  Muonic atom spectroscopy -- the measurement of the x rays emitted during the
formation process of a muonic atom -- has a long standing history in probing
the shape and size of nuclei. In fact, almost all stable elements have been
subject to muonic atom spectroscopy measurements and the absolute charge radii
extracted from these measurements typically offer the highest accuracy
available. However, so far only targets of at least a few hundred milligram
could be used as it required to stop a muon beam directly in the target to form
the muonic atom. We have developed a new method relying on repeated transfer
reactions taking place inside a 100-bar hydrogen gas cell with an admixture of
0.25% deuterium that allows us to drastically reduce the amount of target
material needed while still offering an adequate efficiency. Detailed
simulations of the transfer reactions match the measured data, suggesting good
understanding of the processes taking place inside the gas mixture. As a proof
of principle we demonstrate the method with a measurement of the 2p-1s muonic x
rays from a 5-{\mu}g gold target.
