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Isospin formalism, which describes the neutron and the proton as two states of the same particle, the nucleon, is one of the essential descriptive tools of a broad range of nuclear henomena. The success of the isospin symmetry concept belies its broken nature. Not only the symmetry is broken by the proton-neutron mass difference and the Coulomb interaction, but also by the nucleon-nucleon interaction itself. The investigation of isospin symmetry conservation and breaking effects has revealed a wealth of nuclear structure information. Two of the main consequences of the isospin symmetry breaking (ISB) are the differences in the level schemes of mirror nuclei and the possible mixing of states with different values of isospin quantum number. In this thesis, two experiments performed to study these phenomena are presented. The first experiment was performed at the GANIL laboratory, France. It aimed at the study of the excitation energy differences between analogue states in the mirror nuclei 23 Mg- 23 Na. These differences, called MED, should in principle be only of electromagnetic origin and allow to extract detailed nuclear structure information such as the nature of particle angular-momentum re-coupling along yrast structures (e.g. backbending), the evolution of the nuclear radius or deformation and the identification of pure single-particle configurations. Moreover, systematic studies in the f 7/2 shell have shown that electromagnetic ISB terms are insufficient to reproduce the experimental MED values, and an additional isospin non-conserving term of non-Coulomb origin has to be introduced. While this phenomenon has been extensively studied in the f 7/2 shell, the information in the lower sd space is still scarce and more experimental information is needed. High spin states were populated with the 16 O+ 12 C fusion evaporation reaction and evaporated particles and γ rays, emitted in the de-excitation of the reaction products, were detected with the DIAMANT-NEUTRON WALL-EXOGAM setup. New transitions and levels have been identified in the 23 Mg nucleus and iiithe measured MED have been extended up to J = 15/2 + . The experimental data have been compared with shell model calculations. A new approach, which takes into account the isovectorial effect of the neutron skin on the MED, has been successfully adopted. The second experiment described in this thesis took place in the GSI laboratory, Germany. The scope of the measurement was the study of the degree of isospin mixing in the 2 + states of the 46 Cr- 46 V- 46 Ti, T=1 isospin triplet. If the isospin symmetry holds, no mixing would occur and the Coulomb matrix elements, (proportional to B(E2; 0 + → 2 + )), would vary linearly with the third component of the isospin, T z , along the triplet. Therefore, a precise measurement of the 2 + → 0 + transition probability allows to extract information on the degree of isospin mixing in the N = Z member of the triplet, where T=0 and T=1 states coexist. The experimental setup was composed of the γ-ray tracking array AGATA, the FRS fragment separator and the LYCCA calorimeter. Radioactive beams of the nuclei of interest were produced via the fragmentation of a 58 Ni primary beam. The B(E2; 0 + → 2 + ) value for the proton rich 46 Cr nucleus was deduced from the Coulomb excitation cross section on a gold target. The 46 V value was measured using a differential plunger as target, consisting of a triple stack of gold foils. The Doppler shift of the emitted γ rays after each target is different, resulting in a triple-peak structure, whose shape depends on the lifetime of the state. The B(E2; 0 + → 2 + ) value can then be obtained from the measured lifetime. The 46 Ti nucleus was studied with both techniques in order to have a common reference point. In the first part of this thesis the basic concepts of the isospin formalism are introduced. In the second and third chapter, the data analysis, the results, and the interpretation of the data regarding the two experiments are presented. Finally, in the last section, a summary of the work is given and the future perspectives are discussed. |
