Radioactivity
Radioactivity is defined as the process by which atoms emit particles and high energy rays from their nuclei. Although most elements can be rendered radioactive, the process occurs naturally in only a very few. The modern understanding of radioactivity is that certain elements release radiation as they decay. Such radiation can be recognized in any of three forms: alpha rays, which are positively charged and relatively weak (clothing, or a few sheets of paper, will stop them); beta rays, which can be either positive or negative and are a bit stronger (half an inch of wood will stop them); and gamma rays, which bear no electrical charge yet are by far the strongest. An element which emits alpha or beta rays transforms as it does so, changing into other elements as it releases energy. For example, uranium becomes thorium when it emits alpha particles. Scientists often create "new" radioactive elements by adding or subtracting alpha and beta particles.
Natural radioactivity was discovered in 1896 by the French physicist Henri Becquerel as the result of one of the most fortuitous accidents in science history. The discovery of X-rays by Wilhelm Röntgen piqued the curiosity of Becquerel, who had for years been studying luminescent crystals. Becquerel theorized that Röntgen's "penetrating rays" might be produced by the same mechanism as was luminescence. Using a compound of luminous potassium uranyl sulfate and a set of photographic plates, he was determined to find a connection between these phenomena. After exposing his crystal to sunlight (to trigger the luminescence), he placed it in a darkroom with a photographic plate. He theorized that if after developing the plate it was streaked, then penetrating rays must have been released from the sample. The plate was indeed found to be streaked, and Becquerel was eager to repeat his experiment. However, his next attempt was delayed due to overcast skies. For this reason, he stowed both crystal and plates away in a darkened room until conditions were more favorable. Days later, upon removal of the sample, he noticed that the photographic plate was again streaked, even though the crystal resting upon it had not been exposed to sunlight. The connection to luminescence broken, Becquerel realized that he had discovered a new form of penetrating ray.
In order to isolate the source he used several kinds of crystal, noting that only those that contained uranium would streak the plates. When he used a disk of pure uranium--producing radiation four times as intense as potassium uranyl sulfate--his research was complete, and he presented his findings in May, 1896. Becquerel's discovery initiated the science of nuclear physics and began a scientific revolution, the impact of which is still felt today.
Other scientists took up Becquerel's research where he had left off. Pierre Curie and Marie Curie found first that thorium was also radioactive (a word Marie Curie coined), and later discovered two new radioactive elements, polonium and radium. In England, Joseph J. Thomson and Ernest Rutherford studied the ability of radiation to ionize metal foils. They found that there were two types of radiation: alpha rays and beta rays. In 1900, French physicist Paul Villard (1860-1934) noted a third, more powerful variety called gamma rays. The continuing research led first Rutherford and later Niels Bohr to reassess the construction of the atom.
Different elements decay at different rates—some may last only a fraction of a second, while others may remain radioactive for millions of years. The time it takes for an element to lose half its mass is called its half-life. Most radioactive substances found in nature have very long half-lives, with the notable exception of carbon-14, which is short-lived but leaves an indelible mark as it decays. Scientists use this mark to determine the age of rocks and ancient relics. This method is known as radioactive dating.
Radioactivity has been used for a variety of applications. Radioactive tracers are used in research to study the path of chemicals in plants and animals, as well as to locate areas of disease in humans. Radiation is also a common treatment for cancer, since it can be used to destroy areas of uncontrolled cell growth. However, these same rays, if unchecked, can damage healthy tissue, and people routinely exposed to radiation (especially gamma rays) must use protective shielding to prevent radiation poisoning. Nuclear physicists have also found a wealth of applications for radioactivity. Most important is its use for the production of nuclear power.
The danger of radioactivity to humanity was demonstrated by a disaster at the Chernobyl Nuclear Power Station in Ukraine. On April 26, 1986 one of the nuclear power reactors blew up and sent tons of radioactive debris into the air. Over the next few days the material fell throughout northern Ukraine and even into Europe. Radioactive isotopes became incorporated into the food supply and the drinking water. Ten years later, the incidence of cancer remains higher in populations around Chernobyl and the wildlife also shows signs of genetic damage.
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