Isomeric yield ratios and mass spectrometry of Y and Nb isotopes in the neutron-rich N=60 region: The unusual case of 98Y

The isomeric yield ratio (IYR) of fission products is an observable that carries relevant information about the fragments emerging from the scission of a fissioning nucleus. We report on IYR of 96,98,100 Y and 100,102 Nb, together with the previously reported values for 97 Y and 99 Nb, produced in the 28 MeV α -induced fission of 232 Th at the Ion Guide Isotope Separation On-Line (IGISOL) facility of the University of Jyväskylä. We measured the IYR using two different techniques, the phase-imaging ion-cyclotron-resonance (PI-ICR) and the multiple-reflection time-of-flight mass spectrometry (MR-TOF-MS) methods. Moreover, we measured the masses of the long-lived states in 98,100 Y and 100,102 Nb populated via in-trap β -decay of their precursors. Since the β -decay selectively populates states with a favourable spin-parity, we could identify the measured state and show that the ground state is the low-spin state in the cases of 98 Y and 100 Nb, while it is the high-spin state in the cases of 100 Y and 102 Nb. This measurement confirms the spin-parity assignments of all the nuclei as they are reported in the NUBASE2020 evaluations, disagreeing with the assignment for 100 Y reported in the ENSDF evaluation. Making also use of previously reported data, we observe an anomalously low IYR for the N = 59 isotope 98 Y as compared to other yttrium or neighboring niobium isotopes. This behavior is very rare across the nuclear chart and is posited to be connected to the characteristic shape coexistence of 98 Y, and to the change in the charge radii of the ground and excited states in the N = 58 − 60 region.