Recent Searches for Superheavy Elements in Deep-Inelastic Reactions

摘要

New attempts have been made to synthesize superheavy elements (SHE) by nuclear reactions that may possibly form the products at low excitation energies. Survival of the superheavy elements would then be enhanced because of reduced losses from prompt fission. Classical and diffusion-model calculations of deep-inelastic reactions indicate there should be detectable yields of SHE formed with less than 30 MeV of excitation energy. Accordingly, superheavy elements have been sought in such reactions where targets of exp 248 Cm and exp 238 U have been irradiated with exp 136 Xe and exp 238 U ions. In the most recent experiments, targets of exp 248 Cm metal (3.5 to 7 mg-cm exp -2 ) were bombarded with 1.8-GeV exp 238 U ions from the UNILAC accelerator. The longer-lived SHE and actinides near the target Z were chemically separated, and the yields of a number of isotopes of Bk, Cf, Es, and Fm were measured. An upper limit of 30 nb was obtained for the formation of 1-h exp 259 No. In addition to the off-line chemical recovery and search for SHE, an on-line experiment was performed to detect volatile SHE with half-lives of a minute or more. All experiments to produce and detect superheavy elements were much less than optimum because of premature failures in the Cm-metal targets. The outcome and status of these experiments and the implications of the actinide yields in estimating the chances for forming superheavy elements in the exp 248 Cm + exp 238 U reactions are discussed. 5 figures, 1 table. (ERA citation 06:003165)