ACTINIDE PRODUCTION FROM THE INTERACTIONS OF CA-40 AND CA-44 WITH CH-248 AND A COMPARISON WITH THE CA-48 + CM-248 SYSTEM

Excitation functions have been measured for production of isotopes of Th through Fm in bombardments of Cm-248 with 231 - to 323-MeV Ca-44 ions and for the production of isotopes of Th through Cm in bombardments of Cm-248 With 230- to 291-MeV Ca-40 ions, respectively, using radiochemical methods. Upper production cross section limits were established for nuclides that were not positively detected. The experimental data were compared with the results of previous reaction studies in the systems Ca-40 + Cm-248 and Ca-48 + Cm-248. The half-widths of the Gaussian isotopic distributions were about 2.5 mass numbers for above-target elements and 5 to 5.5 mass numbers for below-target elements in all three systems. The majority of the cross section for the production of above-target nuclides was assigned to predominantly quasielastic reactions, whereas below-target nuclides were formed in deeply inelastic and asymmetric quasifission reactions. The maxima of the isotopic distributions were shown by others to closely follow the minimum of the potential energy surface in experiments with Ca-48, whereas in reactions with Ca-40 and Ca-44 we found that a transfer of 4 to 5 protons in either direction was required to reach the minimum in potential energy. The different neutron numbers of Ca-40, Ca-44, and Ca-48 are only partly reflected in the target-like reaction products. Based on a simple model, excitation energies and the maxima of the excitation functions were calculated and compared with the actual data. The yields for production of the below-target elements Th, U, and Pu were found to be two orders of magnitude smaller in the reaction of Ca-40 with Cm-248 than with Ca-48. The differences for production of above-target elements were largest between the reactions of Ca-44 and Ca-48 ions with Cm-248. Differences in below-target yields have been attributed either to losses of reaction products due to prompt fission, and/or to dynamic effects due to fusion hindrance. The data for above-target elements seem to confirm the presence of a strong isospin driving force, which in combination with favorable reaction energies results in surprisingly high yields in reactions with Ca-44 projectiles.