Threshold effect in the conversion transition of the 90mNb isomer (2.38 ± 0.36 kehV)

Conversion of the mixed M2+E3 transition of the 90mNb isomer state has been investigated in some detail for the case of an isolated niobium atom. A relativistic variant of the Hartree-Fock-Slater method was used in the calculation of the electron wave functions. An extensive variation of the valence band structure for the atomic shell has been performed. It is shown that the observed changes in the isomer decay rate, δλ/λ ∼ 5%, have a natural explanation as a threshold effect; however, the behaviour of the conversion probability near the thresholds, which was founded in the calculation and has its specific features for every particular configuration, does not permit to explain the magnitude of δλ/λ solely as an effect owing to a chemical shift in the orbit binding energy. An expected admixture of the E3-multipole state for the 90Nb41 nucleus is estimated in the framework of the shell model; the result is 1-4%. It is shown that the detected strong retardation of the M2-transition results from a cancellation of the hole transitions (1f5/2)-1 → (1g9/2)-1 in the proton and neutron subsystems