Measurement of the 21+ level lifetime in Sn120 by the Doppler shift attenuation method: Evidence of enhanced collectivity

Abstract

Background: There has been a considerable interest focused on the study of enhancement or suppression in collectivity of the excited ${2}_{1}^{+}$ states in stable Sn isotopes. Independent measurements of Coulomb excitation cross sections and ${2}_{1}^{+}$ level lifetimes report discrepant transition probabilities. Existing estimates for ${2}_{1}^{+}$ lifetime indicate reduced collectivity.Purpose: A reexamination of lifetime of the ${2}_{1}^{+}$ state in the most abundant $^{120}\mathrm{Sn}$ isotope is thus warranted. The same has been carried out in the present work and the result has been used to determine the transition probability as an indicative of the underlying collectivity.Methods: Low-lying levels in the vibrational $^{120}\mathrm{Sn}$ nucleus have been excited by inelastic scattering with $^{32}\mathrm{S}$ beam at ${E}_{\mathrm{lab}}=120$ MeV. Level lifetime measurements have been carried out using the Doppler shift attenuation method, wherein the Doppler affected $\ensuremath{\gamma}$-ray peaks have been analyzed using updated methodologies.Results: From the measured lifetime of the ${2}_{1}^{+}$ state (${E}_{x}=1171$ keV) in $^{120}\mathrm{Sn}, {\ensuremath{\tau}}_{{2}_{1}^{+}}=0.{863}_{\ensuremath{-}0.036}^{+0.029}$ ps, a value of $B(E2;{0}_{\mathrm{g}.\mathrm{s}.}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})=0.{215}_{+0.009}^{\ensuremath{-}0.007}$ is deduced. An estimate of the ${4}_{1}^{+}$ (feeder) level lifetime, ${\ensuremath{\tau}}_{{4}_{1}^{+}}=1.{77}_{\ensuremath{-}0.089}^{+0.084}$ ps, is also reported from lineshape analysis of $\ensuremath{\gamma}$ rays in cascade.Conclusions: An enhancement in collectivity for the ${2}_{1}^{+}$ state is confirmed, following an improved determination of the level lifetime, with reduced uncertainties. The excited ${2}_{1}^{+}$ state is also found to have a nonvanishing moment of inertia, suggesting anharmonic nature of quadrupole vibrations.