Physics
High Energy X-Rays
High energy X-rays are a type of electromagnetic radiation with wavelengths shorter than those of visible light. They are produced by accelerating charged particles, such as electrons, to high speeds and then suddenly decelerating them. High energy X-rays are used in various applications, including medical imaging, industrial inspection, and scientific research due to their ability to penetrate materials and provide detailed information about their internal structure.
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10 Key excerpts on "High Energy X-Rays"
eBook - PDF- Richards, Austin A.(Authors)
- 2011(Publication Date)
One keV is the kinetic energy of an electron that has dropped through a potential difference of 1000 volts (1 kV). This same method of generating x rays is still used today. Typical medical x-ray tube voltages are 100 kV; we would describe the resulting x rays as having energies of 100 keV, corresponding to a wavelength of about 0.01 nm ( 10 -11 m), or about a tenth of the radius of an atom. Electron-volt units are a convenient way to describe their energy, even when the source has nothing to do with voltage, as in the case of thermal x rays. In fact, this descriptive convention applies to all sorts of high-energy particles produced by particle accelerators, radioactive decay, or astrophysical sources. Gamma Rays Gamma rays are electromagnetic waves with extremely short wavelengths, shorter than x rays, and with correspondingly higher photon energies. They are not caused by the rearrangement of electrons within an atom; rather, they are generated by changes in the nuclei of atoms, changes due to nuclear decay processes where a nucleus changes internally and releases energy. Their energies correspond to binding energies of nucleons in the nucleus. We can generate gamma rays by assembling concentrated masses of radioactive material or by using powerful particle accelerators to accelerate electrons to very high energies and then smash them into a target material, causing nuclear processes to occur. The gamma-ray band encompasses all photons with energies above about 100 keV, corresponding to wavelengths of 0.01 nm (10 -11 m) and shorter. What is the difference between x rays and gamma rays? There is no particular boundary between the two (since the naming convention is somewhat arbitrary), but most scientists consider x rays to be produced by electron-atom interactions, whereas gamma rays are in the million electron-volt or higher energy range and are produced by nuclear processes such as radioactive decay.
eBook - PDF- Ronald Kitchen(Author)
- 2001(Publication Date)
- Newnes(Publisher)
In turn these are inclined to be shortened in conversation to ‘soft’ and ‘hard’ radiation. When referring to wavelengths, High Energy X-Rays are also referred to as ‘short wave’ radiation and the low energy end of the spectrum as ‘long wave’ radiation. These should not in any way be confused with the radio broadcasting usage of the terms. From the point of view of the transmitter engineer, the energy of the X-rays is an indicator of the penetrating power in materials, higher energy X-rays requiring thicker shielding for a given material than softer X-rays. This is clearly an important factor in the design of transmitter shielding. In X-ray machines for medical use, metal shields (filters) are used to attenuate selectively so as to maximise the wanted X-ray energies and minimise the unwanted energies. For radio equipment, where we do not want any of the X-rays, there are complications in shielding high power tubes and the intrinsic electronic tube shields do not necessarily reduce the doserates to the levels required by the user. Hence there may be a need for further shielding within the transmitter structure to achieve these ends (see Chapter 11). Both gamma rays and X-rays, being electromagnetic waves, are subject to attenuation in accord-ance with the inverse square law and both may extend for a considerable distance. As those who work with ionising radiation will know, X-rays generated by electronic equipment have one valuable characteristic when compared with radioactive sources, namely that the radiation ceases when the power switch is turned off! Consequently, the first line of defence when carrying out surveys and unexpectedly high X-ray levels are encountered, is either to withdraw to a safe distance or to switch off the equipment, preferably the latter since other people may be unaware of the problem.
