Rafael Escudeiro

Lifetimes of several excited states in the mirror nuclei Cr 47 − V 47 and Mn 49 − Cr 49 , located at the center of the f 7 / 2 shell, were measured using the advanced γ -ray tracking array AGATA. The Doppler shift attenuation method was employed to determine such lifetimes in the subpicosecond range. The reduced transition probabilities obtained from the lifetimes are compared to shell-model calculations in the full f p space. The shell-model-calculated B ( E 2 ) values were underestimated in comparison with experimental data. In particular, large discrepancies are obtained in Mn 49 at low spin. These results suggest an incomplete understanding of low-energy states in the studied nuclei and call for further refinement of the existing nuclear structure theories.

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.