Speaker
Description
Nuclear electromagnetic moments provide sensitive probes of the distribution of charge and magnetization inside the nucleus, yet higher-order moments remain largely unexplored. Current high-resolution laser spectroscopy techniques allow to achieve precisions of the order of 1 MHz [1], giving access to magnetic dipole and electric quadrupole moments. However, as the multipole order increases, the magnitude of the shift in the atomic energy levels decreases rapidly, from ~ GHz for the dipole to hundreds of MHz for the quadrupole and only hundreds of kHz for the octupole, with even smaller shifts expected for higher orders, leaving the octupole and beyond out of reach of standard spectroscopic techniques. The magnetic octupole moment is nevertheless of particular interest, as it offers a rare window on subtle aspects of the nuclear magnetization distribution and thus provides a stringent test for nuclear-structure models.
In this work, we target the magnetic octupole moment of stable $^{209}$Bi through a precision measurement of the hyperfine structure of its atomic ground state $^4S_{3/2}$. With one valence proton outside the doubly magic $^{208}$Pb core, $^{209}$Bi is a benchmark near-single-particle system in which experimental results can be confronted directly with modern nuclear-structure calculations.
To enable this measurement, we have developed at KU Leuven an atomic beam apparatus for laser-radio-frequency double-resonance spectroscopy [2]. Beyond the specific case of $^{209}$Bi, this effort is motivated by the longer-term goal of extending the method to radioactive isotopes, where precision measurements of higher-order moments could provide new structural information far from stability. The setup and analysis procedure were first developed and validated using potassium, allowing the dominant sources of systematic uncertainty to be identified and controlled before moving to bismuth.
We will present the performance of the apparatus, results from the potassium commissioning measurements, and highest-precision hyperfine-structure data to date on $^{209}$Bi. Combined with state-of-the-art atomic structure calculations, these measurements will enable the extraction of a precise magnetic octupole moment for $^{209}$Bi and provide a new benchmark for the description of magnetization properties in heavy nuclei.
[1] P. Campbell, I.D. Moore, and M.R. Pearson, ’Laser spectroscopy for nuclear structure physics’, Progress in Particle and Nuclear Physics, vol. 86, pp. 127-180, 2016.
[2] W.J. Childs, ’Overview of laser-radiofrequency double-resonance studies of atomic, molecular, and ionic beams’, Physics Reports, vol. 211.3, pp. 113-165, 1992.