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X
Ray, penetrating electromagnetic radiation, having a shorter
wavelength than light, and produced by bombarding a target, usually made
of tungsten,
with high-speed electrons (see Electromagnetic
Radiation; Light).
X rays were discovered accidentally in 1895 by the German physicist
Wilhelm Conrad Roentgen while he was studying cathode rays in a
high-voltage, gaseous-discharge tube. Despite the fact that the tube was
encased in a black cardboard box, Roentgen noticed that a
barium-platinocyanide screen, inadvertently lying nearby, emitted
fluorescent light whenever the tube was in operation. After conducting
further experiments, he determined that the fluorescence was caused by
invisible radiation of a more penetrating nature than ultraviolet rays (see
Ultraviolet
Radiation). He named the invisible radiation "X ray" because
of its unknown nature. Subsequently, X rays were known also as Roentgen
rays in his honor.
II.
Nature of X Rays
X
rays are electromagnetic radiation ranging in wavelength from about 100 A
to 0.01 A (1 A is equivalent to about 10-8 cm/about 4
billionths of an in.; see Wave
Motion). The shorter the wavelength of the X ray, the greater is its
energy and its penetrating power. Longer wavelengths, near the
ultraviolet-ray band of the electromagnetic spectrum, are known as soft X
rays (see Spectrum).
The shorter wavelengths, closer to and overlapping the gamma-ray range,
are called hard X rays .
A mixture of many different wavelengths is known as "white" X
rays, as opposed to "monochromatic" X rays, which represent only
a single wavelength. Both light and X rays are produced by transitions of
electrons that orbit atoms, light by the transitions of outer electrons
and X rays by the transitions of inner electrons. In the case of
bremsstrahlung radiation (see below), X rays are produced by the
retardation or deflection of free electrons passing through a strong
electrical field. Gamma rays, which are identical to X rays in their
effect, are produced by energy transitions within excited nuclei.
X
rays are produced whenever high-velocity electrons strike a material
object. Much of the energy of the electrons is lost in heat;
the remainder produces X rays by causing changes in the target's atoms as
a result of the impact. The X rays emitted can have no more energy than
the kinetic energy of the electrons that produce them .
Moreover, the emitted radiation is not monochromatic but is composed of a
wide range of wavelengths with a sharp, lower wavelength limit
corresponding to the maximum energy of the bombarding electrons. This
continuous spectrum is referred to by the German name bremsstrahlung,
which means "braking," or slowing down, radiation, and is
independent of the nature of the target. If the emitted X rays are passed
through an X-ray spectrometer, certain distinct lines are found
superimposed on the continuous spectrum; these lines, known as the
characteristic X rays, represent wavelengths that depend only on the
structure of the target atoms. In other words, a fast-moving electron
striking the target can do two things: It can excite X rays of any energy
up to its own energy; or it can excite X rays of particular energies,
dependent on the nature of the target atom.
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