Do nuclei that are positron emitters lie above or below the band of stability? Which of the following isotopes might be suitable for PET scans? Explain your reasoning and write the equation for the decay:

(a) ¹⁸O; 

(b) ¹³N; 

(c) ¹¹C;

(d) ²⁰F; 

(e) ¹⁵O. 

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.