A positron has the same mass as that of an electron, but the opposite charge. When a positron emitted in a PET scan encounters an electron, annihilation occurs in the body: electromagnetic energy is produced and no matter remains. How much energy (in joules) is produced in the encounter? See Box 10A.1.

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Box 10A.1 WHAT HAS THIS TO DO WITH... STAYING ALIVE? NUCLEAR MEDICINE Nuclear chemistry has transformed medical diagnosis, treat- ment, and research. Radioactive tracers are used to measure organ function, sodium-24 to monitor blood flow, and strontium-87 to study bone growth. However, the most dramatic effect of radioisotopes on diagnosis has been in the field of imag- ing. Technetium-99m (the m denotes a "metastable," reasonably long-lived, state) is the most widely used radioactive nuclide in medicine, especially for bone scans. This isotope is highly active and gives off y-rays that pass through the body. The y-rays cause much less damage than a particles would, and the isotope is so short-lived that the risk to the patient is minimal. Cobalt-60 is used for the gamma knife, a technique that is actually knifeless and that can kill tumor cells in locations such as the brain when surgery is not possible. In gamma-knife ther- apy, about 200 y-rays are focused from different angles onto a tumor with a known shape and location. Each y-ray by itself has a low amplitude and so causes little or no damage to the tissues through which it passes. However, at the intersection of 200 of with fluorine-18 and injected into a cancer patient, the fluorine- bearing compound is preferentially absorbed by the tumor. The positrons given off by the fluorine atoms are quickly annihilated when they meet electrons. The resulting y-rays are detected by a scanner that moves slowly over the part of the body containing the tumor. The growth of the tumor can be estimated quickly and accurately with this technique. A PET imaging facility must be located near a cyclotron so that the positron emitters can be incor- porated into the desired compounds as soon as they are created. Several kinds of cancer therapy use radiation to destroy malignant cells. Boron neutron-capture therapy is unusual in that boron-10, the isotope injected, is not radioactive. However, when boron-10 is bombarded with neutrons, it gives off highly destructive a particles. In boron neutron-capture therapy, the boron-10 is incorporated into a compound that is absorbed pref- erentially by tumors. The patient is then exposed to brief periods of neutron bombardment. As soon as the bombardment ceases, the boron-10 stops generating a particles.