Scientists to schoolchildren

Meeting with Academician Yu.Ts.Oganessian

On 17 February, a group of 8th-10th grade students from school No.1159 in Moscow visited the Museum of History of Science and Technology of JINR, learnt about the history of the city and the Institute and met Academician Yu.Ts.Oganessian.

The initiator of the trip to Dubna and the meeting with the outstanding scientist was a young, enthusiastic chemistry teacher of this school Vladimir Tarasov. He wrote to Yuri Tsolakovich by e-mail, got his consent to the meeting and brought his students. Among them are winners of Olympiads of various levels not only in chemistry, but also in physics, mathematics, computer science and Russian language.

Yuri Oganessian's conversation with the children was in the form of an interview - they received comprehensive answers to their questions from the scientist. V.A.Tarasov prepared for our weekly "Dubna: science, cooperation, progress" the text of this interview and expressed the hope that the meeting with the famous scientist will be remembered by his students for a long time and will influence their choice of chemistry as a future profession.

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What is the basis for the synthesis of the chemical element 120 today?

- The desire to obtain new elements arose long ago, almost 500 years ago, among alchemists. They dreamed of making gold from lead and getting rich in the process. Therefore, lead was heated, melted, subjected to high pressures, poisoned with caustic chemical compounds and poisoned themselves. Nothing worked. If you ever manage to visit the Czech Republic, Prague, you can visit the place where alchemists lived and carried out these experiments. They understood that you cannot spontaneously get another element from one element, you have to put energy into this process. They just didn't know the level of that energy. If they had been able to not just heat or hit lead with a hammer, but to put in energy millions of times greater than that, then maybe something could have worked. So, if you're going to do an experiment, in order to get another element from one element, you have to know how the chemical element or the atom is organized.

And any atom is organized this way: in the middle is a spherical nucleus - small in volume but very dense matter. It's 15 orders of magnitude denser than water. It contains almost all the mass of the atom and all the positive charge. And around this "ball" at a great distance from it, electrons move in circular orbits. If the charge of the nucleus is +1 (and one negatively charged electron in the first orbit), it is hydrogen, if the charge of the nucleus is +2, it is helium (both electrons in the first orbit). If the charge is +3, it is lithium. But the third electron is already in the second orbital. And so on. It's such an openwork construction. If you take the heaviest element in the Earth, uranium, it has an atomic number of 92, that is, the charge of the nucleus is +92 and 92 negatively charged electrons orbit around it. Now, let's imagine that the nucleus is the size of, say, a tennis ball and put it in the middle of Red Square in Moscow. Then, the first 2 electrons will move along the Boulevard Ring, the next orbit will be the Garden Ring, then, the Moscow Ring Road (MKAD) and the last orbit will be somewhere near Stockholm. When alchemists tried to influence the atom, it was by weakly influencing the electrons farthest from the nucleus (in our example, moving near Stockholm). If they had been able to affect the nucleus, then maybe something would have worked.

The way to produce a new element in the laboratory is based on the interaction of two nuclei. Two atoms should be pointed at each other so that their nuclei collide and collide head-on, then they can fuse with each other. But they will not collide and merge so easily, because they are both positively charged, on the contrary, when approaching each other they will be repelled. We have to overcome this repulsion, to accelerate the nucleus-shell to an energy higher than the repulsion energy of the nuclei of the shell and the target. For this purpose, one of the nuclei, let us call it the projectile nucleus, should be accelerated to a speed equal to about 1/10 of the speed of light. It can be done by large electrical machines called charged particle accelerators that produce beams of accelerated projectile nuclei. This beam is directed at a target. So, the target nuclei are at rest and the projectile nuclei collide with them, overcoming electrical repulsion. The fusion reaction is like two drops of water merging. The larger drop absorbs the smaller one. As a result, a new nucleus of total mass and total charge is produced. Around the nucleus, the orbits of electron motion are lined up and a new, heavier atom is produced. Briefly, this is the basis of synthesis. New elements are products of the nuclear reaction of the fusion of two nuclei.

