X-rays, education and history

X-rays are a type of electromagnetic radiation with wavelengths of around 10-10 metres.

When medical X-rays are being produced, a thin metallic sheet is placed between the emitter and the target, effectively filtering out the lower energy (soft) X-rays. This is often placed close to the window of the X-ray tube. The resultant X-ray is said to be hard. Soft X-rays overlap the range of extreme ultraviolet. The frequency of hard X-rays is higher than that of soft X-rays, and the wavelength is shorter. Hard X-rays overlap the range of "long"-wavelength (lower 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. X-ray K-series spectral line wavelengths (nm) for some common target materials.

The basic production of X-rays is by accelerating electrons in order to collide with a metal target. (In medical applications, this 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 soft 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). Here the electrons suddenly decelerate upon colliding with the metal target and if enough energy is contained within the electron it is able to knock out an electron from the inner shell of the metal atom and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted. This process is extremely inefficient (~0.1%) and thus to produce reasonable flux of X-rays plenty of energy has to be wasted into heat which has to be removed.

The spectral lines generated depends on the target (anode) element used and thus are called characteristic lines. Usually these are transitions from upper shells into K shell (called K lines), into L shell (called L lines) and so on. There is also a continuum Bremsstrahlung radiation given off by the electrons as they are scattered by the strong electric field near the high-Z (proton number) nuclei.

X-rays can detect cancer, cysts, and tumors. Due to their short wavelength, in medical applications X-rays act more like a particle than a wave. This is in contrast to their application in crystallography, where their wave-like nature is most important.

To take an X-ray of the bones, short X-ray pulses are shot through a body with photographic film behind. The bones absorb the most photons by the photoelectric process, because they are more electron dense. The x-rays that do not get absorbed turn the photographic film from white to black, leaving a white shadow of bones on the film.

The detection of X-rays is based on various methods. The most commonly known methods are a photographic plate, X-ray film in a cassette, and rare earth screens.

A photographic plate or film is used in hospitals to produce images of the internal organs and bones of a patient. They are also used in industrial radiography processes. Since photographic plates are sensitive to X-rays, they provide a convenient and easy means of recording the image. X-ray film is usually provided as pre-loaded paper cartridges with the film inside a light proof paper envelope. An additional paper coated in a thin layer of lead is often included in contact with the photographic film. The lead reflects the x-rays back through the photo film thus more or less doubling the sensitivity of the assembly. Thus the photographic film has to be used the right way round, and is marked as such. The emulsion is frequently coated on both sides of the film or plate in order to increase the sensitivity further.

The part of the patient to be X-rayed is placed between the X-ray source and the photographic receptor 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. Those areas where the X-rays strike the photographic receptor turn black when it is developed. So where the X-rays pass through "soft" parts of the body such as organs, muscle, and skin, the plate or film turns black.

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