X-ray tube

An x-ray tube functions as a specific energy converter, receiving the electrical energy and converting it into two other forms of energy: x-radiation and heat. Heat is considered the undesirable product of this conversion process; therefore x-radiation is created by taking the energy from the electrons and converting it into photons. This very specific energy conversion takes place in the x-ray tube.

Construction of the x-ray tube

The x-ray tube contains two principal elements:

1. cathode: provides a source of electrons
2. anode: acts as the target for electrons and releases x-rays

Additional components include:

• expansion bellows (provide space for oil to expand)
• tube envelope (evacuated)
• tube housing
• cooling dielectric oil
• rotor
• induction stator
• tube window

CathodeThe cathode and anode are contained in the envelope, which provides vacuum, support and electrical insulation. The envelope is most often createdmade from glass, although some tubes contain envelopes made offormed from ceramic or even metal.

The energy used for this process is provided from the generator, connected by an electrical circuit connected to the system. The generator also needs to convert the electrical energy fromalternating current (AC), from the power systemsupply, into the direct current (DC), being the adequate form to be applied toas needed by the x-ray tube.

The quality and the quantity of the x-radiation are controlled by adjusting the electrical parameters (kV(kV – voltage or the potential applied to the tube, mA – current that flows through the tube) and exposure time, usually a fraction of a second.

To summarise, x-rays are produced in a standard way: by accelerating electrons with a high voltage and allowing them to collide with the focal spot. xX-rays are produced when the electrons are suddenly decelerated upon collision with the metal target. These x-rays are called the “braking radiation” (Bremsstrahlungbremsstrahlung). If the electrons have high energy, they can expel an electron out of the atomic shell of the bombarded atom. Electrons from a higher stateenergy level fill the place of the expelled electron, emitting the x-ray photons with precisequantized (precise) energies, determined by the respective electron energy levels. The x-rays produced in thatthis way are called the “Characteristiccharacteristic x-rays”.