X rays

X rays are electromagnetic waves with wavelengths covering a fairly broad range from about 3 × 10-8 ft (10-8 m) to 3 × 10-11 ft (10-11 m). There is no sharp boundary between x rays and ultraviolet light on the long wavelength side of this range. Similarly, on the short wavelength side, x rays blend into that portion of the electromagnetic spectrum called gamma rays which have even shorter wavelengths. X rays have wavelengths much shorter than visible light, which occurs between 1.2 × 10-6 and 2.1 × 10-6 ft (4 × 10-7 and 7 × 10-7 m), and they also behave quite differently. They are invisible, are able to penetrate substantial thicknesses of matter , and can ionize matter. Since the time of their discovery in 1895 they have been an extremely important tool in the physical and biological sciences and the fields of medicine and engineering .

History

X rays were discovered in Germany in 1895 by Wilhelm Roentgen (1845-1923) quite by accident while he was studying the conduction of electricity through gases at low pressure . The discovery was made when these mysterious "X" rays were observed to light up a fluorescent screen a few meters from the source. Roentgen soon found that these rays were quite penetrating and was actually able to insert his hand between the source and the screen and see on the screen the faint shadow of the bones in his hand. This indicated that more dense materials such as bone absorbed more x rays than less dense material such as human flesh. He soon found that photographic plates were sensitive to x rays and was able to make the first crude x-ray photographs.

Roentgen had been experimenting with what was called a cathode ray discharge tube, i.e., a partially evacuated glass tube with metal electrodes at each end. When a high electrical voltage was applied between the electrodes a discharge took place in the tube. One effect of the discharge was to produce electrons which acquired high velocities as they were attracted to the positive electrode. When they hit this metal electrode the x rays were produced. It was not until 1913 in the United States that W. D. Coolidge (1873-1975) invented the xray tube similar to those still used today. Coolidge removed as much air from the tube as possible and used a hot tungsten filament as the source of electrons. This permitted more careful experiments in which the high voltage applied to the tube and the rate at which electrons hit the target could be varied independently.

Mechanisms for x-ray production

The intensity of x rays from an x-ray tube varies with wavelength. A diagram of the wavelength spectrum from an x-ray tube shows several sharp peaks superimposed on what appears to be a continuous distribution. The peaks and the continuous region are produced by two quite different mechanisms. The continuous spectrum is produced by the incident electrons as they strike and enter the metal target. They are attracted by the positively charged nuclei of the atoms in the target and are suddenly deflected. Electromagnetic theory tells us that when electric charges are accelerated they radiate. Similarly, in the antenna of a radio or TV transmitter, electrons are made to oscillate rapidly back and forth, but in that case the accelerations are not as large and the wavelengths are much larger being measured in meters or centimeters.

A completely different mechanism produces the sharp peaks in the x-ray spectrum. These peaks are at very specific wavelengths and occur at different wavelengths for different targets. This radiation is produced when an incident electron knocks an electron out of one of the inner or low energy levels of the atom. An electron in a higher energy level falls into the vacant level and in the process an x ray is given off. The energy of this x ray is equal to the difference in energy between these two levels. Characteristic x rays have wavelengths ranging from about 10-11 m for uranium to 2.5 × 10-8 m for lithium .

Measuring x-ray wavelengths

Development of the x-ray tube greatly speeded up the detailed study of x rays, the origin and nature of which had finally been discovered by 1912, with the help of the suggestion by the German scientist Max von Laue (1879-1960) that x rays could be diffracted by three-dimensional crystals and thus must be electromagnetic radiation similar to visible light. This new approach was necessary because the wavelengths of x rays are so small that the diffraction gratings used for visible light will not work because the lines on the grating cannot be made with small enough spacings. A "natural" three-dimensional grating in the form of a single crystal of a material such as sodium chloride (salt ) or calcite works very well since the spacing of the atoms in the crystal is roughly the same as the x-ray wavelengths of interest. Intrigued by von Laue's discovery of x-ray diffraction, English physicist W. L. Bragg (1890-1971), working with his father W. H. Bragg (1862-1942), began a series of experiments that culminated in the invention of the xray spectrometer in 1913. This device allowed the Braggs to examine the structure of certain crystals, laying the foundation for the science of x-ray crystallography . Ultimately, the development of the x-ray spectrometer led to important advances in atomic physics and an improved understanding of the periodic table .

In 1913, the English physicist H. G. J. Moseley (1887-1915) used a Bragg-type spectrometer to look at the characteristic x rays from many of the elements. Taking advantage of the regular decrease in wavelength of characteristic x rays as one looks at successively heavier elements, he discovered that it was possible to tell one element from another by looking at the characteristic x rays. He found that elements should be listed in the periodic table in terms of their atomic number , not the atomic weight as had previously been done. He determined, for example, that cobalt should come before nickel even though cobalt had a larger atomic weight. He was also able to predict the existence of several elements, such as scandium and promethium, which were then unknown but later discovered.

Detection of x rays

In order for the Bragg spectrometer to be useful, the reflected rays must be detected. The detection of x rays is always based on their ability to eject electrons from atoms, i.e., to ionize matter. Thus the detector in the first Bragg spectrometer was an ionization chamber which collected the electric charge produced by x rays as they impacted a gas filled chamber. This ionization chamber was soon replaced by the familiar Geiger counter, developed in the study of radioactivity which was discovered at about the same time as x rays.

