Heavy and superheavy element research

Progress in nuclear research has resulted in the discovery of many new and very heavy nuclides during the past few years. From bombardments of 254Es targets with heavy ions, the isotopes 260Md, 261Lr, 262Lr, and 262No have been produced and identified. The nuclide 262No contains more neutrons than any previously known isotope. At the Gesellschaft fuer Schwerionenforschung, Darmstadt, the new elements 108 and 109 have been found, while at the Dubna Laboratory in the USSR, tentative evidence for the discovery of elements 110 as 272[110] has been presented. A major outcome of these discoveries is that, in defiance of all expectations, spontaneous fission is no greater a factor in limiting their nuclear stability than it is for lighter transplutonium model. Theorists now estimate that increasing nuclear stability will be found for nuclei with neutrons numbers ≤ 163 because of a strong neutron subshell at N = 162. Experiments have been made to detect this anticipated gain in stability by searching for 272[109] and 266[107] but, so far, without success. In other advances, spontaneous fission proceeding by two separate modes (bimodal) has been observed for five of the heaviest isotopes of Fm, Md, and No. Shell structure in both the fissioning species and the fragments appears to account for this unique phenomenon. This discovery has brought about renewed interest in the physics of nuclear fission because of new insights it has brought into understanding of the fission process