Bremsstrahlung And Characteristic Radiation PdfBy Chantal B. In and pdf 25.11.2020 at 20:51 8 min read
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Production of X-rays
Table of Contents. X-radiation is created by taking energy from electrons and converting it into photons with appropriate energies. This energy conversion takes place within the x-ray tube. The quantity exposure and quality spectrum of the x-radiation produced can be controlled by adjusting the electrical quantities KV , MA and exposure time, S , applied to the tube.
In this chapter we first become familiar with the design and construction of x-ray tubes, then look at the x-ray production process, and conclude by reviewing the quantitative aspects of x-ray production. An x-ray tube is an energy converter. It receives electrical energy and converts it into two other forms: x-radiation and heat.
The heat is an undesirable byproduct. X-ray tubes are designed and constructed to maximize x-ray production and to dissipate heat as rapidly as possible. The x-ray tube is a relatively simple electrical device typically containing two principle elements: a cathode and an anode. As the electrical current flows through the tube from cathode to anode, the electrons undergo an energy loss, which results in the generation of x-radiation. A cross-sectional view of a typical x-ray tube is shown in below.
The anode is the component in which the x-radiation is produced. It is a relatively large piece of metal that connects to the positive side of the electrical circuit. The anode has two primary functions: 1 to convert electronic energy into x-radiation, and 2 to dissipate the heat created in the process.
The material for the anode is selected to enhance these functions. The ideal situation would be if most of the electrons created x-ray photons rather than heat. The fraction of the total electronic energy that is converted into x-radiation efficiency depends on two factors: the atomic number Z of the anode material and the energy of the electrons. Most x-ray tubes use tungsten, which has an atomic number of 74, as the anode material. In addition to a high atomic number, tungsten has several other characteristics that make it suited for this purpose.
Tungsten is almost unique in its ability to maintain its strength at high temperatures, and it has a high melting point and a relatively low rate of evaporation. For many years, pure tungsten was used as the anode material. In recent years an alloy of tungsten and rhenium has been used as the target material but only for the surface of some anodes. The anode body under the tungsten-rhenium surface on many tubes is manufactured from a material that is relatively light and has good heat storage capability.
Two such materials are molybdenum and graphite. The use of molybdenum as an anode base material should not be confused with its use as an anode surface material. Most x-ray tubes used for mammography have molybdenum-surface anodes. Some mammography tubes also have a second anode made of rhodium, which has an atomic number of This produces a higher energy and more penetrating radiation, which can be used to image dense breast.
The use of a rhenium-tungsten alloy improves the long-term radiation output of tubes. With x-ray tubes with pure tungsten anodes, radiation output is reduced with usage because of thermal damage to the surface. Most anodes are shaped as beveled disks and attached to the shaft of an electric motor that rotates them at relatively high speeds during the x-ray production process.
The purpose of anode rotation is to dissipate heat and is considered in detail in another chapter.. Not all of the anode is involved in x-ray production. The radiation is produced in a very small area on the surface of the anode known as the focal spot. The dimensions of the focal spot are determined by the dimensions of the electron beam arriving from the cathode.
In most x-ray tubes, the focal spot is approximately rectangular. The dimensions of focal spots usually range from 0. X-ray tubes are designed to have specific focal spot sizes; small focal spots produce less blurring and better visibility of detail, and large focal spots have a greater heat-dissipating capacity.
Focal spot size is one factor that must be considered when selecting an x-ray tube for a specific application. Tubes with small focal spots are used when high image visibility of detail is essential and the amount of radiation needed is relatively low because of small and thin body regions as in mammography.
Most x-ray tubes have two focal spot sizes small and large , which can be selected by the operator according to the imaging procedure. The basic function of the cathode is to expel the electrons from the electrical circuit and focus them into a well-defined beam aimed at the anode. The typical cathode consists of a small coil of wire a filament recessed within a cup-shaped region, as shown below. Energy Exchange within an X-Ray Tube.
Electrons that flow through electrical circuits cannot generally escape from the conductor material and move into free space. They can, however, if they are given sufficient energy. In a process known as thermionic emission, thermal energy or heat is used to expel the electrons from the cathode. The filament of the cathode is heated in the same way as a light bulb filament by passing a current through it. This heating current is not the same as the current flowing through the x-ray tube the MA that produces the x-radiation.
