One of the more underrepresented materials in discussions of optics is optical cement—perhaps because it’s just been doing its job for decades. But it is an optical material in the light path that can have its own effects. In one special case of this, researchers from Fermi National Accelerator Laboratory (Batavia, IL) have done a study on the effect of ionizing radiation on some commercially available optically clear epoxies. It should be noted that the Fermi scientists’ interests in optics relate to plastic scintillation detectors, and thus the “optically clear” cements they chose for their study are somewhat different from those usually used for precision optics. They chose plastic-scintillator cements made by Saint-Gobain (Courbevoie, France) and Eljen Technologies (Sweetwater, TX) along with some more general-purpose clear epoxies from System Three (Lacey, WA) and other epoxy manufacturers.
The epoxies were all tested for compatibility with a standard poly(vinyltoluene) scintillator, assembling the scintillators, waiting a year, and then seeing if there were any changes to the scintillator at the epoxy interface: all epoxies passed this test easily. Next, epoxy samples were prepared in two batches separated by one year, with the samples run in a centrifuge before they cured to remove bubbles (samples were 7 mm thick, which is much thicker than the typical 0.1 mm joint thickness). Transmittance over the 190–1100 nm wavelength range was measured before and after exposure to a mixed radiation field at Fermilab’s accelerator. Extrapolating from the 7 mm thickness, measurements showed at least a 5% loss for a 0.1 mm thickness, although two cements did quite a bit better, especially at 400 nm, than the rest. One was difficult to work with, as its silicone-like rubberiness prevented polishing, but the other, made by System Three, was pronounced a good potential candidate for bonding scintillators to light guides or photosensors. Reference: R. J. Tesarek et al., arXiv:2007.02996v1 (July 6, 2020).
