X Ray Production and Properties [Characteristics X-Rays & Continuos X-Rays]

Описание: There are three common mechanisms for the production of X-rays: the acceleration of a charged particle, atomic transitions between discrete energy levels, and the radioactive decay of some atomic nuclei. Each mechanism leads to a characteristic spectrum of X-ray radiation.

In the theory of classical electromagnetism, accelerating electric charges emit electromagnetic waves. In the most common terrestrial source of X-rays, the X-ray tube, a beam of high-energy electrons impinges on a solid target. As the fast-moving electrons in the beam interact with the electrons and nuclei of the target atoms, they are repeatedly deflected and slowed. During this abrupt deceleration, the beam electrons emit bremsstrahlung (German: “braking radiation”)—a continuous spectrum of electromagnetic radiation with a peak intensity in the X-ray region. Most of the energy radiated in an X-ray tube is contained in this continuous spectrum. Far more powerful (and far larger) sources of a continuum of X-rays are synchrotron particle accelerators and storage rings. In a synchrotron, charged particles (usually electrons or positrons) are accelerated to very high energies (typically billions of electron volts) and then confined to a closed orbit by strong magnets. When the charged particles are deflected by the magnetic fields (and hence accelerated via the change in their direction of motion), they emit so-called synchrotron radiation—a continuum whose intensity and frequency distribution are determined by the strength of the magnetic fields and the energy of the circulating particles. Specially designed synchrotron light sources are used worldwide for X-ray studies of materials.

In an X-ray tube, in addition to the continuous spectrum of radiation emitted by the decelerating electrons, there is also a spectrum of discrete X-ray emission lines that is characteristic of the target material. This “characteristic radiation” results from the excitation of the target atoms by collisions with the fast-moving electrons. Most commonly, a collision first causes a tightly bound inner-shell electron to be ejected from the atom; a loosely bound outer-shell electron then falls into the inner shell to fill the vacancy. In the process, a single photon is emitted by the atom with an energy equal to the difference between the inner-shell and outer-shell vacancy states. This energy difference usually corresponds to photon wavelengths in the X-ray region of the spectrum. Characteristic X-ray radiation can also be produced from a target material when it is exposed to a primary X-ray beam. In this case, the primary X-ray photons initiate the sequence of electron transitions that result in the emission of secondary X-ray photons.



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