During tube operation, the cathode is heated to a glowing temperature, and the heat energy expels some of the electrons from the cathode. The anode and cathode are contained in an airtight enclosure, or envelope. The envelope and its contents are often referred to as the tube insert, which is the part of the tube that has a limited lifetime and can be replaced within the housing.
The majority of x-ray tubes have glass envelopes, although tubes for some applications have metal and ceramic envelopes. The primary functions of the envelope are to provide support and electrical insulation for the anode and cathode assemblies and to maintain a vacuum in the tube.
The presence of gases in the x-ray tube would allow electricity to flow through the tube freely, rather than only in the electron beam. This would interfere with x-ray production and possibly damage the circuit. The x-ray tube housing provides several functions in addition to enclosing and supporting the other components. It functions as a shield and absorbs radiation, except for the radiation that passes through the window as the useful x-ray beam.
Its relatively large exterior surface dissipates most of the heat created within the tube. The space between the housing and insert is filled with oil, which provides electrical insulation and transfers heat from the insert to the housing surface. The energy used by the x-ray tube to produce x-radiation is supplied by an electrical circuit as illustrated below.
The circuit connects the tube to the source of electrical energy, that in the x-ray room is often referred to as the generator. As described in another chapter, the generator receives the electrical energy from the electrical power system and converts it into the appropriate form DC, direct current to apply to the x-ray tube. The generator also provides the ability to adjust certain electrical quantities that control the x-ray production process.
The three principle electrical quantities that can be adjusted are the: KV the voltage or electrical potential applied to the tube MA the electrical current that flows through the tube S duration of the exposure or exposure time, generally a fraction of a second The circuit is actually a circulatory system for electrons.
They pickup energy as the pass through the generator and transfer their energy to the x-ray tube anode as described above. The energy that will be converted into x-radiation and heat is carried to the x-ray tube by a current of flowing electrons as shown above.
As the electrons pass through the x-ray tube, they undergo two energy conversions, as illustrated previously: The electrical potential energy is converted into kinetic motion energy that is, in turn, converted into x-radiation and heat. When the electrons arrive at the x-ray tube, they carry electrical potential energy. The amount of energy carried by each electron is determined by the voltage or KV, between the anode and cathode.
For each kV of voltage, each electron has 1 keV of energy. By adjusting the KV, the x-ray machine operator actually assigns a specific amount of energy to each electron. After the electrons are emitted from the cathode, they come under the influence of an electrical force pulling them toward the anode.
This force accelerates them, causing an increase in velocity and kinetic energy. This increase in kinetic energy continues as the electrons travel from the cathode to the anode. As the electron moves from cathode to anode, however, its electrical potential energy decreases as it is converted into kinetic energy all along the way.
Just as the electron arrives at the surface of the anode its potential energy is lost, and all its energy is kinetic. At this point the electron is traveling with a relatively high velocity determined by its actual energy content.
A keV electron reaches the anode surface traveling at more than one half the velocity of light. When the electrons strike the surface of the anode, they are slowed very quickly and lose their kinetic energy; the kinetic energy is converted into either x-radiation or heat. The electrons interact with individual atoms of the anode material, as shown below. Two types of interactions produce radiation.
An interaction with electron shells produces characteristic x-ray photons; interactions with the atomic nucleus produce Bremsstrahlung x-ray photons. The electrons within an atom each have a specific amount of binding energy that depends on the size atomic number, Z of the atom and the shell in which the electron is located. As described in a previous chapter the binding energy is the energy that would be required to remove the electron from the atom.
It is actually an energy deficit rather than an amount of available energy. The binding energy of electrons within an atom plays a major role in the production of characteristic x-radiation as described later. The interaction that produces the most photons is the Bremsstrahlung process. Bremsstrahlung is a German word for "braking radiation" and is a good description of the process.
Electrons that penetrate the anode material and pass close to a nucleus are deflected and slowed down by the attractive force from the nucleus.
The energy lost by the electron during this encounter appears in the form of an x-ray photon. All electrons do not produce photons of the same energy.
Only a few photons that have energies close to that of the electrons are produced; most have lower energies. Although the reason for this is complex, a simplified model of the Bremsstrahlung interaction is shown below. First, assume that there is a space, or field, surrounding the nucleus in which electrons experience the "braking" force.
This field can be divided into zones, as illustrated. This gives the nuclear field the appearance of a target with the actual nucleus located in the center.
An electron striking anywhere within the target experiences some braking action and produces an x-ray photon.
