EXPERIMENTAL SEARCH FOR SUPERHEAVY NUCLEI

摘要

The problem of experimental verification of the existence of the hypothetical domain of superheavy nuclides is one of the fundamental outcomes of the nuclear shell model and it has been extensively discussed in recent years. The properties of heavy nuclides, produced in cold-fusion reactions, are in qualitative (in some cases also quantitative) agreement with the theoretical predictions what concerns the significant influence of structure on their stability when approaching the closed shells with Z=108 and N=162. The status of this region of nuclei is given in detail and, in my opinion, in a very interesting way, in the presentation of Prof. S. Hofmann at this conference. The magic nucleus _(108)~(270)Hs and the daughter nucleus ~(266)Sg, resulting in its α-decay, has been synthesized also in the hot fusion reactions ~(248)Cm(~(26)Mg,4n) and ~(248)Cm(~(22)Ne,4n); these two nuclides undergo α-decay with T_α ～ 2-7 s and 21 s, respectively. It should be noted that without the shell effects, and consequently, in the absence of the fission barrier, these nuclei would undergo spontaneous fission (SF) with T_(SF) ～ 10~(-19) s. Another problem is the existence of even stronger spherical shells beyond ~(208)Pb, in the domain of heavier neutron-rich nuclei with Z = 114 (or possibly 120) and N = 184. Here the nuclei are expected to demonstrate highly enhanced stability with respect to all the radioactive decay modes. They should form the so-called "stability island" of superheavy elements. Since 1998 we make attempts to verify this non-trivial theoretical hypothesis.