The Sources of X – Ray

· Introduction

This article is about the X-ray sources. X-ray are the basic tool of radiologist, the performances of X-ray sources directly effect of radiological techniques. Received the first Nobel Prize in physics in 1901, “in recognition of the extraordinary services he has rendered by the discovery of the remarkable rays subsequently named after him.” Wurzberg Physical-Medical Society, Chairman Albert von Kolliker, whose hand was used to to produce this image, proposed that this new form of radiation be called “Röntgen’s Rays”

The Classic Rontgen mechanism is based on exciting the electron in a material to suitably high energies by bombarding the material with a high energy electron beam. This mechanism, although very widely used, has inherent problems that limit the sources performances. For example, X-rays are emitted in all directions and therefore most of them are wasted only those reaching the imaged object are used for a radiograph. The situation is similar to that a lamp compared with a torchlight, the torchlight may emit less light than the lamp, but it is more effective and less wasteful when we want to illuminate a specific object. A torchlight corrects the lamp problems specifically, lack of collimation by using a focusing mirror. No such mirrors exist for X-ray, therefore they cannot be used to transform a standard sources into a equivalent of a torchlight.

For many radiology applications, the lack of collimation of conventional X-ray sources based on the Rontgen mechanism does not constitute a problem. In fact, conventional sources provide the large field of view which is required by many of such applications and in particular for all routine techniques, however a non-conventional sources emitting a narrow beam in a well-defined direction is required for several novel radiology techniques.

What prevents us from employing other types of sources besides those based on the Rontgen mechanism as we do, for example, for visible light? Unfortunately, sources such as lasers, incandescent lamps, perfectly suitable for other types of electromagnetic radiation, do not work for X-rays.

· Source of X-Ray

X-radiation (composed of X-rays) is a form of electromagnetic radiation. X-rays have a wavelength in the range of 10 to 0.01 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (30 × 1015 Hz to 30 × 1018 Hz) and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays. In many languages, X-radiation is called Röntgen radiation after one of its first investigators, Wilhelm Conrad Röntgen who had originally called them X-rays meaning an unknown type of radiation.

X-rays are primarily used for diagnostic radiography and crystallography. As a result, the term X-ray is metonymically used to refer to a radiographic image produced using this method, in addition to the method itself. X-rays are a form of ionizing radiation and as such can be dangerous.

X-rays span 3 decades in wavelength, frequency and energy. From about 0.12 to 12 keV they are classified as soft X-rays, and from about 12 to 120 keV as hard X-rays, due to their penetrating abilities.

The distinction between X-rays and gamma rays has changed in recent decades. Originally, the electromagnetic radiation emitted by X-ray tubes had a longer wavelength than the radiation emitted by radioactive nuclei (gamma rays). So older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10−11 m, defined as gamma rays. However, as shorter wavelength continuous spectrum “X-ray” sources such as linear accelerators and longer wavelength “gamma ray” emitters were discovered, the wavelength bands largely overlapped. The two types of radiation are now usually defined by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.

Early gas x-ray tubes.

Coolidge/vacuum tubes. Evacuated glass tube. Tungsten cathode at one end heated to around 2000 °C emits electrons through thermionic emission. In a sense, the electrons boil off of the metal surface. But it’s a weird kind of “boiling” since the electrons can never evaporate away. They are always replaced by new ones. If they weren’t we’d wind up with a huge positive charge on the metal surface.

Accelerated by a large potential difference (ranging from a few thousand to nearly a million volts depending on the application) toward a metal anode (a comparatively massive copper heat sink whose target face is cut diagonally and coated with some other metal). With a heated cathode in a high vacuum tube, the electron current may be controlled simply by varying the filament temperature. Then, by varying the voltage across the tube, the penetrating power of the x-rays (a function of the x-ray energy) may be varied. Thus, two important parameters may be controlled independently. More than 99% of the electrons’ energy is converted to heat. This heat must be transferred or the target would melt. Water cooling is one method. In some tubes, the target face in mounted on a small motor.

Modern vacuum x-ray tubes.

Schematic diagram of “an entirely new variety of [x-ray] tube” from William Coolidge’s 1913 patent application. Nearly all contemporary x-ray tubes are variations of the “Coolidge Tube”.

Pic. 1. A vacuum x-ray tube of the type used in dentistry.

X-rays are generated by an X-ray tube, a vacuum tube that uses a high voltage to accelerate electrons released by a hot cathode to a high velocity. The high velocity electrons collide with a metal target, the anode, creating the X-rays. In medical X-ray tubes the target is usually tungsten or a more crack-resistant alloy of rhenium (5%) and tungsten (95%), but sometimes molybdenum for more specialized applications, such as when soft X-rays are needed as in mammography. In crystallography, a copper target is most common, with cobalt often being used when fluorescence from iron content in the sample might otherwise present a problem.

The maximum energy of the produced X-ray photon in keV is limited by the energy of the incident electron, which is equal to the voltage on the tube, so an 80 kV tube can’t create higher than 80 keV X-rays. When the electrons hit the target, X-rays are created by two different atomic processes:

1. X-ray fluorescence: If the electron has enough energy it can knock an orbital electron out of the inner shell of a metal atom, and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted. This process produces a discrete spectrum of X-ray frequencies, called spectral lines. The spectral lines generated depends on the target (anode) element used and thus are called characteristic lines. Usually these are transitions from upper shells into K shell (called K lines), into L shell (called L lines) and so on.

2. Bremsstrahlung: This is radiation given off by the electrons as they are scattered by the strong electric field near the high-Z (proton number) nuclei. These X-rays have a continuous spectrum. The intensity of the X-rays increases linearly with decreasing frequency, from zero at the energy of the incident electrons, the voltage on the X-ray tube.

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