One of the most exciting subjects in contemporary nuclear physics is the study of nuclei at the limits of stability with respect to particle emission. Recently, there has been an intensive experimental activity in measuring proton decay and a large variety of proton emitters were observed in the region of nuclei with 50 minus Z minus 82. Very recently, the proton radioactivity from 117La and 121Pr has been identified and the decay rate deviates significantly from calculations assuming spherical configurations, thus indicating the onset of large deformations in the drip line nuclei below Z = 69. Predictions by the microscopicmacroscopicmass model and by relativistic mean field calculations support these experimental results and suggest the position of the proton drip line. The lifetimes of the proton-decaying isomers are extremely sensitive to the orbital angular momentum ` of the emitted proton and can vary over several orders of magnitude when changing the angular momentum of the occupied orbital. The position of certain orbitals at the Fermi surface depends strongly on the beta2 deformation. It is therefore very important to determine the quadrupole deformation of the nuclei at and beyond the proton drip line, in order to locate the orbitals close to the Fermi surface and to estimate the lifetime of the isomers, which is essential when looking for new proton emitters. The deformation of the nuclei at the proton drip line in the A 130 mass region can be experimentally tested by comparing the lifetimes of proton emitters with theoretical predictions and by investigating the level structure of particle bound nuclei close to the drip line. For Z = 57 the proton drip line is predicted to correspond to the isotope 118La. The lightest La nucleus for which spectroscopic information has been published is 124La . This thesis bescribes an experiment realised to identify for the first time excited states of the doubly-odd nucleus 122 57 La65. Another aspect that make particularly interesting a study of these nuclei is a well-established phenomenon in two-quasiparticle rotational bands in odd-odd nuclei: the signature inversion. Despite the observation of signature inversion in many different nuclei, and numerous theoretical interpretations, a satisfactory explanation of the phenomenon has yet to be found. We want verify if the experimental level scheme of 122La is in agreement with the systematics of the other nuclei in this mass region. The common feature relating the studies of these neutron deficient nuclei is that they are populated with extremely low cross section and require special techniques to be identified. The standard efficient way to achieve this is the use of arrays of high-purity germanium detectors coupled to specific ancillary devices for their selection. Our experiment has been performed at the Legnaro National Laboratory (LNL) where heavy-ion fusion-evaporation reaction 40Ca(@ 200 MeV)+92Mo was performed to establish the scheme of 122La by using the GASP -ray spectrometer in conjunction with the ancillary charged-particle ISIS detector and the Neutron Ring. This experimental set-up led to numerous achievements in the A 130 mass region. Chapter 1 introduces to the physics of the nuclei close and beyond to the proton drip-line. In Chapter 2 we will briey see the various theoretical models created to describe several types of nuclear excitations and discuss in more detail the cranking model. Chapter 3 describes the experimental techniques of -ray spectroscopy and the setup used for the present measurement. In Chapter 4 the steps followed in the sorting of the raw data to produce particle-gated matrices are described, together with the techniques used to select the different reaction channels and to clean the spectra. Chapter 5 shows the systematic features of the rotational bands of the A 130 proton rich odd-odd nuclei. Chapter 6 will be devoted to the construction of the decay scheme and to the spins assignments, performed through the analysis of gated matrices, transition intensities, DCO, and the comparison with the systematics of the La isotopes. Finally, in Chapter 7, the results are discussed in the light of the deduced experimental quantities like single-particle spin alignment, dynamic moments of inertia, signature splitting, and B(M1)=B(E2) branching ratios to assign the Nilsson orbitals to the found bands.
|Titolo:||Nuclear Spectroscopy near the proton drip-line in the Lanthanide region:the 122La nucleus|
|Data di pubblicazione:||2007|
|Appare nelle tipologie:||Tesi di dottorato (Pregresso)|