Introduction
Our super heavy nuclei are located in the upper part of the nuclide cart (Fig. 1) and have Z>102 (the definition is not very precise). They were syntethised in laboratories all over the world starting from 1943. Short historical overview of discoveries in the field is given in table 1.
The nuclide cartAlong with growing number of protons in nucleus, Coulomb repulsion forces cause decreasing of fission barrier. When this number becomes large enough, Z~114-th, the barrier should completely vanish, and an instantenous break up of a nucleus appears. However, Myers and ¦wi±tecki [Mye66] showed in 1966 that closed shells created by quantum effects ensure existence of the barrier even for nuclei with Z>114. Later, theoretical elaborations, based on the shell model, predicted more precisely that the next closed shells should emerge for spherical nuclei at Z=114 and N=184 (neutron number). Such nuclei would be located in the center of the island of stability of super-heavy elements (SHE). Their half-life times, with respect to the spontaneous fission were estimated from few years to many thousands of years. The calculations also revealed, that alpha radioactivity is the main decay mode of those nuclei [Sob66] and one should expect increase of their half-life times for this mode of decay. Experiments targeted on reaching this hypothetic island of stability were started. Production of the super-heavy elements, Z>104, was accomplished by complete fusion reactions in nuclear collisions by laboratories operating heavy ion accelerators: JINR, GSI, LBNL. Such studies were possible because of the impressive progress made in accelerator technology, and new, highly efficient heavy ion sources. Until year 1988, elements with Z up to 112 were discovered in this way [Hof00] Unfortunately, during those studies it turned out that the cross section for super heavy nuclei production in fusion reactions is decreasing quite fast: with every next Z more or less by factor 4 (Fig. 2), reaching for element with Z=112 about 1 pbarn.
Cross section data and extrapolatedvalues for cold fusion reactions
(1n-evaporation channel) [Hof02]
This was a very serious limitation in the synthesis of next elements. Moreover half-life times of the most heaviest ones were becoming as short as few tens of µs. Such small values of half-lifes shaked the belief in existence of island of stability for the SHE. One of possible explanations for these results was that the produced elements were highly neutron deficient isotopes, and they should in fact live quite short. Simply - available combinations of projectiles and targets could not be used to produce more neutron reach nuclei.
Last discoveries (years 1998-2001) made by Dubna-Livermore collaboration - synthesis of element Z=114 in reaction 48Ca+242,244Pu, and Z=116 (year 2002) in reaction 48Ca+248Cm → 296116 (4n) → 292116, [Oga02]although still need to be confirmed by other laboratories, have given a new pulse to search for next heavy elements, and to synthesise new isotopes of already known elements. In case of both reactions half-life times of produced nuclei, for Z=114 it is few seconds, indicate increase of their stability. This brings new hope for existence the island of stability in the region predicted by theory: Z=114, N=184.
In fact all artificial elements beyond fermium were created by complete fusion of heavy ions. Two types of approches have been used in this case: the cold fusion with bismuth or lead targets and projectiles of the most neutron rich isotopes like 64Ni or 70Zn to produce elements 110 and 112 and hot fusion of actinide targets such as 244Pu or 248Cm with 48Ca projectiles to reach elements 114 and 116 [Oga01].
In case of cold
fusion reactions created super heavy
nuclei have low excitation energy E*=10-15 MeV while for hot
excitation energy is E*=30-40 MeV. In both cases only some of them
can survive as a residue and reach their ground state or isometric
states by 1, or 3-4 neutrons emission depending if the reaction
studied is cold
or hot
Most disintegrate immediately
because fission barrier is nearly equal, or even lower then the
neutron binding energy. As a result, cross section for residue
creation is a product of a cross section for fusion reaction and
probability, that in a cooling phase, fission will not occur.
Besides trying to reach island of stability, studies of super-heavy elements production is important for testing theories that describes reaction dynamics in entrance channel [Blo78]and to better understand decay modes of the SHEs. In the first case, more accurate dynamical theory could propose an optimal energy and projectile-target combination for an experiment. In the second case exploration of decay modes gives better knowledge of the shell model parameters.
At present, research on synthesis of super-heavy nuclei is conducted in laboratories such like: LBNL Berkley, GANIL Cean, GSI Darmstadt, FLNR, JINR Dubna, RARF Riken.
Andrzej Wieloch