  We study collisions between neutral, deuterated ammonia molecules (ND$_3$)
stored in a 50 cm diameter synchrotron and argon atoms in co-propagating
supersonic beams. The advantages of using a synchrotron in collision studies
are twofold: (i) By storing ammonia molecules many round-trips, the sensitivity
to collisions is greatly enhanced; (ii) The collision partners move in the same
direction as the stored molecules, resulting in low collision energies. We tune
the collision energy in three different ways: by varying the velocity of the
stored ammonia packets, by varying the temperature of the pulsed valve that
releases the argon atoms, and by varying the timing between the supersonic
argon beam and the stored ammonia packets. These give consistent results. We
determine the relative, total, integrated cross-section for
$\mathrm{ND}_3+\mathrm{Ar}$ collisions in the energy range of 40-140 cm$^{-1}$,
with a resolution of 5-10 cm$^{-1}$ and an uncertainty of 7-15%. Our
measurements are in good agreement with theoretical scattering calculations.
