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High-Resolution Laser Spectroscopy on the Hyperfine Structure of $^{255}$Fm (Z=100)

M. Urquiza-González, M. Stemmler, T. E. Albrecht, B. Bally, M. Bender, S. Berndt, M. Block, A. Brizard, J. S. Andrews, J. Bieron, P. Chhetri, H. Dorrer, C. E. Düllmann, J. G. Ezold, S. Goriely, M. J. Gutiérrez, D. Hanstorp, R. Hasse, R. Heinke, K. Hens, S. Hilaire, M. Kaja, T. Kieck, N. Kneip, U. Köster, A. T. Loria Basto, C. Mokry, D. Münzberg, K. Myhre, T. Niemeyer, S. Péru, S. Raeder, D. Renisch, J. Runke, S. K. Schrell, D. Studer, K. van Beek, J. Warbinek, K. Wendt

Abstract

We report on high-resolution laser spectroscopy of $^{255}$Fm ($T_{1/2} = 20$h), one of the heaviest nuclides available from reactor breeding. The hyperfine structures in two different atomic ground-state transitions at 398.4~nm and 398.2~nm were probed by in-source laser spectroscopy at the RISIKO mass separator in Mainz, using the PI-LIST high-resolution ion source. Experimental results were combined with hyperfine fields from various atomic ab-initio calculations, in particular using MultiConfigurational Dirac-Hartree-Fock theory, as implemented in GRASP18. In this manner, the nuclear magnetic dipole and electric quadrupole moments were derived to be $μ= -0.75(5)~ μ_\textrm{N}$ and $Q_\textrm{S} = +5.84(13)$~eb, respectively. The magnetic moment indicates occupation of the $ν$~7/2[613] Nilsson orbital, while the large quadrupole moment confirms strong, stable prolate deformation consistent with systematics in the heavy actinides. Comparisons with available expectation values from nuclear theory show good agreement, providing a stringent benchmark for the used theoretical models. These results revise earlier data and establish $^{255}$Fm as a reference isotope for future high-resolution studies.

High-Resolution Laser Spectroscopy on the Hyperfine Structure of $^{255}$Fm (Z=100)

Abstract

We report on high-resolution laser spectroscopy of Fm (h), one of the heaviest nuclides available from reactor breeding. The hyperfine structures in two different atomic ground-state transitions at 398.4~nm and 398.2~nm were probed by in-source laser spectroscopy at the RISIKO mass separator in Mainz, using the PI-LIST high-resolution ion source. Experimental results were combined with hyperfine fields from various atomic ab-initio calculations, in particular using MultiConfigurational Dirac-Hartree-Fock theory, as implemented in GRASP18. In this manner, the nuclear magnetic dipole and electric quadrupole moments were derived to be and ~eb, respectively. The magnetic moment indicates occupation of the ~7/2[613] Nilsson orbital, while the large quadrupole moment confirms strong, stable prolate deformation consistent with systematics in the heavy actinides. Comparisons with available expectation values from nuclear theory show good agreement, providing a stringent benchmark for the used theoretical models. These results revise earlier data and establish Fm as a reference isotope for future high-resolution studies.

Paper Structure

This paper contains 3 equations, 3 figures, 3 tables.

Figures (3)

  • Figure 1: a) Section of the nuclear chart showing some of the reactor-accessible nuclides oakridge2015breeding-path, including $^{239}$Pu. Dark arrows show the primary breeding-path, while light arrows show secondary channels. Red arrows indicate the path taken for this work. Fully colored squares highlight isotopes for which nuclear moments are known. The main decay modes are color-coded following the key given on the right. An insert shows the end of the production pathway, indicating the nuclides' half-lives. b) Magnetic dipole moments of even-$Z$ nuclei ranging from plutonium to nobelium NUBASE2020Nothhelfer2022Weber2023Nobelium_Sebastian. Close-lying values are offset horizontally for visualization.
  • Figure 2: Measured HFS spectra for in-source spectroscopy (green) and with the PI-LIST (gray) together with results from Backe et al. Backe2005. For greater clarity, the spectra have an individual vertical offset (dashed lines). Left: spectra for R1 relative to the energy of 25 099.760(14) cm$^{-1}$. Center: spectra for R2 centered relative to the centroid energy of 25 111.760(12) cm$^{-1}$. On the right, the excitation schemes and the auto-ionizing states (AIS) used in this work are shown, along with their energies expressed in wavenumbers (cm$^{-1}$). The atomic configurations of the GS and excited states are also indicated Allehabi2020.
  • Figure 3: Predicted and experimental magnetic moments for some fermium isotopes. Common isotopes also included in Ref. Warbinek2024 are marked as solid symbols.