance of breaking up into fragments from 10% to 20% greater than the normal nucleuses. However, the nucleuses of helium 6 are also preserved because they are hefty. Fragility and gigantism act inversely, one characteristic compensating the other, because of the other, because of the Heisenberg Principle, one of the basic laws of quantum mechanics, the area of physics that seeks to explain the behavior, often apparently without rules, of atomic particles. “Because of the Heisenberg Principle”, says Hussein, “weakly bound particles occupy larger areas in space”. According to him, this compensation mechanism helps to preserve the integrity of the nucleus.

“It would be excellent if there were an increase in fusion when we use exotic nucleuses”, Hussein says. If fusion were to increase, the exotic nucleuses could be seen as donors of neutrons and protons – something that would not only facilitate research but also the applications of physics to medical diagnoses and treatments. In a 111-page article published this month in the Physics Reports magazine, Hussein and another two theoretical physicists from the Federal University of Rio de Janeiro (UFRJ), Felipe Canto and Raul Donangelo, besides Gomes, from the UFF, present the theory that helps to explain experimental results achieved in the particle accelerators of Belgium, France, the United States, Japan, Italy and Brazil. On this equipment, they are trying to reproduce the reactions that originated the Universe and human beings themselves.

Roughly 80% of our body consists of water, made up of two atoms of hydrogen and one of oxygen – all very old. The nucleus of hydrogen, with only one proton, was formed minutes after the Big Bang, the explosion said to have originated the Universe, 14 billion years ago. And the hydrogen atoms – one electron spinning round one proton – were constituted 400 thousand years afterwards.

And only a billion years later, as the Universe was cooling down and expanding, did oxygen, which constitutes 61% of the mass of the human organism, carbon, which accounts for 23%, and all the other heavier chemical elements, such as nitrogen, calcium, phosphorous and iron, begin to form – in the interior of the stars, as a result of the fusion of lighter chemical elements.  Initially loose in space, little by little they joined together in clouds that became so much denser, to the point of originating planets like the Earth and its forms of life.

Even today, hydrogen and helium are being formed in the Sun, oxygen and carbon in larger stars, of the nova kind, and still heavier chemical elements, like sodium, uranium and lead, in the explosions of the supernovas, with a mass thousands of times greater than that of the Sun. Equipment like the Ribras works as if it were a star of the nova kind, forming medium sized nucleuses rich in protons and neutrons.

Besides helium 6, the physicists from USP have now produced nucleuses of lithium 8, with one neutron more than normal lithium, of beryllium 7, with two neutrons less, and of boron 8, with two neutrons less than normal boron. Throwing them against stable and more imposing atoms – like vanadium 51, reproducing experiments already done by other groups, and now with aluminum 27, which had not been done, they discover how the exotic nucleuses can break up.

Other experiments of this kind may perhaps take some time. Although it is new and on a footing with other advanced equipment abroad, the Ribras depends on the Pelletron, a particle accelerator that needs constant maintenance. And it is no longer very easy to find spare parts, which depend on importation, reports Valdir Guimarães while he shows the accelerator’s control room, made up of a mixture of pieces of equipment typical of the 1970s, alongside others, more recent. Soon after the experiments with helium 6, the Ribras stopped working because a component of the Pelletron broke. The physicists believe that the part will be replaced and all the other equipment that it serves will go back to normal before the end of the first half of this year.

The Project
Study of exotic nucleuses with radioactive beams produced at the Pelletron-Linac Laboratory of IF/USP
Thematic Project – Nucleuses of Excellence Program (Pronex)
Alinka Lépine-Szily – IF/USP
R$ 585,000.00 (FAPESP and CNPq)