Optical Čerenkov radiation from microscopic BaSO4grains

Abstract

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 \ensuremath{\sim}0.5 \ensuremath{\mu}m), which constitute the highly reflective interior surface of our light-integration box, emit \ifmmode \check{C}\else \v{C}\fi{}erenkov 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 (\ensuremath{\sim} 2% of the $\ensuremath{\beta}=1$ signal) is consistent with the expected level due to \ifmmode \check{C}\else \v{C}\fi{}erenkov emitting delta rays and/or to diffraction radiation with no residual scintillation.