Final answer:
An X-ray tube operates under a vacuum to allow electrons to travel unimpeded towards the anode where they produce X-rays, and to prevent oxidation of the filament and anode for efficient operation and durability.
Step-by-step explanation:
The X-ray tube is a vacuum to allow electrons to travel without interacting with air molecules, which could scatter and absorb them, decreasing the efficiency of X-ray production. Heinrich Geissler, the pioneer of the cathode ray tube, found that creating a vacuum in the tube was crucial for its operation. In modern X-ray tubes, a vacuum ensures that the energetic electrons ejected from the hot filament can be accelerated without resistance towards the anode, often made of tungsten. When these electrons strike the anode, characteristic X-rays are produced as a result of inner-shell vacancies being filled. For effective generation of X-rays, such as those with energy levels typical of heavy elements like tungsten, a significant accelerating voltage is necessary. For instance, tungsten requires at least 72.5 kV to create these vacancies. Moreover, the vacuum helps prevent the filament and anode from oxidizing, which would lower their efficiency and lifespan.