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| Jurij Colakovič OGANESJAN | Michail ITKIS |
13.03.2005 18:20:56
Transuran s atomovým číslem 114
114 - Uuq - Ununquadium
Výzkumníci společného Spojeného ústavu pro jaderný výzkum v Dubně (JINR, Joint Institute of Nuclear Research) v získali na sklonku roku 1998 prvek s atomovým číslem 114.
V lednu 1999 oznámila skupina z Dubny, že se jí podařilo v reakci 48Ca s terčem z 244Pu syntetizovat 289Uuq (Z = 114). Ze složeného jádra byly přitom emitovány pouze tři neutrony. Izotop se rozpadal třemi po sobě následujícími alfa přeměnami zakončenými spontánním rozštěpením. Z naměřených časových prodlev mezi nimi byly odhadnuty poločasy rozpadu u 289Uuq a jeho dceřiných jader. Ty byly poměrně dlouhé – desítky sekund až minuty. Že bychom se už dostali na dohled k předpověděnému ostrovu stability?
Pracovníci skupiny z Dubna:
Yu. Ts. Oganessian, V. K.
Utyonkov, Yu. V. Lobanov, F. Sh. Abdullin, A. N. Polyakov, I. V.
Shirokovsky, Yu. S. Tsyganov, G. G. Gulbekian, S. L. Bogomolov, B.
N. Gikal, A. N. Mezentsev, S. Iliev, V. G. Subbotin, A. M.
Sukhov, O. V. Ivanov, G. V. Buklanov, K. Subotic, M. G. Itkis, K.
J. Moody, J. F. Wild, N. J. Stoyer, M. A. Stoyer, R. W. Lougheed,
C. A. Laue, Ye. A. Karelin, A. N. Tatarinov.
First postcard from the island of nuclear stability
Recent experiments that took place at the Joint Institute for Nuclear Research, Dubna, near Moscow, reported evidence for element 114, the first inhabitant of a new island of nuclear stability.
Ever since the discovery of neptunium and plutonium almost 60
years ago, physicists have continually sought to synthesize
additional artificial, transuranic elements. Most of these nuclei
are highly unstable, but a fundamental nuclear physics prediction
says that these superheavy elements would eventually reach an
"island of stability" (figure 1).
This intriguing hypothesis, which was proposed more than 30
years ago and has since then been developed intensively, seems to
have received recent experimental confirmation at the Joint
Institute for Nuclear Research in Dubna near Moscow.
In a 34 day bombardment of a heavy target of plutonium-244
by a calcium-48 beam (total dose 5.2 x 1018 ions), an
unusual decay chain was recorded by a position-sensitive detector
array. This decay chain consisted of a heavy, implanted atom,
three sequential alpha decays and a spontaneous fission (SF),
which altogether lasted for about 34 min (figure 2a).
All five of the signals were correlated in time and position. The large values of the alpha-particle energies and the long decay times, in addition to the termination of the sequence by spontaneous fission, provide evidence for the decay of nuclei with large atomic numbers. Under the experimental conditions given, the probability of being able to simulate such a decay chain occurring by random coincidence is significantly small.
Second attempt
The authors consider this to be an excellent candidate for a
decay chain originating from the alpha decay of a parent nucleus
with atomic number 114 and mass 289, produced by the evaporation
of three neutrons from a compound nucleus with a cross-section of
about 1 pb. There are plans to make another attempt at obtaining
a second event in a forthcoming experiment in July 1999 and to
make a final interpretation of the results then.
The experiment was performed in Dubna's Flerov Laboratory of
Nuclear Reactions in November and December of 1998 in
collaboration with the US Lawrence Livermore National Laboratory.
The Dubna gas-filled recoil separator (DGFRS), which is capable
of separating, in flight, the superheavy nuclei evaporation
residues from projectiles and other reaction products, was
employed to extract single atoms.
The beam intensity at the U400 heavy-ion cyclotron was
approximately 4 x 1012 /s at the consumption rate
of 0.3 mg/h of the unique calcium-48 isotope in the ion source.
A follow-up experiment was carried out in March and April (with
the participation of GSI, Darmstadt; RIKEN, Tokyo; and Comenius
University, Bratislava). The objective on that occasion was the
synthesis of a new isotope of element 114 with a mass number 287
in reactions between calcium-48 and plutonium-242. The VASSILISSA
electrostatic recoil separator sifted the reaction products and
recorded the decays of the new nuclides.
The experiment lasted for about 30 days, which involved a
total beam dose of 7.5 x 1018. Two similar events were
recorded as a short decay chain. They consisted of a recoil
nucleus, an alpha particle emitted a few seconds later and final
SF with a half-life of a few minutes (figure 2b). In each case,
all three signals of the decay sequence were correlated in time
and position.
