Muonic atoms are formed when negative muons come to rest and subsequently get captured by a nearby atom, where it behaves like a heavy atomic electron. During the cascade down to the lowest atomic orbital it, Auger electrons and (muonic) x-rays are emitted. Due of its large mass of about 200 times the electron mass, the muon resides 200 times closer to the atomic nucleus. Therefore this exotic atom is and excellent system to study nuclear finite size effects and short range interactions.

We perform high-precision muonic x-ray spectroscopy experiments at the Paul Scherrer Institute in Switzerland on a wide range of nuclei, combining High-Purity Germanium (HPGe) or Magnetic Magnetic Calorimeters (MMC) x-ray detectors with novel target methods and stat-of-the-art digitizing and pulse analysis techniques to achieve the highest accuracy on the transition energies. In addition, the large acceptance of the HPGe array allows us to study transitions with a very low branching ratio.

The most straight forward observable is the nuclear charge radius, typically derived from the 2p-1s transition energy, taking into account all necessary bound-state QED and nuclear polarization contributions. This radius then serves as a benchmark for nuclear structure calculations, laser spectroscopy measurements, or pins down the finite size contribution of precision standard model tests.

We perform high-precision muonic atom spectroscopy with variety of techniques. A few selected transitions in muonic hydrogen and helium are accessible for laser spectroscopy, where the highest accuracy can be reached. For more information, see https://www.agpohl.physik.uni-mainz.de/. The transition energies of the light muonic atoms from helium to neon are in the range of 20-200 keV, where the resolution of a solid state detector is insufficient to precisely determine the nuclear finite size effect. In collaboration with colleagues at the Kirchhoff-Institut für Physik in Heidelberg, we deployed novel Magnetic Metallic Callorimeters (MMCs) at a muon facility for the first time.

The combination of exceptional resolving power and broad energy acceptance makes these devices ideally suited for high-precision exotic-atom spectroscopy measurements. In particular, the Quartet collaboration aims to determine absolute nuclear charge radii from lithium to neon with a relative precision of 0.1%.

First measurements with stable lithium isotopes demonstrated the capability to determine muonic x-ray energies to precision better then 1 eV under the conditions at a secondary muon beamline. An dedicated MMC design improved the the robustness of the detector against various beam induced backgrounds.

In addition to pushing to accuracy of x-ray measurements to 10 ppm or better, we are developing novel target techniques expanding the applicability of muonic x-ray spectroscopy.

These advances are primarily based on a transfer processes from muonic hydrogen to the isotope of interest. By decoupling the stopping of the muon beam, and the formation of the muonic atom, the minimum amount of target material can be as low as a few micro grams, thus allowing to measure less abundant stable isotopes, or even long lived radioactive elements.