  The next generation of "intensity frontier" facilities will bring a
significant increase in the intensity of sub-relativistic beams of $\mu^-$. We
show that the use of these beams in combination with thin targets of $Z\sim 30$
elements opens up the possibility of testing parity-violating interactions of
muons with nuclei via direct radiative capture of muons into atomic 2S
orbitals. Since atomic capture preserves longitudinal muon polarization, the
measurement of the gamma ray angular asymmetry in the single photon
$2S_{1/2}$-$1S_{1/2}$ transition will offer a direct test of parity. We
calculate the probability of atomic radiative capture taking into account the
finite size of the nucleus to show that this process can dominate over the
usual muonic atom cascade, and that the as yet unobserved single photon
$2S_{1/2}$-$1S_{1/2}$ transition in muonic atoms can be detected in this way
using current muon facilities.
