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| X-Rays |
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Bremsstrahlung is the German word for 'slowing down' or 'braking', and here it is used to describe the radiation which is emitted when electrons are decelerated or "braked" when they are fired at a metal target. Accelerated charges give off electromagnetic radiation, and when the energy of the bombarding electrons is high enough, that radiation is in the x-ray region of the electromagnetic spectrum. It is characterized by a continuous distribution of radiation called continuous x-ray spectrum which becomes more intense and shifts toward higher frequencies when the energy of the bombarding electrons is increased. A projectile electron can lose any amount of its kinetic energy in an interaction with the nucleus of a target atom, and the bremsstrahlung radiation associated with the loss can take on a corresponding range of values. For example, an electron with kinetic energy of 70 keV can lose all, none, or any intermediate level of that kinetic energy in a bremsstrahlung interaction; the Bremsstrahlung X-ray produced can have an energy in the range of 0 to 70 keV. Here, 70 keV is the energy that corresponds to the cut off wavelength (smallest wavelength - highest frequency therefore the highest possible energy - use E=hf=hc/l to calculate it). This is different from the production of characteristic x-rays that have specific energies.
Characteristic X-rays are produced by transitions of orbital electrons from outer to inner shells. Bombarding electrons can release electrons from inner energy level orbits. Higher electrons can then fall into the vacancy and if the energy gap between the levels is great enough X-rays will be produced. Since the electron binding energy for every element is different, the characteristic X-rays produced in the various elements are also different. This type of X-radiation is called characteristic radiation because it has precisely fixed,or discrete, energies and that these energies are characteristic of the differences between electron binding energies of a particular element. The effective energy characteristic X-rays increases with increasing atomic number of the target element. The two sharp peaks in the graph are characteristic X-rays which occur when vacancies are produced in K-shell of the atom and electrons drop down from above to fill the gap. The X-rays produced by transitions from L to K levels are called K-alpha x-rays, and those from M to K transition are called K-beta x-rays. Transitions to the L-shell are designated as L x-rays. The graph also shows the "brehmsstrahlung" radiation which forms the base for the two sharp peaks. Example to calculate the emitted x-ray energy: For tungsten, K electrons have binding energies of 69.5 keV, and L electrons are bound by 12.1 keV. A K-shell electron is removed from a tungsten atom and is replaced by an L shell electron. What is the energy of the characteristic X-ray that is emitted (in keV)? Answer: 57.4 keV (the difference in the energies of the shells). Electrons are produced by thermionic emission in the cathode. This is heated by a relatively low voltage supply. At a cathode current of 100 mA, for example, 6 x 1017 electrons travel from the cathode to the anode of the X-ray tube every second. They are accelerated from the cathode to anode across a high voltage. As the kinetic energy of the electrons increases, both the intensity (number of x-rays) and the energy (their ability to penetrate) of the X-rays produced are increased. When these electrons bombard on the heavy metal atoms of the target, they interact with these atoms and transfer their kinetic energy to the target. These interactions occur within a very small depth of penetration into the target. As they occur, the electrons slow down (brake!) and finally come nearly to rest, at which time they can be conducted through the x-ray anode assembly and out into the associated electronic circuitry. The interactions result in the conversion of kinetic energy into thermal energy and electromagnetic energy in the form of X-rays. By far, most of the the kinetic energy is converted into heat. The electrons interact with the outer-shell electrons of the target atoms but do not transfer sufficient energy to these outer-shell electrons to ionize them. Rather, the outer-shell electrons are simply raised to an excited, or higher, energy level. The outer-shell electrons immediately drop back to their normal energy state with the emission of infrared radiation. The constant excitation and restabilization of outer-shell electrons is responsible for the heat generated in the anodes of X-ray tubes. Generally, more than 99% of the kinetic energy of projectile electrons is converted to thermal energy, leaving less than 1% available for the production of X-radiation. In this sense,the X-ray machine is a very inefficient apparatus. The production of heat in the anode increases directly with increasing tube current. Doubling the tube current doubles the quantity of heat produced. Heat production also varies almost directly with varying the high tension voltage too. The efficiency of X-ray production is independent of the tube current. Regardless of what mA is selected, the efficiency of X-ray production remains constant. The efficiency of X-ray production increases with increasing projectile-electron energy. At 60 kVp, only 0.5% of the electron kinetic energy is converted to X-rays; at 120 MeV, it is 70%. Target material
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X-ray and other photographic films are sensitive to the direct action of the x-rays, but the photographic effect can be increased very appreciably, and exposure time can be decreased by the use of an intensifying screen in contact with each side of the film. One form of intensifying screen consists of lead foil, or a thin layer of a lead compound evenly coated on a paper backing. Under the excitation of x-rays of short wavelength and gamma rays, lead is a good emitter of electrons, which expose the sensitive film, thus increasing the total photographic effect. Another form of intensifying screen consists of a powdered fluorescent chemical-- for example, calcium tungstate, mixed with a suitable binder and coated on cardboard or plastic. Its action depends on the fact that it converts some of the x-ray energy into light, to which the film is very sensitive. The decision as
to the type of screen to be used-or whether a screen is to be used at
all-depends on a variety of circumstances, and is made by the radiographer. Several special types of x-ray film have been designed for the radiography of materials. Some types work best with lead screens, or without screens. Other types are intended primarily for use with fluorescent intensifying screens. X-ray films are commonly coated with emulsion on both sides of the support (to double the chance of exposure and therefore decrease the dose) --the superposition of the radiographic images of the two emulsion layers doubles the density and hence greatly increases the speed at which the X-Ray image is formed (halving the dose to the patient). X-ray films coated on one side only (single-coated films) are available for use when the superposed images in two emulsions might cause confusion - when a very detailed image of an area is required. A barium meal is an x-ray examination of the stomach and your oesophagus (gullet). Often pictures of the first part of the small intestine (the duodenum) are also taken. For the test to be successful the stomach should be as empty as possible and so the patient will probably be asked not to eat or drink anything for six hours before the examination. The patient will be asked to swallow some fizzy tablets or granules, with a little water. These will expand the stomach with gas which makes it easier to get a clear view of things. It is very important that the patient does not belch once s/he has taken these. Sometimes s/he if also given an injection of a drug to relax the stomach and stop it moving while the x-rays are taken (this can cause some blurring of vision for an hour or so and if this happens it is best not to drive). The patient is then given a cup of 'barium' to drink. It is actually barium sulphate (a radiopaque - contrast medium) and the mixture used normally contains defoaming agents and a mixture of constituents that make it have excellent coating characteristics. It is often fruit flavoured and is not at all unpleasant. The barium shows up on the x-rays as it strongly absorbs X-rays and therefore outlines the gullet and stomach in the X-ray picture. A number of x-ray pictures will then be taken. This is completely painless. The examination is usually completed within 30 minutes. A barium enema is an x-ray examination which involves filling the large intestine with barium through a tube inserted into the rectum. It is similar to the meal - just inserted into the body the other way round! The patient can eat and drink quite normally once the test is completed. The barium will be passed out with your bowel motions during the next few days, it may make motions paler in colour than normal. The results of the examination will usually be available a few days later.
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