Third example
The spontaneously fissioning emitter (which has a lifetime of
about 1.5 min) had been observed in an earlier experiment that
was performed by the same collaboration in reactions between
calcium-48 and uranium-238. On that occasion, the two observed
spontaneous fission events had tentatively been assigned to the
decay of the new isotope of element 112 with mass number 283 (figure
2c).
In the latest experiment, the same nucleus has been produced
as the daughter product owing to the alpha decay of the mother
nucleus of mass 287 and proton number 114. The atomic numbers of
the synthesized nuclei will be determined chemically. The first
experiment, which is aimed at the chemical separation of element
112, is now being prepared.
The half-lives of the new nuclides are estimated to range
from seconds to tens of seconds. Their daughter nuclei the
decay products live for minutes: almost a million times as
long as the lighter isotopes 110 and 112 with neutron numbers 163
and 165.
This is exactly in line with theoretical predictions. When
approaching the closed 184-neutron shell, the increasing neutron
number should change the shape of the nucleus from elliptical to
spherical. This spherical shell, coming after the 126-neutron
shell in the stable lead-208 nucleus, is so strong that its
influence, according to the calculations, extends even to those
nuclei that have more than 170 neutrons, thus increasing their
lifetime by many orders of magnitude.
From this point of view the properties of the new nuclei,
synthesized in reactions induced by calcium-48, could be
considered a first experimental indication of the existence of
the island of stability of superheavy spherical nuclei.
114 ELEMENT: ANATOMY of
SCIENTIFIC SEARCH
Scientists not only from Russia but from the USA, France, Germany and Japan as well have attempted many times to synthesize 114 element. But everything was in vain, the result was null. So the opinion has become to form that theoretically reasoned heavy ions island of stability is not only unreachable but its existence is a matter of doubt.
But nevertheless it was decided to try "to reach". Four years were spent to enhance considerably - 500 times - experiment sensitivity. The facilities were improved and perfected and much experience gained since last experiment on 114 element synthesis in Darmstadt in 1985. It gave some assurance and hope for success.
To get sensitivity necessary for experiment success it was decided to throw away the old, having world record intensity of the heavy ion beams, facility and to create the new more perfect one.
Statistics and family tieds
The first experiment with high intensity beams was performed in March 1998. That time uranium was irradiated with 48Ca at VASSILISSA facility. During 25 days experiment two events have been observed and they looked very much alike 112 element. It was reported to press and at the international conference in the USA. But it didn't seem to be accepted. The fact is that lifetime of "our" isotope of 112 element was much more than of the one got in Darmstadt. Such result doesn't contradict the theory but the Dubna isotope was 6 neutrones heavier the Darmstadt one and this gave it an extra stability extending lifetime several hundred thousand times. But it is generally accepted to consider a result to be the most reliable if in it either enough number of nuclei has been got (statistics acquired)or not only a nucleus has been synthesized but "genetic" chain of its daughter-nuclei alpha decays has been traced. But in our experiment only spontaneous fission has been observed. Nevertheless our result encouraged us to commence new experiment.
On the "stability mountain" sides
The experiment lasted for 40 days and was completed on the 31st of December 1998. Synthesis was performed at Gas-filled Separator(GFS)in the fission reaction of 48Ca and 244Pu (the heaviest isotope, it has been given by our collaborants of the Livermore National Laboratory of USA; the target has been prepared by our collegue G.V.Buklanov). It was expected to get in the reaction the heaviest of all possible isotope of 114 element with 175 neutrones in the nucleus.
On 25 December some results of the experiment has been analysed. What should be seen in case the nucleus existed after all? What way could it "roll off" the "stability mountain" side? After the first alpha decay 114 element, having lost two neutrons and two protones, becomes 112 one (that is "the daughter" in the genetic chain), after the second alpha decay it becomes 110 one ("the grand-daughter"), after the third -108 one ("the great-granddaughter"),and so on untill spontaneous fission breaks the chain. So it was necessary among the variety of signals to search for decays ended with spontaneous fission.
Elephants bear elephants
Spontaneous fission is a very convenient phenomenon for search: in alpha decays detector energy release is equal approximately to 10 MeV but during spontaneous fission it comes up to 200. So the problem is to find detector registered large signals and "to trace" their prehistory back. It is possible to realise because the separator outlet detector is a position-sensitive device. As a rule certain characteristics of the nuclei passed through the separator such as speed, energy, coordinates and time are being registered. Later during the certain time interval the repeated signals appearance is being traced in the coordinate points where the nucleus arrival has been registered. They are just its decay traces and according to them the "genetic" chain is being arranged.
