Electron Spectroscopy with the SLICES setup

The subject of this thesis is the setup of a new spectrometer for internal conversion elec- tron measurements, SLICES (Spes Low energy Internal Conversion Electron Spectrom- eter), and its first use in an experiment aimed to study the structure of the low-lying states in 106Pd. Electron spectroscopy is a powerful tool in the investigation of fundamental properties of the atomic nucleus, essential in the study of nuclear structure, where empirical nuclear models provide a framework to understand many nuclear phenomena. For instance shape coexistence, that is, following the Poves [55] states, “ a very peculiar nuclear phenomenon consisting in the presence in the same nuclei, at low excitation energy, and within a very narrow energy range, of two or more states (or bands of states) which: (a) have well defined and distinct properties, and, (b) which can be interpreted in terms of different intrinsic shapes.”. Shape coexistence is a widespread feature that is thought to occur in nearly all nuclei [32]. It is associated with the fundamental tendency of nuclei to deform, if not in their ground states, in their excited states. The distinctive character of shape coexistence lies in the interplay between two opposing trends: shell and subshell closures have a stabilizing effect leading to sphericity while residual interactions between protons and neutrons outside closed shells drive the nucleus to deformation. The study of electric monopole transitions between nuclear states having the same spin and parity, the main topic of this thesis, can give an important contribution to clarify the configuration of the nuclear states. Electric monopole (E0) transitions are determined by a change in the radial distribution of the electric charge inside the nucleus, and high E0 transition strengths are expected whenever configurations with different mean-square charge radii mix. In this regard, an enhancement of the monopole strength in transitions between ∆J = 0 states may be considered as a ”signature” for shape coexistence [34]. This conclusion involves also states with J>0, in fact, as recently pointed out in Ref. [67], J π → J π (J 6= 0) transitions in both even and odd nuclei with shape-coexisting configurations can have large E0 components accompanying their M1+E2 allowed multipolarities. E0 transitions play a crucial role in determining nuclear structure properties and provide sensitive tests for nuclear models, developed since the second half of the 20th century, from the formulation of the Shell Model [48] and the Bohr-Mottelson Hamil- tonian [11] to the Interacting Boson Model [33]. The measurement of the monopole strength is complicated by the fact that E0 tran- sitions can proceed solely via internal conversion or pair production ( if the transition energy is larger than 2m0c 2 ). Simultaneous emission of two photons is a higher order process (relative probability ∼ 10−3 ÷ 10−4 ) and is usually neglected. The measurement of conversion electrons is typically achieved using silicon detectors, drifted with lithium (Si(Li)), coupled with a magnetic transport system. The latter allows a high efficiency for electron collection on the detector together with the powerful rejection of background γ-rays. The design of the different components of the magnetic transport system, studied in detail in this thesis, depends also on the electron energies of interest. In each case, before proposing an experiment, Monte Carlo simulations for different configurations have to be performed. For this reason, a GEANT4 simulation has been implemented to provide useful tools to plan future experiments with SLICES, paving the way to experimental campaigns with radioactive beams. Advances in accelerator and ion-source technologies have made it possible to produce Radioactive Ion Beams (RIBs) and have thus opened many horizons to investigate the structure of exotic nuclei. The Selective Production of Exotic Species (SPES) facility [61], currently under construction at INFN Laboratori Nazionali di Legnaro (LNL), aims to produce RIBs using the Isotope Separation OnLine (ISOL) technique with a particular focus on neutron-rich beams in the vicinity of 78Ni and 132Sn, where detailed nuclear structure information is scarce. The development of an experimental setup to perform electron spectroscopy of radioactive beam represents a crucial requirement for the SPES project. To this end, SLICES has been developed to be used in conjunction with the β-decay station that is envisaged at the SPES low energy beam line. Waiting for the future radioactive SPES beams, the commissioning of SLICES has been successfully performed at LNL using the proton beams provided by the Van de Graaf CN accelerator at LNL. The levels of interest have been populated via EC/β + decays of 106Ag produced in the 106Pd(p,n)106Ag reaction. Following the decay of the 106Ag isotope, the 106Pd isotope was populated in its excited states. In the nu- clear de-excitation process, γ-rays and internal conversion electrons are emitted. In our spectroscopic study, these were detected by an HPGe detector and the SLICES spectrometer, respectively. The measurement was performed alternating irradiation and measurement periods using a movable target remotely controlled with a dedicated control system developed in this thesis. Measurements of E0 strengths for transitions between the first excited states 0+ and 2 + in this nucleus have been achieved. The isotope was carefully chosen: some nuclear observables were known with high accuracy, offering the possibility to have a stringent test for the new setup. However, some observables necessary to the 106Pd structure description had never been measured, and, for others, conflicting results were available in the literature. The theoretical interpretation of Palladium isotopes is still controversial: on one hand, their level schemes have been organized into rotational bands, while on the other hand they have been interpreted as vibrational-like nuclei in the framework of Inter- acting Boson Model (IBM). The predictions of this model have been compared to the experimental values for many nuclear observables e.g. excitation energies, static mo- ment and E2/M1 transition strengths but a systematic study of the E0 strengths is still missing. For this reason, in the present work, IBM calculations of E0 transitions have been performed in order to further clarify the level scheme interpretation of 104,106Pd isotopes. A re-analysis of old ICE data on 104Pd has been performed with the intent to extend our study in the Palladium isotopic chain. The thesis is organized as follows: Chapter 1 introduces the general framework of this work, describing the nuclear observables relevant in the considered experiment, the theoretical models used in the description of the nuclei of interest, and the definition of nuclear shape coexistence phenomenon. The spectroscopic information obtained from the measurements of internal conversion electrons are presented in chapter 2, focusing on the experimental techniques involved in this thesis. Finally, a review of the current experimental and theoretical knowledge on the structure of even-even Pd isotopes is provided. Chapter 3 describes the experimental setup, starting from an overview of the SPES facility and its β-decay station, followed by a detailed description of the SLICES spectrometer components, with particular attention to the tests performed for the Si(Li) detector and the design of the magnetic transport system components. The commissioning experiment of SLICES is presented in chapter 4. Since this was the first experiment using SLICES, a detailed description of the setup implemented for the CN accelerator and of the controls developed in this work is provided. The data reduction and the comparison between the obtained results and previous measurements are presented. In chapter 5 the experimental results are interpreted by performing IBM calculations, followed by a description of the ground state and first excited 0+ state in the context of the two-level mixing model. Finally, the outcome of this work is summarized in Chapter 6.