Investigation into the semimagic nature of the tin isotopes through electromagnetic moments

Abstract

A complete set of electromagnetic moments, $B(E2;{0}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+}),\phantom{\rule{0.16em}{0ex}}Q({2}_{1}^{+})$, and $g({2}_{1}^{+})$, have been measured from Coulomb excitation of semimagic $^{112,114,116,118,120,122,124}\text{Sn} (Z=50)$ on natural carbon and titanium targets. The magnitude of the $B(E2)$ values, measured to a precision of $\ensuremath{\sim}4%$, disagree with a recent lifetime study [Phys. Lett. B 695, 110 (2011)] that employed the Doppler-shift attenuation method. The $B(E2)$ values show an overall enhancement compared with recent theoretical calculations and a clear asymmetry about midshell, contrary to naive expectations. A new static electric quadrupole moment, $Q({2}_{1}^{+})$, has been measured for $^{114}\text{Sn}$. The static quadrupole moments are generally consistent with zero but reveal an enhancement near midshell; this had not been previously observed. The magnetic dipole moments are consistent with previous measurements and show a near monotonic decrease in value with neutron number. The $g$-factor measurements in $^{112,114}\mathrm{Sn}$ establish the recoil-in-vacuum method for states with $\ensuremath{\tau}\ensuremath{\sim}0.5$ ps and hence demonstrate that this method can be used for future $g$-factor measurements on proton-rich isotopes toward $^{100}\mathrm{Sn}$. Current theory calculations fail to reproduce the electromagnetic moments of the tin isotopes. The role of 2p-2h and 4p-4h intruders, which are lowest in energy at midshell and outside of current model spaces, needs to be investigated in the future.