Bremsstrahlung X-rays are produced by slowing down of the primary beam electrons by the electric field surrounding the nuclei of the atoms in the sample see Bremsstrahlung animation. Note: Bremsstrahlung X-rays are also referred to as continuum or background X-rays. The primary-beam electrons lose energy and change direction due to inelastic scattering in the sample. Bremsstrahlung X-rays cannot have energies greater than the energy of the electrons in the primary beam so this energy forms the upper energy limit of the X-ray spectrum and is known as the Duane-Hunt limit. Figure : The primary beam electrons are slowed down or deflected by the electric field around the atoms in the specimen.
Bremsstrahlung, for example, accounts for continuous X-ray spectra— i. In generating bremsstrahlung, some electrons beamed at a metal target in an X-ray tube are brought to rest by one head-on collision with a nucleus and thereby have all their energy of motion converted at once into radiation of maximum energy. Other electrons from the same incident beam come to rest after being deflected many times by the positively charged nuclei. Each deflection gives rise to a pulse of electromagnetic energy, or photon , of less than maximum energy. Internal bremsstrahlung arises in the radioactive disintegration process of beta decay , which consists of the production and emission of electrons or positrons, positive electrons by unstable atomic nuclei or the capture by nuclei of one of their own orbiting electrons.
Each type of atom or element has its own characteristic electromagnetic spectrum. In this section, we explore characteristic x rays and some of their important applications. We have previously discussed x rays as a part of the electromagnetic spectrum in Photon Energies and the Electromagnetic Spectrum. That module illustrated how an x-ray tube a specialized CRT produces x rays. Electrons emitted from a hot filament are accelerated with a high voltage, gaining significant kinetic energy and striking the anode. Figure 1. X-ray spectrum obtained when energetic electrons strike a material, such as in the anode of a CRT.
Table of Contents. X-radiation is created by taking energy from electrons and converting it into photons with appropriate energies. This energy conversion takes place within the x-ray tube. The quantity exposure and quality spectrum of the x-radiation produced can be controlled by adjusting the electrical quantities KV , MA and exposure time, S , applied to the tube. In this chapter we first become familiar with the design and construction of x-ray tubes, then look at the x-ray production process, and conclude by reviewing the quantitative aspects of x-ray production.
X-ray fluorescence XRF spectrometry is an elemental analysis technique with broad application in science and industry. XRF is based on the principle that individual atoms, when excited by an external energy source, emit X-ray photons of a characteristic energy or wavelength. By counting the number of photons of each energy emitted from a sample, the elements present may be identified and quantitated. Henry Moseley was perhaps the father of this technique, since he, building on W. In Coster and Nishina were the first to use primary X-rays instead of electrons to excite a sample.
Characteristic x-rays are emitted from heavy elements when their electrons make transitions between the lower atomic energy levels. The continuous distribution of x-rays which forms the base for the two sharp peaks at left is called "bremsstrahlung" radiation. X-ray production typically involves bombarding a metal target in an x-ray tube with high speed electrons which have been accelerated by tens to hundreds of kilovolts of potential.
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The generation of accelerated electron beams in a high-current Z-pinch formed by the implosion of wire cylindrical tungsten arrays on an Angara facility is studied. The most intense characteristic and bremsstrahlung X-ray radiation of fast electrons is recorded from the central region of the pinch at the pin-ching stage. One of the promising directions for the implementation of pulsed thermonuclear fusion is the use of soft X-ray emission for the implosion of spherical thermonuclear targets hereinafter, targets. Currently, the greatest progress has been made in the indirect target compression scheme using high-power soft X-ray emission.
Stationary anode: these are generally limited to dental radiology and radiotherapy systems. Consists of an anode fixed in position with the electron beam constantly streaming onto one small area. Rotating anode: used in most radiography, including mobile sets and fluoroscopy. Consists of a disc with a thin bevelled rim of tungsten around the circumference that rotates at 50 Hz.
Characteristic X-rays are emitted when outer- shell electrons fill a vacancy in the inner shell of an atom , releasing X-rays in a pattern that is "characteristic" to each element. Characteristic X-rays were discovered by Charles Glover Barkla in ,  who later won the Nobel Prize in Physics for his discovery in Characteristic X-rays are produced when an element is bombarded with high-energy particles, which can be photons, electrons or ions such as protons. When the incident particle strikes a bound electron the target electron in an atom, the target electron is ejected from the inner shell of the atom. After the electron has been ejected, the atom is left with a vacant energy level , also known as a core hole.