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Physicists working at the Georgi Flerov Laboratory in the Dub-na- based Joined Institute for Nuclear Research have scored an outstanding result-they were the world's first research team to synthesize the superheavy 116th element of the Mendeleyev Periodic Table. The laboratory's research supervisor, RAS Corresponding Member Yuri Oganesyan, supplies the details.
Looking at the map of chemical isotopes that lists the number of protons and neutrons in the atomic nucleus, we may identify three regions. The largest one, a "land of stability" so to speak, is composed of elements ordered from hydrogen to bismuth.* The heaviest elements are located within a small "peninsula of stability", with only thorium and uranium discovered here on earth. The point is that isotopes of the subsequent transuranium elements are radioactive and subject to alpha and beta decays. Their lifetime decreases at a logarithmic rate with an increase in the atomic number. For instance, from uranium (92nd element) to fermium (100th element) the stability of nuclei drops by more than 20 orders, and then this downturn keeps increasing even more. Why?
Here we come to deal with yet another radioactivity, the spontaneous nuclear fission. Although this kind of fission is but a rare phenomenon for uranium (it was discovered by K. Petrjacque and G. Flerov in 1940), it is quite natural for the subsequent elements.
All the way back in 1939 the Danish physicist Niels Bohr (Nobel prize winner, honorary member of the USSR Academy of Sciences) and George Wheeler of the United States predicted such kind of possibility. They hypothesized that a process like that could occur if nuclear matter possessed the characteristics of structureless matter- say, as if it were a drop of charged liquid. Deformed under the action of electric forces, it could have its potential energy increase up to a certain limit (barrier) and then fall irreversibly with the further deformation until this "droplet" split in two.
Actually, the uranium barrier keeps the nucleus from fission for as long as 10 16 years. With the heavier elements, in the nuclei of which the Coulomb forces are much higher and the probability of fission rises significantly, the barrier goes down and then disappears altogether. By Bohr's and Wheeler's estimates, such a phenomenon can affect the elements with the atomic numbers of 104 to 106.
However, Dr. Oganesyan goes on to say, in 1964 his lab researchers discovered - quite unexpectedly - yet another half-life period in heavy nuclei. So one and the same element may have two single-type decays of different probability or two lifetimes. For uranium, as said above, this is 10 16 years (according to Petrjacque and Flerov) in the first case, and it is very short, a mere 0.3 ms - in the other. Such half-lives imply two states of the nucleus which are responsible for fission. This situation does not square with the analogy of a charged liquid droplet. A body may possess two states unless it is amorphous and structureless. This is fully true of the nuclei of transuranium elements.
Nuclear physicists had to grapple with a rather difficult problem - explain why uranium has a double-humped barrier and how the structure of its nucleus changes as a result of deformation. It was predicted in theory that in heavy elements the structure should be fully manifest where the droplet analogy was irrelevant, and there would appear a structural barrier so-called. In that case a nucleus should live on for a long time, sometimes even for a very long time. This nontrivial theoretical conclusion led us actually to the prediction of the third hypothetical region of stability for superheavy elements mapped a good way off from isotopes known to us.
This idea may be proved or disproved only experimentally. So all major laboratories of the world launched appropriate experiments. Yet the results were negative, among them those obtained in the United States, France and Germany. Dubna physicists also attempted to synthesize heaviest elements with atomic numbers 114 and 116. Our
* See: Ye. Molchanov, "Searching for 'Islands of Stability'", Science in Russia, No. 3, 1999 . - Ed.
aim was to obtain atoms with nuclei having a significant excess of neutrons. Only this way (says the theory) could we increase the lifetime of superheavy nuclei and have the hypothetical "island of stability" well within our reach.
In their first experiments various research centers all over the world (including those in the Soviet Union) attempted to irradiate the source material (uranium, plutonium) with a powerful neutron flux on upgraded reactors. Yet this mode of synthesis (fusion) failed to deliver on fermium - its isotope of mass 258 had a lifetime of only 0.3 ms. The entire chain of sequential neutron capture broke up at the 24th step (which means that 24 neutrons were trapped instead of the required 60). Attempts made by our American colleagues to obtain superheavy elements in nuclear blasts, that is in a powerful pulsed neutron flux, likewise resulted in the formation of an isotope of element 100 with a mass of 257.
Dead-end prospects of the neutron method, Dr. Oganesyan points out, made us think of an essentially new method of fusion first tried in the mid-1950s. The idea boiled down to this: have two heavy nuclei collide and fuse into an entity with a total mass of the components. But for this purpose one of the two nuclei had to be accelerated to a speed equal to about 0.1 of that of light. This could be achieved only with the aid of heavy ion accelerators having a very high flux power.
Synthesizing a neutron-excess nucleus is a stupendous job no one has ever accomplished. In our Dubna laboratory we decided to use reactions in which a large excess of neutrons was preassigned both in the "target" and in the "missile nucleus". In this latter capacity nuclei of calcium-48 were chosen. It is a stable isotope with an atomic number 20. Yet this is a very rare isotope (ordinary calcium contains only 0.18 percent of it) exceedingly hard to isolate. Yet we did the job. Thereupon we could, by irradiating uranium, plutonium or curium, get into the coveted region promising a rise in the stability of nuclei of superheavy elements and, consequently, achieve a dramatic increase in their lifetime.
