Origin of the electric hexadecapole isomer in

We present a shell-model analysis of 93Mo to investigate the un­usual behavior of its 21/2+ isomer - a prominent candidate for nuclear excita­tion by electronic capture. This state is unique as its decay is dominated by a slow electric hexadecapole E4 transition, while the typically much faster elec­tric quadrupole E2 decay path is energetically forbidden. We investigate the microscopic origin of this phenomenon by examining in detail the structure of the wave functions of the initial and final states, and the E4 transition matrix elements. This analysis of 93Mo is contrasted with that of its particle-hole con­jugate, 99Cd, where such an E4 transition is absent.