Exotic nuclei are like movie stars: their behavior is not always easy to explain. However, physicists at the University of São Paulo (USP) are using a low-energy particle accelerator to scan how these strange creatures react – the nuclei, not the actors – during collisions, in the hope of learning more about the structure of the particles and the forces that govern the Universe.
In three recent studies, Rubens Lichtenthäler Filho’s group, from the Institute of Physics at USP, has closely followed the reactions of an unusual chemical element, helium-6. This unstable nucleus, made up of six sub-atomic particles, two with a positive charge (protons) and four without a charge (neutrons), two neutrons more than the more common and stable form of helium, live less than a second. By smashing a beam of these particles against a tin target, physicists obtained scattered data (and their remains) that can help understand how the shape of the nucleus of helium-6 affects its ability to fuse with other atoms. Known as nuclear fusion, this phenomenon is important, for example, to understand how the heavier chemical elements of the Cosmos are created in supernova explosions, highly dense stars whose nuclear fuel has been exhausted.
In fact, particle physics does not only live from high energy. Although experiments at giant facilities, such as the European LHC (Large Hadron Collider), the world’s largest particle accelerator, attract more attention, the truth is that most of the nuclear reactions in the Universe, including those that generate the heaviest elements in the supernovae, have much more modest energy levels, and a substantial part of them remains as unexplained as ever.
The mysteries of these interactions are on the main agenda of the physicists of the Pelletron, the accelerator installed in the 1970’s at USP by the physicist Oscar Sala, a former scientific director and president of FAPESP. The equipment is undergoing comprehensive renovation and modernization that should breathe new life into national nuclear research. One of the specialties of the Pelletron is the study of exotic nuclei by means of the Ribras (Portuguese acronym for Radioactive Ion Beams in Brazil), coordinated by Lichtenthäler Filho. Installed in the Pelletron, the Ribras is the only equipment in the southern hemisphere capable of producing beams of exotic nuclei.
But what, after all, are these exotic nuclei? They get this name when their nuclear composition makes them unstable, with a very short existence. This occurs when there is a major imbalance between the number of protons and neutrons. Helium, for example, the second most abundant element in the Universe, has two protons in the nucleus. However, it can have one neutron (Helium-3), two (helium-4), or more. Only versions with more than two neutrons are unstable. Helium-5 is the rarest of them and, once formed, lasts a ridiculously small fraction of a second: in the order of 10-22 seconds. Helium-6, on the other hand, has a half-life, the time in which half the sample undergoes radioactive decay and becomes another element, of 0.8 seconds.
One of the curiosities of exotic nuclei is that they have shapes and sizes that are incompatible with their mass, defined by the sum of protons and neutrons (see Pesquisa FAPESP issues 99 and 120). “As protons and neutrons have roughly the same mass, it was assumed that lithium-6, which is stable and has three protons and three neutrons, and helium-6 with two protons and four neutrons, but unstable, had almost the same volume. However, it’s not what happens,” explains Lichtenthäler Filho. “Helium-6 has a halo, as if there were a neutron cloud around the nucleus, making its volume much greater,” he adds. This changes the way nuclear interactions occur, making them more frequent and powerful. Roughly speaking, the higher the volume, the greater the chance of collision.
How these interactions are altered is something that, so far, the theory has not predicted completely. Therefore, the experiments are fundamental to understanding what is happening with these swollen atomic nuclei.
The Ribras was installed in 2004, but the joy of the physicists was short-lived. The following year an accident severely damaged the Pelletron, almost eliminating its production capacity. “In an experiment in April 2005, the metal indium was accidentally used to stick two parts in the vacuum system right next to the accelerator tube,” says Alinka Lépine-Szily, director of the Pelletron since 2007. “In addition to being a good conductor, indium is a metal with a very low evaporation temperature.”
The accelerator remained switched on all night after that operation without closing the valve that isolates the accelerator tube. Result: the indium evaporated, entered the system and condensed on the ceramic walls inside the tube. When the device was restarted, there were sparks everywhere. To make matters worse, uncontrolled electricity converted the gas of the accelerator tank into corrosive acids, which damaged another critical element of the system, the load currents.
