The X-ray tube envelope that encloses a vacuum is made of:

• A. Gadolinium alloy
• B. Pyrex glass
• C. Kevlar composite
• D. Tungsten alloy

Answer: (B)

Holding a high vacuum inside the tube while enduring repeated heating is essential for the X-ray tube envelope. Pyrex glass, a borosilicate glass, is used because its very low coefficient of thermal expansion minimizes stress during heating and cooling, reducing the chance of cracks or seal failure. It also forms a reliable vacuum seal with metal components, helping maintain the near-perfect vacuum needed to prevent arcing and gas ionization. Additionally, it provides sufficient X-ray transparency for the photons to exit the tube with minimal attenuation. The other materials don’t fit as the envelope: gadolinium alloy isn’t used for vacuum envelopes, Kevlar is a polymer that would outgas and fail to maintain vacuum at operating temperatures, and a tungsten alloy is suited for the filament or target, not the envelope itself due to poor vacuum performance and excessive absorption.