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X-ray performance and interaction with matter

Clicks:Updated:2015-11-27 10:11:28

X-radiation (composed of X-rays) is a form of electromagnetic radiation.Most X-rays have a wavelength ranging from 0.01 to 10 nanometers,corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz) and energies in the range 100 eV to 100 keV.X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays.In many languages,X-radiation is referred to with terms meaning Röntgen radiation,after Wilhelm Röntgen,who is usually credited as its discoverer,and who had named it X-radiation to signify an unknown type of radiation.Spelling of X-ray(s) in the English language includes the variants x-ray(s),xray(s),and X ray(s).

X-ray photons carry enough energy to ionize atoms and disrupt molecular bonds.This makes it a type of ionizing radiation,and therefore harmful to living tissue.A very high radiation dose over a short amount of time causes radiation sickness,while lower doses can give an increased risk of radiation-induced cancer.In medical imaging this increased cancer risk is generally greatly outweighed by the benefits of the examination.The ionizing capability of X-rays can be utilized in cancer treatment to kill malignant cells using radiation therapy.It is also used for material characterization using X-ray spectroscopy.

Hard X-rays can traverse relatively thick objects without being much absorbed or scattered.For this reason,X-rays are widely used to image the inside of visually opaque objects.The most often seen applications are in medical radiography and airport security scanners,but similar techniques are also important in industry (e.g.industrial radiography and industrial CT scanning) and research (e.g.small animal CT).The penetration depth varies with several orders of magnitude over the X-ray spectrum.This allows the photon energy to be adjusted for the application so as to give sufficient transmission through the object and at the same time good contrast in the image.

X-rays have much shorter wavelength than visible light,which makes it possible to probe structures much smaller than what can be seen using a normal microscope.This can be used in X-ray microscopy to acquire high resolution images,but also in X-ray crystallography to determine the positions of atoms in crystals.

Interaction with matter
X-rays interact with matter in three main ways,through photoabsorption,Compton scattering,and Rayleigh scattering.The strength of these interactions depend on the energy of the X-rays and the elemental composition of the material,but not much on chemical properties since the X-ray photon energy is much higher than chemical binding energies.Photoabsorption or photoelectric absorption is the dominant interaction mechanism in the soft X-ray regime and for the lower hard X-ray energies.At higher energies,Compton scattering dominates.

Photoelectric absorption
The probability of a photoelectric absorption per unit mass is approximately proportional to Z3/E3,where Z is the atomic number and E is the energy of the incident photon.This rule is not valid close to inner shell electron binding energies where there are abrupt changes in interaction probability,so called absorption edges.However,the general trend of high absorption coefficients and thus short penetration depths for low photon energies and high atomic numbers is very strong.For soft tissue photoabsorption dominates up to about 26 keV photon energy where Compton scattering takes over.For higher atomic number substances this limit is higher.The high amount of calcium (Z=20) in bones together with their high density is what makes them show up so clearly on medical radiographs.

A photoabsorbed photon transfers all its energy to the electron with which it interacts,thus ionizing the atom to which the electron was bound and producing a photoelectron that is likely to ionize more atoms in its path.An outer electron will fill the vacant electron position and produce either a characteristic photon or an Auger electron.These effects can be used for elemental detection through X-ray spectroscopy or Auger electron spectroscopy.

Compton scattering
Compton scattering is the predominant interaction between X-rays and soft tissue in medical imaging.Compton scattering is an inelastic scattering of the X-ray photon by an outer shell electron.Part of the energy of the photon is transferred to the scattering electron,thereby ionizing the atom and increasing the wavelength of the X-ray.The scattered photon can go in any direction,but a direction similar to the original direction is a bit more likely,especially for high-energy X-rays.The probability for different scattering angles are described by the Klein–Nishina formula.The transferred energy can be directly obtained from the scattering angle from the conservation of energy and momentum.

Rayleigh scattering
Rayleigh scattering is the dominant elastic scattering mechanism in the X-ray regime.Inelastic forward scattering gives rise to the refractive index,which for X-rays is only slightly below 1.

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