X-ray

:In the NATO phonetic alphabet, X-ray represents the letter X.

Detectors

Photographic plates

The detection of X-rays is based on various methods. The most commonly known method are a photographic plate and a fluorescent screen.

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The X-ray photographic plate is frequently used in hospitals to produce images of the internal organs and bones of a patient. The part of the patient to be X-rayed is placed between the X-ray source and the photographic plate to produce what is a shadow of all the internal structure of that particular part of the body being X-rayed. The X-rays are blocked by dense tissues such as bone and pass through soft tissues. Where the X-rays strike the photographic plate it turns black when it is developed. So where the X-rays go through "soft" parts of the body like organs and skin the plate turns black. Contrast compounds containing barium or iodine can be injected in the artery of a particular organ. The contrast compounds strongly block the X-rays and hence the circulation of the organ can be more readily seen.

Related Topics:
Photographic plate - Hospital - Barium - Iodine

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Another method of detecting X-rays is a fluorescent plate. In modern hospitals a special plastic sheet is used in place of the photographic plate. The plastic sheet is read by a scanning laser beam. The resultant image is then stored in a computer.

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The plastic sheet can be used over and over again.

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Geiger counters

Initially, most common detection methods were based on the ionisation of gases, as in the Geiger-Müller counter: a sealed cylinder with a polymer window contains a gas, and a wire, and a high voltage is applied between the cylinder (cathode) and the wire (anode). When an X-ray photon enters the cylinder, it ionizes the gas which becomes conducting, creating a current flow (a kind of flash); this peak of current is detected and is called a "count".

Related Topics:
Geiger-Müller counter - Cathode - Anode

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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 spectrum; such a feature requires a diffracting crystal to first separate the different photons, the method is called wavelength dispersive X-ray spectroscopy (WDX or WDS).

Related Topics:
Energy spectrum - Diffracting - Wavelength dispersive X-ray spectroscopy - WDX

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Scintillators

Some materials such as NaI can "convert" an X photon to a visible photon; an electronic detector can be built by adding a photomultiplier. These detectors are called "scintillators" or "scintillation counters".

Related Topics:
Photomultiplier - Scintillators - Scintillation counters

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Semiconductor detectors

Since the 1970s, new semiconductor diode detectors have been 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". More recently, cadmium telluride (CdTe) and its alloy with zinc, cadmium zinc telluride detectors have been developed with an increased sensitivity. This allows lower doses of X-rays to be used.

Related Topics:
1970s - Semiconductor diode detector - Silicon - Germanium - Lithium - Peltier effect - Nitrogen - Energy dispersive X-ray spectroscopy - EDX - X-ray fluorescence - Cadmium telluride - Cd - Zinc - Cadmium zinc telluride

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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, very strictly speaking, 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.http://www.orau.org/ptp/articlesstories/invisiblelight.htm 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

Related Topics:
Eye - Röntgen's

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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.

Related Topics:
Experiment - Ionizing radiation - Rhodopsin - Phosphorescence

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