XRAYS

What are X-rays….When high energetic electrons are made to strike a metal target, an electromagnetic radiation comes out a large part of this radiation has wavelength of the order of 0.1mm(nearly 1A0) and is known as X-rays. X-rays was discovered by German

physicist W.C.Roentgen in 1895. He found that photographic film wrapped

light-tight in a black paper near cathode ray tube. He named these rays as x-rays.

X-rays

Frequencies: >3 x 1016 Hz.Wavelengths: <10 nm.Quantum energies: >124 eV.

Characteristics of X-rays X-rays are not deflected by electric and magnetic fields. They are highly penetrating and can pass through many solids. The transparency depends on the density of the material. They effect a photographic plate and even more effective than light. They cause florescence in many materials eg. Bariumplatinocyanide, tungsten.

Continued from characteristics of X-rays…..

They have destructive effect on living tissues. Exposure of human body to X-rays causes reddening of the skin and sores. When X-rays fall on heavy metals, they produce secondary x-rays. Under suitable conditions X-rays are reflected and refracted like ordinary light.

Production of X-rays Commonly used X-ray tube was designed by Coolidge in 1916 and is known as Coolidge tube. It emits a large number of electrons called thermions. Velocity of these thermions depends upon the potential difference between cathode or anti cathode. Cathode is surrounded by molybdenum maintained at negative potential difference between A and B.

Basic diagram of Coolidge tube

Characteristics of an anticathode

It should have high atomic weight. High melting point to withstand high temperature developed as most of energy of impinging electrons converted into heat. High thermal conductivity. Low vapour pressure at high temperature.

Characteristics of anticathode

Only tantalum, tungsten and platinum. But tungsten is the best and widely used material.

Minimum wavelength of X-ray photon

When the electron hitting the target is brought to rest by the force of attraction between this electron and the nucleus , the entire K.E(.5mv2) is completely transformed into X-ray photon of the maximum frequency(Vmax)or minimum wavelength( λmin )

ko = .5mv2=hVmax

Ko =hc/ λmin

λmin =hc/ko

Relation between potential difference and wavelength: Let V be the potential difference

between m cathode and anti-cathode then,

Ko=.5mv2=eV

eV=hc/ λmin

λmin=hc /eV

Since h,c,e are the constants, therefore

λminœ 1/V

Types of X-rays on the basis of energyHard X-raysSoft X-raysHard X-rays: X-rays of higher energy

is known as hard X-ray. If kinetic energy of the accelerating

electrons increased then cut off wavelength will decrease further resulting into production of X-rays of lower wavelength which are known as hard X-rays.

Soft X-rays X-rays of longer wavelength are

known as soft X-rays. These have low energy. Soft X-rays have energies in the

0.09 to 2.5 keV range

Characteristic X-rays

Atom consist of nucleus surrounded by negatively charged electrons. Electrons in the innermost orbit is attracted by nucleus strongly with greatest force and large amount of energy is required to detach it. Electron in outermost orbit require small amount of energy to detach it from atom.

Continued from characteristic X-rays

When fast moving energetic electron from cathode fall on anti cathode it will penetrate deep into the surface of the target and knock out tightly bound electron from higher orbits jump to occupy the position and difference in the energy is radiated in the form of X-rays known as characteristic x-rays.

Continued from characteristic X-rays

Electrons make transitions between lower atomic energy levels in heavy elements. They have definite energies since they have energies determined by the atomic energy levels.

Continuous X-rays When fast moving Energetic electron

penetrate deep into the interiors of the atoms of the target and are attracted by attractive forces of the nuclei.

Due to these forces electron may deflect from the original paths and electrons are retarded i.e. electron loose its energy.

This loss in the energy is given in the form of electromagnetic radiation known as X-rays.

Diagram representing characteristic and continuous X-ray

Uses of the X-rays Application in industry and

Engineering1) X-rays are used to identify

manufacturing defects in tyres, tennis balls and are also used to check defects in diamond.

2) X-rays are used to check flaws in welding joints, insulating material.

3) It can be used to analyze the structure of alloys from diffraction pattern.

Medical Applications

X-rays for the diagnosis of many diseases that cannot be identified by pathological test. It is used to find fractures in the bones, diseased organs and presence of foreign matter in the body. X-rays can be used to cure many types of skin diseases, malignant sores,internal cancer and tumours.

Application in scientific research

X-rays are used to study the structure of the crystalline solids , atoms and alloys. X-rays diffraction pattern are used for analyzing the structure of organic complex molecules. It can be used for the determination of the atomic number of the element and for identification of the chemical elements.

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