  We describe the creation and characterisation of a velocity tunable,
spin-polarized beam of slow metastable argon atoms. We show that beam velocity
can be determined with a precision below 1 \% using matter-wave interferometry.
The profile of the interference pattern was also used to determine the velocity
spread of the beam, as well as Van der Waals co-efficient for the interaction
between the metastable atoms and the multi-slit silicon nitride grating. The
Van der Waals co-efficient was determined to be $C_3$=1.84$\pm$0.17\,a.u., in
good agreement with values derived from spectroscopic data. Finally, the spin
polarization of the beam produced during acceleration of the beam was also
measured, demonstrating a spatially uniform spin polarization of 96 \% in the
m=+2 state.
