Production of X-rays

Shaping of the cathode and hooded anode to minimize focal spot size is a speciahzed art. Apparent focal spots of 1 mm (as viewed from the detectors) are typical for baggage systems. Tube currents are commonly in the range of 

Scattering events produce a continuous spectrum of X-ray energies, but the flux tapers off to zero as X-ray energies approach the apphed voltage, E /e, where e is 

X-ray energy, E, is related to the frequency, V, of the radiation by the well-known Planck equation 

The quality of synchrotron beams is usually characterized by brilliance, which is defined as brightness divided by the product of the source area (in mm ) and a fraction of a useful photon energy, i.e. bandwidth [see, for example, J. Als-Nielsen and D. McMorrow, Elements of modern x-ray physics, John Wiley Sons, New York (2001)]. 

In general, there is no principal difference in the diffraction phenomena using the synchrotron and conventional x-ray sources, except for the presence of several highly intense peaks with fixed wavelengths in the conventionally obtained x-ray spectrum and their absence, i.e. the continuous distribution of photon energies when using synchrotron sources. Here and throughout the book, the x-rays from conventional sources are of concern unless noted otherwise. 

Target Filter Incident beam liKa) Km Filter thickness for KKd) 500 Km 1 in trans. beam KKd) trans. I(Ka) incident 

All x-ray tubes contain two electrodes, an anode (the metal target) maintained, with few exceptions, at ground potential, and a cathode, maintained at a high negative potential, normally of the order of 30,000 to 50,000 volts for diffraction work. X-ray tubes may be divided into two basic types, according to the way in which electrons are provided gas tubes, in which electrons are produced by the ionization of a small quantity of gas (residual air in a partly evacuated tube), and filament tubes, in which the source of electrons is a hot filament. 

These resemble the original x-ray tube used by Roentgen. They are now obsolete. Filament Tubes 

The size and shape of the focal spot of an x-ray tube is one of its most important characteristics. Within limits, it should be as small as possible in order to concentrate the electron energy into a small area of the target and so produce an x-ray source of high intensity. 

All x-ray tubes have a maximum power rating which cannot be exceeded without injury to the tube. This limit is fixed by the amount of heat that can be dissipated by the target and is usually stated by the manufacturer in terms of the maximum allowable tube current (in mA) for a given tube voltage (in kV). 

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Fig. 18. Production of x-ray fluorescence K and E series from molybdenum atom.

Equation (1) demonstrates that the analyst must choose a beam energy that exceeds the critical excitation energy for the species to be analyzed. In general, a value of f/> 2 is required to achieve adequate efficiency in the production of X rays. 

Other considerations are that the source material, which forms a target for high-energy electron bombardment leading to the production of X-rays, should be a good conductor- to enable rapid removal of heat - and should also be compatible with UHV. 

For efficient production of X-rays by electron bombardment, exciting electron energies that are at least an order of magnitude higher than the line energies must be used, so that in Mg and A1 sources accelerating potentials of 15 kV are employed. Mod-... 

Why are barium- and iodine-based materials selected for contrast media The production of X-ray images depends on the differences between the X-ray absorbing power of various tissues. This difference in absorbing power is called contrast and is directly dependent on tissue density. To artificially enhance the ability of a soft tissue to absorb X-rays, the density of that tissue must be increased. The absorption by targeted soft tissue of aqueous solutions of barium sulfate and iodized organic compounds provides this added density through the heavy metal barium and the heavy nonmetal iodine. 

Fig. 11.7. X-ray intensity as a function of the electronic density of the plasma and the laser strength parameter. The process of nonlinear Thomson scattering for the production of X-ray emission can be observed for the highest laser intensities and along the laser axis (ao = 5.6 for the two first figures)...

According to the classical and other theories of the production of x-radiation, the radiation should be polarized in the plane containing the direction of motion of the electrons by which it was produced. In this respect the experiments on mercury vapor radiation substantiate the conclusions of the theory in part only. Radiation from the mercury vapor appears to be polarized in the proper direction, but only partially polarized. In a recent note to the Academy4 Sommerfeld has generalized his interesting theory of the production of x-rays so as to include the possibility of their being only partially polarized and also the possibility of the projection of radiation in the direction of motion of the electrons. 

X-rays are produced by allowing a stream of electrons to strike against a metal plate. Fig. 17 shows an X-ray tube used for the production of X-rays. 

Figure 2. Production of X-rays and Auger electrons by electron...

We might expect, therefore, that the waves whose wavevectors originate on the 8 branch would be absorbed due to the production of x-rays, whereas the waves whose wavevectors originate on the a branch would pass through the crystal unattenuated when the crystal is set at the exact Bragg angle. 

Figure 5-6 A simplified representation of the production of X-rays by bombardment of a solid target with a high-energy beam of electrons.

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