What material is most commonly used to make the envelope surrounding the x-ray tube?

The envelope surrounding the x-ray tube is most commonly made of Pyrex. Pyrex is a type of glass known for its ability to withstand high temperatures and thermal shock, making it ideal for the harsh conditions present in an x-ray tube, where high voltage and heat are generated during operation.

Pyrex envelopes provide excellent structural integrity, ensuring that the vacuum seal necessary for the efficient operation of the tube is maintained. The material's properties help prevent any contamination or degradation from the radiation produced, ensuring the longevity and safety of the x-ray equipment.

While regular glass can also be used, it may not withstand the high temperatures and pressures as effectively as Pyrex. Materials like PVC and ceramic are not typically used for x-ray tube envelopes due to their lower temperatures or mechanical strength capabilities under the operating conditions of an x-ray system.