Compare glass envelope tubes with metal envelope tubes in terms of heat handling, vacuum stability, and durability.

• A. Glass envelopes are common and inexpensive but can outgas and fail with heat; metal/ceramic envelopes tolerate higher heat, have better vacuum stability, and offer greater durability.
• B. Metal envelopes are always superior in every aspect.
• C. Glass envelopes never outgas.
• D. There is no difference between envelope types.

Answer: (A)

Envelope material affects heat handling, vacuum stability, and durability in X-ray tubes. Glass envelopes are common and inexpensive, which is why you’ll see them in many lower-power tubes. But when glass heats up during operation, it tends to outgas, releasing gas molecules that can degrade the vacuum and raise the risk of arcing or vacuum failure. Glass can also soften or crack under high heat or thermal stress, limiting how much heat the tube can safely carry.

Metal and ceramic envelopes are designed to withstand higher temperatures. They maintain a more stable vacuum because their seals and internal surfaces release far fewer gases at operating temperatures, and the construction helps resist vacuum degradation during thermal cycling. They also offer greater durability due to stronger mechanical strength and better resistance to thermal shock and vibration, which is important for high-power or demanding duty cycles.

Therefore, the described comparison—glass envelopes being common and inexpensive but prone to outgassing and heat-related failure, versus metal/ceramic envelopes handling more heat, offering better vacuum stability, and being more durable—captures the practical differences well.