X-ray Tube for X-ray Generation

X-ray Tube


X-ray: X-rays are short wave-length electromagnetic radiation in the range of 0.06 to 100 Angstroms. X-ray is produced by the deceleration of high-energy electrons. They can also be produced by suitable electronic means.

X-rays can be obtained by the following three processes:

  • bombardment of a metal target with high-energy electrons.
  • exposure of a substance to a primary X-ray beam.
  • the decay process of radioactive material.

X-rays are produced if electrons from a heated cathode and accelerated to a voltage of 100 kV strike a metal target (anode). During this process, a part of the energy of the electrons is converted to X-rays.

Continuous spectrum of frequencies produced by X-rays up to a certain maximum determined by the anode Potential. The higher the anode potential, the higher is the maximum frequency or the lower is the minimum, wavelength of X-rays produced. The minimum wavelength λ0 of X-rays produced is given by, \lambda _{0}=\frac{12398}{\nu }, where λ0 and υ units of Angstroms and volts respectively.

X-ray Generation

X-rays are generated when electrons of high velocity are incident on a target material of high atomic number. Most of the energy of an incident electron beam which strikes the target is lost in the form of the heat energy. However, a small but definite fraction of electrons lose their kinetic energy to the target atoms. This loss of kinetic energy is responsible for the generation of X-rays.

Some of the electrons penetrate deep into the surface and forces even inner-most shell electrons to come out by giving up a part of the kinetic energy of the electron. The vacancies so generated in the atomic shell may be taken up by electrons residing at nearby shells. Thus electronic transitions take place between the energy level of the two shells, E1 and E2 respectively. This energy difference E_{1}-E_{2}=\frac{hc}{\lambda }, where λ is the length, h the Planck’s constant and c the velocity of light. If the value of λ corresponds to that of X-ray region of the spectrum, then X-rays are emitted. The value of λ depends on the target used. The spectra of X-rays consists of sharp lines, the characteristics of target material. The X-radiation emitted when an electron falls to the L-shell from outer shells are called L-lines or L-series. Similarly, the K lines, refer to the fall of electrons to the K shell respectively.

Besides the above form of X-radiation emission, a few fast-moving electrons may penetrate deep inside the target material and attracted by the attractive forces of the positively charged nuclei. As a result, the electron gets deflected from its original trajectory. They are decelerated (i.e., velocity decreases) and their energy is reduced also. The energy loss during retardation is given in the form of electromagnetic radiation of wavelengths corresponding to the X-ray region of the spectrum. The X-ray produced will have a continuously changing wavelength lying between λmin and λmax.

If an electron of mass m and velocity υ1 changes the velocity from υ1 to υ2, the change in kinetic energy is

\Delta E=\frac{1}{2}m\nu _{1}^{2}-\frac{1}{2}m\nu _{2}^{2}

The energy of the emitted photon will be equal to the loss in kinetic energy and thus

h\nu =\frac{1}{2}m\nu _{1}^{2}-\frac{1}{2}m\nu ^{2}

In above equation, if υ2, the final velocity is zero, then

h\nu_{max} =\frac{h_{c}}{\lambda _{min}}=\frac{1}{2}m\nu _{1}^{2}

\lambda _{min} =\frac{h_{c}}{\frac{1}{2}m\nu _{1}^{2}}=\frac{h_{c}}{eV_{1}}

Where V1 is the potential to which the electron has been accelerated \left ( eV_{1}=\frac{1}{2}m\nu _{1}^{2} \right ).

X-ray Tube for X-ray Generation

The most widely used and common source of X-rays is the X-ray tube is shown in the figure below. It consists of a source of electrons, the filament cathode, and the target material mounted inside an evacuated tube. The filament material is normally tungsten form in which electron emission takes place by the method of thennionic emission (emission from a heated filament). The filament voltage required for the emission of electrons.

