Muonic atom spectroscopy is a well-established technique for measuring absolute nuclear charge radii with high precision and has been successfully applied across a wide range of the nuclear chart, from the proton and the lightest nuclei to medium-mass, heavy, and radioactive isotopes. Precise nuclear charge radii provide important benchmarks for modern ab initio nuclear theory and serve as essential nuclear-structure input for high-precision atomic calculations, including tests of QED and the determination of fundamental constants.
However, despite several decades of work on muonic atom spectroscopy, precise measurements remain unavailable for most of the light nuclei from lithium to neon, largely because their muonic x-ray energies of approximately 20每200 keV fall into a technologically challenging range for precision spectroscopy. The QUARTET collaboration aims to close this gap using cryogenic metallic magnetic calorimeters (MMCs), which combine record resolving power with broadband detection efficiency and thereby provide access to precision spectroscopy of these light muonic atoms.
This talk presents the first high-resolution broadband x-ray spectroscopy measurement of muonic lithium using MMCs, performed at the Paul Scherrer Institute. With this approach, an energy resolution of about 20 eV FWHM was achieved at the 米Li 2p-1s transitions near 19 keV, enabling their energies to be measured with sub-eV precision. This allowed a new extraction of the absolute nuclear charge radii of 6-Li and 7-Li in combination with atomic theory calculations, providing the first independent confirmation of the previously accepted values from electron scattering data while improving their precision by more than a factor of four. Beyond the charge-radius measurement itself, the measured spectra showcase the new level of detail accessible with MMCs and illustrate how the improved experimental resolution provides access to previously unresolved features of the muonic x-ray spectrum and cascade dynamics. The measurement thus demonstrates the potential of this detector technology for precision spectroscopy of muonic atoms and establishes a basis for its application in future experimental campaigns.