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.
When medical X-rays are being produced, a thin metallic sheet is placed between the emitter and the target, effectively filtering out the lower energy (soft) X-rays. This is often placed close to the window of the X-ray tube. The resultant X-ray is said to be hard. Soft X-rays overlap the range of extreme ultraviolet. The frequency of hard X-rays is higher than that of soft X-rays, and the wavelength is shorter. Hard X-rays overlap with the range of "long"-wavelength (lower energy) gamma rays, however the distinction between the two terms in medicine depends on the source of the radiation, not its wavelength; X-ray photons are generated by energetic electron processes, gamma rays by transitions within atomic nuclei.
The basic production of X-rays is by accelerating electrons in order to collide with a metal target. (In medical applications, this 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. )
In the X-ray tube the electrons suddenly decelerate upon colliding with the metal target and if the electron has enough energy it can knock out an electron from the inner shell of the metal atom and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted. This process is extremely inefficient (~0.1%) and thus to produce reasonable flux of X-rays plenty of energy has to be wasted into heat which has to be removed. Radiographs obtained using X-rays can be used to identify a wide spectrum of pathologies. Due to their short wavelength, in medical applications, X-rays act more like a particle than a wave. This is in contrast to their application in crystallography, where their wave-like nature is most important.
