  Muons are a fascinating probe to study nuclear properties. Muonic atoms can
easily be formed by stopping negative muons inside a material. The muon is
subsequently captured by the nucleus and, due to its much higher mass compared
to the electron, orbits the nucleus at very small distances. During this atomic
capture process the muon emits characteristic X-rays during its cascade down to
the ground state. The energies of these X-rays reveal the muonic energy level
scheme, from which properties like the nuclear charge radius or its quadrupole
moment can be extracted. While almost all stable elements have been examined
using muons, probing highly radioactive atoms has so far not been possible. The
muX experiment has developed a technique based on transfer reaction inside a
high pressure hydrogen/deuterium gas cell to examine targets available only in
microgram quantities.
