The framework of this Thesis is the study of nuclear structure and reaction dynamics through gamma-spectroscopy measurements in fusion-evaporation reactions. The gamma emission associated with the Dynamical Dipole (DD) mode and the Giant Dipole Resonance (GDR) is a very good tool to study the role of the nuclear interaction in compound-nucleus formation and decay as is discussed in this Thesis with two examples, namely a measurement of DD gamma emission in a N/Z-asymmetric fusion reaction and of GDR decay from a self-conjugate compound nucleus (CN). In fact, DD oscillation is ruled by the symmetry term of the nuclear force that drives the proton and neutron densities toward a configuration which minimizes the potential energy of the system. The hindrance of isospin-forbidden GDR gamma decay from a self-conjugate CN is due to a partial restoration of isospin symmetry at high nuclear temperature, since the excited compound-nucleus lifetime is too short for the relatively weak Coulomb interaction to mix states with different isospin. Two experiments are discussed, both performed at Laboratori Nazionali di Legnaro with the GARFIELD-HECTOR array. This array includes a gamma detector with good efficiency and timing performance, a detector for evaporation residues in order to select the fusion-evaporation channel and a detector for light charged particles accounting for the other components of the decay. The first one is the HECTOR array of BaF2 scintillators, the second one is a group of PHOSWICH scintillators and the third one is the GARFIELD DE-E array of microstrip gas chambers coupled to CsI scintillators. The results obtained with the N/Z-asymmetric 16O+116Sn reaction at beam energies of 8.1, 12 and 15.6 MeV/u display a “rise and fall” trend of the Dynamical Dipole multiplicity as a function of beam energy, with the maximum at beam energy of 12 MeV/u. Data are compared with theoretical calculations of gamma emission performed applying the Bremsstrahlung formula to the dipole oscillation obtained with a Stochastic Mean Field approach based on Boltzmann-Nordheim-Vlasov transport equation. Theoretical calculations display a rather flat dependence on beam energy which calls for further investigations. For the analysis of isospin mixing in 80Zr* a modified version of standard Statistical Model calculation has been used, which includes the appropriate isospin physics in CN population and decay. Theoretical models predict a decrease of isospin mixing with increasing CN temperature and existing measurements confirm a strong hindrance of first-step gamma decay from the self-conjugate CN due to the selection rules for E1 transitions forbidding I=0 to I=0 transitions. The degree of isospin mixing obtained for 80Zr* is consistent with the systematics at finite temperature and with the expected temperature dependence.

NUCLEAR COLLECTIVE MODES AT FINITE TEMPERATURE AS A PROBE OF NUCLEAR STRUCTURE AND DYNAMICS / A.m. Corsi ; Supervisore: Franco Camera ; Coordinatore: Marco Bersanelli. Università degli Studi di Milano, 2010 Dec 10. 23. ciclo, Anno Accademico 2010. [10.13130/corsi-anna-maria_phd2010-12-10].

NUCLEAR COLLECTIVE MODES AT FINITE TEMPERATURE AS A PROBE OF NUCLEAR STRUCTURE AND DYNAMICS

A.M. Corsi
2010

Abstract

The framework of this Thesis is the study of nuclear structure and reaction dynamics through gamma-spectroscopy measurements in fusion-evaporation reactions. The gamma emission associated with the Dynamical Dipole (DD) mode and the Giant Dipole Resonance (GDR) is a very good tool to study the role of the nuclear interaction in compound-nucleus formation and decay as is discussed in this Thesis with two examples, namely a measurement of DD gamma emission in a N/Z-asymmetric fusion reaction and of GDR decay from a self-conjugate compound nucleus (CN). In fact, DD oscillation is ruled by the symmetry term of the nuclear force that drives the proton and neutron densities toward a configuration which minimizes the potential energy of the system. The hindrance of isospin-forbidden GDR gamma decay from a self-conjugate CN is due to a partial restoration of isospin symmetry at high nuclear temperature, since the excited compound-nucleus lifetime is too short for the relatively weak Coulomb interaction to mix states with different isospin. Two experiments are discussed, both performed at Laboratori Nazionali di Legnaro with the GARFIELD-HECTOR array. This array includes a gamma detector with good efficiency and timing performance, a detector for evaporation residues in order to select the fusion-evaporation channel and a detector for light charged particles accounting for the other components of the decay. The first one is the HECTOR array of BaF2 scintillators, the second one is a group of PHOSWICH scintillators and the third one is the GARFIELD DE-E array of microstrip gas chambers coupled to CsI scintillators. The results obtained with the N/Z-asymmetric 16O+116Sn reaction at beam energies of 8.1, 12 and 15.6 MeV/u display a “rise and fall” trend of the Dynamical Dipole multiplicity as a function of beam energy, with the maximum at beam energy of 12 MeV/u. Data are compared with theoretical calculations of gamma emission performed applying the Bremsstrahlung formula to the dipole oscillation obtained with a Stochastic Mean Field approach based on Boltzmann-Nordheim-Vlasov transport equation. Theoretical calculations display a rather flat dependence on beam energy which calls for further investigations. For the analysis of isospin mixing in 80Zr* a modified version of standard Statistical Model calculation has been used, which includes the appropriate isospin physics in CN population and decay. Theoretical models predict a decrease of isospin mixing with increasing CN temperature and existing measurements confirm a strong hindrance of first-step gamma decay from the self-conjugate CN due to the selection rules for E1 transitions forbidding I=0 to I=0 transitions. The degree of isospin mixing obtained for 80Zr* is consistent with the systematics at finite temperature and with the expected temperature dependence.
10-dic-2010
Settore FIS/04 - Fisica Nucleare e Subnucleare
Heavy-Ion Fusion Reactions ; Compound Nucleus ; Giant Dipole Resonance ; Dynamical Dipole ; Symmetry Energy ; Isospin Mixing
CAMERA, FRANCO
BERSANELLI, MARCO RINALDO FEDELE
CAMERA, FRANCO
Doctoral Thesis
NUCLEAR COLLECTIVE MODES AT FINITE TEMPERATURE AS A PROBE OF NUCLEAR STRUCTURE AND DYNAMICS / A.m. Corsi ; Supervisore: Franco Camera ; Coordinatore: Marco Bersanelli. Università degli Studi di Milano, 2010 Dec 10. 23. ciclo, Anno Accademico 2010. [10.13130/corsi-anna-maria_phd2010-12-10].
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/150144
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