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Unlike visible light, which gives a relatively stable view of the universe, the X-ray universe is unstable. It features stars being torn apart by black holes , galactic collisions, and novae, and neutron stars that build up layers of plasma that then explode into space. An X-ray laser device was proposed as part of the Reagan Administration 's Strategic Defense Initiative in the s, but the only test of the device a sort of laser "blaster" or death ray , powered by a thermonuclear explosion gave inconclusive results.

For technical and political reasons, the overall project including the X-ray laser was de-funded though was later revived by the second Bush Administration as National Missile Defense using different technologies. Phase-contrast X-ray imaging refers to a variety of techniques that use phase information of a coherent x-ray beam to image soft tissues.

It has become an important method for visualizing cellular and histological structures in a wide range of biological and medical studies.

Penetration depth - Wikipedia

There are several technologies being used for x-ray phase-contrast imaging, all utilizing different principles to convert phase variations in the x-rays emerging from an object into intensity variations. A disadvantage is that these methods require more sophisticated equipment, such as synchrotron or microfocus x-ray sources, X-ray optics , and high resolution x-ray detectors.

X-rays with high photon energies above 5—10 keV, below 0. The term X-ray is metonymically used to refer to a radiographic image produced using this method, in addition to the method itself. Since the wavelengths of hard X-rays are similar to the size of atoms, they are also useful for determining crystal structures by X-ray crystallography.

There is no consensus for a definition distinguishing between X-rays and gamma rays. One common practice is to distinguish between the two types of radiation based on their source: X-rays are emitted by electrons , while gamma rays are emitted by the atomic nucleus. Some measurement techniques do not distinguish between detected wavelengths. However, these two definitions often coincide since the electromagnetic radiation emitted by X-ray tubes generally has a longer wavelength and lower photon energy than the radiation emitted by radioactive nuclei.

Thus, gamma-rays generated for medical and industrial uses, for example radiotherapy , in the ranges of 6—20 MeV , can in this context also be referred to as X-rays. 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 period 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. 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 provide good contrast in the image. X-rays have much shorter wavelengths than visible light, which makes it possible to probe structures much smaller than can be seen using a normal microscope. This property is used in X-ray microscopy to acquire high resolution images, and also in X-ray crystallography to determine the positions of atoms in crystals.

Depth sensitive X‐ray diffraction as a probe of buried half‐metallic inclusions

X-rays interact with matter in three main ways, through photoabsorption , Compton scattering , and Rayleigh scattering. The strength of these interactions depends 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.

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. 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 x-ray or an Auger electron. These effects can be used for elemental detection through X-ray spectroscopy or Auger electron spectroscopy. Compton scattering is the predominant interaction between X-rays and soft tissue in medical imaging.

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 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 is the dominant elastic scattering mechanism in the X-ray regime.

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X Rays: Atomic Origins and Applications

Whenever charged particles electrons or ions of sufficient energy hit a material, X-rays are produced. X-rays can be generated by an X-ray tube , a vacuum tube that uses a high voltage to accelerate the electrons released by a hot cathode to a high velocity. The high velocity electrons collide with a metal target, the anode , creating the X-rays. 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.

When the electrons hit the target, X-rays are created by two different atomic processes:. So, the resulting output of a tube consists of a continuous bremsstrahlung spectrum falling off to zero at the tube voltage, plus several spikes at the characteristic lines. The voltages used in diagnostic X-ray tubes range from roughly 20 kV to kV and thus the highest energies of the X-ray photons range from roughly 20 keV to keV. Both of these X-ray production processes are inefficient, with only about one percent of the electrical energy used by the tube converted into X-rays, and thus most of the electric power consumed by the tube is released as waste heat.

When producing a usable flux of X-rays, the X-ray tube must be designed to dissipate the excess heat.

Depth sensitive X‐ray diffraction as a probe of buried half‐metallic inclusions

A specialized source of X-rays which is becoming widely used in research is synchrotron radiation , which is generated by particle accelerators. Its unique features are X-ray outputs many orders of magnitude greater than those of X-ray tubes, wide X-ray spectra, excellent collimation , and linear polarization. Short nanosecond bursts of X-rays peaking at keV in energy may be reliably produced by peeling pressure-sensitive adhesive tape from its backing in a moderate vacuum.

This is likely to be the result of recombination of electrical charges produced by triboelectric charging. The intensity of X-ray triboluminescence is sufficient for it to be used as a source for X-ray imaging. X-rays can also be produced by fast protons or other positive ions. The proton-induced X-ray emission or particle-induced X-ray emission is widely used as an analytical procedure.

X-rays are also produced in lightning accompanying terrestrial gamma-ray flashes. The underlying mechanism is the acceleration of electrons in lightning related electric fields and the subsequent production of photons through Bremsstrahlung. X-ray detectors vary in shape and function depending on their purpose. Imaging detectors such as those used for radiography were originally based on photographic plates and later photographic film , but are now mostly replaced by various digital detector types such as image plates and flat panel detectors.

For radiation protection direct exposure hazard is often evaluated using ionization chambers , while dosimeters are used to measure the radiation dose a person has been exposed to. X-ray spectra can be measured either by energy dispersive or wavelength dispersive spectrometers. For x-ray diffraction applications, such as x-ray crystallography , hybrid photon counting detectors are widely used.

Projectional radiography is the practice of producing two-dimensional images using x-ray radiation. Bones contain much calcium , which due to its relatively high atomic number absorbs x-rays efficiently. This reduces the amount of X-rays reaching the detector in the shadow of the bones, making them clearly visible on the radiograph. The lungs and trapped gas also show up clearly because of lower absorption compared to tissue, while differences between tissue types are harder to see. Projectional radiographs are useful in the detection of pathology of the skeletal system as well as for detecting some disease processes in soft tissue.

Some notable examples are the very common chest X-ray , which can be used to identify lung diseases such as pneumonia , lung cancer , or pulmonary edema , and the abdominal x-ray , which can detect bowel or intestinal obstruction , free air from visceral perforations and free fluid in ascites. X-rays may also be used to detect pathology such as gallstones which are rarely radiopaque or kidney stones which are often but not always visible.

Traditional plain X-rays are less useful in the imaging of soft tissues such as the brain or muscle. One area where projectional radiographs are used extensively is in evaluating how an orthopedic implant , such as a knee, hip or shoulder replacement, is situated in the body with respect to the surrounding bone. This can be assessed in two dimensions from plain radiographs, or it can be assessed in three dimensions if a technique called '2D to 3D registration' is used.

This technique purportedly negates projection errors associated with evaluating implant position from plain radiographs. Dental radiography is commonly used in the diagnoses of common oral problems, such as cavities. In medical diagnostic applications, the low energy soft X-rays are unwanted, since they are totally absorbed by the body, increasing the radiation dose without contributing to the image.


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Hence, a thin metal sheet, often of aluminium , called an X-ray filter , is usually placed over the window of the X-ray tube, absorbing the low energy part in the spectrum. This is called hardening the beam since it shifts the center of the spectrum towards higher energy or harder x-rays. To generate an image of the cardiovascular system , including the arteries and veins angiography an initial image is taken of the anatomical region of interest. A second image is then taken of the same region after an iodinated contrast agent has been injected into the blood vessels within this area.

These two images are then digitally subtracted, leaving an image of only the iodinated contrast outlining the blood vessels.

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