eBook - ePubX-Ray Imaging
Fundamentals, Industrial Techniques and Applications
- Harry E. Martz, Clint M. Logan, Daniel J. Schneberk, Peter J. Shull(Authors)
- 2016(Publication Date)
- CRC Press(Publisher)
β1 , etc.K-series x-rays are of utmost technological importance because they are of the highest energy and most penetrating. The highest-energy K-series x-ray is created when the vacancy is filled with a free (unbound) electron. The K-series x-rays increase in energy with increasing atomic number. The maximum for Fe is 7 keV, while that for W is 70 keV. Tables of characteristic energies are widely available and are easily found on the Internet, e.g., http://xdb.lbl.gov/Section1/Sec_1-2.html . Characteristic x-ray radiation is typically superimposed on the bremsstrahlung radiation, as shown in Figure 8.9 in Chapter 8 .4.2.1.3Accelerating Charge
We know in general that free charges (those not bound within an atom) emit electromagnetic radiation when accelerated. That much is true for charges changing speed along a straight line within a linear accelerator, sailing around in a circle inside a cyclotron, or simply oscillating back and forth in a radio antenna—if a charge moves nonuniformly, it radiates. A free charged particle can spontaneously absorb or emit a photon, and an increasing number of important devices, ranging from the free-electron laser to the synchrotron radiation generator, utilize this mechanism on a practical level. For further discussions on these types of sources, see Section 8.4 .4.2.2THE NUCLEUS AND γ-RAY AND X-RAY GENERATIONγ-rays, by definition, are high-energy electromagnetic radiation that arise from the decay of a radioactive nucleus. Often people think that x-rays are lower in energy than γ-rays. This is not true. They only differ by definition, i.e., origin of the electromagnetic radiation. It is very important to point out that x-rays can be higher in energy or equal to γ-rays and visa versa.
eBook - PDF- John A. S. Adams, Paolo Gasparini(Authors)
- 2013(Publication Date)
- Elsevier(Publisher)
The characteristics of the interaction of the electromagnetic radiation with matter, mainly the phenomena of emission of electrons from metals when struck by high frequency radiations (photoelectric emission) and the scattering of such radiations by electrons, required a further modification of the theory. Emission and scattering could be explained only if one assumed that radiation, in addition to being absorbed and emitted in an integral number of quanta, propagates through space as discrete quanta, or photons. Each photon has an energy, E, which is a function of the characteristic frequency (or wave length) of the radiation, i.e.: E=hv = hc/λ (1.1) where is the frequency and λ the wave length of the radiation; c is the velocity of light, and h is the Planck constant. When the frequency is expressed in sec"^ and the energy expressed in ergs, the Planck constant has the value: h = 6.62554(±0.00015)· 10 -2 '7 erg sec Gamma and X-ray energies are usually expressed in electron volts (eV). One electron volt is the energy acquired by a charged particle carrying unit electronic charge when it is accelerated through a potential difference of one volt. One eV is equivalent to 1.602 · 10" erg. One eV is associated through the Planck constant with a photon of wave length 1.2395 . The energies of natural X and gamma rays are of the order of 10^-10^ eV, and they are usually expressed in keV (10^ eV) or in MeV (10^ eV). X and gamma rays are classically differentiated according to their wave lengths: electromagnetic radiations with frequencies between 10^^ and 10^^ sec" ^ (corresponding to wave lengths between 10"^ and 10"^^ cm and to energies between 40 keV and 4 MeV) are called gamma rays; those with frequencies ranging from 10^^ to 10^^ sec"^ (corresponding to wave lengths from 10"^ to 10"^ cm and to energies from 40 eV to 40 keV) are called X-rays. It has been found more meaningful and unambiguous to distinguish X from gamma rays on the basis of where they originate.
- John L. Ball, Adrian D. Moore, Steve Turner(Authors)
- 2012(Publication Date)
- Wiley-Blackwell(Publisher)
The electrons so liberated are known as photoelectrons . The photoelectric effect and the ioni-sation resulting from exposure to X-and gamma radiation are discussed further in Section 17.2.2. With one notable exception, we have now covered most aspects of the proper-ties of light which impinge on diagnostic imaging. The major omission is optics , the study of the way light is reflected and refracted by lenses and mirrors and the way it is diffracted when passing through narrow apertures. We refer the reader to sources such as Muncaster (1993) for a general treatment of geometri-cal optics, to Jenkins & White (2001) for a mathematical approach and to Ball & Price (1995) for a largely non-mathematical treatment of areas of optics partic-ularly relevant to radiographers. In the next chapter we begin our study of the high-energy radiation produced by X-ray tubes on which much of the work of a radiographer relies. Chapter 16 X-Rays Production of X-rays Interactions at the target X-ray spectrum K-, L-and M-series emissions Maximum photon energy and minimum wavelength Quality and intensity of X-rays Factors affecting X-ray tube output We are now in a position to be able to study X-rays: how they are produced and the properties they possess. 16.1 Production of X-rays There are two types of events which can lead to X-ray production: (1) An electron travelling at high speed may experience a sudden change in its direction of motion. (2) An electron in an atom may undergo a transition from a high-energy state to a lower energy state. Most of the output of an X-ray tube is the result of the first type of event taking place, but in some circumstances (e.g. in mammography equipment) the second type of event may contribute significantly to the total X-ray output. The X-ray tube is a device which is designed to produce fast-moving electrons and cause them to deviate violently. In Chapter 12 we studied the construction of the X-ray tube in detail.
