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

Simone Cannarozzo, Stephan Pomp, Anu Kankainen, Iain Moore, Marek Stryjczyk, Ali Al-Adili, Andreas Solders, Ville Virtanen, Tommi Eronen, Zhihao Gao, Zhuang Ge, Arthur Jaries, Mattias Lantz, Maxime Mougeot, Heikki Penttilä, Andrea Raggio, Jouni Ruotsalainen

TL;DR

This work addresses the isomeric yield ratios (IYRs) of fission fragments in the neutron-rich $N=60$ region, focusing on the unusual case of $^{98}$Y. The authors measure IYRs for $^{96,98,100}$Y and $^{100,102}$Nb from 28 MeV $\alpha$-induced fission of $^{232}$Th at IGISOL, using two high-resolution mass-spectrometry techniques, PI-ICR and MR-TOF-MS, complemented by selective in-trap $\beta$-decay mass measurements to fix the ordering of isomeric states and test $NUBASE2020$ against ENSDF. They find an anomalously low IYR for $^{98}$Y relative to neighbors and demonstrate that the ground-state spins for $^{98}$Y and $^{100}$Nb are low-spin, while those for $^{100}$Y and $^{102}$Nb are high-spin, clarifying data disagreements for $^{100}$Y. The results are interpreted in terms of shape coexistence in $^{98}$Y and associated changes in mean-square radii in the $N=58-60$ region, illustrating how IYR measurements can probe nuclear shape evolution in neutron-rich fission fragments.

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 $α$-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 $β$-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.

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

TL;DR

This work addresses the isomeric yield ratios (IYRs) of fission fragments in the neutron-rich region, focusing on the unusual case of Y. The authors measure IYRs for Y and Nb from 28 MeV -induced fission of Th at IGISOL, using two high-resolution mass-spectrometry techniques, PI-ICR and MR-TOF-MS, complemented by selective in-trap -decay mass measurements to fix the ordering of isomeric states and test against ENSDF. They find an anomalously low IYR for Y relative to neighbors and demonstrate that the ground-state spins for Y and Nb are low-spin, while those for Y and Nb are high-spin, clarifying data disagreements for Y. The results are interpreted in terms of shape coexistence in Y and associated changes in mean-square radii in the region, illustrating how IYR measurements can probe nuclear shape evolution in neutron-rich fission fragments.

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 Y and Nb, together with the previously reported values for Y and Nb, produced in the 28 MeV -induced fission of 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 Y and Nb populated via in-trap -decay of their precursors. Since the -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 Y and Nb, while it is the high-spin state in the cases of Y and 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 Y reported in the ENSDF evaluation. Making also use of previously reported data, we observe an anomalously low IYR for the isotope 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 Y, and to the change in the charge radii of the ground and excited states in the region.

Paper Structure

This paper contains 6 sections, 1 equation, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Experimental methods
  3. IYR measurement with the MR-TOF-MS for $^{96}$Y
  4. IYR measurement with the PI-ICR technique at JYFLTRAP
  5. Mass spectrometry of $^{98,100}$Y and $^{100,102}$Nb with selective in-trap beta decay
  6. Results and discussion

Figures (4)

  • Figure 1: In black, IYR of Y ($Z=39$) and Nb ($Z=41$) isotopes produced by the 28 MeV $\alpha$-particle induced fission for $^{233}$Th as a function of neutron number N. In red, the changes in the mean-square charge radii $\delta\langle r^2 \rangle$ (data from Ref. ANGELI201369_rms) compared to $^{89}$Y and $^{93}$Nb for yttrium and niobium isotopes, respectively. Hollow markers correspond to $\delta\langle r^2 \rangle$ of the isomeric states (data taken from Ref. Cheal).
  • Figure 2: MR-TOF-MS mass spectrum of fission products with mass $A = 96$ produced by the $^{232}$Th($\alpha$,f) fission reaction. The top plot shows the complete spectrum, while the bottom plot presents a zoom in of the nuclei of interest, i.e. the isomeric states of $^{96}$Y. The time of flight of $^{96}$Y was 28.254 ms after 864 revolutions.
  • Figure 3: PI-ICR images for the studied Y and Nb isotopes. In each figure, the ground state (gs) and excited state (exc) have been indicated as well as the phase accumulation time.
  • Figure 4: IYR of Y and Nb isotopic series compared to bibliographic data from different fissioning systems retrieved from the nuclear data library EXFOR EXFORExperimentalNuclear. The reason for the disagreement on the IYR of $^{99}$Nb is not clearly identifiable, but might be related to using an outdated decay scheme in the $\gamma$-spectroscopy measurements. Data taken from Refs. Kudo1998KU01Campbell1997CAZWTanikawa199353RakAM_tofGao2023_iyrsGoto1999GO06Troutner1963TR01Denschlag1977_exfor20878.1Weis1981WE20TipnisBugrov1985_exfor40895.1BailLietz1986_exfor22017.1Martin