Nuclear structure and statistical decay properties of closed and near closed shell nuclei
Appears in the following Collection
- Fysisk institutt 
AbstractThis dissertation presents the measurement of level densities and y-ray strength functions of selected warm nuclei in the vicinity of closed and near closed shells. These quantities are important for their application in stellar evolution models in astrophysics and in the design and operation of nuclear reactors. The particle beams are impinged on to the target nuclei at the Oslo Cyclotron Laboratory (OCL). Neutron pick-up and inelastic scattering reactions are used to excite nuclei to high temperatures. Then, the excited states decay down to the ground state by y-emissions. The resulting y-rays in coincidence with the outgoing charged particle are detected and stored. The energy distribution of the first y-rays in each cascade contains the information of level density and y-ray strength function.
The experimental results are compared with different theoretical models over the entire energy region. It is found that level densities and y-ray strength functions in closed and near closed shell nuclei displayed bump structures. These structures can be explained in terms of shell closure effects and breaking of Cooper pairs. In lead nuclei bump structures of the y-ray strength function are found to be more pronounced for completely closed shell nuclei than their neighboring nuclei. The y-ray strength functions of vanadium, scandium and titanium nuclei have shown a low-energy y-ray enhancement, which can not presently be explained by theory. The thermodynamic properties like entropy, temperature, and specific heat capacity of nuclei are deduced from the extracted level densities. The difference of entropy between even-even and even-odd closed shell nuclei is found to be lower than for mid-shell nuclei.
List of papers.
1. A.C. Larsen, R.Chankova, M. Guttormsen, F. Ingebretsen, T. Lönnroth, S. Messelt, J. Rekstad, A. Schiller, S. Siem, N.U.H. Syed, A. Voinov, and S.W. Ødegård, Microcanonical entropies and radiative strength functions of 50,51V, Phys. Rev. C 73, 064301 (2006).
2. A.C. Larsen, M. Guttormsen, R.Chankova, F. Ingebretsen, T. Lönnroth, S. Messelt, J. Rekstad, A. Schiller, S. Siem, N.U.H. Syed, and A. Voinov, Nuclear level densities and gamma-ray strength functions of 50,51V, Phys. Rev. C 73, 064301 (2007). Phys. Rev. C 76, 044303, (2007).
3. A.V. Voinov, S.M. Grimes, A.C. Larsen, C.R. Brune, M. Guttormsen, T. Massey, A. Schiller, S. Siem, and N.U.H. Syed, Level densities of 44Sc and 47Ti from different experimental techniques, Phys. Rev. C 77, 4034613 (2008).
4. N.U.H. Syed, M. Guttormsen, F. Ingebretsen, A.C. Larsen, T. Lönnroth, J. Rekstad, A. Schiller, S. Siem, and A. Voinov, Level densities and gamma-decay properties of closed shell Pb nuclei, Phys. Rev. C 79, 024316 (2009).
5. N.U.H. Syed, Extraction of thermal and electromagnetic properties in 45Ti, submitted to the Oslo Collaboration (2008).