A: When exposed to Gamma or X-ray radiation most optical materials will change colour, typically dark brown, resulting in almost total loss of transmission. Fortunately, there are some standard optical glass that can be stabilized against the transmittance loss caused by ionizing radiation by adding cerium to the composition. The added cerium changes the intrinsic colour of the glass from white to a yellowish tint. The goal of creating a cerium-doped glass, is to optimize the cerium content to achieve high stability against radiation while keeping the tint colouring to a minimum.
Read technical note: https://www.resolveoptics.com/radiation-resistance-what-does-it-mean/