Nucleon transfer reactions in the 90Zr+208Pb system

Varga Pajtler, Maja (2014) Nucleon transfer reactions in the 90Zr+208Pb system. Doctoral thesis, Faculty of Science > Department of Physics.

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Abstract

Multinucleon transfer reactions have been studied in the 90Zr+208Pb system close to the Coulomb barrier energy in the Prisma + Clara set-up. Light binary partners have been detected in the Prisma magnetic spectrometer, while the coincident γ rays have been detected in the Clara γ-array. Trajectory reconstruction in the spectrometer provides the identification of the light fragments in their charge, mass and velocity. These allow, together with the coincidences with the Clara array, to attribute electromagnetic transitions and yields to each of the transfer channels. In the 90Zr+207Pb multinucleon transfer reactions, outgoing particles with the atomic number Z = 30 − 40 and mass number A = 60 − 94 were recognized. The level schemes of the presented isotopes have been constructed and updated based on systematics with neighbouring nuclei, and data from literature. In this thesis, we focused on γ spectra of Zr (together with their heavy partner, Pb), Y and Sr isotopic chains. The γ spectrum of each isotope is presented, together with recognized γ rays. Based on these observations, level schemes are constructed. In some cases, new γ rays were observed, and if possible, γ transitions were suggested. The Prisma + Clara spectrometers offer the possibility to measure yields of different transfer products, and to identify nuclei with good mass and charge resolution, even in this rather heavy ion collision. In addition, the semi-classical models which were thus far a good basis for the description of heavy ion collisions in somewhat lighter systems, can be tested for this heavy system. The evidence shows that the most important degrees of freedom are surface vibrations/rotations and single particles, and their coupling. It is through the excitation of these modes that energy and angular momentum are transferred from the relative motion to the mass and vcharge rearrangement and intrinsic excitations of the partners of the collision. The experimental set-up used in the present measurement allowed us to investigate excited states, and to see if their underlying structure reflects the significance of the same degrees as in lighter nuclei. By the study of the Pb isotopes (the heavy binary partner of Zr), the effect of evaporation which strongly affects the primary yields has been discussed. The isotopes produced after the evaporation of one or two neutrons were recognized through their γ rays, while their relative strengths allowed to quantitatively estimate the the effect of the evaporation for the first time in such a heavy system. In our experiment, we were searching for excitation of the 0+ states that were recognized as pairing vibration states, and that were strongly excited in the (t, p) reactions. At the level of the accumulated statistics, we could not undoubtedly confirm the excitation of those states. However, we observed that the important part of the yield is in the excitation of the states whose underlying structure could be connected with the pairing, especially the states of the phonon-fermion coupled nature. We discussed such coupling scheme in odd Zr and Sr isotopes, taking into account the similarities in excitation energies of the odd and even isotopes, as well as the decay patterns and the decay strengths of the involved states. The analysis showed evidences of the particle-vibration coupling. In some isotopes, our conclusions about the importance of the particle-vibration coupling were partially obscured by the fact that the structure of some states of interest display a rather mixed configurations.

Item Type: Thesis (Doctoral thesis)
Keywords: multinucleon transfer reactions, thin-target experiment, nuclear structure, spectra, single-particle states, pairing vibration, phonon-fermion coupling, yrast states, neutron evaporation
Supervisor: Milin, Matko and Szilner, Suzana
Date: 21 October 2014
Number of Pages: 235
Subjects: NATURAL SCIENCES > Physics
Divisions: Faculty of Science > Department of Physics
Depositing User: Gordana Stubičan Ladešić
Date Deposited: 28 Nov 2014 12:29
Last Modified: 20 Jan 2017 14:36
URI: http://digre.pmf.unizg.hr/id/eprint/3402

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