Опубликовано 01 сентября 2005 года
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X rays are one of the most useful forms of energy. They were discovered in 1895 by Wilhelm C. Roentgen, a German physicist. Roentgen called the rays X rays because at first he did not understand what they were. X is a scientific symbol for the unknown.
Scientists now know that X rays are a kind of electromagnetic radiation, which also includes visible light, radio waves, and gamma rays. X rays and visible light, for example, have many characteristics in common. X rays travel at the speed of light--186,282 miles (299,792 kilometers) per second. Both X rays and light travel in straight lines in the form of related electric and magnetic energy, called electromagnetic waves. In addition, X rays darken photographic film in much the same way that light does.
However, X rays and light differ in terms of wavelength, the distance between two crests of an electromagnetic wave. The wavelengths of X rays are much shorter than those of light. For this reason, X rays can penetrate deeply into many substances that do not transmit light. The penetrating power and other characteristics of X rays make them extremely useful in medicine, industry, and scientific research.
X rays can cause biological, chemical, and physical changes in substances. If the rays are absorbed by a plant or animal, they may damage or even destroy living tissue. For this reason, X rays can be dangerous. In human beings, an overdose of X rays may produce cancer, skin burns, a reduction of the blood supply, or other serious conditions. Dentists and radiologists (physicians who work with X rays) must take special care not to overexpose their patients or themselves to the rays.
In nature, X rays are produced by the sun, other stars, pulsars, and certain other heavenly bodies. Most X rays from sources in space are absorbed by the atmosphere before they reach the earth.
Machine-made X rays are produced chiefly with X-ray tubes, a principal part of X-ray machines. Devices that accelerate atomic particles also produce X rays. Betatrons and linear accelerators are examples of such devices.
Use of X rays
In medicine, X rays are widely used to make radiographs (X-ray pictures) of the bones and internal organs of the body. Radiographs help physicians detect abnormalities and disease conditions, such as broken bones or lung disease, inside a patient's body. Dentists take X-ray pictures to reveal cavities and impacted teeth (see TEETH [Dental checkups]).
A radiograph is made by passing a beam of X rays through the patient's body onto a piece of photographic film. The bones absorb more of the rays than do muscles or other organs, and so the bones cast the sharpest shadows on the film. Other parts of the body allow more X rays through than the bones do and cast shadows of varying density. The shadows of the bones show up clearly as light areas on a radiograph, and the organs are seen as darker areas. Radiologists can see a patient's organs actually functioning by means of an X-ray device called a fluoroscope. The rays cause a special screen in the fluoroscope to fluoresce (glow) when they strike it.
Sometimes a harmless substance is injected into the body to make certain organs stand out clearly on a radiograph or fluoroscopic image. For example, a doctor may give a patient a solution of barium sulfate to swallow before making an intestinal X ray. The barium sulfate absorbs X rays, and so the intestines show up clearly on the X-ray image.
X rays are widely used to treat cancer. They kill cancer cells more readily than they kill normal cells. A cancerous tumor can be exposed to a limited dose of X rays. In many cases, the X rays eventually destroy the tumor but do less damage to nearby healthy tissue.
X rays also serve other purposes in medicine. For example, they are used to sterilize such medical supplies as plastic or rubber surgical gloves and syringes. These materials would be damaged by exposure to intense heat and cannot be sterilized by boiling.
In industry, X rays are used to inspect products made of various kinds of materials, including aluminum, steel, and other cast metals. Radiographs reveal cracks and other defects in these products that are not visible on the surface. X rays are also used to check the quality of many mass-produced products, such as transistors and other small electronic devices. Some metal detection devices work by means of X rays. They include the scanners used at airports to check for weapons in luggage.
Manufacturers treat certain kinds of plastics with X rays. The rays cause a chemical change in these substances that makes them stronger. Powerful X rays have been used to help control an insect pest called the blowfly. Male blowflies cannot produce young after being exposed to X rays. In addition, X rays have been used to cause mutations (changes in cell structure) in barley. Mutated barley has produced new varieties of the grain. Some of these varieties can be raised in poor soil that cannot support regular barley.
In scientific research. X rays have been used to analyze the arrangement of atoms in many kinds of substances, particularly crystals. The atoms in crystals are arranged in planes, with regular spacing between each plane. When a beam of X rays travels through a crystal, the planes of atoms act as tiny mirrors that diffract (spread out) the rays into a regular pattern. Each type of crystal has a different diffraction pattern. Scientists have learned much about the arrangement of atoms in crystals by studying the various diffraction patterns. The study of how crystals diffract X rays is known as X-ray crystallography. Scientists also use X rays to help analyze the structure and makeup of many complex chemical substances, such as enzymes and proteins.
Archaeologists have used X rays to examine ancient objects that are covered by a heavy crust of dirt or corrosion. This method allows researchers to see an image of the object without attempting to remove the crust, which could damage the specimen. X rays also are used to reveal a painting covered by other paintings.
Characteristics of X rays
Electromagnetic radiation with short wavelengths has higher energy than radiation with long wavelengths. X rays have some of the shortest wavelengths and highest energies among all the kinds of electromagnetic radiation. The wavelengths of X rays range from about 1/100 of an angstrom unit to 100 angstrom units. An angstrom unit equals about 4/1,000,000,000 of an inch (0.00000001 centimeter). By comparison, the wavelengths of visible light range from about 4,000 to 7,000 angstrom units. The World Book article on ELECTROMAGNETIC WAVES has a diagram that compares X rays with other kinds of electromagnetic radiation.