The accelerator spent the following two years more switched off than on, and when it was in operation it worked with an energy level far lower than normal. Ideally, the Pelletron operates at 8 million volts, but this voltage has not been fully recovered. Bringing the Pelletron back to the forefront of science has not been the simplest of tasks. Imported from the United States, the USP accelerator is the oldest of its kind. It is located in an eight-story tower at the Institute of Physics and is of a vertical design. The atoms that produce the beams used by the physicists start from the top, where they gain electrons and have a negative charge. Therefore, they are attracted by the accelerator terminal, located midway between the eighth floor and the ground, with energy up to 8 million volts. When they pass the terminal, the nuclei lose their electrons as they pass through a thin sheet of carbon. They are left, therefore, with a positive charge and gain additional momentum. When they reach the ground they are manipulated by means of a magnetic field to make a 90 degree turn and are then diverted to one of seven available lines – each plugged into a different instrument.
Advanced age alone would not be a problem for the accelerator, according to Alinka. “In Australia, they have the second oldest, which is only two years newer than ours, and it’s in wonderful condition.” For Alinka, the problem here is lack of constant and sufficient funds to maintain the machine. However, after the accident of 2005 the opportunity arose to make up on lost time.
Improvements and refurbishment carried out on the accelerator after 2007, with funding from FAPESP, such as the Roberto Ribas project, gradually improved its performance. Changes were made, from the installation of a simple flow meter on the compressor that exchanges the gas, to more radical ones, such as the use of resistors to reduce the sparks inside the apparatus. “There was a legend that it was very efficient and that the gas was completely replaced after 40 minutes,” says Alinka. “We installed the meter and found that seven to eight hours were needed for a complete exchange.”
In the original system, small metal plates with needles very close to them, separated by millimeters, transmitted the electrical current between the ends and the terminal. However, the electric current converted the gas used in the accelerator into corrosive acids, which attacked the load current and hindered the functioning of the machine. “We managed to get $146,000 to buy resistors and we’re installing them,” says Alinka. “We should complete the refurbishment in about two months` time.”
With the new system, the accelerator should again reach 8 million volts of energy. There is also the prospect of using it to certify the quality of satellite electronic circuits. “The idea is to shoot a beam against the circuits to simulate the conditions they will face in space,” says Alinka. Even on its worst days, the Pelletron never stopped generating data. With the resumption of normal activities, it will tend to stand out in the field of exotic nuclei, in which it is one of the pioneers, and in the collision analysis of lighter stable nuclei, such as lithium, with three protons.
Among recent works, Derberson de Sousa, a master’s degree student of Dirceu Pereira, helped determine the density of the stable lithium-6 and lithium-7 isotopes, by making them collide with tin targets. The data from the work, one of the most important published in 2010 in Nuclear Physics, is fundamental for predicting how the nucleus reacts in a collision with other atoms.
Pereira has been working with the Pelletron since it was installed in the 1970s. Thirteen years ago, he collaborated with Luiz Chamon, Mahir Hussein, Diogenes Galetti and Marco Antonio Ribeiro in the development of a theoretical model called the Potential of São Paulo, which helps to explain how atomic nuclei interact when they are close to colliding. “We’re talking about a distance of around 10-13 cm [one-tenth of a trillionth of a centimeter],” he explains.
On this scale, the strong nuclear force that holds protons and neutrons together in the nucleus begins to operate. To describe how quantum phenomena linked to it unfold in these collisions between atoms is complex. In experiments, the physicists obtain more data in order to bring harmony to the models, so they are closer to reality.
The density study of lithium removed barriers that had prevented scientists from obtaining these measures without any ambiguity. It enabled them, for example, to reconcile the experimental results with those obtained in certain theoretical models. This was yet another test for the Potential of São Paulo. So far, the model has survived. However, who knows what will happen in the next Pelletron experiment.
1. From the origin of the elements to technological applications: exploring the nature of accelerated atoms (nº 2005/04719-3); Type Thematic Project; Coordinator Roberto Ribas – IF/USP; Investment R$ 543,834.76 and US$313,349.54 (FAPESP).
2. Study of exotic nuclei with radioactive beams produced in the Pelletron-Linac (nº 2003/10099-2); Type Thematic Project – Centers of Excellence Program (Pronex); Coordinator Alinka Lépine-Szily – IF/USP; Investment R$ 418,392.64 (FAPESP/CNPq).
3. Study of nuclear properties with exotic nuclei beams (nº 2001/06676-9); Type Thematic Project; Coordinator Rubens Lichtenthäler Filho – IF/USP; Investment R$ 474,927.53 (FAPESP).
4. Study of nuclear phenomena involving stable and exotic nuclei (nº 1998/14946-1); Type Equipment Program Multiple users; Coordinator Dirceu Pereira – IF/USP; Investment R$ 2,490,621.88 (FAPESP).