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An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz to 60 EHz). X-rays are primarily used for diagnostic medical imaging and crystallography. X-rays are a form of ionizing radiation and as such can be dangerous.
X-rays with a wavelength approximately longer than 0.1 nm are called soft X-rays. At wavelengths shorter than this, they are called hard X-rays. Hard X-rays overlap the range of long-wavelength (low energy) gamma rays, however the distinction between the two terms 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 detection of X-rays is based on various methods. Most commonly known is the photographic plate, well known from its use in hospitals. The X-rays blacken the photographic plate (negative), it is black where the X-rays go through ("soft" parts of the body like organs and skin) and white where the X-rays are stopped ("hard" parts like bones, or contrast product containing iodine injected in blood). Another method is to use a fluorescent plate, e.g. sodiumSodium is the chemical element in the periodic table that has the symbol Na Natrium in Latin) and atom number 11. Sodium is a soft, waxy, silvery reactive metal belonging to the alkali metals that is abundant in natural compounds (especially halite). iodide NaI. These methods give no information about the energy of the X-ray photons, just their spatial density.
Initially, most common detection methods were based on the ionisation of gases, as in the Geiger-Müller counterA Geiger counter measures ionizing radiation. Geiger counters can detect photons, alpha, beta, and gamma radiation, but not neutrons. The sensor is a Geiger-Muller tube, a gas-filled tube that briefly conducts electricity when a particle or photon of radi: a sealed cylinder with a polymer window contains a gas, and a wire, and a high voltage is applied between the cylinder ( cathodeThe electrode of an electrochemical cell at which reduction occurs is referred to as the cathode . In an electrolytic cell the cathode is negatively charged and in a galvanic cell the cathode is positively charged. The oppositely charged electrode in that) and the wire ( anodeAn anode is the positive electrode in an eletrolytic system or circuit. Literally, the path through which the electrons ascend out of an electrolyte solution. In electrochemistry, the anode is where oxidation occurs and is also the negative discharge plat). When an X-ray photon enters the cylinder, it ionises the gas which becomes conducting, creating a current flow (a kind of flash); this peak of current is detected and is called a "count".
When the high voltage between anode and cathode is decreased, the detector is no longer saturated, and the height of the current peak is proportional to the energy of the photon; it is thus called a "proportional counter". Most of times, the cylinder is not sealed but is constantly fed with "fresh gas", is thus called a "flow counter". This proportionality property allows filtering the "interesting" peaks from the noise and other photons, but the resolution in energy is not enough to determine the energy spectrumThe noun spectrum (plural: spectra has a variety of meanings. Ghosts Originally a spectrum was what is now called a spectre, i. a phantom or apparition. Spectral evidence is testimony about what was done by spectres of persons not present physically, or h; such a feature requires a diffractingIn physics, diffraction is a wave phenomenon: the apparent bending and spreading of waves when they meet an obstruction. Diffraction occurs with electromagnetic waves, such as light and radio waves, and also in sound waves and water waves. Diffraction als crystal to first separate the different photons, the method is called wavelength dispersive X-ray spectroscopyThe Wavelength dispersive X-ray spectroscopy is a method used to determine the energy spectrum of a X-ray radiation. It is mainly used in chemical analysis, in a X-ray fluorescence spectrometer, or in an electron microprobe (Castaing probe). The X-rays em (WDX or WDS).
Some materials such as NaI can "convert" an X photon to a visible photon; an electronic detector can be build by adding a photomultiplier. These detectors are called " scintillators" or " scintillation counters".
Since the 1990s, new detectors based on semiconductors were developed ( silicon or germanium doped with lithium, Si(Li) or Ge(Li)). X-ray photons are converted to electron-hole pairs in the semiconductor, and are collected to detect the X-rays. When the temperature is low enough (the detector is cooled by Peltier effect or best by liquid nitrogen), it is possible to directly determine the X-ray energy spectrum; this method is called energy dispersive X-ray spectroscopy (EDX or EDS); it is often used in small X-ray fluorescence spectrometers. These detectors are often called "solid detectors".
It is commonly thought that X-rays are invisible to the human eye, and for almost all everyday uses of X-rays this may seem true, however, it is actually false. In special circumstances, X-rays are in fact visible to the "naked eye". An effect first discovered by Brandes in experimentation a short time after Röntgen's landmark 1895 paper; he reported, after dark adaptation and placing his eye close to an X-ray tube, seeing a faint "blue-gray" glow which seemed to originate within the eye itself.[1] Upon hearing this, Röntgen reviewed his record books and found he in fact, also saw the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen saw the same blue glow seeming to emanate from the eye itself, but thought his observations were spurious due to the fact that he only saw the effect when he used one type of tube. Later he realized that the tube which created the effect was the only one which produced X-rays powerful enough to make the glow plainly visible and the experiment was thereafter repeated readily. The fact that X-rays are actually faintly visible to the dark-adapted naked eye has largely been forgotten today is probably due to the lack of desire to repeat what we would now see as a recklessly dangerous and harmful experiment with ionizing radiation. It is not known what the exact mechanism in the eye is which produces the visibility and it could be due to either conventional detection (excitation of rhodopsin molecules in the retina), direct excitation of retinal nerve cells, or secondary detection via, for instance, X-ray induction of phosphorescence in the eyeball and then conventional retinal detection of the secondarily produced visible light.
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