If we want to produce, for example, element 112, we can choose as target atoms the heaviest natural element - uranium, its atomic number is 92. And as a projectile, we will take a nucleus with atomic number 20 (this is the nucleus of calcium atom) and accelerate it to about 1/10 of the speed of light. If the two nuclei with numbers 92 and 20 fuse, we get the element 112. It would seem that in a similar way you can move forward in the synthesis of the element 114, taking a heavier target from the element 94 - plutonium. But there is no such element on Earth because our planet is very old. Earth, Mars, Venus are the planets that orbit the Sun. And the entire solar system began 4.5 billion years ago. At that time, plutonium was slightly less than uranium, but its lifetime was much shorter than the age of the Earth. And only those elements that had a lifetime longer than the age of the Earth have survived, we call them stable elements (from hydrogen to uranium).

How long would it take to synthesize one chemical element?

- As I understand it, you want to ask what is the probability of producing an element, what is the yield of what we are engaged in. Suppose that the nuclei have fused and a nucleus of a future, heavier element is produced for a moment. This heavier nucleus is very hot. It will immediately split in two. In order to preserve the new nucleus, it should be cooled. It can only be cooled by emitting its neutral particles, neutrons that can carry away all the heat energy destructive to the new nucleus. Unfortunately, in relation to fission, the emission of neutrons has a very low probability. At best, only one ten-millionth of them survive. The heavier the element we want to synthesize, the less likely it is to survive. We were happy if we could get one atom in one day's work. We learned to work with one atom a week. Five atoms of element 118 were produced in five months.

Hard work!

- But at some point, I thought it was not the business to work with such rare atoms. How to move on? After all, it is extremely interesting to determine the chemical properties of a new element! But how can this be done on individual atoms? Apart from their small number, superheavy nuclei also live very short lives: a second, a tenth of a second, a hundredth of a second. How do you learn the chemistry of an element in a hundredth of a second? Before you can think, I'm not talking about stirring a solution in a test tube, the nucleus of the atom has disintegrated! It's difficult in liquid chemistry. You can move on to gas phase chemistry. But definitely, the number of atoms of superheavy elements has to be increased substantially.

I remember well that 10-12 years ago, when not all superheavy elements had been discovered yet and not all of them had received their official names, my colleagues turned to me with a question: what is the next step? Further, I told them, I do not know, but one question should be answered. We have been preparing for our experiments for almost 10 years and we have been carrying out our round-the-clock work on the calcium-48 ion beam for the 14th year already. Please, tell me, if we had started work on the beam not since 1999 but say, in 2014, how long would it have taken us? The first answer was, "We don't know." Then I say, if no one knows how to answer this question, let's try to find the answer together. Let's put on the palm of our left hand everything that we have learned over the past 14 years: that nuclei-drones can be accelerated, the probability that they can be fused with target nuclei, how many of them survive, how they disintegrate and so on. And on the palm of the right hand we will put what is not related to the elements, but is a consequence of scientific and technological progress during these 15 years. How the knowledge-intensive technologies we use have changed, what new materials have appeared, how computing, computers, others have advanced. Now, let's join both hands and determine how many times faster we could do all our work? It turned out (you won't believe it), 100 times faster. It amazed me myself! After all, it follows from this: everything that is exceptional today may become banal in 15 years. Therefore, there is no point in holding on to the existing "value" with both hands. Life moves at such a pace that everything changes rapidly and sometimes we don't even feel it.

But if we have estimated correctly, another question arises: what are we engaged in today? If our research can be implemented 100 times faster, why are we laying out on an old rig? One more element, what's next? After all, to go further, we will have to throw away everything old anyway. They say to me, "How can we throw it away? That's what gave us the discovery of the five elements of the Periodic Table. The heaviest, superheavy elements. "Our accelerator and equipment were the best in the world!" Of course, everything was so in reality, but with these "world records" there is no road. Our records are not enough for the whole opened field of research of the island of stability of superheavy elements. In the same way I made a report at the Scientific Council of JINR, there is a discovery of new elements, but there is no way further. Then we addressed our proposal to Directorate of the Institute to establish a new laboratory that with my light hand was called the Superheavy Element Factory. This name was to remind us all the time that we should change our thinking and approach to all our work so that the investigation of the properties of superheavy elements would not be an exclusive endeavor. We need to make sure that it is somewhat ordinary but qualitative scientific research.

