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.