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.
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