Mayorov, Dmitriy A (2015-04). Synthesis of Shell-Stabilized Nuclides in Fusion-Evaporation Reactions Induced by 48Ca, 50Ti, or 54Cr Projectiles. Doctoral Dissertation.
Thesis
Production cross sections of nuclides in the vicinity of the closed neutron shell at N = 126 were measured in heavy-ion fusion reactions with 154Gd, 159Tb, 162Dy, and 165Hotargets. Even-Z beams of 48Ca, 50Ti, and 54Cr were chosen for this systematic survey. The resulting shell-stabilized residues vary in deformation and fissility, and the effect of these properties on the magnitude of the cross section is examined. Experimental data were collected at Texas A&M University Cyclotron Institute using the Momentum Achromat Recoil Spectrometer, which was operated as a particle separator. The measured cross sections cover a range from 12 mb to < 1 ub, decreasing in reactions induced by projectiles with successively higher Z. Model calculations, describing the progression of the fusion reaction from projectile-target collision to the ground-state product, were performed by dividing the process into three discrete steps of capture, fusion, and survival. The standard calculations overestimate the measured excitation functions by 0.5-2 orders of magnitude. The predictions are rectified by incorporating collective enhancement of level density into the model, suggesting that the fission probabilities in the deexcitation process of the compound nucleus exceed initial predictions. Hence, a rather weak influence of shell-stabilization on the production cross section of spherical nuclei is deduced. For 48Ca, 50Ti, and 54Cr reactions on the same target, the change in production-cross section is found to strongly depend on the difference between the fission barrier and neutron separation energy of the products and less so on the entrance channel.
