A. Korichi

GABRIELA: A new detector array for γ-ray and conversion electron spectroscopy of transfermium elements

With the aid of the Geant4 Monte Carlo simulation package a new detection system has been designed for the focal plane of the recoil separator VASSILISSA situated at the Flerov Laboratory of Nuclear Reactions, JINR, Dubna. GABRIELA (Gamma Alpha Beta Recoil Investigations with the ELectromagnetic Analyser VASSILISSA) has been optimised to detect the arrival of reaction products and their subsequent radioactive decays involving the emission of α- and β-particles, fission fragments, γ- and X-rays and conversion electrons. The new detector system is described and the results of the first commissioning experiments are presented.

Lifetimes of shears bands in 199Pb

Abstract Lifetimes of states within the two strongest shears bands in 199 Pb have been measured using Doppler-shift techniques. The deduced B (M1) values confirm the proposed proton-particle neutron-hole structure of these bands. They show a decrease with increasing spin as expected within the tilted-axis cranking model. The B (E2) values confirm the low quadrupole collectivity and small deformation predicted by the calculations. The results give further support to the interpretation of the dipole bands in terms of the shears mechanism.

Evidence for octupole vibration in superdeformed 196 Pb

Abstract Evidence for a low-lying octupole vibrational excitation in the superdeformed energy minimum of 196 Pb is presented. High-spin states in this nucleus have been populated in the 170 Er( 30 Si,xa04n) 196 Pb reaction. The in-beam γ -ray decay has been investigated using the EUROBALLxa0IV Ge-detector array. The linear polarization of γ -ray transitions connecting an excited superdeformed band with the yrast superdeformed band has been measured from which E1 multipolarity is determined. These E1 transitions, which compete with the highly collective in-band E2 transitions, are strongly enhanced. This is an important step towards establishing that low-lying octupole vibrations are a wide spread phenomenon in the A =190 region of superdeformation.

Excitation energies and spins of the yrast superdeformed band inHg191

The excitation energies and spins of the levels in the yrast superdeformed band of {sup 191}Hg have been determined from two single-step {gamma} transitions and the quasicontinuum spectrum connecting the superdeformed and normal-deformed states. The results are compared with those from theoretical mean-field calculations with different interactions. A discussion of pairing in superdeformed states is also included.

Two-proton high-K oblate structures in 194Pb

Two regular dipole gamma -ray cascades have been observed in 194Pb. A comprehensive level scheme related to these structures has been built and first members of rotational bands built on K=8 and K=11 two-proton oblate states have been identified. From the excitation energies, spin values, behaviours of the moments of inertia and alignments, and from the comparison with the isotone 192Hg, multiparticle configurations can be attributed to three high-spin structures of 194Pb built respectively on the 4966 keV, 4334 keV and 6198 keV levels: ( pi 9/2(505) pi 13/2(606))(vi13/2)2, ( pi 9/2(505) pi 13/2(606))(vf5/2vi13/2), ( pi 9/2(505) pi 13/2(606)) (vf5/2(vi13/2)3). Within this interpretation, the regular dipole bands correspond to the presence of two or three i13/2 decoupled neutrons.

Two proton high-spin excitations and dipole bands in192Hg

The192Hg nucleus was populated in the160Gd(36S, 4n) reaction at a beam energy of E=159 MeV. Emittedγ-rays were detected with the EUROGAM array comprising 43 Compton-suppressed large volume Ge detectors. The level scheme of192Hg has been extended up to an excitation energy of E=10.4 MeV and spin I=34 ħ. Two new structures, made of competing ΔI=1 and ΔI=2 transitions have been observed and their connexions with the low-lying levels established. Their lowest levels are located at 6.304 MeV and 6.879 MeV excitation energy. The experimental results are compared with mean-field HF+BCS calculations. It is proposed that the new structures originate from deformation-aligned quasi-proton excitations π(i13/2 * h9/2)K=11 andπ (h9/2)K=82, coupled to rotation-aligned quasi-neutron ν(i13/2)n and quasi-proton π(h11/2)2 excitations.

Observation and lifetime of the first excited superdeformed band in Hg-192

An excited superdeformed (SD) band has been observed in 192Hg with the UROGAM spectrometer. It is interpreted as a two quasi-neutron excitation in the 192Hg SD nucleus. Nuclear level lifetimeshave been measured using the Dopple-shift attenuation method. An average transition quadrupole moment of 19.5 ± 1.5 b has been obtained which is similar to the value measured in the yeast SD band. These results indicate that the deformation in the 192Hg SD nucleus is stable despite changes in the occupancy of high-N-orbitals.

Relative spins and excitation energies of superdeformed bands in {sup 190}Hg: Further evidence for octupole vibration

An experiment using the Eurogam phase II {gamma}-ray spectrometer confirms the existence of an excited superdeformed (SD) band in {sup 190}Hg and its very unusual decay into the lowest SD band over 3--4 transitions. The energies of the transitions linking the two SD bands have been firmly established, and their angular distributions are consistent with a dipole character. Comparisons with calculations using random-phase approximation indicate that the excited SD band can be interpreted as an octupole-vibrational structure.

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

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 48Ca + 207,208Pb → 255,256No* and 48Ca + 209Bi → 257Lr* reactions were studied.

Detailed study of magnetic rotation in 196Pb

Abstract High-spin states in 196 Pbxa0have been populated in two different heavy-ion-induced reactions using thin and Au-backed targets, respectively. Gamma-ray coincidences were measured using the EUROBALL spectrometer array. Five new magnetic-rotational bands have been found. The previously known four bands have been extended and some of their levels have been reordered. With one exception, all the bands have been connected to lower-lying states and excitation energies, spins and parities have been determined. From the systematic properties of the bands and from comparison to tilted-axis cranking calculations, arguments for configuration assignments to the bands are derived. It is suggested that the bands are based on proton 11 − and 8 + particle states coupled to various neutron–hole excitations.