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

Abstract

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 $\hat{I}\pm$-induced fission of ${}^{232}$Th at the Ion Guide Isotope Separation On-Line (IGISOL) facility of the University of Jyv{ä}skyl{ä}. 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 ${\hat{I}}^2$-decay of their precursors. Since the ${\hat{I}}^2$-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.

Submitted to Physics Letters B (corrected version)