Structure des noyaux lourds et R&D associée (Pôle Physique Nucléaire)

par Rikel Chakma (Pôle Physique Nucléaire, groupe SDF)

vendredi 27 nov. 2020, 10:00 → 12:00 Europe/Paris

100/0-MXX - Salle des Conseils (IJCLab)

Lien de connexion / Link :

 https://ijclab.zoom.us/j/95214782916?pwd=Smh1T2tPOWZoL3RyUWJFclUrZDBGUT09

Merci de laisser fermés camera et micro de votre ordinateur pendant la soutenance.
Please keep your camera and microphone switched off throughout the defense. 

Résumé

La quête des noyaux les plus lourds entend répondre à un ensemble d'interrogations fondamentales: jusqu'où s'étend la charte des noyaux et le tableau de Mendeleev a-t-il une limite supérieure ? Récemment, l'IUPAC a reconnu la découverte des éléments 113, 115, 117 et 118 mais ces résultats ne sont pas sans controverse, en particulier en ce qui concerne l'identification en Z. Les éléments super lourds sont aussi un laboratoire unique pour étudier la structure et la dynamique nucléaire sous l'influence de forces de Coulomb très intenses. Pour étendre la région de noyaux accessibles aux études spectroscopiques, des développements en terme de séparation mais aussi de détection sont nécessaires. Cet axe de recherche se déroule essentiellement en Russie au FLNR-JINR (Flerov Laboratory for Nuclear Reactions, Joint Institute for Nuclear Research, Dubna). Des expériences ont également lieu à JYFL (Jyvaskyla, Finlande), à ANL et au GANIL (Caen). Les expériences en Russie ont lieu auprès du nouveau séparateur d'ions de recul SHELS et de son système de détection au plan focal : GABRIELA. Le sujet de thèse proposé consiste en 2 volets : -Investiguer la possibilité d'identifier les particules au plan focal de séparateurs d'ions de recul avec un développement innovant en termes de détecteurs et d'analyse de formes d'impulsions. - Participer à la campagne d'expérience en Russie et analyser les données de décroissance de noyaux de No (Z=102) et Rf (Z=104).

Abstract

Characterization of the multi-detector GABRIELA and decay spectroscopy of 255Rf and 251No 

One unanswered question that lies at the crossroad of physics and chemistry is: what is the limit of the periodic table. Nuclear theories suggest the existence of the so-called ``island of stability", inhabited by long-lived superheavy elements. These elements can materialize only because of strong quantum shell stabilizing effects. Thus, superheavy elements form a unique laboratory for studying nuclear structure and dynamics under the influence of very large Coulomb forces between the numerous protons in the nucleus. However, a theoretical challenge lies in the prediction the exact position of this island as different models predict the position of the next shell closures at Z = 114, 120, or 126, and N = 172, 184 beyond the closed spherical shells at Z = 82 and N = 126 rather inharmoniously. To better understand the behavior of nuclear matter for extreme values of proton and neutron numbers and constrain nuclear models it is, therefore, necessary to investigate the nature and sequence of states in lighter, more accessible, transactinide nuclei through spectroscopic studies. In this work, states of 255Rf were populated through the fusion-evaporation reaction 50Ti(207Pb,2n)255Rf using an intense 50Ti beam provided by the U400 cyclotron of FLNR in Dubna. The evaporation residues were separated from the beam and background of other reaction products using the recoil separator SHELS and implanted into the implantation detector of the GABRIELA setup. The GABRIELA multidetector array allows to perform time and position correlations between the implanted nuclei and their subsequent decays and is sensitive to the emission of gamma rays, internal conversion electrons, alpha particles and fission products. To interpret the experimental decay spectra, the GABRIELA multidetector was characterized using Geant4 simulations, which were validated with calibration data. In particular, the impact of summing on the gamma-ray- and electron-detection efficiencies was investigated and showed that simulations are vital for interpreting experimental results obtained using compact and efficient setups like GABRIELA. A novel method to estimate the implantation depth profile of the evaporation residues was devised, which is essential especially for internal-conversion-electron spectroscopy. 

To be able to use Geant4 for elements heavier than Fm (Z = 100), the source code was modified and the Fluorescence and the Auger emission data were extrapolated to allow accurate radioactive decay simulations up to the element Rf. Using 257Rf experimental data obtained in Dubna and the known decay scheme of the 21/2+ high-K isomer, the functionality of the modified Geant4 code was validated. Simulations were then used to study the fine structure alpha decay of 255Rf and derive branching ratios to states in 251No. The gamma-decay branching ratios and internal conversion coefficients for transitions in 251No were also extracted. In 255Rf, two new high-K isomeric states were identified and the existence of a low-lying spin isomer was confirmed. Quantitative and qualitative comparisons of the experimental spectra to simulations have allowed to establish the likely decay schemes and assign possible quasiparticle configurations for all 3 isomers.