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SUPERHEAVY ELEMENTS

The newly constructed Berkeley Gas-Filled Separator was vital for the synthesis of element 118. Its strong magnetic fields focused the superheavy nuclei and separated them from the beam and all other reaction products. Nuclear treasure island

1999 looks to be a vintage year for have particularly favourable production rates. Now that this route has been signposted, similar reactions could be possible: new ele­ "superheavy" nuclei.These heavier-than- ments and , tests of nuclear stability and mass models, and a new understanding of nuclear reactions for the production of isotopes are a 20th-century heavy elements. postcript to the Periodic Table. The 118-, identified at Berkeley, contains 118 and 175 in its nucleus. Less than 1 ms after its creation, it Soon after the experiments at Dubna, which synthesized ele­ decays by emitting an alpha particle, leaving behind an isotope of ment 114 and made the first footprints on the beach of the "island nucleus 116, containing 116 protons and 173 neutrons.This alpha- of nuclear stability" (see pl9), two new superheavy elements have decays in turn to an isotope of element 114.The chain of successive been discovered at the Lawrence Berkeley National Laboratory. is observed until at least element 106. Element 118 and its immediate decay product, element 116, were Vital to the experiment was the newly constructed Berkeley Gas- manufactured at Berkeley's 88 inch cyclotron by fusing targets of filled Separator (BGS). Another important factor was the versatility of -208 with an intense beam of 449 MeV -86 ions. the 88 inch cyclotron, in operation since 1961 and recently Although both new nuclei almost instantly decay into lighter ones, upgraded by the addition of a high-performance ion source. the decay sequence is consistent with theories that have long pre­ It is incongruous that this new transuranic nucleus was discov­ dicted the for nuclei with approximately 114 pro­ ered at Berkeley only a few months after the death of Glenn tons and 184 neutrons. Seaborg, co-discoverer at Berkeley of and nine other ele­ Theorist Robert Smolanczuk, visiting from the Soltan Institute for ments heavier than uranium, the heaviest naturally occurring Nuclear Studies in Poland, had calculated that this reaction should nucleus (CERN Courier May p35). •

18 CERN Courier September 1999 SUPERHEAVY ELEMENTS

Recent experiments that took place at the Joint Institute for Nuclear Research, Dubna, First near Moscow, reported evidence for element 114, the first inhabitant of a new island of nuclear stability. postcard Ever since the discovery of and plutonium almost 60 years ago, physicists have continually sought to synthesize addi­ tional artificial, transuranic elements. Most of these nuclei are highly unstable, but a fundamental prediction says that these superheavy elements would eventually reach an "island of from the 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 island of a -48 beam (total dose 5.2 x 1018 ions), an unusual was recorded by a position-sensitive detector array.This decay chain consisted of a heavy, implanted atom, three sequential alpha decays and a (SF), which altogether lasted for about 34 min (figure 2a). nuclear 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 stability being able to simulate such a decay chain occurring by random coincidence is significantly small.

Fig. 1: Another brick in the wall. The Periodic Table of elements, showing the position of the new element, 114, which can be synthesized by bombarding an isotope of plutonium with another of calcium.

115?: : H7? 58 59 63 64 68 69 70 Til m Tfo Dy4fjH

CERN Courier September 1999 19 SUPERHEAVY ELEMENTS

Dubna gas-filled recoil separator 244 48 The increasing a) Pu+ Ca (DGFRS), which is capable of separating, in flight, the super­ number 3n heavy nuclei evaporation resi­ 289114 292114 should change the V dues from projectiles and other reaction products, was employed shape of the nucleus to extract single atoms. from elliptical to The beam intensity at the 285 112 U400 heavy-ion cyclotron was SpneriCai approximately 4xl012/s - at 8.67 MeV > 242 48 the consumption rate of 0.3mg/h of the unique calcium-48 iso­ 15.4 mmyf^ b) Pi* + Ca tope in the ion source. A follow-up experiment was carried out in March and April (with 3n 287 290 the participation of GSI, Darmstadt; RIKEN, Tokyo; and Comenius 114 114 8.83 MeV University, Bratislava). The objective on that occasion was the syn­ 1.6 min thesis of a new isotope of element 114 with a 287 in '10.29 MeV 1.32 s reactions between calcium-48 and plutonium-242.The VASSILISSA electrostatic recoil separator sifted the reaction products and 190 MeV recorded the decays of the new . 16.5 min g'^* 195 MeV 1 The experiment lasted for about 30 days, which involved a total SF - 9.3min X \ beam dose of 7.5 x 1018.Two similar events were recorded as a short SF 3a 287 2% decay chain. They consisted of a recoil nucleus, an alpha particle 114 114 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 '2.31 MeV(escape) • 14.4 s sequence were correlated in time and position.

Third example 165 MeV1 The spontaneously fissioning emitter (which has a lifetime of about • 3.8 min * * 1.5 min) had been observed in an earlier experiment that was per­ SF c) 23gU + 48Ca formed by the same collaboration in reactions between calcium-48 and uranium-238. On that occasion, the two observed spontaneous T =3.ot^min* SF fission events had tentatively been assigned to the decay of the new isotope of element 112 with mass number 283 (figure 2c). 190 MeV In the latest experiment, the same nucleus has been produced - 3.0 min JC * SF as the daughter product owing to the alpha decay of the mother nucleus of mass 287 and 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. 212 MeV - 0.9 min * \ The half-lives of the new nuclides are estimated to range from SF seconds to tens of seconds.Their daughter nuclei - the decay prod­ Fig. 2: The different decays of element 114 and its daughter ucts - live for minutes: almost a million times as long as the lighter isotopes. isotopes 110 and 112 with neutron numbers 163 and 165. This is exactly in line with theoretical predictions. When approach­ Second attempt ing the closed 184-neutron shell, the increasing neutron number The authors consider this to be an excellent candidate for a decay should change the shape of the nucleus from elliptical to spheri­ chain originating from the alpha decay of a parent nucleus with cal. This spherical shell, coming after the 126-neutron shell in the 114 and mass 289, produced by the evaporation of stable lead-208 nucleus, is so strong that its influence, according to three neutrons from a compound nucleus with a cross-section of the calculations, extends even to those nuclei that have more than about 1 pb.There are plans to make another attempt at obtaining a 170 neutrons, thus increasing their lifetime by many orders of mag­ second event in a forthcoming experiment in July 1999 and to make nitude. a final interpretation of the results then. From this point of view the properties of the new nuclei, synthe­ The experiment was performed in Dubna's Flerov Laboratory of sized in reactions induced by calcium-48, could be considered a Nuclear Reactions in November and December of 1998 in collab­ first experimental indication of the existence of the island of stabil­ oration with the US Lawrence Livermore National Laboratory. The ity of superheavy spherical nuclei. •

20 CERN Courier September 1999 Demanding Technology..

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