Fortunately only several spontaneous fission large signals have been registered. Twice fission has happened in a thousandth of a second after nuclei had been implanted in the detectors. These are well known nuclei (so called spontaneously fissioning isomers, they were discovered at our laboratory in 1962) - Americium-244, they appear in such quantity that separator is not able to sift them entirely. But those nuclei have given evidence that everything worked well.
And at last both spontaneous fission fragments appeared, and earlier - four signals more. First-recoil nucleus, in 30 seconds-the first alpha decay, in 15 minutes- the second alpha decay, in 2 minutes-the third and at last in 17 minutes- the spontaneous fission signal. That means that all decay chain takes 34 minutes- giant time for nuclear physics! Judging by 112 that lives 15 minutes or 110 that lives 2 minutes it is possible to say that theoretical prediction of stability island existence is being confirmed. And in spite of the fact that we did not reach the peak but only the offshoots of stability mountain the nucleus already senses this giant shell influence and takes more stable spherical shape. Because of it its lifetime increases greatly. This is not an exception to the rule, this is the rule. That's why the reliability of new element synthesis is confirmed by the following "genetic" chain: 114 element lives for half a minute, its "daughter" - for 15 minutes, "the grand-daughter" - for 2 minutes, "the great-granddaughter" - for 17 minutes, that means that long-livers bear long-livers, as well as elephants bear elephants.
Let them repeat
114 element was discovered in Dubna, but it might happened in Darmstadt or in Berkly as well because the scientists approached the result together, sharing experience, information, taking into consideration mistakes and results of each other. "114 element is of tremendous importance for superheavy elements physics research,"-said Prof. Giorso from Berkly. The result is very important in deed so its reliability must be very high.
114 element synthesis repetition is in active preparation in Berkly now. They have got some extra financing, staff, made principally the same gas filled separator, new ion source. The experiment is scheduled for this Autumn. The same experiment is scheduled for this Summer in Dubna.
It's for the benefit to study landscape
It was decided to repeat the experiment. But the conditions have been changed: instead of 244Pu was used 242 Pu made in Russia; the experiment was performed at VASSILISSA facility by the other team and in different collaboration (with GSI of Germany and RIKIEN of Japan); only high intensity 48Ca beams were left. New and lighter (by 2 mass units)isotope was expected to have such characteristics: it should be a short chain of 114 (living for a few seconds) and daughter, more stable 112 element (living for a few minutes).
Experiment at VASSILISSA lasted since 2 March till 4 April: during these 32 days two events of production and decay of 114 element have been observed. In both cases they were short chains indeed: new isotope of 114 existed for a few seconds, then alpha decay followed, produced 112 element existed for 3 minutes then its spontaneous fission followed.
So as a result of two different experiments 6 long-livers nuclei have been observed:114, 112, 110, 108 elements during the first experiment and 114 and 112 during the second one. This test seems more valuable because if we indeed got to the stability island then to confirm its existence it's necessary not only "to jump" on the same place but "to walk" along it making sure that this is the right sort of landscape.
Hundred million years old nuclei may exist somewhere
And further there are two ways to go, each attractive in its own way. If new elements lifetime is measured by minutes then it is possible to study their chemical properties. This year it is planned to study properties of 112 element (analogous to mercury). This is the beginning of the large program on superheavy elements. The scientists from many laboratories of Germany, France, Swiss, the USA and Japan are expected to take part.
The second way is to irradiate with 48Ca beams Americium instead of Plutonium, for example, getting the chain of 115, 113, 111 elements or Curium, waiting for 116, 114, 112 nuclei... In short to move along the stability island synthesising new elements, determining their radioactive and chemical properties.
If during nucleosynthesis the nuclei with 184 neutrones (at stability peak) had been produced and had the lifetime longer than 100 million years they could survive in nature till our days. So there is the reason to return to search for superheavy elements in nature. G.N.Flerov devoted ten years of his active life to such experiments. That time 114 element (analogous to lead) was considered the most long-lived and so lead materials were mainly investigated. High sensitivity had been reached but nothing had been observed. Our theory predicts and it follows out of our experiments that 108 element, analogous to osmium or to be more exact its isotope having about 180 neutrons, must be the most stable. But there was no search for superheavy elements among osmium minerals yet.
That's why it's so important to know chemical properties of superheavy elements: ekaosmium, ekaplatinum, ekamercury - because search for any rare element in nature is based on enrichment. In other words success in superheavy elements properties research may serve as a basis for search for these elements in nature, underground, away from space beams where it is possible to watch natural decay of superheavy nuclei...