Our Dubna experiment involved an isotope of plutonium (element 94) with a mass of 244 ("target"), and calcium-48, the "missile" bombarding the "target". The nuclear fusion reaction was to produce element 114, much more stable than lighter elements obtained in other reactions.
It was all right with the "target" and the "missile". But the accelerator posed problems because of the all too rigorous standards. To begin with, it was to sustain a calcium-48 flux dozens of times superior to what had been obtained before; it was to generate high-intensity accelerated ions and expend the least of costly substances.
So it took us five years to built the required setup, and it was commissioned at Dubna in 1998. It combined a very low expenditure of the substance (0.3 mg/h) with a rate of 5 x 10 12 ions/s. Thus we could carry out experiments a hundred and thousand times as sensitive as those conducted in the previous 25 years.
What was the gist of our experiment? Obtaining a flux of calcium ions, we targeted it at plutonium-244 (this isotope was donated by the Livermore National Laboratory of the United States). We hoped that the fusion of two nuclei would produce atoms of a new element; these atoms, knocked out from the target, would keep moving together with the flux. Next, they had to be separated from calcium-48 ions and other reaction products by means of a separator where a transverse electric field was induced. Since the rates of the nuclei were not equal, the flux came to be arrested in the absorber, and the heavy nuclei of element 114 circumvented it and kept moving along the curvilinear trajectory and ultimately reached the detector identifying the heavy nuclei and registering their decay.
Now, what did we want? If the "island-of-stability" hypothesis held for superheavy elements and their nuclei proved stable with respect to spontaneous fission, we would have the alpha-decay. That is to say, a nucleus should eject an alpha-particle composed of two protons and two neutrons and move into a daughter nucleus. In our reaction element 114 was to move into element 112 and so forth. Yet the "vertex" of stability proved elusive in a succession of alpha-decays, and finally what we got was a "sea of instability" in which spontaneous fission predominated. A vivid picture unfolded before our very eyes: each of the escaping alpha- particles released an energy of about 10 MeV, with the energy of spontaneous fission equal to 200 MeV. Here the decay chain broke up.
This experiment that continued non-stop for three months confirmed our theoretical predictions. With a superheavy nucleus in the detector, a 9.87 MeV alpha-particle was registered one second after the arrest of the flux (in fact, the analogous time in the heaviest nucleus synthesized previously was equal only to 0.0002 s). Thereupon, 10.3 seconds after that, another alpha-particle escaped, its energy equal to 9.21 MeV; and in yet another 14.5 a spontaneous fission took place.
All that occurred on the 25th of June 1999. And on the 25th of October of the same year we repeated this experiment with much success; the results of both experiments proved absolutely identical in thirteen parameters. The probability of random coincidences of signatures imitating such decay in the detector is thus equal only to 10 -16 .
What we saw was this: isotopes of elements 114, 112 and 110 (in that order) were acted upon by structural forces forming an "island of stability" of superheavy elements. This stability, if compared with isotopes of the selfsame elements having 6 to 8 neutrons less, increases by five orders.
But, Dr. Oganesyan says, we were in no hurry to end our experiments
and made an attempt at synthesizing element 116; as target we took an isotope of the 96th element, curium, with a mass of 248 (this unique substance was obtained on a powerful reactor at the Research Institute of Atomic Reactors in the town of Dmitrovgrad) plus the selfsame calcium-48. The observable chains of element 116 decays, we figured, would be yet another proof that element 114 was synthesized indeed. To make assurance double sure, we planned to switch off all power units in the laboratory with the release of the first alpha-particle so as to exclude any background effects.
That's what happened indeed. An alpha-particle of energy 10.56 MeV escaped 47 ms after a heavy nucleus got into the detector, and all the power units were off. Then the process developed quite smoothly and concurred with the previous experiment in all its parameters. As I have already said, this event (synthesis of element 116) took place on July 19, 2000; it was restaged on the 2nd and 8th of May 2001. There was no doubt at all: Dubna is the birthplace of the superheavy elements, 116 and 114.
Now let us collate our theoretical forecasts with the experimental data. In theory, for element 116 the lifetime was to increase by five orders with an increase in the number of neutrons in the nucleus from 166 to 176; the actual increase, however, was six orders of magnitude. We could observe the same picture for subsequent isotopes as well. So from now on they could be studied by radiochemistry methods, which makes it possible to conduct experiments on verifying Dmitry Mendeleyev's basic law on unification of chemical properties in laboratory columns. What concerns the superheavy elements, we can now well assume that element 112 is related to cadmium and mercury; element 114-to tin, lead and so one. This simple extrapolation to hitherto unknown elements can now be verified experimentally.
In conclusion, Dr. Oganesyan called attention to the following aspect: theoretically, there should be an "island of stability" in very heavy elements whose nuclei are neutron-rich. Since experiments showed an excess of stability over the computed values even on the "slopes" of this "island", it is not ruled out that the "vertex" could have chemical elements with a lifetime of millions of years - elements found somewhere in stellar formations.
Yu. Oganesyan, "A New Region of Nuclear Stability", Vestnik RAN (RAS Bulletin), Vol. 7, 2001
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