New lifetime measurements for the 2⁺₁ level in ^112,120Sn by the Doppler-shift attenuation method

The tin (Sn; Z = 50) isotopes constitute the longest chain of semi-magic even-even nuclei between the ¹⁰⁰Sn (N = 50) and ¹³²Sn (N = 82) double-shell closures, seven of which, ^{112,114,116,118,120,122,124}Sn, are stable. These isotopes have become a prototypical benchmark of extensive microscopic theory and experiment, reflected in the large number of studies investigating the decay of their low-lying 2⁺₁ excited state. The transition characteristics are inferred through the B(E2; 0⁺_g.s →2⁺₁) values, which, in principle, are contingent on the lifetime of the corresponding level, and are the most direct and unambiguous test of the collective nature of the transitions. There has been a considerable interest focused on the study of enhancement or suppression in collectivity of the excited 2⁺₁ state in the stable Sn isotopes. Independent experiments on Coulomb excitation, heavy-ion scattering and 2⁺₁ level lifetime measurements report discrepant transition probabilities, with the lifetime estimates indicating significantly reduced collectivity. A re-examination of the same has been carried out in the present work on two of the stable isotopes, ^112,120Sn. Low-lying levels in the ^112,120Sn isotopes have been excited by inelastic scattering with heavy ion beams. Level lifetime measurements have been carried out using the Doppler shift attenuation method, wherein the Doppler affected γ-ray peaks from the decay of the 2⁺₁ level in each isotope have been analyzed using updated methodologies, and corresponding B(E2; 0⁺_g.s → 2⁺₁) values become indicative of the underlying collectivity. The present results are discrepant with respect to existing lifetime estimates, but in compliance with systematic measurements of the B(E2;0⁺_g.s. →2⁺₁) values in the stable Sn isotopes, by pure Coulomb excitation as well as heavy-ion induced inelastic collisions. The results are also found to be in good agreement with generalized seniority model as well as state-of-the-art Monte Carlo shell model (MCSM) calculations. (author)