X-ray tube Created 2026-10-18 Updated 2026-10-20
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An X-ray tube accelerates electrons from a negative cathode to a positive target anode. A heated cathode supplies electrons by thermionic emission; the evacuated envelope limits collisions before the target. This differs from the gas-discharge tube in the Röntgen X-ray discovery experiment.[ref]
Figure 1.
From accelerated electrons to a measured X-ray transmission signal
. Original schematic of a modern heated-cathode X-ray tube and a narrow-beam measurement, not Röntgen's apparatus. Electrons stop in the target; emerging X-rays continue toward the detector. Apertures of a collimator select a small angular range and reject much scattered radiation. The two reference readings use identical geometry. Arrow widths and component sizes are schematic. Details.
In bremsstrahlung, an electron changes velocity in the target's electric fields, emitting a photon while losing kinetic energy. Different encounters allow a continuous range of transferred energies. This is distinct from the discrete electronic transitions producing Characteristic X-rays.
Changing the accelerating voltage changes the available kinetic energy per electron and the X-ray tube cutoff. Changing the number of arriving electrons changes the number of opportunities for photon emission. These are distinct controls: a larger electric current does not by itself give each electron more energy. For an original idealized budget, and tube electric current give electrical power and arrival rate , where is the elementary charge. This is input power, not emitted X-ray power; obtaining the latter requires a conversion efficiency.
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