Here, I cannot but mention the great role of JINR Directors Academicians V.G.Kadyshevsky, A.N.Sisakyan and V.A.Matveev, as well as members of the Scientific Council - famous scientists from many countries that immediately understood the essence of the project and unanimously supported it. In 2012, the forest was cut down, the land was dug up and in six years, a new building with a new heavy ion accelerator, three experimental halls with new equipment was constructed. In short, we established a new advanced laboratory with the prospect of working with 100 times more precious superheavy atoms than before. Today, we are almost at that level. But it's still not enough for us. Such a life! When you graduate from school and institute, the productivity of the factory here will probably be increased by another 10 times.

What is the basis for the island theory of the stability of chemical elements in the periodic system?

- This is a slightly different question. Here, in the foreground is our knowledge that has accumulated in the 114 years since Rutherford's discovery of the atomic nucleus that gave rise to the new science of nuclear physics. But this knowledge is still not enough to understand the nuclear forces, the so-called strong interaction that is necessary to build a rigorous theory, like the theory of electricity. And it is extremely necessary to understand how the world is organized, where its limits are, how many elements there can be, where and how the Periodic Table ends. We need to know and understand what this dense nuclear matter at the centre of the atom is. Our compatriot Georgy Gamov, a graduate of Leningrad University, believed back in 1928 that the nucleus is like a drop of liquid, has a well-defined spherical shape, is incompressible and has a huge density, 15 orders of magnitude greater than that of water. This was the first theory of the atomic nucleus that was called the droplet prototype of the nucleus. We, dealing with heavy nuclei, are always looking for an answer to the question: how big can a nuclear droplet be? In fact, if there are no other forces counteracting the well-known surface tension forces compressing the droplet into a spherical shape, there is nothing preventing the size of the droplet, it can be quite large. We see that at the tip of a pipette, the droplet enlarges only to a certain size, then breaks off and falls downward because of the earth's gravity. It is not difficult to realize that the largest drop size is reached at the moment when the force of surface tension holding the drop at the tip of the pipette is equal to the force of Earth's gravity - the weight of the drop. I remember that the first astronauts would pour water out of a bottle and it would hang in weightlessness as a large ball of water. The astronaut would break this ball into small balls with the palm of his hand and push them into his mouth. He drank water in this way! Now, they probably drink it differently.

Returning to the nucleus of the atom, we should recall that it is positively charged. Electrical repulsive forces are trying to stretch the spherical nucleus. It is also easy to realize that as long as the surface tension is greater than the electric repulsion, the nucleus will occur.

According to this Gamow droplet prototype, the limiting element would be the element with atomic number 100. When I started working in G.N.Flerov's group at the Institute of Atomic Energy in Moscow, we aimed at the synthesis of the element 102. Before us, such an attempt was made by Swedish scientists, but their data were refuted by American physicists from Berkeley (USA). Then it turned out that the Berkeley data also needed revision. The final results were obtained in Dubna, nine years after the beginning of work in Stockholm, when the largest at that time accelerator of heavy ions U-300 was launched in JINR. In the difficulties of synthesizing the element 102, it seemed that we really approached the edge of the abyss, where nuclei become completely unstable. And then, at new accelerators in Dubna and Berkeley, elements 103, 104, 105, then 106... were synthesized. And afterwards, the idea arose that the nucleus is not a classical liquid that has no structure, not an amorphous body. There should be an internal structure in nuclear matter. Then there was the impression that all the atomic nucleus theorists literally outlined the task and found this structure because of which elements with atomic number more than 100 can occur. Moreover, far beyond uranium, near element 114 and its neighbors, a whole area (island) of such heavy and long-lived elements was predicted. The occurrence of such islands is in fact, a manifestation of the internal structure of nuclear matter. Just as in a solid body, such as carbon, there can be a pressed powder (structureless soft pencil lead) or a hard diamond with a clearly expressed structure.

But the fact that the amazing survivability of atomic nuclei predicted by the theory while advancing to the area of increasingly heavier elements was due to the structural properties of nuclear matter required experimental proof. And this proof came in the form of the results of direct experiments on the synthesis of superheavy elements, closed the seventh row of the Periodic Table.

Article was prepared by Deputy Director of the JINR Museum Nadezhda KAVALEROVA