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The Discovery Of Radioactivity:
The Dawn of the Nuclear Age
Fran Slowiczek, Ed.D and Pamela M. Peters, Ph.D.
One hundred years ago, a group of scientists unknowingly ushered in the Atomic Age. Driven by curiosity, these men and women explored the nature and functioning of atoms. Their work initiated paths of research which changed our understanding of the building blocks of matter; their discoveries prepared the way for development of new methods and tools used to explore our origins, the functioning of our bodies both in sickness and in health, and much more. How did our conceptions of atomic properties change? How has that change affected our lives and our knowledge of the world?
Atoms and Elements: A Beginning
Elements are the building blocks of matter. The smallest particle of an element that still retains the identity of that element is the atom. All atoms of a given element are identical to one another, but differ from the atoms of other elements. Ancient Greeks first predicted the existence of the atom around 500 BC. They named the predicted particle 'atomos,' meaning "indivisible."
In 1803, John Dalton (1766-1844) proposed a systematic set of postulates to describe the atom. Dalton's work paved the way for modern day acceptance of the atom. But scientists of his day considered the atom to be merely a subordinate player in chemical reactions, an uninteresting, homogeneous, positively charged "glob" that contained scattered electrons. That premise remained unchallenged until the end of the nineteenth century, when a series of brilliant discoveries opened the door on the atomic science of the twentieth century. Working concurrently and often collaboratively, three pioneering scientists helped release the genie of the atom.
Antoine Henri Becquerel
Becquerel, a French physicist, was the son and grandson of physicists. Becquerel was familiar with the work of Wilhelm Conrad Roentgen on December 22 1895, "photographed" his wife's hand, revealing the unmistakable image of her skeleton, complete with wedding ring. Roentgen's wife had placed her hand in the path of X-rays which Roentgen created by beaming an electron ray energy source onto a cathode tube. Roentgen's discovery of these "mysterious" rays capable of producing an image on a photographic plate excited scientists of his day, including Becquerel. Becquerel chose to study the related phenomena of fluorescence and phosphorescence. In March of 1896, quite by accident, he made a remarkable discovery.
Becquerel found that, while the phenomena of fluorescence and phosphorescence had many similarities to each other and to X-rays, they also had important differences. While fluorescence and X-rays stopped when the initiating energy source was halted, phosphorescence continued to emit rays some time after the initiating energy source was removed. However, in all three cases, the energy was derived initially from an outside source.
In March of 1896, during a time of overcast weather, Becquerel found he couldn't use the sun as an initiating energy source for his experiments. He put his wrapped photographic plates away in a darkened drawer, along with some crystals containing uranium. Much to his Becquerel's surprise, the plates were exposed during storage by invisible emanations from the uranium. The emanations did not require the presence of an initiating energy source--the crystals emitted rays on their own! Although Becquerel did not pursue his discovery of radioactivity, others did and, in so doing, changed the face of both modern medicine and modern science.
The Curies: Lives Devoted to Research
Working in the Becquerel lab, Marie Curie and her husband, Pierre, began what became a life long study of radioactivity. It took fresh and open minds, along with much dedicated work, for these scientists to establish the properties of radioactive matter. Marie Curie wrote, "The subject seemed to us very attractive and all the more so because the question was entirely new and nothing yet had been written upon it."
Becquerel had already noted that uranium emanations could turn air into a conductor of electricity. Using sensitive instruments invented by Pierre Curie and his brother, Pierre and Marie Curie measured the ability of emanations from various elements to induce conductivity. On February 17, 1898, the Curies tested an ore of uranium, pitchblende, for its ability to turn air into a conductor of electricity. The Curies found that the pitchblende produced a current 300 times stronger than that produced by pure uranium. They tested and recalibrated their instruments, and yet they still found the same puzzling results. The Curies reasoned that a very active unknown substance in addition to the uranium must exist within the pitchblende. In the title of a paper describing this hypothesized element (which they named polonium after Marie's native Poland), they introduced the new term: "radio-active."
After much grueling work, the Curies were able to extract enough polonium and another radioactive element, radium, to establish the chemical properties of these elements. Marie Curie, with her husband and continuing after his death, established the first quantitative standards by which the rate of radioactive emission of charged particles from elements could be measured and compared. In addition, she found that there was a decrease in the rate of radioactive emissions over time and that this decrease could be calculated and predicted. But perhaps Marie Curie's greatest and most unique achievement was her realization that radiation is an atomic property of matter rather than a separate independent emanation.
Despite the giant step forward which science had now taken in it's understanding of radioactivity, scientists still understood little of the structure of the atom. This understanding awaited the work of Ernest Rutherford.
Ernest Rutherford and the Atom
In 1911, Rutherford conducted a series of experiments in which he bombarded a piece of gold foil with positively charged (alpha) particles emitted by radioactive material. Most of the particles passed through the foil undisturbed, suggesting that the foil was made up mostly of empty space rather than of a sheet of solid atoms. Some alpha particles, however, "bounced back," indicating the presence of solid matter. Atomic particles, Rutherford's work showed, consisted primarily of empty space surrounding a well-defined central core called a nucleus.
In a long and distinguished career, Rutherford laid the groundwork for the determination of atomic structure. In addition to defining the planetary model of the atom, he showed that radioactive elements undergo a process of decay over time. And, in experiments which involved what newspapers of his day called "splitting the atom," Rutherford was the first to artificially transmute one element into another--unleashing the incredible power of the atom which would eventually be harnessed for both beneficial and destructive purposes.
Taken together, the work of Becquerel, the Curies, Rutherford and others, made modern medical and scientific research more than a dream. They made it a reality with many applications. A look at the use of isotopes reveals just some of the ways in which the pioneering work of these scientists has been utilized.
Applications: Isotopes in Research and Medicine
Scientists can now create radioactive forms of common elements, called isotopes. Each isotope has a fixed rate of decay which can be characterized by its half-life, or the length of time that it takes half of the radioactive atoms in a sample to decay. Because each isotope decays at a unique and predictable rate, different isotopes can be used for a variety of purposes. For example, isotopes play an important role in modern medicine. They can be ingested and traced in their path through the body, revealing biochemical and metabolic processes with precision. These isotropic "tracers" are currently used for practical diagnosis of disease as well as in research.
The dating of radioactive carbon has helped to define the history of life on this planet. Any living organism takes in both radioactive and non-radioactive carbon, either through the process of photosynthesis or by eating plants or eating animals that have eaten plants. When the animal dies, however, uptake of carbon stops. As a result, radioactive carbon atoms are not replaced as they decay, and the amount of this material decreases over time. The rate of decrease is predictable and can be described with accuracy, vastly increasing our ability to date the biological events of our planet.
Conclusion: The Contradictions of Radioactivity
Radiation is a two edged sword: its usefulness in both medicine and anthropological and archaeological studies is undisputed, yet the same materials can be used for destruction. Human curiosity drove inquiring scientists to harness the power of the atom. Now humankind must accept the responsibility for the appropriate and beneficial uses of this very powerful tool.
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