L’IPHC | Thèses et stages Master » Les Doctorants de l’IPHC / The PhD Students of the IPHC » Pages pro » Anciens doctorants » GOASDUFF Alain
Dernière mise à jour dimanche 1er juillet 2012, par
Institut Pluridisciplinaire Hubert Curien - IPHC
Département de Recherche Subatomique - DRS
Centre National de la Recherche Scientifique - CNRS
UMR 7178 CNRS/Université De Strasbourg - UDS
23 rue du loess, 67037 Strasbourg cedex 2, France
Contact : Alain GOASDUFF
Titre de la thèse : États intrus dans les noyaux de la couche sd : de 1p-1t à np-nt dans les isotopes de Si
Période : 2009-2012
Encadrant(s) : Sandrine COURTIN
Nuclear shell-model and γ-spectroscopy I worked with a new full 1hω interaction for the sd-nuclei which allows us to reproduce positive and negative parity states with an overall good agreement for all sd-nuclei. This interaction was used to predict lifetimes in N=20 nuclei especially in S, P and Si isotopes. Those predicted lifetimes fall in the range of differential Doppler shifted method. Using the AGATA demonstrator, a high resolution segmented HPGe detector array, coupled to the large acceptance magnetic spectrometer PRISMA (LNL, Legnaro) we measured low lying state lifetimes of neutron rich isotopes of S, P and Si. Moreover, these isotopes and in particular the Si isotopes are at the boundaries of the so called island of inversion where higher order excitations appear in low lying states wave functions. Indeed recent shell-model calculations have shown that 2hω excitations represent more than half of the wave functions of the first states in the 34Si. I am in charge of the analysis of the PRISMA part of the experiment and the Si isotopes for the Strasbourg-Paisley collaboration.
Nuclear reaction and structure Going beyond the actual capabilities of shell-model calculations, the high excitation energy state structures of 28Si were investigated. 28Si is a unique nucleus in the sd shell, indeed it displays a rich variety of structures : several deformations at low energy as well as the appearance of collective states, cluster states, when increasing the excitation energy. To explore those large deformations, we used the radiative capture process between heavy ions, in which the compound nucleus decays solely by γ-ray emission. Those states corresponding to nhω excitations cannot be reproduced by actual shell-model calculation but decay though γ-ray emissions to low lying states. γ spectroscopy is therefore very interesting to understand the transition between cluster structure and mean-field structure in the nuclear system. To reduce the number of open channels, reactions were performed at resonance energies below the Coulomb barrier. Using the high efficiency BGO array associated to the 0° DRAGON spectrometer (TRIUMF, Vancouver) full γ-spectra of the explored resonances were observed. For this experiment, I was in charge of the complete analysis of the complex experimental data and their comparisons with numerical simulations of the complete detection setup. These were performed with GEANT3. I included the γ-ray angular distribution in the simulations in order to better reproduce the recoil acceptance. I also extracted radiative fusion cross sections at these two sub-barrier energies which are more important than what was previously observed, because of the previously unobserved feeding of states around 11 MeV.
Nuclear reactions The two previous part build up my Ph.D. Thesis. Complementary to my thesis, I had the opportunity to work more deeply on reaction mechanisms themselves. Multi-nucleon transfer and deep inelastic reactions are proved to be useful tools to populate neutron rich exotic nuclei to perform spectroscopic studies. In the case of multi-nucleon transfer, reaction probabilities seem to exhibit an enhancement for even number of neutrons compared to theoretical calculations. I have been involved in an experiment to test if this enhancement comes from complexity of computing the transfer mechanism or indicate the presence of correlation effects. I’m also involved in fusion cross sections measurements at energies close and below the Coulomb barrier, where the fusion cross section is sensitive to the nuclear potential in the inner side of the Coulomb barrier.
Keywords : Shell Model, Collective Models, γ-spectroscopy, nuclear structures, nuclear reactions, radiative capture, multi-nucleon transfer, AGATA, PRISMA, DRAGON