Background: The semimagic Sn ($Z=50$) isotopes have been subject to many nuclear-structure studies. Signatures of shape coexistence have been observed and attributed to two-proton–two-hole (2p-2h) excitations across the $Z=50$ shell closure. In addition, many low-lying nuclear-structure features have been observed which have effectively constrained theoretical models in the past. One example are so-called quadrupole-octupole coupled states (QOC) caused by the coupling of the collective quadrupole and octupole phonons.

Purpose: Proton-scattering experiments followed by the coincident spectroscopy of $\gamma$ rays have been performed at the Institute for Nuclear Physics of the University of Cologne to excite low-spin states in ${}^{112}\mathrm{Sn}$ and ${}^{114}\mathrm{Sn}$ to determine their lifetimes and extract reduced transition strengths $B\left(\mathrm{\Pi }L\right)$.

Methods: The combined spectroscopy setup SONIC@HORUS has been used to detect the scattered protons and the emitted $\gamma$ rays of excited states in coincidence. The novel $(p,p^{\prime }\gamma )$ Doppler-shift attenuation (DSA) coincidence technique was employed to measure sub-ps nuclear level lifetimes.

Results: Seventy-four (74) level lifetimes $\tau$ of states with $J=0–6$ were determined. In addition, branching ratios were deduced which allowed the investigation of the intruder configuration in both nuclei. Here, $sd$ IBM-2 mixing calculations were added which support the coexistence of the two configurations. Furthermore, members of the expected QOC quintuplet are proposed in ${}^{114}\mathrm{Sn}$ for the first time. The $1^{-}$ candidate in ${}^{114}\mathrm{Sn}$ fits perfectly into the systematics observed for the other stable Sn isotopes.

Conclusions: The $E2$ transition strengths observed for the low-spin members of the so-called intruder band support the existence of shape coexistence in ${}^{112,114}\mathrm{Sn}$. The collectivity in this configuration is comparable to the one observed in the Pd nuclei, i.e., the 0p-4h nuclei. Strong mixing between the $0^{+}$ states of the normal and intruder configuration might be observed in ${}^{114}\mathrm{Sn}$. The general existence of QOC states in ${}^{112,114}\mathrm{Sn}$ is supported by the observation of QOC candidates with $J\ne 1$.

• Received 4 March 2018

DOI:https://doi.org/10.1103/PhysRevC.97.054319