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Accueil > Séminaires > Séminaires passés > Structure of heavy nuclei and associated R&D


Structure of heavy nuclei and associated R&D

Jeudi 18 Juin 2015 à 14h00 C.S.N.S.M. - Bât. 108 - Salle de réunion

One of the key goals of modern nuclear physics is to extend the borders of the nuclear chart. The fission barrier calculated within the macroscopic liquid drop model fails to explain the stability of nuclei with a number of protons Z≥90. Transfermium elements (Z≥100) have a vanishing liquid-drop barrier and are solely stabilized by quantum shell effects. These quantum shell effects give rise to a prediction of a so-called “Island of Stability” - a cluster of spherical, stable or very long-lived super-heavy elements (SHE). The theoretical predictions for the position of these nuclei are very model-dependent : Z=114, 120 or 126 for protons and N=172 or 184 for neutrons (see, e.g., [1]). However, it is extremely difficult to check these predictions since the production cross-sections of the SHE are extremely low (giving a few events per month). Luckily, the quantum states responsible for the enhanced stability for spherical SHE are also active for the lighter deformed (rubgy ball shaped) transfermium (Z 100-105) nuclei. These nuclei have much larger cross-sections, and thus give access to valuable data constraining theory.
We produce these transfermium nuclei in fusion-evaportaion reactions. The ion beam (O, Ne, Ar, Ca, Ti) hits a target (such as Pb, Bi, U) and fuses to form a compound nucleus which de-excites by evaporating a few protons and/or neutrons. The nuclei of interest are selected from other reaction products with SHELS (Separator for Heavy ELement Spectroscopy) on the basis of their velocity and charge state. The nuclei of interest are then implanted into a DSSD (Double-sided Silicon Strip Detector) at the focal plane of SHELS.
The subsequent de-excitation and decay of the nucleus is registered by the detection system GABRIELA (Gamma Alpha Beta Recoil Investigations with ELectromagnetic Analyzer) [2]. It consists of a DSSD, a box of silicon strip detectors and an array of BGO-shielded germanium crystals. GABRIELA is sensitive to various kinds of decays : spontaneous fission, alpha and beta decay, de-excitation via gamma rays or internal-conversion electrons, X rays etc. Thus we are able to perform complete spectroscopy of the nuclei of interest and establish their level schemes.
SHELS and GABRIELA were developed within an IN2P3-JINR collaboration (CSNSM-IPHC-FLNR) and are installed at the JINR, Dubna.
In this work, the spectroscopy of 251Fm and 221Th will be presented. Some details on the experimental setup, its recent upgrade and commissioning will be given.

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Renseignements CSNSM : Hélène le Sueur/ Stéphanie Roccia 01 69 15 52 25/52 15