Optical Čerenkov radiation from microscopic BaSO4grains

In the analysis of an experiment to determine the response of scintillators to relativistic heavy ions, we have discovered that the BaS${\mathrm{O}}_4$ grains (radius ∼0.5 μm), which constitute the highly reflective interior surface of our light-integration box, emit Čerenkov radiation with an index of refraction intermediate to that of crystalline BaS${\mathrm{O}}_4$ and to that of the air-crystal volume-averaged reflective coating. The small amount of radiation emitted below threshold (∼ 2% of the $\beta =1\mathrm{}$ signal) is consistent with the expected level due to Čerenkov emitting delta rays and/or to diffraction radiation with no residual scintillation.