  We study the properties of a cryogenic buffer gas source that uses a low
temperature two-stage buffer gas cell to produce very slow beams of ytterbium
monofluoride molecules. The molecules are produced by laser ablation inside the
cell and extracted into a beam by a flow of cold helium. We measure the flux
and velocity distribution of the beam as a function of ablation energy, helium
flow rate, cell temperature, and the size of the gap between the first and
second stages of the cell. We also compare the velocity distributions from
one-stage and two-stage cells. The one-stage cell emits a beam with a speed of
about 82 m s$^{-1}$ and a translational temperature of 0.63 K. The slowest
beams are obtained using the two-stage cell at the lowest achievable cell
temperature of 1.8 K. This beam has a peak velocity of 56 m s$^{-1}$ and a flux
of $9 \times 10^9$ ground state molecules per steradian per pulse, with a
substantial fraction at speeds below 40 m s$^{-1}$. These slow molecules can be
decelerated further by radiation pressure slowing and then captured in a
magneto-optical trap.
