  Plastic scintillators are widely used in high-energy and medical physics,
often for measuring the energy of ionizing radiation. Their main disadvantage
is their non-linear response to highly ionizing radiation, called ionization
quenching. This nonlinearity must be modeled and corrected for in applications
where an accurate energy measurement is required. We present a new experimental
technique to granularly measure the dependence of quenching on
energy-deposition density. Based on this method, we determine the parameters
for four commonly used quenching models for two commonly used plastic
scintillators using protons with energies of 30 MeV to 100 MeV; and compare the
models using a Bayesian approach. We also report the first model-independent
measurement of the dependence of ionization quenching on energy-deposition
density, providing a purely empirical view into quenching.
