X-rays (aka Röntgen rays, after their discoverer Wilhelm Conrad Röntgen) are an ionizing type of electromagnetic radiation in the frequency range of 3×1016 Hz to 3 × 1019 Hz. They can be divided into the more energetic hard x-rays (3×1018 Hz to 3 × 1019 Hz) adjacent to gamma rays, and into soft x-rays (3×1016 Hz to 3 × 1018 Hz), adjacent to ultraviolet light.
They are widely used for structural investigations in all parts of materials science, though care has to be exerted for uses on living tissue, since ionizing radiation can cause cell damage; see acute radiation syndrome.
Principles of X-ray applications
Imaging using X-rays
The most common use of X-rays is in diagnostic imaging. These applications have a source of X-rays, possibly a means of focusing the radiation beam, mechanisms for positioning the source and subject in respect to another, and a device for recording the X-ray beam after it passed through the subject.
X-ray imaging depends on the principle that different materials are more or less opaque to X-rays, and thicker sections of the same material will be more opaque than thinner sections. For example, bone is more opaque than soft body tissue, and thick bone is more opaque than thin bone. In other words, different parts of the subject differently attenuate the X-ray beam. Most often, the image is presented in "negative" form, with whiter parts showing more and the darker parts showing less attenuation.
Non-imaging X-rays used in evaluating materials
X-rays used to affect materials and tissue
It is increasingly common to have images read at a distance from the imaging equipment, distance ranging from an office to a continent away. A standard method for transmitting these images is digital imaging and communications in medicine (DICOM).
Practical X-ray machines for diagnostic imaging internal body parts, at a minimum, an #X-ray source, mechanisms both for positioning the X-ray source and the patient, and a means of that records the signals that have gone through the body to form an image.
While they follow the basic principles described here, more advanced medical X-ray techniques, such as X-ray computed tomography, have sufficient differences and refinements that a sub-article is needed for the details.
- X-ray diffraction : A non-destructive analytical technique which reveal information about the crystallographic structure, chemical composition, and physical properties of materials and thin films, using x-rays.
X-ray fluorescence spectroscopy
X-ray fluorescence spectrosopy is a nonimaging testing methods, applied to nonliving materials, in which the X-rays cause certain atoms in the material to generate, through a special case of the photoelectric effect, energy in a different part of the electromagnetic spectrum than X-rays. 
- Digital X-Ray Machine and Camera System, Lattice Semiconductor Corporation
- X-ray Fluorescence Spectroscopy (XRF), Amptek
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