GABRIELA setup for nuclear spectroscopy of the transfermium element isotopes at the VASSILISSA separator

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PROPERTIES OF ATOMIC NUCLEI Experiment

GABRIELA Setup for Nuclear Spectroscopy of the Transfermium Element Isotopes at the VASSILISSA Separator* A. G. Popeko1)** , A. V. Belozerov1) , Ch. Briancon ¸ 2) , V. I. Chepigin1) , O. Dorvaux3), K. Hauschild2), A. P. Kabachenko1), A. Korichi2), A. Lopez-Martens2) , O. N. Malyshev1), ˇ Sˇ aro ´ 4), A. V. Shutov1), A. I. Svirikhin1), and A. V. Yeremin1) Yu. Ts. Oganessian1) , S. Received December 12, 2005

Abstract—The JINR (Dubna)–IN2P3 (France) Collaboration project aimed at the α, β, γ spectroscopy of the transfermium element isotopes using heavy-ion beams of the U-400 cyclotron and the modernized recoil separator VASSILISSA was launched at Dubna. During the first full-scale experiment in September– October 2004, nobelium and lawrencium isotopes produced in the 48 Ca + 207,208 Pb → 255,256 No∗ and 48 Ca + 209 Bi → 257 Lr∗ reactions were studied. PACS numbers : 29.30.-h, 23.20.-g, 23.40.-s, 23.60.+e DOI: 10.1134/S1063778806070143

1. INTRODUCTION In recent years, γ spectroscopy of heavy nuclei has been intensively developed in two directions. The first one is the study of properties of excited nuclei via prompt γ-ray spectroscopy using efficient 4π Ge arrays placed around the target. Experiments of this type, based on the recoil decay tagging (RDT) method, allow one to evaluate such important quantities as Ex , J π , transition rates, deformations, and momenta of inertia for nuclei which have production cross sections on the order of 1 µb. The pioneering in-beam γ-ray spectroscopic studies of the transfermium nucleus 254 No were performed at the ANL (Ar¨ gonne, USA) [1, 2] and at the University of Jyvaskyl a¨ (Finland) [3]. The second direction is γ and/or β spectroscopy of evaporation residues (ER) and their decay products implanted into the focal-plane detector of a recoil separator. The properties of the excited states in 256,257 Db, 255 Rf, and their decay products 252,253 Lr and 251 No have been studied for the first time at the GSI (Germany) using the recoil separator SHIP and ∗

The text was submitted by the authors in English. FLNR, JINR, Dubna, Russia. 2) CSNSM, IN2P3–CNRS, Orsay Campus, France. 3) IReS, IN2P3–CNRS, Strasbourg, France. 4) Department of Physics, Comenius University, Bratislava, Slovakia. ** E-mail: [email protected] 1)

a large-size Ge “clover” detector placed downstream of the focal-plane detector array [4]. The first phase of the JINR–IN2P3 Collaboration project “Nuclear Structure and Reaction Mechanism Studies Towards Superheavy Elements: γ−β Spectroscopy in Very Heavy Nuclei at Z ≈ 104” is devoted to the focal-plane spectroscopy. High intensity and quality beams of the FLNR U-400 cyclotron, availability of unique radioactive targets, and the electrostatic separator VASSILISSA [5, 6] make it possible to study heavy (neutron-rich) nuclei having formation cross sections of higher than 1 nb. In this paper, we describe the new setup and present some first experimental data for illustration. 2. EXPERIMENTAL SETUP The recoiling products (ERs) from a heavy-ionindu