X-rays (aka Röntgen rays, after their discoverer Wilhelm Conrad Röntgen) are an ionizing type of electromagnetic radiation in the frequency range of 3×1016Hz to 3 × 1019Hz. They can be divided into the more energetic hard x-rays (3×1018Hz to 3 × 1019Hz) adjacent to gamma rays, and into soft x-rays (3×1016Hz to 3 × 1018Hz), 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
 Medical Applications
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).
 Medical diagnosis
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.
 Medical treatment
 X-ray astronomy
 X-ray diffraction
 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. 
 Adverse effects
- ↑ Digital X-Ray Machine and Camera System, Lattice Semiconductor Corporation, http://www.latticesemi.com/solutions/marketsolutions/medical/digitalxraymachinecameras.cfm
- ↑ X-ray Fluorescence Spectroscopy (XRF), Amptek, http://www.amptek.com/xrf.html
- ↑ Mettler FA, Huda W, Yoshizumi TT, Mahesh M (2008). Effective doses in radiology and diagnostic nuclear medicine: a catalog.. Radiology 248 (1): 254-63. DOI:10.1148/radiol.2481071451. PMID 18566177. Research Blogging.
- ↑ Brenner DJ, Hall EJ (2007). Computed tomography--an increasing source of radiation exposure.. N Engl J Med 357 (22): 2277-84. DOI:10.1056/NEJMra072149. PMID 18046031. Research Blogging.
- ↑ Einstein AJ, Henzlova MJ, Rajagopalan S (2007). Estimating risk of cancer associated with radiation exposure from 64-slice computed tomography coronary angiography.. JAMA 298 (3): 317-23. DOI:10.1001/jama.298.3.317. PMID 17635892. Research Blogging.
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