  Plastic scintillators are a versatile and inexpensive option for particle
detection, which is why the largest particle physics experiments, CMS and
ATLAS, use them extensively in their calorimeters. One of their challenging
aspects, however, is their relatively low radiation hardness, which might be
inadequate for very high luminosity future projects like the FCC-hh. In this
study, results on the effects of ionizing radiation on the optical properties
of plastic scintillator samples are presented. The samples are made from two
different matrix materials, polystyrene and polyvinyltoluene, and have been
irradiated at dose rates ranging from $2.2\,$Gy/h up to $3.4\,$kGy/h at room
temperature. An internal boundary that separates two regions of different
indices of refraction is visible in the samples depending on the dose rate, and
it is compatible with the expected oxygen penetration depth during irradiation.
The dose rate dependence of the oxygen penetration depth for the two matrix
materials suggests that the oxygen penetration coefficient differs for PS and
PVT. The values of the refractive index for the internal regions are elevated
compared to those of the outer regions, which are compatible with the indices
of unirradiated samples.
