X Ray Definition and Properties (X Radiation)

What You Need to Know About X-Rays

Dense tissue scatters x-rays, producing an image, while the radiation passes through soft tissue.
Dense tissue scatters x-rays, producing an image, while the radiation passes through soft tissue. Tetra Images / Getty Images

X-rays or x-radiation are part of the electromagnetic spectrum with shorter wavelengths (higher frequency) than visible light. X-radiation wavelength ranges from 0.01 to 10 nanometers, or frequencies from 3×1016 Hz to 3×1019 Hz. This puts the x-ray wavelength between ultraviolet light and gamma rays. The distinction between x-ray and gamma rays may be based on wavelength or on radiation source. Sometimes x-radiation is considered to be radiation emitted by electrons, while gamma radiation is emitted by the atomic nucleus.

German scientist Wilhelm Röntgen was the first to study x-rays (1895), although he was not the first person to observe them. X-rays had been observed emanating from Crookes tubes, which were invented circa 1875. Röntgen called the light "X-radiation" to indicate it was a previously unknown type. Sometimes the radiation is called Röntgen or Roentgen radiation, after the scientist. Accepted spellings include x rays, x-rays, xrays, and X rays (and radiation).

The term x-ray is also used to refer to a radiographic image formed using x-radiation and to the method used to produce the image.

Hard and Soft X-Rays

X-rays range in energy from 100 eV to 100 keV (below 0.2–0.1 nm wavelength). Hard x-rays are those with photon energies greater than 5-10 keV.  Soft x-rays are those with lower energy. The wavelength of hard x-rays is comparable to the diameter of an atom. Hard x-rays have sufficient energy to penetrate matter, while soft x-rays are absorbed in air or penetrate water do a depth of about 1 micrometer.

Sources of X-Rays

X-rays may be emitted whenever sufficiently energetic charged particles strike matter. Accelerated electrons are used to produce x-radiation in an x-ray tube, which is a vacuum tube with a hot cathode and a metal target. Protons or other positive ions may also be used. For example, proton-induced x-ray emission is an analytical technique. Natural sources of x-radiation include radon gas, other radioisotopes, lightning, and cosmic rays.

How X-Radiation Interacts With Matter

The three ways x-rays interact with matter are Compton scattering, Rayleigh scattering, and photoabsorption. Compton scattering is the primary interaction involving high energy hard x-rays, while photoabsorption is the dominant interaction with soft x-rays and lower energy hard x-rays. Any x-ray has sufficient energy to overcome the binding energy between atoms in molecules, so the effect depends on the elemental composition of matter and not its chemical properties.

Uses of X-Rays

Most people are familiar with x-rays because of their use in medical imaging, but there are many other applications of the radiation:

In diagnostic medicine, x-rays are used to view bone structures. Hard x-radiation is used to minimize absorption of low energy x-rays. A filter is placed over the x-ray tube to prevent transmission of the lower energy radiation. The high atomic mass of calcium atoms in teeth and bones absorbs x-radiation, allowing most of the other radiation to pass through the body. Computer tomography (CT scans), fluoroscopy, and radiotherapy are other x-radiation diagnostic techniques. X-rays may also be used for therapeutic techniques, such as cancer treatments.

X-rays are used for crystallography, astronomy, microscopy, industrial radiography, airport security, spectroscopy, fluorescence, and to implode fission devices. X-rays may be used to create art and also to analyze paintings. Banned uses include x-ray hair removal and shoe-fitting fluoroscopes, which were both popular in the 1920s.

Risks Associated with X-Radiation

X-rays are a form of ionizing radiation, able to break chemical bonds and ionize atoms. When x-rays were first discovered, people suffered radiation burns and hair loss. There were even reports of deaths. While radiation sickness is largely a thing of the past, medical x-rays are significant source of man-made radiation exposure, accounting for about half the total radiation exposure from all sources in the U.S. in 2006. There is disagreement about the dose that presents a hazard, partially because risk depends on multiple factors. It is clear x-radiation is capable of causing genetic damage that can lead to cancer and developmental problems. The highest risk is to a fetus or child.

Seeing X-Rays

While x-rays are outside the visible spectrum, it's possible to see the glow of ionized air molecules around an intense x-ray beam. It's also possible to "see" x-rays if a strong source is viewed by a dark-adapted eye. The mechanism for this phenomenon remains unexplained (and the experiment is too dangerous to perform). Early researchers reported seeing a blue-gray glow that seemed to come from within the eye.