GAMMA SPECTROSCOPY OF TRANSFERMIUM ELEMENTS AT THE VASSILISSA SET UP

Abstract

In recent years gamma spectroscopy of heavy nuclei has been very intensively developed in two directions. The first was the development of very efficient 4 π Ge arrays which has allowed the properties of excited nuclear states to be determined via prompt γ-ray spectroscopy around the target. This type of experiment allows one to gain access to such important quantities as Ex, J , transition rates, deformations, and moments of inertia which can be compared with theoretical predictions. The Recoil Decay Tagging (RDT) method has made it possible to perform in-beam γ-ray and electron-spectroscopy in nuclei which have production cross sections at the order of 1 μb, or even less. The pioneering in-beam γ-ray spectroscopic studies in the transfermium nucleus No were performed at the ANL, Argonne USA [1] and the University of Jyvaskyla, Finland [2]. The ground state rotational band was observed up to a spin of 20 h. Recent experiments performed at Jyvaskyla have probed nuclei in the Fm Lr region having formation cross sections down to 200 nb. The second direction is the decay and/or isomeric γ and/or electron spectroscopy of the evaporation residues implanted into the focal plane detectors of recoil separators and their daughter products. Such experiments have been performed at GSI, Germany using the recoil separator SHIP and a large size Ge detector placed close to the focal plane Si detector array. The decay properties of the Db and Rf evaporation residues were studied and results were obtained for the first excited states in the Lr and No daughter isotopes via γ-ray spectroscopy [3]. A IN2P3 JINR collaboration project ”Nuclear structure and reaction mechanism studies towards Super Heavy Elements: γ – electron Spectroscopy in very heavy nuclei at Z ≈ 104” was launched last year in Dubna. It is aimed at the study of nuclear spectroscopy of the transfermium elements using high intensity heavy ion beams of the U–400 cyclotron, exotic targets and recoil separator VASSILISSA (FLNR JINR). The first phase of our project is devoted to the focal plane spectroscopy. The FLNR U–400 cyclotron is capable of producing very intense beams, which can be used on unique radioactive rotating targets, and allows one to reach very heavy (neutron rich) nuclei having formation cross sections of higher than 1 nb with appropriate statistics. The sensitivity and selectivity needed for these experiments requires: • A stable, high intensity and good quality beam of the U–400 cyclotron.