Rays X

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.