Answer to Question #4073 Submitted to "Ask the Experts"

Lead-glass windows provided in an x-ray room are transparent to light, but how are they made to be radiopaque?

Lead-glass windows incorporate lead in the form of lead oxide into the glass during manufacture. The lead oxide has a small negative effect on the transmission of visible light through the glass; it does somewhat increase the index of refraction of the glass for visible light (this effect has little significance in a flat window, but it is used to advantage in high-lead content cut crystal glassware to increase the “color” through bending of the light and enhancing separation of different wavelengths).

The small effect on visible light transmission has to do with the electronic configuration of lead in the glass not being such as to favor strong absorption of energy from the visible light photons. We might note that x rays and other ionizing photons have energies typically 10,000 to millions of times larger than visible light photons. As such they may interact with different electrons within the atoms of material than do visible light photons; when x rays are attenuated by lead, the attenuation is a result of the x rays transferring their energy to tightly bound inner-shell electrons of the lead atoms. These inner-shell electrons are not affected at all by the much-lower-energy visible light.

The lead-glass effectiveness in stopping x rays stems from the additional facts that (1) the attenuation of ionizing photons, of which x rays are an example, increases dramatically as the atomic number of the attenuating material increases, (2) the attenuation of such photons increases as the mass density of the attenuating material increases, and (3) the attenuation of such photons also increases dramatically as the energies of the ionizing photons decrease. The lead has a high atomic number, 82, much higher than the normal constituents in glass; it also has a high atomic mass that increases somewhat the density of the glass. Finally, x rays used in medicine, especially diagnostic x rays, often have relatively low energies as far as ionizing radiation goes, and this makes for strong attenuation. The lead glass used in windows often has amounts of lead added that make the window equivalent in attenuation to 1 to 2 mm of pure lead. You are probably aware that lead is also incorporated into aprons that are worn by x-ray technologists and others to reduce dose from x rays. These aprons typically have amounts of lead equivalent to 0.25 to 0.5 mm of lead.

I hope this answer is helpful to you.

George Chabot, PhD, CHP