X-ray tube

[1] The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation.

The positive ions bombard the cathode of the tube to release electrons, which are accelerated toward the anode and produce X-rays when they strike it.

Vaporized tungsten condenses on the inside of the envelope over the "window" and thus acts as an additional filter and decreases the tube's ability to radiate heat.

[6] The range of photonic energies emitted by the system can be adjusted by changing the applied voltage, and installing aluminum filters of varying thicknesses.

The number of emitted X-ray photons, or dose, are adjusted by controlling the current flow and exposure time.

The cathode was concave so that the electrons were focused on a small (~1 mm) spot on the anode, approximating a point source of X-rays, which resulted in sharper images.

[citation needed] To operate, a DC voltage of a few kilovolts to as much as 100 kV was applied between the anodes and the cathode, usually generated by an induction coil, or for larger tubes, an electrostatic machine.

[citation needed] The glass envelope of the tube would blacken with usage due to the X-rays affecting its structure.

[citation needed] In the Coolidge tube, the electrons are produced by thermionic effect from a tungsten filament heated by an electric current.

In one common type of end-window tube, the filament is around the anode ("annular" or ring-shaped), the electrons have a curved path (half of a toroid).

[citation needed] What is special about side-window tubes is an electrostatic lens is used to focus the beam onto a very small spot on the anode.

The anode is specially designed to dissipate the heat and wear resulting from this intense focused barrage of electrons.

[citation needed] A considerable amount of heat is generated in the focal spot (the area where the beam of electrons coming from the cathode strike to) of a stationary anode.

[citation needed] The major drawback of solid-anode microfocus X-ray tubes is their very low operating power.

[11][12] In the case with a 10 μm electron-beam focus a metal-jet-anode microfocus X-ray source may operate at 30-60 W. The major benefit of the increased power density level for the metal-jet X-ray tube is the possibility to operate with a smaller focal spot, say 5 μm, to increase image resolution and at the same time acquire the image faster, since the power is higher (15-30 W) than for solid-anode tubes with 10 μm focal spots.

Any vacuum tube operating at several thousand volts or more can produce X-rays as an unwanted byproduct, raising safety issues.

In the late 1960s it was found that a failure in the HV supply circuit of some General Electric TVs could leave excessive voltages on the regulator tube, causing it to emit X-rays.

The same failure mode was also observed in early revisions of Soviet-made Rubin TVs equipped with GP-5 voltage-regulator tube.

The models were recalled and the ensuing scandal caused the US agency responsible for regulating this hazard, the Center for Devices and Radiological Health of the Food and Drug Administration (FDA), to require that all TVs include circuits to prevent excessive voltages in the event of failure.

These other technologies, such as LED, LCD and OLED, are incapable of producing x-rays due to the lack of a high voltage transformer.