An X-ray picture (radiograph), taken by Wilhelm Röntgen in 1896, of His wife's hand]] X-rays (or Röntgen rays) are a form of electromagnetic radiation with a wavelength in the range of 10 to 0.01 nanometers, corresponding to frequencies in the range 30 to 30 000 PHz (1 PHz = 1015 Hertz). X-rays are primarily used for diagnostic radiography and crystallography. X-rays are a form of ionizing radiation and as such can be dangerous. In many languages it is called Röntgen radiation after one of the first investigators of the X-rays, Wilhelm Conrad Röntgen.
The unit of measure for an X-ray is called a rem, abbreviated "r". A rem is a relatively large amount, and therefore exposure to X-rays for medical use is often measured in mrems (or millirems).
The average person living in the United States is exposed to approximately 360 mrem annually from background sources alone.
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