The Development of Artificial Radioactivity

Overview

In 1934, the French chemists Frédéric and Irène Joliot-Curie produced radioactivity artificially. In other words, they produced radioactivity in elements that are not naturally radioactive. One important use of artificial radioactivity is in the diagnosis and treatment of disease. In addition, the Joliot-Curies' breakthrough helped lead to the discovery of nuclear fission.

Background

All atoms of an element have the same number of protons. The number of neutrons in the atoms of an element, however, may vary. Atoms of the same element that have different numbers of neutrons are called isotopes. For example, carbon-12 and carbon-14 are isotopes of carbon. (Carbon-12 has six protons and six neutrons, and carbon-14 has six protons and eight neutrons.)

Radioactivity is a property of certain unstable isotopes. The nuclei of radioactive isotopes emit matter and/or energy. This emission is called radioactive decay. The atoms that result from radioactive decay may also be unstable. If so, the decay process will continue until a stable isotope is reached. Radioactivity occurs naturally in some elements such as uranium and polonium.

In the early 1900s, scientists began attempts to produce radioactivity in elements that are not normally radioactive. These efforts proved unsuccessful. Later in the century, the husband and wife team of Frédéric (1900-1958) and Irène (1897-1956) Joliot-Curie began researching radioactivity. Irène was the daughter of Pierre (1859-1906) and Marie Curie (1867-1934). Pierre and Marie had won the Nobel Prize for Physics in 1903 for their discovery of radioactivity. Marie won a second Nobel Prize in 1911 for her isolation of the radioactive element radium. In 1918, Irène became her mother's assistant at the Institut du Radium at the University of Paris, and she began to investigate the alpha rays emitted by polonium. (Alpha rays consist of two protons and two neutrons; they are one type of radiation.) In 1925, Frédéric Joliot was hired at the lab as an assistant, and the following year, he and Irène were married.

The Joliot-Curies continued to study the radioactive properties of polonium. At the time, only very small amounts of radioactive materials were available for use in experiments. Only about 300 grams (10.6 ounces) of radium were available worldwide, for instance. For this reason, many of the properties of newly discovered radioactive elements had yet to be examined. One piece of equipment Frédéric and Irène used in their research was a Wilson cloud chamber. This device contains gas that is saturated with moisture. When a charged particle passes though the chamber, droplets of liquid condense along its path. The cloud chamber allowed the Joliot-Curies to see the paths made by the particles emitted from radioactive elements. Although these pathways lasted for only fractions of a second, Frédéric developed a way to photograph them. Another tool the Joliot-Curies used to detect radiation was the Geiger counter. A Geiger counter emits a click when a radioactive particle passes through it. It can be used to determine how much radiation a sample is emitting.

Because they worked in Marie Curie's lab, Frédéric and Irène had access to a relatively large supply of polonium as compared to many other researchers. However, even this amount proved to be insufficient. Because some types of radioactive phenomena occurred very rarely or extremely quickly, experiments often had to be repeated over and over again. When the Joliot-Curies finally obtained enough pure polonium, they began using the alpha particles given off by its decay to bombard metals such as beryllium. They found that hitting beryllium with alpha particles caused the beryllium to emit particles as well. As soon as the polonium was removed, however, the emission from beryllium stopped. After they bombarded a sample of aluminum, however, they noticed that a nearby Geiger counter continued to click even after the polonium had been removed; the sample was continuing to emit radioactive particles on its own. In other words, the Joliot-Curies had produced radioactivity artificially.

The radioactive material that resulted from the bombardment of aluminum was an isotope of phosphorus not found in nature. This short-lived isotope gave off gamma rays (radiation emitted as energy) and beta particles (radiation in the form of high-speed electrons), and it decayed into a stable form of silicon. The Joliot-Curies also bombarded atoms of boron and magnesium with alpha particles. These collisions resulted in the production of radioactive isotopes of nitrogen and aluminum, respectively. The Joliot-Curies won the Nobel Prize for chemistry in 1935 for their discovery of artificial radioactivity.

Impact

One way in which scientists use artificially radioactive isotopes is to monitor chemical reactions either in laboratory experiments or inside organisms. Radioactive isotopes used in this way are called tracers. Early work with tracers included that by Frédéric Joliot-Curie and Antoine Lacassagne (1884-1971) to show that radioactive iodine is taken up by the thyroid gland. The thyroid gland is located at the base of the throat and produces hormones that help control metabolism—the chemical reactions that take place in an organism. Thyroid hormones contain the element iodine, and the thyroid gland takes up this element from the bloodstream in order to produce these hormones.

Today, small amounts of radioactive iodine are sometimes given to patients suspected of having certain thyroid problems. Normally, this tracer would soon make its way to the thyroid gland, where it could be detected on x-ray film. If the tracer fails to accumulate in the thyroid, however, doctors know that the gland is not working properly.

Artificially radioactive phosphorus is also used as a tracer. Small amounts can be given to plants and animals. When these organisms are released into the wild, scientists can find them again by detecting their radioactive "tag." Tracers have been used to study the flying patterns of insects, for example.

