  We consider a measurement of the gravitational acceleration of antimatter,
gbar, using muonium. A monoenergetic, low-velocity, horizontal muonium beam
will be formed from a surface-muon beam using a novel technique and directed at
an atom interferometer. The measurement requires a precision three-grating
interferometer: the first grating pair creates an interference pattern which is
analyzed by scanning the third grating vertically using piezo actuators.
State-of-the-art nanofabrication can produce the needed membrane grating
structure in silicon nitride or ultrananoscrystalline diamond. With 100 nm
grating pitch, a 10% measurement of gbar can be made using some months of
surface-muon beam time. This will be the first gravitational measurement of
leptonic matter, of 2nd-generation matter and, possibly, the first measurement
of the gravitational acceleration of antimatter.
