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Lighting Gallery
Odysee

X-ray Tubes

Electrically X-rays can be emitted with high-speed electrons interacting with atoms. The efficiency isn't high and more heat is generated than x-rays, but the x-ray energy is controllable. Acceleration voltages over 20kV are generally necessary to start x-ray emission. Production of x-rays starts at less than 10kV but these are generally shielded as glass is opaque to these. Some valves, used in high voltage circuits have warnings printed on them for x-ray emission, as on this PD500 triode. It is also possible, although damaging, to use some valves to produce x-rays in cold cathode mode.

Early x-ray tubes were used with cold cathodes and relied on controlling small gas reservoirs to regulate the low pressure to control the x-ray emission. Thermionic emission and better vacuums bought more control and reduced the voltage required for later x-ray tubes.


Different anode materials could also be used for x-ray production, although all have to be good at conducting heat away from the target point. A focused electron beam and spot target is essential to produce a point source for image clarity. Some use a beryllium window for its transparency to x-rays.

The diagram shows the main internal parts of a typical x-ray head assembly.

O:- The entire housing contains oil for its electrical insulating and thermal cooling properties.

N:- A rubber diaphragm allows the oil to expand and contract without increasing the internal pressure.

T: - X-ray tube body is wholly contained in the assembly housing except for the window where the x-rays emerge.

K:- Two cathode heaters are provided. Each have different electron beam properties.

A:- Target anode. The anode in most modern x-ray tubes rotate. This reduces the effect of heating on the anode surface by the incident electron beam.

R:- The rotor and bearings are part of the anode assembly and are fully contained inside the evacuated X-ray tube body.

S:- Stator coils are mounted on the outside of the tube. Their magnetic field drives the rotor and therefore spins the anode when operational.


The images on the right are taken at differing energies, upper is a flower, the lower is a transmitting valve. Both images are shown on a green fluorescent screen.

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