A. Hürstel

Heavy Element Spectroscopy At JYFL

A series of experiments to study the properties of transfermium nuclei have been performed at the Department of Physics of the University of Jyvaskyla. The experiments are carried out by a large group of collaborating institutes. These studies have been rendered possible by the coupling of various state‐of‐the‐art detector systems to the RITU gas‐filled recoil separator. In‐beam conversion‐electron and gamma‐ray measurements have been made using the SACRED silicon and JuroGam germanium‐detector arrays, respectively. The introduction of the GREAT spectrometer and TDR data acquisition system have greatly improved the quality of the data obtained at RITU. A brief overview of the instrumentation used and highlights from recent experiments are presented.

EXPERIMENTAL MEASUREMENT OF THE DEFORMATION THROUGH THE ELECTROMAGNETIC PROBE: SHAPE COEXISTENCE IN EXOTIC KR AND SR ISOTOPES

The light krypton isotopes were studied in a series of Coulomb excitation experiments using radioactive beams at GANIL. The static quadrupole moments found in these experiments give firm experimental evidence for the shape coexistence scenario that is based on theoretical calculations and on the systematics of low-lying excited 0+ states. The experimental results are interpreted within a phenomenological two-band mixing model. Configuration mixing calculations based on triaxial Hartree-Fock-Bogolyubov calculations with the Gogny D1S effective interaction have been performed and compared to experimental data.

Shape Coexistence In Light Krypton Isotopes

Coexisting states of prolate and oblate shapes were studied via Coulomb excitation of radioactive 74Kr and 76Kr beams at safe energies at GANIL. The results for the transitional E2 matrix elements for several yrast transitions in both isotopes are in contradiction with the B(E2) values extracted from the lifetimes that are found in the literature. This motivated a new lifetime measurement which showed strong deviations from the previously published values, but is consistent with the results from Coulomb excitation. Having resolved these discrepancies and knowing the B(E2) values with high precision allows to extract the diagonal matrix elements from the Coulomb excitation data. The results confirm the proposed shape coexistence scenario.

Gamma‐ray Spectroscopy Of Very Neutron‐Deficient Bi Isotopes

Prompt and delayed γ rays from in 191,193Bi have been identified using the recoil‐decay tagging (RDT), isomer tagging, and recoil gating techniques, resulting in extensive level schemes for both nuclei. Excitation energies of the isomeric 13/2+ states have been established and rotational bands based on them have been observed. The nearly spherical 9/2− ground‐state bands appear to be crossed by more deformed low‐lying structures. TRS calculations have been performed and B(M1)/B(E2) ratios have been extracted for the observed strongly‐coupled bands. In 193Bi, a 3 μs isomer has been interpreted as a h9/2 proton coupled to the 12+ state of the 192Pb core.

Spectroscopy of odd‐proton nuclei in the region of 254No

Two rotational bands have been found in the transfermium nuclei 251Md and 255Lr, the latter being the heaviest nucleus so far studied in‐beam. Both are assigned to a [521]1/2− Nilsson state by comparison to theory. The experiments have been carried out in the Accelerator Laboratory of the University of Jyvaskyla (JYFL), where the array of germanium detectors JUROGAM was used in conjunction with the recoil‐seperator RITU and the focal‐plane setup GREAT for gamma‐spectroscopic studies.

Spectroscopy of very neutron-deficient 187,189Bi isotopes

Shape coexistence is well known to occur in nuclei, in particular near closed shells [1], where particle-hole excitations across the shell gap can create deformed intruder states. In the neutron-deficient lead isotopes (Z = 82), deformed structures appear at low excitation energy. The isotope 188Pb [2] shows for example a triple shape coexistence with oblate and prolate excited 0+ states that compete with the spherical ground state. The study of the odd-proton single-particle excitations in Bi isotopes allows to obtain information on the orbitals involved in the different shapes observed in this mass region.