Speaker
Ms
Elisa Rapisarda
(IKS Katholieke Universiteit Leuven)
Description
The mass region of neutron-deficient mercury and lead isotopes near the midshell (N=104) is well known for the phenomenon of shape coexistence. In neutron-deficient, even-even 180-188Hg isotopes an oblate (beta2~-0.15) ground state band is found to coexist with an excited prolate (beta2~0.25) band at low spin and low-excitation energies. This band is built on top of a deformed excited 0+ state, which is interpreted as resulting from proton excitations across the Z=82 closed shell. Such intruder states have been found to be a widely occurring structural feature of nuclei at and near closed shell.
The low-lying coexisting states in 180,182,184Hg have been studied at ISOLDE, CERN through the beta+/EC decay of 180,182,184Tl as part of a systematic alpha, beta, and beta-delayed fission study of neutron-deficient thallium isotopes. The beta+/EC decay is a very simple but still powerful tool which allows to effectively populate low-lying not-yrast states in the daughter nucleus, normally not easily accessible with other techniques, thus providing complementary information to the ones from in-beam gamma-spectroscopy studies and from alpha-decay studies from the Pb parent nuclei.
Mass-separated Tl beams, produced at ISOLDE, CERN, in the bombardment of 238U by 1.4 GeV protons and selectively laser ionized, were implanted on a carbon foil mounted on a rotating wheel. The implantation foil was surrounded by two Si detectors for alpha, and electron detection while gamma rays were detected with two high-resolution Ge detectors.
By means of unambiguously Si-gamma and gamma-gamma coincidences, a detailed level scheme of the coexisting states has been built-up as well as a detailed description of their decay properties (gamma intensities, E0 component of 2+ -->2+ transitions).
The newly observed or better energy-determined 02+, 22+, 23+ states in 180,182Hg follow well the general trend of the prolate band. They confirm that the minimum of the parabolic behavior in excitation energy of the prolate band occurs in 182Hg, as expected. The exceptionally large E0 component observed in the 21+ -->22+ transition in 184Hg (23+/-5) confirm that the two states are strongly mixed and they have different deformation.
Isomerism is well-known in the heavier thallium isotopes and the population of low-spin states as well as high-spin states (up to 8+ in 182,184Hg) in the beta decay points to similar features in the lighter thallium isotopes.
The information gathered can be combined with the ones obtained with different techniques, such as in-beam gamma and conversion-electron spectroscopy, Coulomb excitation on post-accelerated radioactive ions (recently performed at ISOLDE), lifetime measurements and laser spectroscopic studies to get a deeper knowledge of the shape-coexistence phenomenon.
Primary author
Ms
Elisa Rapisarda
(IKS Katholieke Universiteit Leuven)
