Valery Chumakov, June 8, 2016, 22:46 – REGNUM
Especially for IA REGNUM. The magazine “The World of Science», № 5/6, 2016
The superheavy elements
30 December 2015 g . The International Union of pure and applied chemistry (IUPAC) finally approved the fact of opening four new chemical elements with atomic numbers 113, 115, 117 and 118. Now, the seventh period of the periodic table of the six elements is fully completed in accordance with the periodic law.
The six elements of the seventh period – 113, 114, 115, 116, 117, 118 – were synthesized for Nuclear Research Joint Institute (JINR) at the accelerator complex U400 Laboratory of Nuclear reactions. GN Flerova in collaboration with physicists and chemists of the US National Laboratory in Livermore, California, Oak Ridge, Tennessee, and Vanderbilt University. Experiments were carried out in Dubna (Moscow Region), under the leadership and with the direct participation of Academician Yuri Ts Oganesyan, of the supervisor of the Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research. He told our magazine, as was the long-awaited search elements.
Supervisor Yu.Ts. JINR Laboratory of Nuclear Reactions Hovhannisyan
One of the fundamental scientific questions – where is the boundary of the material world, or how many items can accommodate the periodic table
A big step in the search for an answer to this question was made by our compatriot Dmitry Ivanovich Mendeleyev . It was he who first attempted to classify all the natural elements, and realized that their chemical properties fit into a certain pattern, now known as the periodic law of DI Mendeleev.
Note that when the Mendeleev periodic created his system in his possession was only 63 elements. And he, I think, too, thought about how many of them may be beyond that. This question and to this day has not lost its relevance in modern science
In 1911, Ernest Rutherford proposed a planetary model of the atom:. Positively charged nucleus, containing almost the whole mass of the atom and all the positive charge and the electrons that moving on a huge distance from that core. When two years later the great Niels Bohr calculated, it looks like a simple hydrogen atom (a proton, around which revolves an electron) can be considered the year of birth of nuclear physics. Further, as is well known, it has developed a truly rapid pace.
People tend to desire to know the boundaries of their dwelling, including the boundaries of the material world around us. Later it turned out that the planetary model of the atom Rutherford – Bohr’s stable and works well enough until the atomic numbers 173-175. But the problem is that long before achieving these numbers is no longer living nucleus of an atom. So the question is, where is the border, should be redirected from atomic physics to nuclear physics.
It should be noted that atomic physics based on quantum electrodynamics – an exact science. This theory allows to calculate a huge nuclear power station or a small chip, and everything will work perfectly as it follows from the fundamental laws of electrodynamics. In nuclear physics, unfortunately, this is still far. It is still unknown the nature of nuclear forces, by means of which interact with each other protons and neutrons in the nucleus. The theory of the nucleus is still in the making. However, if we still want to define the boundary of existence of nuclei, we have to understand or assume the look of the core, and what are the basic properties of nuclear matter
The kernel -. Drop
Our compatriot George Gamow, a graduate of Leningrad University, worked with the famous scientist Abram Ioffe in Leningrad physico-technical Institute and then the rest of his life in the United States, in 1928, he expressed a surprising suggestion that the nucleus like a drop of charged liquid. In the late 1920s, it was already known that the nucleus has a small size, high density, well-defined edges, a spherical shape, and it is incompressible. In fact, very similar to a drop of charged liquid.
The assumption that in the microcosm of the object can be identified as a macroscopic body (visible drop of liquid), was brave enough. And the fact that this analogy to work, hard to believe. But the “drip” model has proved very fruitful. She allowed Gamow to create a theory of alpha decay and explain the phenomenon of radioactivity, which was discovered in 1898 by Henri Becquerel. Trickle model allowed Carl Weizsacker withdraw his famous formula for the calculation of the potential energy of the nucleus. Finally, when in 1939 in Berlin, it was discovered nuclear fission, Niels Bohr and John Wheeler, based on “drop model” created fission theory.
