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 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. interaction with matter attenuation length of x-rays in water showing the oxygen absorption edge at 540 ev, the energy−3 dependence of photoabsorption, as well as a leveling off at higher photon energies due to compton scattering. the attenuation length is about four orders of magnitude longer for hard x-rays (right half) compared to soft x-rays (left half). 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. 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 x-ray 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 more likely, especially for high-energy x-rays. the probability for different scattering angles is 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. production whenever charged particles (electrons or ions) of sufficient energy hit a material, x-rays are produced. production by electrons characteristic x-ray emission lines for some common anode materials. anodematerialatomicnumberphoton energy wavelength kα1kβ1kα1kβ1 w 7459.367.20.02090.0184 mo 4217.519.60.07090.0632 cu 298.058.910.1540.139 ag 4722.224.90.05590.0497 ga 319.2510.260.1340.121 in 4924.227.30.05120.0455 spectrum of the x-rays emitted by an x-ray tube with a rhodium target, operated at 60 kv. the smooth, continuous curve is due to bremsstrahlung, and the spikes are characteristic k lines for rhodium atoms. 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 medical x-ray tubes the target 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 softer 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. the maximum energy of the produced x-ray photon is limited by the energy of the incident electron, which is equal to the voltage on the tube times the electron charge, so an 80 kv tube cannot create x-rays with an energy greater than 80 kev. when the electrons hit the target, x-rays are created by two different atomic processes: characteristic x-ray emission (x-ray electroluminescence): if the electron has enough energy, it can knock an orbital electron out of the inner electron shell of the target atom. after that, electrons from higher energy levels fill the vacancies, and x-ray photons are emitted. this process produces an emission spectrum of x-rays at a few discrete frequencies, sometimes referred to as spectral lines. usually, these are transitions from the upper shells to the k shell (called k lines), to the l shell (called l lines) and so on. if the transition is from 2p to 1s, it is called kα, while if it is from 3p to 1s it is kβ. the frequencies of these lines depend on the material of the target and are therefore called characteristic lines. the kα line usually has greater intensity than the kβ one and is more desirable in diffraction experiments. thus the kβ line is filtered out by a filter. the filter is usually made of a metal having one proton less than the anode material (e.g. ni filter for cu anode or nb filter for mo anode). bremsstrahlung: this is radiation given off by the electrons as they are scattered by the strong electric field near the high-z (proton number) nuclei. these x-rays have a continuous spectrum. the frequency of bremsstrahlung is limited by the energy of incident electrons. 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 150 kv and thus the highest energies of the x-ray photons range from roughly 20 kev to 150 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. 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 15 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. production by fast positive ions 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. for high energies, the production cross section is proportional to z12z2−4, where z1 refers to the atomic number of the ion, z2 refers to that of the target atom. an overview of these cross sections is given in the same reference. production in lightning and laboratory discharges 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. this produces photons with energies of some few kev and several tens of mev. in laboratory discharges with a gap size of approximately 1 meter length and a peak voltage of 1 mv, x-rays with a characteristic energy of 160 kev are observed. a possible explanation is the encounter of two streamers and the production of high-energy run-away electrons; however, microscopic simulations have shown that the duration of electric field enhancement between two streamers is too short to produce a significant number of run-away electrons. recently, it has been proposed that air perturbations in the vicinity of streamers can facilitate the production of run-away electrons and hence of x-rays from discharges. detectors 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. medical uses patient undergoing an x-ray exam in a hospital radiology room a chest radiograph of a female patient, demonstrating a hiatal hernia since röntgen's discovery that x-rays can identify bone structures, x-rays have been used for medical imaging. the first medical use was less than a month after his paper on the subject. up to 2010, five billion medical imaging examinations had been conducted worldwide. radiation exposure from medical imaging in 2006 made up about 50% of total ionizing radiation exposure in the united states. projectional radiographs main article: projectional radiography plain radiograph of the right knee projectional radiography is the practice of producing two-dimensional images using x-ray radiation. bones contain a high concentration of 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.