Radiation and Madame Curie

In 1896, French scientist Henri Becquerel (1852-1908) discovered that uranium darkened photographic plates through several thick layers of paper. Marie Curie (1867-1934) and Pierre Curie (1859-1906), her hus-

In 1896, French scientist Henri Becquerel (1852-1908) discovered that uranium darkened photographic plates through several thick layers of paper. Marie Curie (1867-1934) and Pierre Curie (1859-1906), her hus-

Madam Curie 2nd Nobel Prize

Figure 2.3 Marie Curie (1867-1934). This portrait was made in 1911, when Curie received her second Nobel Prize.

Figure 2.2 Isotopes of Hydrogen. The three isotopes differ only in the number of neutrons present.

Figure 2.3 Marie Curie (1867-1934). This portrait was made in 1911, when Curie received her second Nobel Prize.

Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition band, discovered that polonium and radium did likewise. Marie Curie coined the term radioactivity for the emission of energy by these elements. Becquerel and the Curies shared a Nobel Prize in 1903 for this discovery.

Marie Curie (fig 2.3) was not only the first woman in the world to receive a Nobel Prize but also the first woman in France even to receive a Ph.D. She received a second Nobel Prize in 1911 for inventing radiation therapy for breast and uterine cancer. Curie crusaded to train women for careers in science, and in World War I, she and her daughter, Irène Joliot-Curie (1897-1956), trained physicians in the use of X-ray machines.

In the wake of such discoveries, radium was regarded as a wonder drug. Unaware of its danger, people drank radium tonics and flocked to health spas to bathe in radium-enriched waters. Marie herself suffered extensive damage to her hands from handling radioactive minerals and died of radiation poisoning at age 67. The following year, Irène and her husband, Frédéric Joliot (1900-1958), were awarded a Nobel Prize for work in artificial radioactivity and synthetic radioisotopes. Apparently also a martyr to her science, Irène died of leukemia, possibly induced by radiation exposure.

Many forms of radiation, such as light and radio waves, have low energy and are harmless. High-energy radiation, however, ejects electrons from atoms, converting atoms to ions; thus it is called ionizing radiation. It destroys molecules and produces dangerous free radicals and ions in human tissues. Examples of ionizing radiation include ultraviolet rays, X rays, and three kinds of radiation produced by nuclear decay: alpha (a) particles, beta (P) particles, and gamma (y) rays.

An a particle is composed of two protons and two neutrons (equivalent to a helium nucleus), and a P particle is a free electron. Alpha particles are too large to penetrate the skin, and P particles can penetrate only a few millimeters. They are relatively harmless when emitted by sources outside the body, but they are very dangerous when emitted by radioisotopes that have gotten into the body. Strontium-90 (90Sr), for example, has been released by nuclear accidents and the atmospheric testing of nuclear weapons. It settles onto pastures and contaminates cow's milk. In the body, it behaves chemically like calcium, becoming incorporated into the bones, where it emits P particles for years. Uranium and plutonium emit electromagnetic y rays, which have high energy and penetrating power. Gamma rays are very dangerous even when emitted by sources outside the body.

Each radioisotope has a characteristic physical halflife, the time required for 50% of its atoms to decay to a more stable state. One gram of 90Sr, for example, would be half gone in 28 years. In 56 years, there would still be 0.25 g left, in 84 years 0.125 g, and so forth. Many radioisotopes are much longer-lived. The half-life of 40K, for example, is 1.3 billion years. Nuclear power plants produce hundreds of radioisotopes that will be intensely radioactive for at least 10,000 years—longer than the life of any disposal container yet conceived. The biological half-life of a

Chapter 2 The Chemistry of Life 59

radioisotope is the time required for half of it to disappear from the body. This is a function of both physical decay and physiological clearance from the body. Cesium-137, for example, has a physical half-life of 30 years but a biological half-life of only 17 days. Chemically, it behaves like potassium; it is quite mobile and rapidly excreted by the kidneys.

There are several ways to measure the intensity of ionizing radiation, the amount absorbed by the body, and its biological effects. To understand the units of measurement requires a grounding in physics beyond the scope of this book, but the standard international (SI) unit of radiation exposure is the sievert3 (Sv), which takes into account the type and intensity of radiation and its biological effect. Doses of 5 Sv or more are usually fatal. The average American receives about 3.6 millisieverts (mSv) per year in background radiation from natural sources and another 0.6 mSv from artificial sources. The most significant natural source is radon, a gas that is produced by the decay of uranium in the earth and that may accumulate in buildings to unhealthy levels. Artificial sources include medical X rays, radiation therapy, and consumer products such as color televisions, smoke detectors, and luminous watch dials. Such voluntary exposure must be considered from the standpoint of its risk-to-benefit ratio. The benefits of a smoke detector or mammogram far outweigh the risk from the low levels of radiation involved. Radiation therapists and radiologists face a greater risk than their patients, however, and astronauts and airline flight crews receive more than average exposure. U.S. federal standards set a limit of 50 mSv/year as acceptable occupational exposure to ionizing radiation.

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