First observation of a shape isomer and a low-lying strongly-coupled prolate band in neutron-deficient semi-magic 187Pb

Prompt and delayed γ-ray spectroscopy of the neutron-deficient, semi-magic isotope 187Pb has been performed using the recoil-decay and isomer-decay tagging techniques at the Argonne Gas-Filled Analyzer. A new 5.15(15)-μs isomeric state at only 308 keV above the spherical 3/2− ground state is identified and classified as a shape isomer. A strongly-coupled band is observed on top of the isomer, which is nearly identical to the one built on the prolate 7/2−[514] Nilsson state in the isotone 185Hg. Based on this similarity and on the result of the potential-energy surface calculations, the new isomer in 187Pb is proposed to originate from the same configuration. The retarded character of the 308-keV (7/2−)→3/2gs− transition with a deduced B(E2)=5.6(2)×10−4 W.u. can be well explained by the significant difference between the prolate parent and spherical daughter configurations, leading to the shape isomerism. The excitation energy of the isomer is surprisingly low, being roughly half of the excitation energies of the known 0+ intruder bandheads in the neighboring 186,188Pb isotopes. The combined results of the present work and the previous α-decay and laser spectroscopy studies present evidence for triple shape coexistence at low energy in the negative-parity configurations of 187Pb, which is well reproduced by the potential-energy surface calculations

Spectroscopy of 187Tl: shape coexistence

The prompt and delayed $$\gamma$$-ray spectra of 187Tl was studied via the 142Nd(50Cr, 3p2n)187Tl fusion-evaporation reaction at a beam energy of 255 MeV. An enhanced level scheme of 187Tl was established. The collective bands with one-quasiparticle configurations from the $$2f_{7/2}$$, $$1h_{9/2}$$, $$1h_{11/2}$$and $$1i_{13/2}$$orbitals and high-lying structures with possible three-quasiparticle configurations are investigated in terms of the tilted axis cranking covariant density functional theory. At low excitation energy, the rotational bands with one-quasiparticle configurations reflect coexistence of three shapes: prolate, triaxial, and oblate. The possible shapes of two microsecond isomers at high excitation energy are proposed

Total Kinetic Energy Measurements for Spontaneous Fission of 255, 256, 258^{255,\,256,\,258}Rf

International audienceSpontaneous fission of 255Rf, 256Rf and 258Rf was studied at SHIP in GSI Darmstadt. The isotopes were produced in fusion–evaporation reactions 50Ti + 207,208Pb and 50Ti + 209Bi (compound nuclei 257Rf, 258Rf and 259Db, respectively) and implanted into the focal plane detector of the SHIP setup. The deficit in the measured fragments energies was evaluated as a function of implantation depth of evaporation residues in the silicon detector. This correction was applied to obtain the mean total kinetic energies of 255Rf, 256Rf and 258Rf

Decay Spectroscopy of Heavy Isotopes at SHIP Using the COMPASS Focal Plane Detection Set-up

International audienceThe new COMPASS detection system designed and developed at GSI, Darmstadt was employed in the focal plane of the SHIP velocity filter on-line during a period of commissioning. The isotope 254No was initially measured for control purposes, following which the nuclei, 227,228,230U, 229Np, and 229,230Pu were synthesized. The obtained data from α-decay spectroscopy is evaluated and compared with previous measurements

Decay Spectroscopy of Heavy Isotopes at SHIP Using the COMPASS Focal Plane Detection Set-up

International audienceThe new COMPASS detection system designed and developed at GSI, Darmstadt was employed in the focal plane of the SHIP velocity filter on-line during a period of commissioning. The isotope 254No was initially measured for control purposes, following which the nuclei, 227,228,230U, 229Np, and 229,230Pu were synthesized. The obtained data from α-decay spectroscopy is evaluated and compared with previous measurements

First observation of the decay of the 13/2+ isomer in 183Hg and B(M2) systematics of neutron transitions across the nuclear chart

The decay of the 13/2+ isomeric state in 183Hg was observed for the first time following the α decay of the 13/2+ isomer in 187Pb produced in the 142Nd(50Cr, 2p3n) reaction. Using α−γ delayed coincidence measurements, the half-life of this isomer was measured to be 290(30) μs. This isomer is proposed to deexcite by an unobserved low-energy M2 transition to the known 9/2− member of a strongly prolate-deformed 7/2−[514] band, followed by a 105-keV M1 transition to the bandhead. A lower limit of B(M2) ≥ 0.018 W.u. was deduced for the unobserved transition. The presumed retardation is proposed to be due to the notable shape change between the initial, nearly spherical, and the final, strongly deformed, states. A similar scenario is also considered for the 13/2+ isomer in 181Hg, suggesting both are cases of shape isomers. The B(M2) systematics of neutron transitions across the nuclear chart is discussed