Possibly the most complete accomplished up until now, this study reveals that launching an exotic nucleus at extremely high speed against the nucleus of another atom does not increase the probability of both fusing together with the shock. Nor does it decrease the probability. This super atomic collision generates another form of interaction: the common atomic nucleus receives from this type of nucleus, called exotic, its surplus neutrons, which probably orbit in its surroundings forming a type of cloud, as the data published on the 14th of October in Nature informed.
“This result does not mean mean that we are returning to square one, but, on the contrary, we have come away from it”, says the physicist Alinka Lépine-Szily, from the University of São Paulo (USP), the co-author of the study in Nature. “The theoretical models that had indicated a greater probability of nuclear fusion occurring in these cases will have to be revised, now with a base of detailed information.” Anyone who does not have a passion for the beauty of physics could even believe that this discovery is no more than a simple detail. But it’s not. Nuclear fusion is the energy source of the stars, just like the sun.
In the interior of stars fusion occurs because the gravitational force exerts a pressure which brings the nuclei one to another. Part of the energy released escapes in the form of radiation and makes life on earth possible. And as well, the fusion of the atomic nuclei of chemical elements lighter and simpler – such as hydrogen made up only of a particle with a positive charge (proton) – give rise to the nuclei of larger and heavier atoms, examples being helium, lithium and carbon.
The interest to understand and to dominate nuclear fusion came about at the beginning of the last century, almost 2,500 years after the Greek philosopher Leucipo postulated that matter was made up of atoms. At the end of the decade of the 1930’s, just before the start of World War II, the German physicist Hans Bethe verified that the fusion of the nuclei of two atoms of hydrogen liberated energy. In this phase of political turbulence and economic instability, this physics phenomenon then became seen as a possible source of energy, alternative to fossil fuels – especially coal and crude oil.
The understanding of how particles behave in the nuclei of atoms would also give to humanity a power of destruction never before seen, with the use of fusion to produce extremely powerful nuclear weapons, such as the hydrogen bomb or H bomb – as the atomic bombs, such as that launched against Japan, are produced based on an opposing phenomenon, nuclear fission, in which the nucleus of large atoms break apart, releasing energy. In the H bomb, the union of the nuclei of deuterium – a particular form of hydrogen whose nucleus contains one proton and one neutron – produces the chemical element helium, in a transformation similar to that observed in the interior of the sun.
On being combined, these nuclei lose less than 1% of their mass, which transforms itself into a true mountain of energy, as forecast by one of the best know physics equations, developed by Albert Einstein, E = mc2. This formula indicated that the energy (E) produced in a nuclear reaction corresponds to the mass (m) lost multiplied by the velocity of light (c) elevated to the power of two – hence the value becomes so elevated.
However, it is not so simple to repeat here what happens in the heart of the stars. In the core of these celestial bodies the gravitational pressure and the temperatures are so elevated that distinct atomic nuclei get close to the point of joining one another, overcoming the force of repulsion. It is even possible to artificially reach such high temperatures, but the consumption of energy is so high that it practically turns fusion impossible from the economic point of view – just to have an idea, it is necessary to explode an atomic bomb in order to initiate the fusion of the nuclei in an H bomb.
In 1985, the team of the physicist Isao Tanihata, from the Nuclear Physics Center of Japan, noted that the exotic nuclei of lithium, known as lithium 11 (11 Li), containing eight neutral particles, was more bulky than expected. The reason is that two of its four excessive neutrons do not remain combined in the nucleus, but form a cloud of neutrons – in nature, the lithium nucleus contains only four neutrons along with its three protons.
In these exotic nuclei, which last for less than a second after being created, some of these neutral particles remain further away, forming a type of cloud or halo, as the physicists say. Quickly it was imagined that, with less coherence, the exotic nuclei would facilitate fusion. As well as this, since they presented a greater mass, it was supposed that the force of attraction between the nuclei would begin to act at greater distances and, in this manner, compensate for the force that repels the particles with the same electrical charge – positive, in the case of the protons of the atomic nuclei.
The paradox of helium 6 (6 He)
An international team coordinated by Atsumasa Yoshida, from Japan and Cosimo Signorini, from Italy, attempted to prove the greater probability of exotic nuclear fusion in experiments with beryllium 11 (11 Be) (with four protons and seven neutrons), but the results were unsuccessful. Another test carried out by James Kolata, from the University of Notre Dame, in Indiana, United States, revealed the opposite: nuclear fusion occurred more easily with helium 6 (6 He). With these results it was impossible to reach a conclusion. In an attempt to unmask the doubt, Jean Luc Sida, from the Atomic Energy Commission in France, brought together an international group – formed by Belgian, French, Italian, Polish and Brazilian physicists – in order to carry out an experiment that would be more complete and that would have a more detailed analysis that any previous one.
Making use of the particle accelerator at the Cyclotron Research Center at Louvain-la-Neuve, the physicists launched nuclei of helium 6 (6 He) against the much larger nuclei of uranium 238 (238 U) – something like serving a tennis ball at velocities close to that of light against a football field. If everything was to go well and the helium would facilitate complete fusion, nuclei should be created of a chemical element that is even larger and heavier: named plutonium 244 (244 Pu), with 94 protons and 150 neutrons. Almost instantaneously after the fusion, the plutonium would undergo fission and would divide itself into two other chemical elements, emitting radiation. At the same time, as would be verified, there would be the emission of alpha particles, formed from two protons and two neutrons, identical to the nuclei of helium 4 (4 He), characteristics of nuclear reactions.
The initial analysis of the data, carried out by Riccardo Raabe, the first author of the study published in Nature, showed that in reality the helium 6 (6 He) had brought about a higher number of fissions than helium 4 (4 He). But this was only part of the information. All that left to do was to check what had happened at the start of this process of transformations and had brought about fission – all nuclear fusion is followed by fission, but not all fission is caused by the fusion of atomic nuclei. When he evaluated the path that the alpha particles had taken until the detectors and the energy with which they arrived there, the group, in which Alinka participated, verified that they had resulted from the loss of two neutrons of helium 6 (6 He) – those that had formed the halo – to the nucleus of uranium 238 (238 U), which, then suffered fission. It was then clear: in a large part of the collisions, instead of fission the transference of neutrons occurred.
Is this what happened with helium 6 (6 He)? During the transfer, the two neutrons that go to the uranium could have broken off and been liberated, continuing to exist as helium 4 (4 He). Alinka intends o carry out an in depth study, at USP itself, of these reactions that compete with fusion. At the beginning of this year in the Physics Institute, equipment that integrates into the Ribras Project and is capable of producing beams of exotic nuclei (see Pesquisa FAPESP No 99 of May 2004 ), came into operation. “We can now do here what previously was only possible abroad.”
A study of exotic nuclei with radioactive beams produced in the Pelletron-Linac laboratory (nº 03/10099-2); Modality Thematic Project and Pronex; Coordinator Alinka Lépine-Szily – USP; Investment R$ 600,723.48 (FAPESP and CNPq)