eBook - PDF- Patrick N. McDermott, Colin G. Orton(Authors)
- 2018(Publication Date)
- Medical Physics Publishing(Publisher)
The energy of these photons will be converted into random motion of the atoms in the target; i.e., heat. Another possibility is that an outer shell electron can be ejected from the atom (i.e., the atom is ionized). The ejected electron will move through the target material; it may collide with other atoms, giving up some of its energy to the random motion of these atoms—heat again. Eventually it will find another atom that is missing an outer shell electron, and it will “recombine” with that atom, emitting a low- energy photon in the process. The energy of this photon will ultimately be absorbed, contributing once again to heat production. Occasionally a high-energy bombarding electron will eject an inner shell electron (e.g., K or L shell). There will then be a vacancy that can be filled by outer shell electrons dropping down (see Figure 5.1). Inner shell electrons are very tightly bound. The energy released when outer shell electrons drop down to fill an inner shell vacancy will be high, particularly for a material with a high atomic number. Many of the photons emitted in the downward cascade will be of sufficiently high energy to be classified as x-rays. Some of the higher-energy x-rays may be able to escape from the target without being absorbed. These x-rays will have discrete energies (monoenergetic spectral lines), which are characteristic of the atomic species from which they are emitted and are unique for each element. The emission of characteristic x-rays Figure 5.1 On the left a high-energy bombarding electron ejects an electron from the K shell of an atom. On the right an L shell electron drops down to occupy the vacancy in the K shell. This, in turn, leaves a vacancy in the L shell, which is now filled by an M shell electron and so on. The result of this downward cascade of electrons is the emission of x-rays with dis- crete energies that are characteristic of the particular element.
eBook - PDF- K. Kirk Shung, Michael Smith, Benjamin M.W. Tsui(Authors)
- 2012(Publication Date)
- Academic Press(Publisher)
The polychromatic beam after traversing a medium contains less photons in the lower energy range, causing the effective energy of the beam to increase. This phenomenon is called beam hard-ening. II. e eratio a Detectio o f X-rays A. X-ra y e eratio X-rays are generated when electrons with high energy strike a target made from materials like tungsten or molybdenum. The high-energy electrons can interact with the nuclei of the tungsten atoms producing the so-called general radiation or white radiation or Bremsstrahlung, or they can interact with the orbital electrons producing the characteristic radiation. 1. W it e Ra iatio When an electron that is negatively charged passes near the positively charged nu-cleus, the electron is attracted toward the nucleus and then deflected from its orig-inal path. The electron may lose some energy or may not. If it does not, the pro-cess is called elastic scattering and no X-ray photons will be produced. If it does lose energy, the process is called inelastic scattering and the energy lost by the electron is emitted in the form of an X-ray photon. The radiation produced in this way is called white radiation, which is depicted in Fig. 7. The probability of the electron to lose energy is increased as the atomic number of the atom increases. The electrons striking the target can interact with a number of nuclei before being stopped and the electrons may carry different energies. Therefore, the energies of the X-ray photons generated by the process of general radiation are distributed over a wide range, as shown in Fig. 8. 12 C H A P T E R 1 X -r a y E 1 > Fi u re7 Deflection of high-energy electron by nucleus produces white radiation. 2. C aracteristi c Ra iatio When the electrons striking the target interact with orbital electrons in inner shells, characteristic radiation results. This process is very similar to that de-scribed in photoelectric effect. Figure 8 shows the typical X-ray spectrum gener-ated at a tungsten target.
