Titre Contribution à l’étude de la structure de 78Ni : Etude de la dérive monopolaire des états de particule individuelle neutron dans l’espace de valence naturel de 78Ni. Contribution to 78Ni structure study : Probing the neutron single particle monopole drift in the 78Ni natural valence space Supervisor: David VERNEY, IPN Nuclear Structure Group (NESTER) [email protected] Context: Evolution of Magic Numbers in exotic nuclei, the 78Ni case. The modification of shell gaps far from stability raises doubts about one of the firmest paradigms of nuclear structure : the universality of magic numbers throughout the nuclear chart. Nuclei are more satble and difficult to excite at particular neutron or proton numbers 8, 20, 28, 50 …, the so‐called magic numbers. In recent years, evidence has surfaced pointing to changing shell structure with a varying number of protons and/or neutrons. These findings furnish a stringent test for modern nuclear structure models and have important astrophysical implications. From a theoretical point of view, the reasons for this shell evolution are not well established and different scenarios are under consideration: variation in the mean field when approaching the neutron drip‐lines as well as specific components (pairing, tensor interaction…) in the residual interaction, to name a few. A unique opportunity to study these shell effects is offered by the region of 78Ni nucleus, which has 28 protons and 50 neutrons—both magic numbers in stable nuclei. 78Ni has a 14‐neutron excess over the heaviest stable nickel nuclide, and would be—provided 28 and 50 retain their ‘magic’ character—
the most neutron rich example of doubly magic nucleus in the whole nuclide chart with an extreme N/Z ratio of 1.79 (to be compared to N/Z=1.54 for 208Pb or 1.64 for 132Sn). So far, only a dozen of 78Ni could be successfully synthesized and identified with most advanced techniques of production of rare isotopes, using high energy beam fragmentation. The production and observation of 78Ni was reported only twice in the history of nuclear physics and this region of the nuclide chart remains extremely hard to reach experimentally. The only way presently available to study the doubly magic nature of the 78Ni region is to study some relevant cases as close as possible to it. PhD research program The light odd‐neutron N=51 nuclei constitute the most interesting cases to study the neutron single particle evolution towards 78Ni. Low‐lying states in N=51 isotones may naturally be understood in terms of single‐particle configurations and core‐particle coupled states. Our group will perform in year 2011 an experiment at Legnaro National Laboratory (Italy) to determine the nature of the low‐lying yrast or quasi yrast 7/2+ states in 32 < Z < 40, Figure 1 AGATA demonstrator at Legnaro National Lab (Italy) odd‐neutron N=51 nuclei in order to assess their collective or ν1g7/2 single‐particle origin and better constrain the relative position of the latter with respect to other neutron single particle states above a 78Ni core. The structure of the low lying states in odd N=51 isotones will be investigated using the Recoil distance Doppler‐shift (RDDS) method by use of a (so‐called) plunger device. The neutron‐rich nuclei will be produced in deep‐
inelastic, multi‐nucleon transfer and induced fission reactions with the 82Se+ 238U system with an incident 82Se beam energy of 570 MeV delivered by the Legnaro Tandem+ALPI accelerator. The plunger will be combined with the AGATA gamma‐ray spectrometer and the PRISMA fragment spectrometer. If the Legnaro 2011 experiment schedule is compatible, then the student will participate in the plunger‐AGATA experiment at Legnaro during his/her “pre‐thèse” training program. He/she will take in charge during his/her PhD all the data analysis of the experiment. In addition, he PhD student will be in charge of the interpretation of the data in the framework of the nuclear shell model and core‐particle coupling model in close collaboration with the theoretical groups involved in the field. Last, the PhD student will participate in complementary beta‐decay experiments at ALTO which will be scheduled within the next three years.