Which material is used for the vacuum envelope of an x-ray tube?

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

Answer: (C)

The vacuum envelope must hold a high-quality vacuum while withstanding the intense heat of the electron beam and allowing x-rays to pass with minimal attenuation. Pyrex glass (borosilicate) fits this well because it has very low thermal expansion and excellent thermal shock resistance, so it won’t crack from heating. It also maintains a good vacuum seal and can be manufactured with a thin window section to transmit x-rays efficiently. The other materials don’t meet these needs: gadolinium alloy isn’t used for vacuum enclosures, Kevlar is a protective fiber material not suitable for a vacuum tube envelope, and tungsten alloy is chosen for the anode target due to its high melting point, not for the envelope. So Pyrex glass is the appropriate choice for the vacuum envelope.