Many of the special characteristics of X rays result from their short wavelengths and high energy. The behavior of X rays can be compared with the behavior of visible light. For example, X rays can penetrate matter more deeply than light can because their energy is much higher than the energy of light. Also, X rays cannot be reflected easily by a mirror, as light can. Because of their high energy, X rays usually penetrate the mirror instead of being reflected by its surface.
X rays do not refract (bend) much when they travel from one material into another, as light does when it travels from air into a glass lens. Light is refracted by a lens because the light waves interact with the electrons in the atoms of the lens. But X rays have such short wavelengths that they pass through many substances without interacting with the electrons.
X rays are absorbed by a substance when they strike electrons in the atoms of the substance. The number of electrons in an atom equals its atomic number (see ATOM [The atomic number]). Therefore, substances that have atoms of a high atomic number generally absorb more X rays than do substances with atoms of a low atomic number. Lead, which has an atomic number of 82, absorbs more X rays than most substances do. It is often used to make X-ray shields. Beryllium, which has an atomic number of 4, absorbs relatively few X rays. X-ray absorption also depends on the density of the substance and on other complex factors. High-density substances absorb more X rays than do low-density substances.
If the X rays absorbed by a substance have enough energy, they knock electrons out of the atoms of the substance. Whenever an electrically neutral atom gains or loses electrons, it becomes an electrically charged particle called an ion. This process is called ionization. Ionization causes the many kinds of biological, chemical, and physical changes that make X rays both useful and dangerous.
How X rays are produced
X rays are produced whenever high-energy electrons suddenly give up energy. Machines produce the rays by accelerating electrons to extremely high speeds and then crashing them into a piece of solid material called a target. There, the electrons rapidly slow down because they collide with atoms in the target, and part of their energy is changed into X rays. Physicists call such X rays bremsstrahlung--from the German word for braking radiation.
Some of the high-energy electrons knock other electrons out of their normal positions in the atoms of the target. When these dislodged electrons fall back into place, or are replaced by others, other X rays may be produced. Physicists call such X rays characteristic X rays. Bremsstrahlung has a wide range of wavelengths, but each characteristic X ray has a particular wavelength, depending on the electronic structure of the atom it came from (see ATOM [How scientists study atoms]).
X rays are produced by high-vacuum X-ray tubes for many medical and industrial uses. Such tubes consist of an airtight glass container with two electrodes--one positive and one negative--sealed inside (see ELECTRODE). The cathode (negative electrode) has a small coil of wire. The anode (positive electrode) consists of a block of metal. In most X-ray tubes, the anode and cathode consist of tungsten or a similar metal that can withstand high temperatures.
When an X-ray tube is in operation, an electric current flows through the cathode, causing it to glow white-hot. The heat releases electrons from the cathode. At the same time, an extremely high voltage is applied across the cathode and the anode. This high voltage forces the free electrons to travel at extremely high speeds toward the anode, which serves as the target. The electrons move easily through the space between the cathode and the target because the tube contains almost no air to block their motion. When the electrons strike the target, X rays and heat are produced.
The X rays are given off in many directions from the target. But most of them are absorbed by the tube housing, a metal case that surrounds the tube. One side of the housing has a small window through which a narrow beam of X rays escapes. The beam can be aimed at whatever object is to be X-rayed. The tube housing has a lead lining to absorb stray X rays. It also may contain oil or water to insulate and cool the tube.
The voltage across the cathode and target of an X-ray tube determines the energy, or penetrating power, of the rays it produces. A high voltage slams the electrons into the target at a higher energy level than does a low voltage. The X rays become more penetrating as the speed of the electrons increases. The voltage can be either raised or lowered by means of a control box.
The voltage in most X-ray tubes ranges from about 20,000 to 250,000 volts. Such voltages produce X rays powerful enough for most medical purposes. However, voltages of 300 million electronvolts (300 MeV) or higher can be achieved in betatrons and linear accelerators. X rays produced by these machines are used for medical and other scientific research.
After Roentgen discovered X rays in 1895, he experimented with them and soon demonstrated most of their characteristics. The discovery caused a sensation among scientists and the public. Within a few months, doctors were using X rays to examine broken bones.
In 1896, the American inventor Thomas A. Edison improved the fluoroscope so it could be used to view X-ray images. During the next 17 years, various scientists and inventors refined the X-ray tube. In 1913, the American physicist William D. Coolidge devised a way to make a more efficient X-ray tube. Modern X-ray tubes are similar to the type developed by Coolidge.
In the 1970's, radiologists began to use new processes for recording X-ray pictures. One process, called xeroradiography, records the image on a sheet of clear plastic instead of on photographic film. Xeroradiography is less expensive and requires less X-ray exposure than the old process. In another process, called digital imaging, detectors measure the X rays that pass through the body and send this information to a computer. The computer converts the data into an image that is displayed on a television screen. The image is stored on a magnetic disk.
Digital imaging is used in the computed tomographic scanner, or CT scanner. The CT scanner is an X-ray machine that makes a cross-sectional view of a patient's body. A CT scanner shoots a pencil-thin beam of X rays at the body from many angles. Detectors measure the rays that pass through, and a computer converts the many views obtained into a single, cross-sectional image. CT scanners enable physicians to see detailed pictures of organs and tissues with greatly improved contrast.
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