Introduction to Xray
X Rays, electromagnetic radiation similar to that of visible light, but of much shorter wavelength. Virtually all matter is, to some degree, transparent to X rays. For this reason, X rays make it possible to see through flesh, bones, metals, and other substances through which visible light cannot pass. X rays are an invaluable tool in medicine, technology, and scientific research.
X rays are sometimes called Roentgen rays after Wilhelm Konrad Roentgen, who discovered them in 1895. Roentgen called the phenomenon X rays because the symbol X stands for the unknown, and he was unable to determine the nature of the radiation. Not until 1912 was it learned that X rays are electromagnetic radiation. X rays are measured in units called rads and roentgens. A physician who specializes in the diagnosis and treatment of disease by X rays is called a radiologist or a roentgenologist.
Properties of X Rays
Like radio waves and other electromagnetic radiation, X rays have a wide range of wavelengths. Strong, deeply penetrating, and highly destructive rays with short wavelengths are called hard X rays. Those with longer wavelengths and less penetrating power—the type used in medical and dental diagnosis—are known as soft X rays.
X rays can penetrate some substances more easily than others. For example, they penetrate flesh more easily than bone, and bone more easily than lead. Thus they make it possible to see bones within flesh and a bullet embedded in bone. The ability of X rays to penetrate depends not only on their wavelength, but also on the density and thickness of the substance.
X rays affect photographic film in the same way as light rays do. An X-ray photograph is made by passing a beam of X rays through the subject onto photographic film. In a more recent technique, called xeroradiography, an electrostatically charged metal plate is substituted for photographic film. When X rays pass through an object and strike the plate, they discharge it in proportion to the density of the object. To bring out the electrostatic image thus formed, the plate is sprayed with a powder that adheres to the charged area. The powder gathers more thickly in heavily charged areas than in lightly charged spots, producing a detailed picture.
X rays cause certain substances to glow, or fluoresce.
Uses of X Rays
Within a few weeks after Roentgen made his discovery, X rays were being used in medicine. Since then they also have had important uses in science and industry.
X rays help dentists detect diseases of the teeth. Doctors use X rays to locate bullets and other foreign objects within the body; to guide them in setting broken bones; and to detect cancer, ulcers, kidney stones, and other abnormalities.
Various types of X-ray scanners have been developed that allow highly detailed views of a particular section of the body. One type, known as a CT (computerized tomography) scanner, sends narrow beams of X rays at various angles through a patient's body. The information obtained from the X rays is processed by a computer to produce an image of a cross-section of the body. The image shows much more detail than an ordinary X-ray picture. A section of the body can be studied in three dimensions by producing a series of adjacent cross-sectional images.
X rays can halt the growth of cells and even destroy them altogether. They are therefore used to destroy benign and malignant tumors. X rays have also been used in the treatment of leukemia and bursitis.
X rays are used to inspect canned goods and other packaged products. A conveyor carries the goods past a beam of X rays. If a container is improperly filled, or if it contains a foreign substance, the X rays set off an alarm or set into action a device that removes the container from the conveyor. X rays are similarly used to separate beryl from granite and to inspect airplane and automobile parts, rubber goods, plastics, metal castings, and a variety of other products.
When used as a target in an X-ray tube, every element gives off X rays of specific wavelengths. These characteristic rays are used to analyze metal alloys, paint pigments, and other substances.
Scientists have learned much about the structure of matter by means of X rays. Among other things, they have learned how atoms are arranged in crystals. The average wavelength of X rays is about equal to the distance between the atoms in crystals. Crystals therefore act as diffraction gratings for X rays. That is, they scatter X rays in a pattern that shows the positions of their atoms.
When the patterns of specific crystalline substances are known, technicians can use X rays to analyze substances of which they are a part. Petroleum products, metal alloys, and other substances are thus analyzed.
Customs officers and airport security personnel use X rays in examining luggage and packages to check for weapons or smuggled articles. X rays show whether pearls are natural or cultured, and whether gemstones are natural or synthetic. X rays have also been used to learn whether paintings attributed to noted painters are authentic. Sometimes they have revealed changes made in the original work, or an earlier painting under the one that appears on the surface.
Dangers of X Rays
Because X rays can kill living cells, they must be used with extreme care. When improperly used they can cause severe burns, cancer, leukemia, and cataracts. They can speed aging, reduce immunity to disease, and bring about disastrous changes in the reproductive cells. Lead screens, sheets of lead-impregnated rubber, and leaded glass are used to shield patients and technicians from undesired radiation.
The effect of X radiation is cumulative. That is, a number of minor doses over a number of years is equivalent to a large dose at one time.
How X Rays Are Produced
The most common device for producing X rays is a vacuum tube called a Coolidge tube. It contains two electrodes: a cathode, which is heated to emit electrons, and an anode, which supports a piece of metal called the target. When a high voltage is applied across the two electrodes, the electrons stream toward the target and strike it at high speed, producing X rays. The target is usually made of tungsten or some other metal with a high melting point because the bombardment of the electrons also generates a large amount of heat.
When the electrons strike the target, X rays are produced by two processes. One process occurs as the electrons are slowed down and deflected by the atoms of the target; the kinetic energy (energy of motion) of the electrons is converted directly into X-ray radiation. The radiation, called bremsstrahlung, contains X rays of all wavelengths above a certain minimum wavelength. The higher the voltage of the tube, the greater the kinetic energy of the electrons that strike the target, and the shorter the minimum wavelength of the X rays that can be produced.
The other process involves the electrons of the atoms that make up the target. The electrons can be thought to be arranged about the nucleus, or core, of the atom in a number of shells, or layers, one within the other. The larger the shell, the greater the energy of the electrons that form it. When a high-speed electron from the cathode strikes an atom of the target, it sometimes knocks an electron from out of one of the inner shells. The ejected electron is immediately replaced by an electron from a larger (higher-energy) shell. The energy lost by the electron's dropping down to a shell of lower energy is given off as an X ray.
A variety of astronomical objects are sources of X rays produced naturally by physical processes involving very hot gases or high-energy particles.