Schematic diagram of an X-ray tube, X-ray Tube
Schematic diagram of an X-ray tube

The anode consists of a heavy block of copper with the target material plated on or embedded inside the surface of the copper. The anode voltage with respect to the cathode is kept high. The electrons are accelerated to a voltage of the order of 20,000 to 25,000 volts. The heater current can be controlled by means of the rheostat Rh as shown in the figure. The high-velocity electrons impinge upon the metallic anode producing X-rays. The anode surface is tilted with respect to the flow of electron current so that the resulting X-rays are emitted at the desired angle with respect to the electron beam. The accelerating voltage between the target and the cathode decides the energy of the emitted X-rays (or wavelength). The short wavelength limit (λmin) for an accelerating voltage is well defined and is independent of the target material. It is apparent that the λmin for the tungsten target for a beam voltage of 35 kV for the target spectrum.

Properties of X-rays

The important properties of X-rays are as follows:

  1. X-rays are electromagnetic waves of very short wavelengths. They are invisible to the human eyes and they travel in straight lines with velocity equal to the velocity of light.
  2. x-rays exhibit interference, diffraction, and polarization properties.
  3. Under typical conditions, they may be reflected and refracted just like ordinary light.
  4. X-rays do not exhibit the property of being deflected in electric and/or magnetic field.
  5. They exhibit the property of light emission (fluorescence) in various materials like barium, cadmium,  tungstate, zinc sulfide, etc.
  6. X-rays while passing through a gaseous medium ionize the gas (i.e., produce positive and negatively charged particles).
  7. X-rays can also produce secondary X-rays while incident on heavy metals.
  8. One very important property of X-rays is that they can pass through materials that are opaque to ordinary light like flesh, wood, paper, thin metal sheets, etc. X-rays are widely used in the medical field.
  9. X-rays when incident on certain metals can also exhibit the property of photoelectric effect i.e., liberate photoelectrons from the metal.
  10. The X-rays have also some harmful effects on living tissues. When passed through the human body, they may cause redeeming of the skin, sores, and series of injuries to the human tissues. They may also kill the white blood corpuscles (WBC) while passing through the human body.

Applications of X-rays

X-rays find widespread use and numerous practical applications due to their excellent and distinctive properties. They are widely used in various branches of industry, engineering, medicine, scientific research etc.

Industrial Applications of X-ray

X-rays find widespread use in the following fields:

  1. to analyze and test the homogeneity of welded joints, insulating materials.
  2. to detect cracks in structures crack detection of larger and relatively costly objects such as metal castings, welded assemblies, motor car body or an airplane or a complicated structure, etc.
  3. X-ray diffraction is a standard technique for the detection of the crystal forms of composite materials and alloys. Analysis of alloys like cobalt-nickel steel, bronze, duraliminium, porcelain insulators is tested by X-rays.
  4. X-rays can be used to detect cracks in heavy steel plates, steel ropes, etc. used in modem bridge construction or in machine tools.
  5. The molecular structure of materials like cellulose, plastic, rubber can be studied using X-ray methods.
  6. X-ray image when projected on a fluorescent screen called fluoroscopy and this technique is used for inspection of fruits before packing, canned foods, etc.

Medical Applications of X-ray

The medical applications of X-rays have made it very popular even to the layman. In the medical field, X-rays are used for two purposes (a) Radiography and (b) X-ray therapy.

Radiography: The applications include taking X-ray photography on the film of human interiors. It is an invaluable aid to a medical practitioner for the detection of fractures, tumors, diseased organs, foreign matter, stones inside the human body, etc. The computerized tomography (CT) scanner depends on its operation on X-rays.

X-rays therapy: X-rays have curative effects on the human body. X-rays are useful for the treatment of some types of skin diseases like cancer, tumor, etc. They can be used to kill diseased tissues inside the human body. Both soft X-rays (with lower penetrating power) and hard X-rays (with higher penetrating power) are used. It may be mentioned in this connection that hard X-rays are produced when the beam voltage in an X-ray tube is very high while soft X-rays are produced when the beam voltage is comparatively low. Hard X-rays can destroy tumors deep inside the human body.

Scientific Research of X-ray

In scientific research, X-rays find application in:

  1. investigations of the structure of crystalline solids and alloys.
  2. analysis of the structure of complex organic molecules.
  3. determination of the atomic number and identification of various chemical elements.

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