eBook - PDF- David Dowsett, Patrick A Kenny, R Eugene Johnston(Authors)
- 2006(Publication Date)
- CRC Press(Publisher)
3 X-ray production and properties: fundamentals Introduction 71 X-ray tube design 74 The X-ray spectrum 83 Electrical characteristics 89 X-ray tube rating 90 Keywords 95 3.1 INTRODUCTION On Friday 8 November 1895 Wilhelm Conrad Röntgen (1845–1923; German physicist) while experimenting with high voltages using a Crooke’s tube (an evacu-ated tube with electrodes inserted) noticed that invis-ible radiation was being produced that penetrated the soft tissue of his hand revealing skeletal structure. He called the radiation ‘X-rays’: X denoting their un-known origin. Within a year of their discovery X-rays were being used for medical imaging. In 1901 Röntgen received the first Nobel Prize for physics. In 1913 W.D. Coolidge (1873–1975; USA engineer) produced the electrically heated cathode tube: the forerunner of all modern X-ray tubes. 3.1.1 X- and gamma radiation The principle of X-ray generation is relatively simple. A beam of high energy electrons from a heated fila-ment (situated in a cathode assembly) bombards a positively charged heavy metal target, the anode . The electrons mostly react with the target’s orbital electrons producing heat (99%): the remaining elec-trons interact with the target nuclei giving a continuous X-ray spectrum made up of many photon energies; a poly-energetic spectrum. X-ray photon energy from the tube can be controlled by varying the electrical supply high voltage. The diagnostic range has a peak energy (kV p ) from 40 to 140 kV p ; mammography uti-lizes a lower energy from 20 to 30 kV p . Figure 3.1a shows that an X-ray tube can be treated as a electronic diode since the electrons only travel in one direction, having a heated filament at one end which acts as the source of electrons and an anode at the other. Electrons emitted by the filament are accelerated across a vacuum by applying a high voltage, colliding with the anode to produce X-radiation.
eBook - PDF- M. I. Pergament(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
As in the previous case, the energy of the emitted photon will be equal to the difference in the energy states of the atom before and after relaxation. Note that if the vacancy, say, on the level K , is filled by an elec-tron from (say) level L , the latter level in turn becomes a vacancy, which will then be filled by electrons from still higher levels. Radiation gener-ated by an atom undergoing such a cascade of transitions is known as the 272 ◾ Methods of Experimental Physics characteristic radiation . Its discrete spectrum consists of a relatively small number of spectral lines, making it a very useful tool for identification of the chemical composition of an object under investigation. This is illus-trated in Figure 10.1, showing an example of the transitions responsible for generation of the K and L series spectra in a copper atom. The physical mechanisms listed above form the basis of artificial sources of X-ray radiation. We will mainly consider those sources, which are used by experimental physicists. The oldest but still a very common device (although it is used mostly in medicine) is the Roentgen, or X-ray tube, depicted in Figure 10.2, which shows the principal elements of this device: a heated cathode, an electron emitter, a cooled anode, and power supplies. The upper limit for the generated X-ray spectrum is determined by the applied voltage, and the lower bound is determined by the trans-mission through the walls of the tube. This is a simple, reliable, and rea-sonably compact device. Its efficiency in transforming electrical energy n 4 4 .
- Michael D. Morris(Author)
- 1993(Publication Date)
- CRC Press(Publisher)
9 X-Ray Emission Imaging Barry M. Gordon and Keith W. Jones Brookhaven National Laboratory, Upton, New York I. INTRODUCTION Shortly after the discovery of x-rays, Barkla [l] established that elements emit-ted groups of high and low energy x-rays under electron bombardment. In 1913 Moseley [2], in a systematic study of this phenomenon, showed a monotonically increasing relationship between the energy of these K-and L-shell x-rays and atomic number of the targets, as shown in Fig. 1. The detection and measure-ment of the characteristic x-rays became the basis of a technique for elemental analysis. The subject of this chapter is the application of the excitation of a sam-ple target and the imaging of the elemental distributions of the target at small spatial resolutions using the characteristic fluorescence x-rays. While the technique of x-ray fluorescence analysis was established with Moseley's discovery and was applied very early, the sensitivity of the technique has steadily improved with technological advances, and its applicability has wid-ened. In particular the introduction of nuclear techniques has led to the attain-ment of small spatial resolutions and low detection limits. The influence of the nuclear sciences is shown in the sometimes misleading use of nuclear units to describe the atomic process of ionization, such as quoting cross section values in barns. One liberty taken in this chapter is the specification of x-ray energy E in kiloelectronvolts (keV), not frequency, wavenumbers, or wavelength units. The conversion is E = 12.3985 fA., where A. is wavelength (A). X-ray fluorescence analysis and imaging, in particular, have been applied in a wide variety of fields including the biomedical, geochemical, and material sci-ences. The technique is usually nondestructive and can be performed in air or inert atmospheres. For the most part, the technique has been limited to elemental 303
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