31-05-2012, 11:32 AM
X-RAYS
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INTRODUCTION
X-Rays are penetrating electromagnetic radiation, having a shorter wavelength than light, and are produced by bombarding a target, usually made of tungsten, with high-speed electrons. X-rays were discovered accidentally in 1895 by the German physicist Wilhelm Conrad Roentgen while he was studying cathode rays in a high-voltage gaseous-discharge tube. Despite the fact that the tube was encased in a black cardboard box, Roentgen noticed that a barium platinocyanide screen, placed nearby by chance, emitted fluorescent light whenever the tube was in operation. After conducting further experiments, he determined that the fluorescence was caused by invisible radiation of a more penetrating nature than ultraviolet radiation. He named the invisible rays “X-rays” because of their unknown nature. Subsequently, X-rays were known also as Roentgen rays in his honour.
NATURE OF X-RAYS
X-rays are electromagnetic radiation ranging in wavelength from about 10 nm to 0.001 nm. The shorter the wavelength of the X-ray, the greater is its energy and its penetrating power. Longer wavelengths, near the ultraviolet-ray band of the electromagnetic spectrum, are known as soft X-rays. The shorter wavelengths, closer to or overlapping the gamma-ray range, are called hard X-rays . X-rays forming a mixture of many different wavelengths are known as “white” X-rays, as opposed to “monochromatic” X-rays, which represent only a single wavelength. Both light and X-rays are produced by transitions from orbit to orbit of electrons in atoms, light by the transitions of outer electrons and X-rays by the transitions of inner electrons.
Using crookes tubes
The first X-ray tube was the Crookes tube, a partially evacuated glass bulb containing two electrodes, named after its designer, the
British chemist and physicist Sir William Crookes. When an electric current passes through such a tube, the residual gas is ionized and positive ions, striking the cathode, eject electrons from it. These electrons, in the form of a beam of cathode rays, bombard the glass walls of the tube and produce X-rays. Such tubes produce only soft X-rays of low energy.
USING CATHODE RAY-TUBE
An early improvement in the X-ray tube was the introduction of a curved cathode to focus the beam of electrons on a heavy-metal target, called the anode. This type generates harder rays of shorter wavelengths and of greater energy than those produced by the original crookes tube, but the operation of Such tubes is erratic because the X-ray production depends on the gas preSsure within the tube
X-RAY DIFFRACTION:
X-rays may be diffracted by passage through a crystal or by reflection (scattering) from it, because the crystal consists of regular lattices of atoms that serve as fine diffraction gratings. The resulting interference patterns may be photographed and analyzed to determine the wavelength of the incident X-rays or the spacing's between the crystal atoms, whichever is the unknown factor.