Abrupt structural transition in exotic molybdenum isotopes unveils an isospin-symmetric island of inversion

Abstract

Abstract Like electrons in atoms, protons and neutrons in nuclei occupy orbitals in a shell structure with energy gaps at magic numbers. Radioactive-beam experiments revealed the disappearance of magic numbers in some neutron-rich isotopes. In these nuclei, configurations involving particles excited across the shell gap gain correlation energy, becoming the ground state. Neutron-rich regions of the nuclear chart that exhibit this property are known as “Islands of Inversion”. Here we present the lifetime measurement of the first 2 + states in 84 Mo ( N  =  Z ) and 86 Mo ( N  =  Z  + 2) revealing an unexpected sharp structural change between them defining the edge of the region of deformation around 80 Zr. Similarly to the neutron-rich N  = 40 Island of Inversion near 64 Cr where cross-shell excitations dominate, we identify this region as an Island of Inversion with symmetrical proton and neutron excitations that we term “Isospin-Symmetric Island of Inversion”. Three-nucleon forces are suggested to drive Mo isotope structural changes.