Study of shape coexistence in lead and thallium isotopes

The β⁺;/EC decays of mass-separated ¹⁹⁹;Bi and ¹⁹³;Bi were studied at the Oak Ridge Holifield Heavy-Ion Research Facility. In these experiments the heavy- ion fusion reactions ¹⁸⁷;Re(¹⁶;O,4n) and ¹⁸⁵;Re(¹⁶;O,8,n) were used. Time-sequenced spectra of γ-rays, X-rays, conversion electrons and γγt, γXt, eγt, eXt coincidence data have been obtained. The decay schemes of ^199l;Bi consisting of 67 excited states and 216 transitions and of ¹⁹³;Bi consisting of 41 excited states and 61 transitions in were constructed Several very-converted transitions interpreted to have E0 multipole admixtures, an indicator of shape coexistence, were found to de-excite positive-parity levels at 1705 and 1754 keV in ¹⁹⁹;Bi and levels 757 and 1023 keV higher than the isomer state in ¹⁹³;Bi. One possible deformed state with negative parity was found at 739 keV in ¹⁹³;Bi.

The systematics of the lower excited states in odd-mass Pb isotopes were discussed. The good agreement were found for p_3/2;, f_5/2; and i_13/2; energies as com- pared with the theoretical deformed harmonic-oscillator potential predictions. The systematics of shape coexistent states in odd-mass Pb isotopes were discussed in both positive and negative parities states and compares was made with even-mass isotopes. The odd-A states were interpreted in terms of a single neutron weakly coupled with the spherical and oblate deformed even-core. The Nilsson model and Total Routhian Surface calculations were used to studied the nuclear shape of Pb isotopes.

As a by-product of the ¹⁹³;Bi experiment, a decay scheme of ^{193;Pb with 56 excited states and 86 transitions was established. Two bands of πh9/2;-;[505] and πh_7/2;[514]were expanded. The particle-rotor model was applied to calculate level energies and and transition rates. The calculation with ε₂;=-0.145 and γ= 0°ree; gave a good fit to the experimental relative intensities for K=9/2 band.}