X Ray Production and Tube

History: Over a century ago in 1895, Roentgen discovered the first example of ionizing radiation, x-rays. The key to Roentgens discovery was a device called a Crooke’s tube, which was a glass envelope under high vacuum, with a wire element at one end forming the cathode, and a heavy copper target at the other end forming the anode. When a high voltage was applied to the electrodes, electrons formed at the cathode would be pulled towards the anode and strike the copper with very high energy. Roentgen discovered that very penetrating radiations were produced from the anode, which he called x-rays.

X-rays are produced due to sudden deceleration of fast moving electrons when they collide and interact with the target anode. In this process of deceleration more than 99% of the electron energy is converted into heat and less than 1% of energy is converted into X-ray. Whenever electrons of high energy strike a heavy metal target, like tungsten or copper. When electrons hit this material, some of the electrons will approach the nucleus of the metal atoms where they are deflected because of there opposite charges (electrons are negative and the nucleus is positive, so the electrons are attracted to the nucleus). This deflection causes the energy of the electron to decrease, and this decrease in energy then results in forming an x-ray.
X-rays are produced by two main mechanisms; Characteristic and Bremsstrahlung x rays.


Bremsstrahlung x rays: bremsen "to brake" and Strahlung "radiation", i.e. "braking radiation" or "deceleration radiation") is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon, thus satisfying the law of conservation of energy. This process of slowing down due to collision results in the release of x radiation.
Characteristic X Rays: When a fast moving electron collides with a K-shell electron, the electron in the K-shell is ejected (provided the energy of incident electron is greater than the binding energy of K-shell electron) leaving behind a 'hole'. This hole is filled by an outer shell electron (from the L-shell, M-shell etc) with an emission of a single X-ray photon, called characteristic radiation, with an energy level equivalent to the energy level difference between the outer and inner shell electron involved in the transition.

As opposed to the continuous spectrum of bremsstrahlung radiation, characteristic radiation is represented by a line spectrum. As each element has a specific arrangement of electrons at discrete energy levels then it can be appreciated that the radiation produced from such interactions is 'characteristic' of the element involved.

Definitions

Cathode: is the negative terminal of the x ray tube.It is tungsten filament and when current is flown through it, the filament gets heated and start emitting its surface electrons by the process called thermionic emission.
Anode: It is the positive terminal of the tube. It is made of tungsten disc in ordinary diagnostic x ray tube and molybdenum in mammography x ray tube.
Kilovoltage: High voltage is applied between cathode and anode. High voltage of kilovolt range (1000 volts) causes electrons to move towards the positive terminal of the tube at a velocity of half the velocity of light.
Focal spots: Not all of the anode is involved in x-ray production. The radiation is produced in a very small area on the surface of the anode known as the focal spot. The dimensions of the focal spot are determined by the dimensions of the electron beam arriving from the cathode. In most x-ray tubes, the focal spot is approximately rectangular. The dimensions of focal spots usually range from 0.1 mm to 2 mm.
Envelope: The anode and cathode are contained in an airtight enclosure, or envelope. The envelope and its contents are often referred to as the tube insert, which is the part of the tube that has a limited lifetime and can be replaced within the housing. The majority of x-ray tubes have glass envelopes, although tubes for some applications have metal and ceramic Envelopes.The primary functions of the envelope are to provide support and electrical insulation for the anode and cathode assemblies and to maintain a vacuum in the tube. The presence of gases in the x-ray tube would allow electricity to flow through the tube freely, rather than only in the electron beam. This would interfere with x-ray production and possibly damage the circuit.
Housing: The x-ray tube housing provides several functions in addition to enclosing and supporting the other components. It functions as a shield and absorbs radiation, except for the radiation that passes through the window as the useful x-ray beam. Its relatively large exterior surface dissipates most of the heat created within the tube. The space between the housing and tube insert is filled with oil, which provides electrical insulation and transfers heat from the insert to the housing surface.
X Ray Circiut: The circuit connects the tube to the source of electrical energy, that in the x-ray room is often referred to as the generator. the generator receives the electrical energy from the electrical power system and converts it into the appropriate form (DC, direct current) to apply to the x-ray tube.  The generator also provides the ability to adjust certain electrical quantities that control the x-ray production process.
The three principle electrical quantities that can be adjusted are the:

• KV (the voltage or electrical potential applied to the tube)
• MA (the electrical current that flows through the tube)
• S (duration of the exposure or exposure time, generally a fraction of a second)