Nuclear Charge Radius of ${}^9$Be from Muonic Atom Spectroscopy Using a Microcalorimeter

The $2p\to 1s$ transition energy in muonic ${}^9$Be was measured using a metallic magnetic calorimeter, resulting in $E_{2p\to 1s}=33391.48(34)$eV. The result is 30 times more precise than the previous best measurement and enables the extraction of the corresponding nuclear charge radius $r_c($${}^9$Be$)=2.5506(51)$fm. It is $2.4$ times more precise than the commonly used value based on electron scattering and differs from it by $2.3$ times the combined uncertainties. This measurement represents the first determination of a nuclear charge radius using muonic x-ray spectroscopy with microcalorimeters.