Nuclear fission and fusion in a random-walk model
Sammanfattning: This dissertation deals with theoretical descriptions of nuclear fission and synthesis of superheavy elements via fusion. The associated shape evolutions are treated using a random-walk approach where both the potential energy and the nuclear level density influence the dynamics. The dissertation consists of seven original research papers, and an introductory part providing background information and some additional details of the studies.Paper I contains results for fission-fragment neutron multiplicities in 235U(n,f) using an energy partition based on shape-dependent microscopic level densities.Paper II gives results regarding the energy dependence of fission-fragment neutron multiplicities in 235U(n,f), using the same method as in Paper I.Paper III presents calculations of fission-fragment mass and total-kinetic-energy distributions following fission of the fermium isotopes 256,258,260Fm at low excitation energies. A transition from asymmetric fission in 256Fm to symmetric fission in 258Fm is obtained with a correlated large change in total kinetic energy.Paper IV provides results of fission-fragment mass and total-kinetic-energy distributions following fission of even-even nuclei in the region 74 ≤ Z ≤ 126 and 92 ≤ N ≤ 230. An island of asymmetric fission is obtained in the superheavy region where the heavy fragment is found to be close to 208Pb and a corresponding light fragment.Paper V presents calculations of neutron multiplicities from fission fragments with specified mass numbers for events having a specified total fragment kinetic energy in 235U(n,f). With increasing neutron energy a superlong fission mode is found to grow increasingly prominent.Paper VI studies the persistence of the symmetric super-short fission mode versus both particle number and excitation energy of even fermium isotopes 254–268Fm.Paper VII investigates the shape dynamics in the fusion process in production of superheavy elements and how this competes with quasifission.
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