The purpose of the trip is suggested by the same shell theory of the structure of the nucleus. This is the region of superheavy elements with a suitable (and very large) number of neutrons and protons, the legendary “Island of Stability”. Calculations say that some of the local “inhabitants” may no longer exist for fractions of microseconds, but for many orders of magnitude longer. “In a certain approximation, they can be considered as water droplets,” explained to us Academician of the Russian Academy of Sciences Yuri Oganesyan. – Up to lead, spherical and stable nuclei follow. They are followed by a peninsula of moderately stable nuclei – such as thorium or uranium – which is pulled out by a shoal of highly deformed nuclei and breaks off into an unstable sea … But even further, beyond the strait, there may be a new region of spherical nuclei, superheavy and stable elements numbered 114, 116 and further ”. The lifetime of some elements on the “Island of Stability” can last for years, even millions of years.
Island of stability
Transuranium elements with their deformed nuclei can be created by bombarding targets made of uranium, thorium or plutonium with neutrons. By bombarding them with light ions accelerated in an accelerator, one can consistently obtain a number of elements even heavier – but at some point the limit will come. “If we consider different reactions – the attachment of neutrons, the attachment of ions – as different ‘ships’, then all of them will not help us reach the ‘Island of Stability,’” continues Yuri Oganesyan. “This will require a larger“ ship ”and a different design. As a target it will be necessary to use neutron-rich nuclei of artificial elements heavier than uranium, and they will need to be bombarded with large, heavy isotopes containing many neutrons, such as calcium-48.
Only a large international team of scientists was able to work on such a “ship”. The engineers and physicists of the Electrokhimpribor plant have isolated an exceptionally rare 48th isotope from natural calcium, which is contained here in an amount of less than 0.2%. Targets made of uranium, plutonium, americium, curium, californium were prepared at the Dimitrograd Research Institute of Atomic Reactors, at the Livermore National Laboratory and at the Oak Ridge National Laboratory in the USA. Well, key experiments on the synthesis of new elements were carried out by Academician Oganesyan at the Joint Institute for Nuclear Physics (JINR), in the Flerov Laboratory of Nuclear Reactions. “Our accelerator in Dubna worked 6-7 thousand hours a year, accelerating calcium-48 ions to about 0.1 speed of light,” explains the scientist. – This energy is necessary so that some of them, hitting the target, overcome the forces of Coulomb repulsion and merge with the nuclei of its atoms. For example, element 92, uranium, will give the nucleus of the new element 112, plutonium 114, and californium 118. ”
Yuri Oganesyan, Academician of the Russian Academy of Sciences, Scientific Director of the Flerov Laboratory of Nuclear Reactions, JINP (Dubna)
“The search for new superheavy elements allows us to answer one of the most important questions of science: where is the border of our material world?”
“Such nuclei should already be quite stable and will not decay immediately, but will sequentially eject alpha particles, helium nuclei. And we know how to register them very well, ”continues Hovhannisyan. A superheavy nucleus will eject an alpha particle, turning into an element two atomic numbers lighter. In turn, the daughter nucleus will lose an alpha particle and turn into a “granddaughter” – four more lighter, and so on, until the process of sequential alpha decay ends with random appearance and instant spontaneous fission, the death of an unstable nucleus in the “Sea of Instability”. Based on this “genealogy” of alpha particles, Oganesyan and his colleagues traced the entire history of the transformation of nuclides obtained in the accelerator and outlined the near shore of the “Island of Stability”. After half a century of sailing, the first people landed on it.
Already in the first decade of the 21st century, the fusion reactions of actinides with accelerated calcium-48 ions have synthesized atoms of elements with numbers from 113 to 118, lying on the shore of the “Island of Stability” far from the “mainland”. Their lifetime is already orders of magnitude longer than that of their neighbors: for example, element 114 is stored not for milliseconds, like the 110th, but for tens and even hundreds of seconds. “Such substances are already available for chemistry,” says academician Oganesyan. – This means that we are returning to the very beginning of the journey and now we can check whether the Periodic Law of Mendeleev is observed for them. Will the 112th element be analogous to mercury and cadmium, and the 114th – analogous to tin and lead? The very first chemical experiments with the isotope of the 112th element (copernicium) showed: apparently, there will be. The nuclei of copernicium, ejected from the target during the bombardment, the scientists directed into a long tube, including 36 paired detectors, partially covered with gold. Mercury easily forms stable intermetallic compounds with gold (this property is used in the ancient technique of gilding). Therefore, mercury and atoms close to it should settle on the gold surface of the very first detectors, while radon and atoms close to noble gases can get to the end of the tube. Obediently following the Periodic Law, Copernicus proved to be a relative of mercury. But if mercury became the first known liquid metal, then copernicium may be the first gaseous: its boiling point is below room temperature. According to Yuri Hovhannisyan, this is only a faint beginning, and superheavy elements from the “Island of Stability” will open up a new, bright and unusual field of chemistry for us.
But for now we have lingered at the foot of the island of stable elements. It is expected that the 120th and the following nuclei may be truly stable and will exist for many years, or even millions of years, forming stable compounds. However, it is no longer possible to obtain them with the help of the same calcium-48: there are no sufficiently long-lived elements that could combine with these ions to give nuclei of the required mass. Attempts to replace calcium-48 ions with something heavier so far have also failed. Therefore, for a new search, the navigational scientists raised their heads and looked at the heavens.
Space and factory
The original composition of our world did not differ in variety: in the Big Bang, only hydrogen appeared with small impurities of helium – the lightest of atoms. All other respected participants in the periodic table appeared in nuclear fusion reactions, in the interiors of stars and in supernova explosions. Unstable nuclides quickly decayed, stable ones, like oxygen-16 or iron-54, accumulated. It is not surprising that heavy unstable elements such as americium or copernium cannot be found in nature.