Nuclear charge radius of 9 Be from muonic atom spectroscopy using a microcalorimeter

The 2p→1s transition energy in muonic ⁹Be was measured using a metallic magnetic calorimeter, resulting in E2p→ 1s=33\,391.48(34)\,eV. The result is 30 times more precise than the previous best measurement and enables the extraction of the corresponding nuclear charge radius rc(^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.