Abstract
There are only 3 methods for the production of heavy and superheavy (SH) nuclei, namely, fusion reactions, a sequence of neutron capture and beta(-) decay and multinucleon transfer reactions. Low values of the fusion cross sections and very short half-lives of nuclei with Z>120 put obstacles in synthesis of new elements. At the same time, an important area of SH isotopes located between those produced in the cold and hot fusion reactions remains unstudied yet. This gap could be filled in fusion reactions of 48 Ca with available lighter isotopes of Pu, Am, and Cm. New neutron-enriched isotopes of SH elements may be produced with the use of a 48 Ca beam if a 250 Cm target would be prepared. In this case we get a real chance to reach the island of stability owing to a possible beta(+) decay of 291 114 and 287 112 nuclei formed in this reaction with a cross section of about 0.8 pb. A macroscopic amount of the long-living SH nuclei located at the island of stability may be produced by using the pulsed nuclear reactors of the next generation only if the neutron fluence per pulse will be increased by about three orders of magnitude. Multinucleon transfer processes look quite promising for the production and study of neutron-rich heavy nuclei located in upper part of the nuclear map not reachable by other reaction mechanisms. Reactions with actinide beams and targets are of special interest for synthesis of new neutron-enriched transfermium nuclei and not-yet-known nuclei with closed neutron shell N=126 having the largest impact on the astrophysical r-process. The estimated cross sections for the production of these nuclei allows one to plan such experiments at currently available accelerators.
Highlights
For the more asymmetric 48Ca induced fusion reactions rather constant values of the cross sections for the production of SH elements up to Z=118 were found [3]. This unusual behavior of the cross sections has been predicted and explained in [4, 5] by the relatively slow decrease of the fusion probability and by the increasing survival probability of compound nuclei (CN) owing to increasing values of their fission barriers caused by the larger shell corrections as the CN approach the neutron and proton closed shells in the region of the island of stability
Extension of the area of known isotopes of SH elements is extremely important for better understanding of their properties and for developing the models which will be able to predict well the properties of SH nuclei located beyond this area
We found that the optimal beam energy for the production of neutron-rich isotopes of SH elements in multinucleon transfer reactions with heavy actinide nuclei is very close to the energy needed for these nuclei to reach the contact configuration
Summary
Due to the bending of the stability line toward the neutron axis, in fusion reactions of stable nuclei one may produce only proton rich isotopes of heavy elements. This unusual (at first sight) behavior of the cross sections has been predicted and explained in [4, 5] by the relatively slow decrease of the fusion probability (in contrast to the more symmetric “cold” fusion reactions) and by the increasing survival probability of compound nuclei (CN) owing to increasing values of their fission barriers caused by the larger shell corrections as the CN approach the neutron and proton closed shells in the region of the island of stability These predictions have been fully confirmed by the experiments performed in Dubna and later in Berkeley [6] and at GSI [7, 8]. The elements (with Z > 120) being synthesized in such a way might be already beyond this natural time limit for their detection (see the right panel of Fig. 1)
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