Toutes les versions de cet article : English , français
The synthesis and study of new elements (with very large numbers of protons Z) is one of the most challenging prospects in nuclear structure research. Elements beyond Fermium (proton number Z=100 and up to the heaviest element discovered with Z=118) owe their existence solely to quantum effects. These effects are enhanced when the neutron and proton configurations coincide with shell-like structures, which result from the underlying mean field. One of the longest standing, as yet unverified, predictions of nuclear theory is that, for the transfermium nuclei, magicity may be assigned to the neutron number N=184 and proton number Z=114. Note however, that recent calculations suggest that the spherical shell closure would be reached at Z=126, rather than at Z=114. The additional stability, which arises from the shell closure, results in a greater probability that the nuclei in this region may exist for a sufficiently long time to be observed. This region has thus been called an "Island of Stability". Its discovery would represent a major triumph for nuclear science.
However, searching for the limits of the Table of Mendeleev and probing the extension of the nuclear chart is not only a scientific challenge, it is also an experimental one. Indeed, the last discovered elements (Z=111, 112, 113, 114, 115, 116 and 118) have production cross-sections which are extremely small and the identification of a handful of events requires long irradiation times (many months). Luckily, equally important information on the structure of Super Heavy Elements (SHE) can come from the study of lighter deformed transfermium (Z 100-106). The cross-section for the population of these nuclei is many orders of magnitude higher than for Z > 110 so that detailed spectroscopy becomes possible.
Such studies are presently systematically carried out worldwide in only a few laboratories : ANL & LBNL (USA), JYFL (Jyväskylä, Finland), GSI (Darmstadt, germany) and recently, by means of our Franco-Russian collaboration, FLNR-JINR (Dubna, Russia).