  We have designed, constructed, and commissioned a cryogenic helium buffer-gas
source for producing a cryogenially-cooled molecular beam and evaluated the
effect of different cell geometries on the intensity of the produced molecular
beam, using ammonia as a test molecule. Planar and conical entrance and exit
geometries are tested. We observe a three fold enhancement in the NH$_3$ signal
for a cell with planar-entrance and conical-exit geometry, compared to that for
a typically used `box'-like geometry with planar entrance and exit. These
observations are rationalized by flow-field simulations for the different
buffer-gas cell geometries. The full thermalization of molecules with the
helium buffer-gas is confirmed through rotationally-resolved REMPI spectra
yielding a rotational temperature of 5 K.