Applications of x rays

The uses of x rays in the fields of medicine and dentistry have been extremely important. X-ray photographs utilize the fact that portions of the body such as bones and teeth with higher density are less transparent to x rays than other parts of the human body. X rays are widely used for diagnostic purposes in these fields. Examples might include the observation of the broken bones and torn ligaments of football players, the detection of breast cancer in women, or the discovery of cavities and impacted wisdom teeth.

Since x rays can be produced with energies sufficient to ionize the atoms making up human tissue , it is not surprising that x rays can be used to kill these cells. This is just what is done in some types of cancer therapy in which the radiation is directed against the malignancy in the hope of destroying it while doing minimal damage to nearby normal tissue. Unfortunately, too much exposure of normal tissue to x rays can cause the development of cancer, a fact that was learned too late for many of the early workers in this field. For this reason, great care is taken by physicians and dentists when taking x rays of any type to be sure that the exposure to the rest of the body is kept at an absolute minimum.

A relatively new technique for using x rays in the field of medicine is called computerized axial tomography , producing what are called CAT scans. These scans produce a cross-sectional picture of a part of the body which is much sharper than a normal x ray. This is because a normal x ray, taken through the body, often shows organs and body parts superimposed on one another. To produce a CAT scan, a narrow beam of x rays is sent through the region of interest from many differentangles and a computer is used to reconstruct the cross-sectional picture of that region.

Moseley found that various natural elements can be identified by measuring the energy of their characteristic x rays. This fact makes a useful form of elemental analysis possible. If x rays of sufficient energy impact a sample of unknown composition, electrons will be knocked out of the atoms of the various elements in the sample and characteristic x rays will be given off by these atoms. Measurement of the energy of these x rays permits a determination of the elements present in the sample. This technique is known as x-ray fluorescence analysis. It is often used by chemists to perform a nondestructive elemental analysis and by law enforcement agencies when it is necessary to know what elements are present in a sample of hair or blood or some other material being used as evidence in a criminal investigation.

X rays are used in business and industry in many other ways. For example, x-ray pictures of whole engines or engine parts can be taken to look for defects in a nondestructive manner. Similarly, sections of pipe lines for oil or natural gas can be examined for cracks or defective welds. Airlines also use x-ray detectors to check the baggage of passengers for guns or other illegal objects.

In recent years an interesting new source of x rays has been developed called synchrotron radiation. Many particle accelerators accelerate charged particles such as electrons or protons by giving them repeated small increases in energy as they move in a circular path in the accelerator. A circular ring of magnets keeps the particles in this circular path. Any object moving in a circular path experiences an acceleration toward the center of the circle, so the charged particles moving in these paths must radiate and therefore lose energy. Many years ago, the builders of accelerators for research in nuclear physics considered this energy loss a nuisance, but gradually scientists realized that accelerators could be built to take advantage of the fact that this radiation could be made very intense. Electrons turn out to be the best particle for use in these machines, called electron synchrotrons, and now accelerators are built for the sole purpose of producing this radiation which can be adjusted to produce radiation anywhere from the visible region up to the x ray region. This synchrotron radiation, from which very intense beams at nearly one wavelength can be produced, is extremely useful in learning about the arrangement of atoms in various compounds of interest to biologists, chemists, and physicists.

One of the more important commercial applications of synchrotron radiation is in the field of x-ray lithography , used in the electronics industry in the manufacture of high density integrated circuits. The integrated circuit chips are made by etching successive layers of electric circuitry into a wafer of semiconducting material such as silicon. The details of the circuitry are defined by coating the wafer with a light sensitive substance called a photoresist and shining light on the coated surface through a stencil like mask. The pattern of the electric circuits is cut into the mask and the exposed photoresist can easily be washed away leaving the circuit outlines in the remaining photoresist. The size of the circuit elements is limited by the wavelength of the light-the shorter the wavelength the smaller the circuit elements. If x rays are used instead of light, the circuits on the wafer can be made much smaller and many more elements can be put on a wafer of a given size, permitting the manufacture of smaller electronic devices such as computers.

Resources

books

Dyson, Norman A. X Rays in Atomic and Nuclear Physics. White Plains, NY: Longman, 1973.

Hewitt, Paul. Conceptual Physics. Englewood Cliffs, NJ: Prentice Hall, 2001.

Young, Hugh. University Physics. Reading, MA: Addison-Wesley, 1999.

Robert L. Stearns

KEY TERMS

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Bragg x-ray spectrometer

—A device using a single crystal with regularly spaced atoms to measure the wavelengths of x rays.

Continuous x-ray spectrum

—The x rays produced by the electrons in an x-ray tube as they strike the target and are suddenly deflected. A broad range of wavelengths is produced.

Synchrotron radiation

—Electromagnetic radiation from electron accelerators called synchrotrons that can range from the visible region to the x-ray region.

X-ray fluorescence analysis

—A method of detecting the presence of various elements in an unknown sample by observing the characteristic x rays given off by the sample when excited by sufficiently energetic x rays.

X-ray tube

—Evacuated tube in which electrons moving at high velocities are made to hit a metal target producing x rays.