In addition, short-lived phosphorus tracers are sometimes injected into the bloodstream of a patient who has had a heart attack. Cells containing radioactive phosphorus become attached to damaged muscle in the heart. The radioactive cells then show up on x rays that doctors can use to determine the location and severity of the attack. Radioactive phosphorus is also attracted to brain tumors. By passing a special type of Geiger counter over a patient's head, doctors can locate both the radioactive phosphorus and the tumor.

Artificially radioactive isotopes can be used in the treatment of disease as well as in its diagnosis. For example, an artificially radioactive isotope of cobalt is used to treat some types of cancer patients. The radiation from the cobalt damages cancerous cells and may prevent the spread of the disease. However, the radiation also damages healthy cells, so this isotope must be used in precisely controlled amounts.

Besides its medical applications, artificial radioactivity also provided scientists with additional opportunities for studying radioactive decay and the changes that occur in nuclei. The discoveries of the Joliot-Curies inspired the Italian physicist Enrico Fermi (1901-1954) to hypothesize that artificial radioactivity could be produced by using neutrons rather than alpha particles. Unlike alpha particles, which have a positive charge, neutrons are electrically neutral. Fermi believed neutrons might be more effective at creating artificial radioactivity because they would not be repelled by the positively charged nuclei of the bombarded atoms. Through trial and error, he found that he obtained the best results by slowing the neutrons down before they hit their target atoms. Fermi found that these slow neutrons were quite effective in causing radioactivity, and he produced numerous new isotopes as a result.

When Fermi bombarded uranium atoms with slow neutrons, he got a radioactive, yet somewhat mystifying, result. Some scientists incorrectly concluded that a previously unknown element with an atomic number greater than uranium's had been produced. However, in 1938, the German physicists Otto Hahn (1879-1968), Fritz Strassmann (1902-1980), and Lise Meitner (1878-1968) realized that when a uranium atom is bombarded with slow neutrons, it breaks apart into an atom of barium and an atom of krypton. This type of reaction, in which one atom splits into two lighter atoms, is called nuclear fission. Meitner realized that some of the mass of the uranium atom was converted to energy during the process of fission. Even though the amount of mass involved was very small, the amount of energy released was enormous. It was later determined that the fission of one kilogram (2.2 pounds) of uranium will produce as much energy as the burning of three million kilograms (3,307 tons) of coal. Scientists soon recognized that this energy could be a powerful tool if it could be harnessed.

Fermi believed that if neutrons were among the products of nuclear fission, they could be used to begin a chain reaction. In other words, the neutrons released by the fission of one atom of uranium could cause the fission of other atoms of uranium, which would release more neutrons, and so on. In 1939, Leo Szilard (1898-1964), Herbert L. Anderson, and Frédéric Joliot-Curie showed that this was true. Experiments indicated that 2.5 neutrons per uranium atom are produced during fission.

Advances in nuclear chemistry followed in quick succession, in large part because of World War II (1939-1945). The Joliot-Curies had openly published their work on artificial radioactivity. As the Nazis came to power in Europe, however, Frédéric and Irène began to worry about the implications of their recent research should it fall into the wrong hands. For this reason, when they wrote a paper in 1939 that discussed how a nuclear reactor could be built, they placed it in a sealed envelope at the Académie des Sciences. This work remained secret until 1949, after the war was over. Despite the concerns of some scientists, both sides in the war raced to be the first to develop nuclear fission as a weapon. In 1942, scientists achieved the first self-sustaining chain reaction of nuclear fission. Later that same year, the first atomic bomb was tested. (The explosive energy of such bombs comes from nuclear fission.) In 1945, the United Stated dropped the first atomic bombs used in warfare on the Japanese cities of Hiroshima and Nagasaki.

STACEY R. MURRAY

THE HIDDEN DANGERS OF SCIENTIFIC DISCOVERY

Today, researchers who work with radioactive materials take special precautions not to be exposed to radiation, which can seriously damage cells and tissues over time. When radioactivity was first discovered in 1896, however, scientists were not aware that radiation could be harmful. Marie and Pierre Curie's initial investigations of radioactivity were conducted in a poorly equipped lab. Because it had no fume hoods to draw away dangerous gases, they breathed in the radioactive radon gas given off by the radium with which they worked. Whenever a spill occurred, they merely mopped it up with a cloth. Their fingers were constantly burned and cracked from contact with radioactive materials. Their daughter Irène was probably exposed to radiation from childhood since her parents wore their work clothes home from the lab. Marie Curie and Frédéric and Irène Joliot-Curie all died as a result of complications arising from long-term radiation exposure.

Radiation safety began to be a concern about 20 years after its initial discovery. In 1915, a British scientific society proposed the use of lead or wooden shields around radioactive materials and limits to the number of hours that researchers could work with such chemicals. Scientists soon developed methods for measuring the amount of radiation to which a person was being exposed, and by 1923, maximum daily limits of radiation exposure were being suggested.

STACEY R. MURRAY