According to Bohr and Wheeler, in charged droplet occurs confrontation between two forces: the surface tension, which squeezes a drop and gives it a spherical shape, and the Coulomb forces, which it is stretched as positively charged protons repel each other. The spherical drop these forces are balanced. With the addition of protons in the nucleus, we change a little surface tension, but definitely we increase the destructive forces of Coulomb repulsion. As a result, as evident from the calculations, “drop” will be deformed. Up to a certain limit such deformation it is not terrible: the surface tension force of its return to the initial spherical shape. But upon reaching a certain critical strain she has a waist – “neck”. Further increase of more stretches the “neck” and, finally, a drop breaks into two approximately equal parts. In English “division» – division . But Lisa Meitner, which, I believe, were the first to explain the process of nuclear fission, said: “No, it’s not like division , nuclear fission process is more like fission – division biological cells. ” That is how a cell divides: first, it is stretched, then there is the neck, and then the cell is broken into two parts. Uranus – the heaviest element that has been preserved in the Earth since the formation of the solar system. Otto Hahn and Fritz Strassmann from the Institute of Chemistry of the Kaiser Wilhelm Society in Berlin for the first time observed the formation of fission fragments when irradiated uranium layers of neutron flux. They have shown that it is necessary to add the uranium nucleus and a neutron in the nucleus to make an energy of about 6 MeV, as the core is divided into two parts. This phenomenon is explained by the classical (drip) theory of division.
But the theory of Bohr and Wheeler followed and that uranium and no extra energy can spontaneously (spontaneously) to split into two parts. Spontaneous fission of uranium – a very rare process having a purely quantum-mechanical nature. Bohr and Wheeler predicted a huge time of the breakup. If the section of the critical strain (the top of the fission barrier) until the gap to the two fragments fission process takes only 10 -19 seconds, there are expected to take more than 10 20 years. Age of the universe is less than this time, the collapse of almost a million times!
In the first experiment, the American physicist and chemist Willard Libby spontaneous fission of uranium were found. From this experiment indicated that the time of the fission of uranium – more than the 10 15 s, which was not surprising, since it is not contrary to the predictions of the “dropping” of fission theory
<. strong> The Fourth kind radioactivity
I still do not quite understand what guided Igor Kurchatov, when offered the two young scientists – former students Georgy Flerov and Konstantin Petrzhak – to do the study of spontaneous uranium fission. He did not believe Libby did not believe the predictions of the Bohr and Wheeler?
But the two young men set to work with great enthusiasm. And soon we discovered the spontaneous fission of uranium.
Our heroes have decided to measure in the experiment are not accompanying the fission of uranium radiation (neutrons) as did Libby, and directly register the fission fragments. Fragments need to be detected against the background of 10 million. Alpha particles. Many spent and widely used methods by the time proved to be unsuitable. As the fission fragments of the detector is proportional to the ionization chamber, for which it was necessary to create a special wide-band amplifier with a gain has been selected approximately 2 × 10 6 . Power, of course, was the lamp and camera layers uranium total area of 1 sq. M. see (and then 6 thousand. sq. cm) was remarkably similar to the variable capacitance of the old radio.
Measurements in Leningrad PTI carried out only at night, so as not to interfere with the trams, which create an electrical spark and background. Young physicists trained himself to sit for hours in a dark room, staring at the screen of the oscilloscope, which was always visible ripples – signals from uranium alpha particles. But sometimes in the stockade, bursts of high signal from the fission fragments. Spontaneous fission! And the signal coming every half an hour, sometimes an hour. They sat for hours waiting when will these signals again. And they appeared …
At the first meeting, where the results were presented vigils, scientific elite was extremely skeptical. Few believed that the uranium undergoes spontaneous fission with a probability of only ten times smaller than the experimental limit received Libby, but a million times higher than that predicted by the great Bohr. There were some remarks: “Clearly, young people are enthusiastic, they feel that they have made the greatest discovery. They may not yet know that there are cosmic rays, which can cause the fission of uranium, uranium and many in the chamber. Where to watch Kurchatov? »
working, they are less arguing To the credit of the people, but most do. It was decided to test the robustness of results to deadly hypothesis about the division of cosmic radiation. Commissar Kaganovich had written a letter asking to give them a little room on the underground metro stations, to repeat their experiments in the new environment. And the young men with their strange baggage arriving from Leningrad to the capital of the USSR, to the deepest on the then “Dinamo” station (it seems 32 m underground) repeat experiments on the spontaneous fission of uranium. They were worn all day in Moscow, looking forward to the night when the last train will stop and it will be possible with one in the morning to five in the morning to take measurements. spontaneous fission effect completely repeated, although the flux of cosmic rays at this depth weakened by almost a thousand times
The following is what is written in their report in 1940:.
«So, it can be argued that the established spontaneous pulse effect is due to acts of uranium fission. This process represents a new type of radioactivity, fundamentally different from previously known types of radioactivity with the emission of alpha and beta particles.
The difference between the experimentally observed time uranium life and said Bohr and Wheeler explained by the fact that the formula passage of a particle through the barrier very sensitive to the selected height and width of the barrier, and the choice of these values to a large extent arbitrary.
