College Physics

Figure 24.20This shadow X-ray image shows many interesting features, such as artificial heart valves, a pacemaker, and the wires used to close the sternum. (credit: P. P.

Urone)

The use of X-ray technology in medicine is called radiology—an established and relatively cheap tool in comparison to more sophisticated

technologies. Consequently, X-rays are widely available and used extensively in medical diagnostics. During World War I, mobile X-ray units,

advocated by Madame Marie Curie, were used to diagnose soldiers.

Because they can have wavelengths less than 0.01 nm, X-rays can be scattered (a process called X-ray diffraction) to detect the shape of molecules

and the structure of crystals. X-ray diffraction was crucial to Crick, Watson, and Wilkins in the determination of the shape of the double-helix DNA

molecule.

X-rays are also used as a precise tool for trace-metal analysis in X-ray induced fluorescence, in which the energy of the X-ray emissions are related

to the specific types of elements and amounts of materials present.

Gamma Rays

Soon after nuclear radioactivity was first detected in 1896, it was found that at least three distinct types of radiation were being emitted. The most

penetrating nuclear radiation was called agamma ray (γray)(again a name given because its identity and character were unknown), and it was

later found to be an extremely high frequency electromagnetic wave.

In fact,γrays are any electromagnetic radiation emitted by a nucleus. This can be from natural nuclear decay or induced nuclear processes in

nuclear reactors and weapons. The lower end of theγ-rayfrequency range overlaps the upper end of the X-ray range, butγrays can have the

highest frequency of any electromagnetic radiation.

Gamma rays have characteristics identical to X-rays of the same frequency—they differ only in source. At higher frequencies,γrays are more

penetrating and more damaging to living tissue. They have many of the same uses as X-rays, including cancer therapy. Gamma radiation from

radioactive materials is used in nuclear medicine.

Figure 24.21shows a medical image based onγrays. Food spoilage can be greatly inhibited by exposing it to large doses ofγradiation, thereby

obliterating responsible microorganisms. Damage to food cells through irradiation occurs as well, and the long-term hazards of consuming radiation-

preserved food are unknown and controversial for some groups. Both X-ray andγ-raytechnologies are also used in scanning luggage at airports.

876 CHAPTER 24 | ELECTROMAGNETIC WAVES

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