Abstract
The nucleus is studied by means of -delayed electron-conversion spectroscopy at ALTO. The radioactive beam is produced through the isotope separation on line photofission technique and collected on a movable tape for the measurement of and emission following decay. An electric monopole transition, which points to a 639(1) keV intruder state, is observed for the first time. This new state is lower than the level in , and provides evidence of shape coexistence close to one of the most neutron-rich doubly magic nuclei discovered so far, . This result is compared with theoretical estimates, helping to explain the role of monopole and quadrupole forces in the weakening of the gap at . The evolution of intruder states towards is discussed.
- Received 26 January 2016
DOI:https://doi.org/10.1103/PhysRevLett.116.182501
© 2016 American Physical Society