An X-ray is a form of high-energy electromagnetic radiation. Most X-rays have wavelengths ranging from 0.01 to 10 nm, corresponding to the frequencies in the range of 30 petahertz to 30 exahertz (3×1016 to 3×1019 Hz). They have energies in the range of 100eV to 100keV. X-rays wavelengths are shorter than those of UV rays and typically longer than those of gamma rays.
X-ray radiation is also known as Roentgen radiation, after the German scientist William Roentgen, who discovered it on November 8, 1895. He named it X-radiation to signify a type of radiation that was unknown at the time.
Properties of X-Ray
- The X-radiations are invisible and travel in straight lines without transference of matter.
- They are transverse electromagnetic rays and are non-electrical in nature.
- They are reflected, refracted, diffracted, and polarized just like light.
- They ionize gases and affect the electrical properties of solids and liquids.
- They are differentially absorbed by the matter.
- The X-rays are produced by the impact of cathode rays upon matter and are characterized by a wide range of wavelengths (10-4 angstroms to 1000 angstroms).
- X-rays are capable of acting photochemically. There are also capable of damaging living cells and producing genetic mutations.
- If the energy of the X-rays is above 1 MeV, they are capable of producing electron-positron pairs.
- The X-rays are emitted in the form of a continuous spectrum whose short-wavelength limit is determined by the voltage on its tube.
- They also exhibit a line spectrum which is the characteristic of the element of the anode.
- The absorption spectra of X-radiations are the characteristic of chemical elements.
- X-rays show dual properties of matter and radiation.
- They are diffracted by the crystal in accordance with Bragg’s law.
- The total reflection of X-rays is used to construct X-ray microscopes.
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Production of X-Ray
X-rays are emitted whenever the matter is bombarded by high-velocity electrons. The matter suddenly stops the swiftly moving electrons and in doing so lessens their kinetic energy. This loss of kinetic energy becomes the energy of the X-rays.
The basic principle of the generation of X-rays demands the following parts in the apparatus: a source of electrons, a target to impact the electrons, and a means of applying the potential difference between the cathode and the target which will accelerate the electrons to the appropriate velocity during the passage between the cathode and the target.
In the generation of X-rays, the gas tube, Coolidge tube, and Betatron are used. The Coolidge tube is an independent source of electrons. There is not enough gas present to enable the passage of the circuit. The tube has just 1 micrometer of Mercury. Electrons are emitted from a hot wire cathode.
The Cathode is a tungsten filament. A metal cup makes the electron beam narrow. When the higher energy electrons strike the target, there is a generation of X-rays. The whole system is surrounded by a coolant such as water. The cathode is surrounded by Molybdenum, which has the capacity to repel electrons.
X-rays are produced in the target in the direction perpendicular to the electron beam. The kinetic energy of the electrons is emitted in the form of X-rays.
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X-ray Spectra
Continuous X-ray Spectra
The spectrum of X-rays is continuous it includes radiations of all possible levels from a lower limit to a higher limit continuously just like visible light. The production of X-rays involves electrons tracking the target material which is of a very high atomic number.
But most of the energy of the incident electron which hits the target goes into the heating of the target. Since the atoms of the target material have a very high atomic number is very difficult to penetrate near the nucleus. A few very fast electrons that are incident on the target are able to parrot rate deep inside the atoms and are attracted towards the nucleus.
This attraction causes the electrons to deviate from their incident direction. This causes decceleration and seems the electron is a charged particle and it is accelerates there is an emission of electromagnetic radiation. The electron decelerates continuously.
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Characteristic X-ray Spectra
This consists of a definite and well-defined wavelength of X-rays which are superimposed on the continuous x-ray spectra. Unlike continuous x-ray spectra, the characteristic spectra will depend on the material of the target. The characteristic spectra ranges in the form of different groups.
When the incident electrons with the target material some of the very fast moving electrons having an extremely high velocity of about one 10th the velocity of light will be able to penetrate the atoms of the target materials and knockout it tightly bound electrons even from the most inner shells in that atom. The shells are the K and the L shells.
When this happens the energy difference between the two levels is liberated in the form of X-rays of very short wavelength. If the transitions happen from the K shell then it will lead to one of the K lines so like K-alpha, or K-beta. Here the wavelength of the imitate radiation depends on the material of the target. This spectrum is known as characteristic spectra.
The X-ray spectra will be different for different elements. The characteristics spectra are not single lines but their careful analysis will show that they have a fine structure. There are there he can be dead with by understanding the different quantum numbers associated.
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Auger transition
When a K shell electron is ejected from an atom by electron impact or absorption of an X-ray Photon vacancy in the K shell will be filled by the transition of an electron from an outer shell. For example, an L shell electron will go into the K level. Such transitions are usually followed by the emission of a characteristic X-ray.
But in some cases, the extra energy in such a transition will be absorbed by another yellow shell and hence will be emitted. Thus an electron will be ejected. Transitions resulting in the emission of two electrons from the same atom are called Auger transitions.
It was first discovered by Pierre Auger in a cloud chamber photograph. During this type of transition, there will be no electromagnetic radiation emitted. The transition is radiation less.
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