We express our sincere gratitude to the head of prof. IV . Kurchatov, to outline all the basic control experiments and to take a direct part in the discussion of the results »
So, 42 years after the discovery of alpha radioactivity of a new type of radioactive decay of nuclei was detected – spontaneous fission. It was sent a short message to the American magazine Physical Review . Then experiments stopped the war.
Far beyond uranium …
It quickly became clear that this will be a war not of infantry and cavalry, and machinery and equipment. It was known that Germany is trying to create a formidable weapon that uses the energy of the fission of uranium, which in the nucleus of uranium accumulated enormous energy and its release is not comparable to any other means of pulsed energy developments.
Nuclear physics has attracted a lot of attention when the warring governments . The development issue huge amounts of money have been invested. Very quickly, in 1943 in the US and in 1946 we have the first nuclear reactors were built. They began to produce plutonium – following the uranium, but an artificial element.
The table of elements found in the earth, ending the 92-th element – uranium. Plutonium – 94th, a man-made. Nevertheless, in the twentieth century. this artificial element to turn out many hundreds of tons of special facilities, not only for nuclear weapons, but also for energy – as a neutron source for many other purposes.
During the long campaign, not only the plutonium produced in a nuclear reactor. The fewer there are synthesized the following elements of the periodic table: 95th (americium), 96th (curium), 97th (berkelium), 98th (California). Number of transuranic elements greatly decreases with increasing atomic number. At the same time their lifetime decreases sharply. In fact, uranium is 4.5 billion years old lives, plutonium -. 25 thousand years, the americium decays faster, half-life of the longest-lived isotope of the element – 8 thousand years.. Hundredth element (farm) lives a year 102nd (nobelium) – second. 104th member – milliseconds
So to answer the question where the limit of the periodic table, according to the above, it would seem, it is not difficult: very close.. Especially since the dropping of the theory of the division indicated that the periodic table would be limited to only the first hundred members. The fact that we have synthesized the 104th, there is little difference. Further progress to heavier nuclei quickly lead us to a critical situation where meaningless to talk about the item. We get a kernel that will break before any orbital electrons around it. This will happen when a half-life of this nucleus is less than 10 -14 seconds. A similar pattern can be expected for nuclei 106 th or 108 th elements. Therefore, the above trend of a sharp drop in the lifetime of nuclei with increasing atomic number should be regarded as a good agreement with the predictions of the droplet model of the nucleus.
This was the case until 1962, when, in our laboratory, quite by accident, in another experiment aimed at the synthesis of element 104, discovered the spontaneous fission of very short (14 ms) half-life. It soon turned out that this is not the 104th, not 100 minutes or even 98 minutes, and 95-th element – americium. But isotopes of element 95 then were already known and studied good. We knew that the isotope americium-242, which was observed in our experiment, the period of spontaneous fission exceeds 10 15 years. And then – the second value of the half-life of just 14 milliseconds. A strange fact: Two half-life of two or probability of spontaneous fission of the same core! Apparently, the error, we decided
In search of the causes of this error appeared another isotope americium -. Americium-244 with a period of spontaneous fission of 0.9 milliseconds. Then it turned out that another 33 isotopes of transuranic elements have two, and sometimes three half-life, a very long one, and the other – much shorter. The same isotope uranium-238, with a period of spontaneous fission, which Flerov and Petrzhak identified as the 10 16 years, has a second half-life of 0.3 microseconds. The difference is 30 orders
Two half-life (or two different probabilities of the same type of decay, in this case – spontaneous fission!) Can be only in case of the nucleus of the two states, of which division occurs. But it is not in any way fit in view of the drop. Two states may only be the case if the body is not amorphous but has an internal structure. This has been perhaps the most vivid example of that nuclear material is not a complete analog of charged liquid droplets. Trickle analogy – it’s kind of the approach in the description of the properties of the nucleus. A more rigorous description requires it to take into account the structural features.
Now, turning to the question of the boundary of the masses of the nuclei, we will have to reconsider the previously expressed opinions. With the growth of the mass and charge of the nuclei we inexorably approaching the critical point. When the Coulomb repulsion forces will be comparable to the surface tension forces, or will exceed them, even small amendments to the nuclear binding energy will play an important role. This role in the balance “to be or not to be” played by structural adjustment, which will be discussed below.
The first expedition to the heavyweights
it has long been observed that among the known today about 3,000 nuclear core has more than associated with the calculation in the liquid-drop model, and there are others – relating weaker. We are naturally interested in the first, more connected.
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