Abstract
Background: The semimagic Sn () 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 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 rays have been performed at the Institute for Nuclear Physics of the University of Cologne to excite low-spin states in and to determine their lifetimes and extract reduced transition strengths .
Methods: The combined spectroscopy setup SONIC@HORUS has been used to detect the scattered protons and the emitted rays of excited states in coincidence. The novel Doppler-shift attenuation (DSA) coincidence technique was employed to measure sub-ps nuclear level lifetimes.
Results: Seventy-four (74) level lifetimes of states with were determined. In addition, branching ratios were deduced which allowed the investigation of the intruder configuration in both nuclei. Here, IBM-2 mixing calculations were added which support the coexistence of the two configurations. Furthermore, members of the expected QOC quintuplet are proposed in for the first time. The candidate in fits perfectly into the systematics observed for the other stable Sn isotopes.
Conclusions: The transition strengths observed for the low-spin members of the so-called intruder band support the existence of shape coexistence in . 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 states of the normal and intruder configuration might be observed in . The general existence of QOC states in is supported by the observation of QOC candidates with .
2 More- Received 4 March 2018
DOI:https://doi.org/10.1103/PhysRevC.97.054319
©2018 American Physical Society