  We demonstrate a method for probing interaction effects in a thermal beam of
strontium atoms using simultaneous measurements of Rydberg EIT and
spontaneously created ions or electrons. We present a Doppler-averaged optical
Bloch equation model that reproduces the optical signals and allows us to
connect the optical coherences and the populations. We use this to determine
that the spontaneous ionization process in our system occurs due to collisions
between Rydberg and ground state atoms in the EIT regime. We measure the cross
section of this process to be 0.6 $\pm$ 0.2 $\sigma_{\rm{geo}}$, where
$\sigma_{\rm{geo}}$ is the geometrical cross section of the Rydberg atom. This
results adds complementary insight to a range of recent studies of interacting
thermal Rydberg ensembles.
