Radioactive … involves the spontaneous disintegration of an atomic nucleus. As the nucleus …, various particles as well as high-energy electromagnetic waves are … . Radioactivity is not a new … . Radioactive materials have always been present everywhere in the … —in the atmosphere, on land, and in the oceans—but until the latter years of the 19th century the phenomenon of … had escaped scientific attention because it usually exists at such low levels that its effects are difficult … .
As a subject of scientific interest, radioactivity came to the fore when the French scientist Antoine-Henri Becquerel (1852–1908) … a series of experiments whose purpose was to investigate X-rays. In these experiments he wrapped … photographic plates in … paper so that no light could reach them and placed certain materials near the wrapped plates. He discovered that when he placed compounds … the metal uranium near the plates, they emitted rays that passed through the paper and exposed the plates. He found that he could take primitive pictures with these … . By placing a coin between the material emitting the rays and the photographic plate, for example, he found that the coin blocked the rays. The result was that the plate was exposed everywhere except for the area beneath the coin. Although he misinterpreted the nature of the phenomenon he had discovered, his results attracted the attention of the most successful husband-and-wife team in the history of science, the French scientist Pierre Curie (1859–1906) and the Polish-born scientist Marie Curie (1867–1934). (Marie Curie later coined the term radioactivity.)
The phenomenon observed by Becquerel was a weak one because natural uranium emits few radioactive particles per unit time. Scientists describe this situation by saying that natural uranium has a low specific activity. Intrigued by Becquerel’s experiments, Marie Curie began to investigate the mineral pitchblende, which has a higher specific activity than uranium.
While Pierre Curie concentrated on identifying the physical properties of radiation, Marie sought to isolate the elements responsible for emitting the higher levels of radiation. Her work culminated in the discovery of two new elements, polonium and radium. The element radium was of particular interest because compared to uranium, radium has a very high specific activity; in other words, a unit mass of radium emits a great deal more radioactivity per unit time than does a unit mass of natural uranium.
Physicists of the time were astonished by what Marie Curie had discovered. Not only did the radium continuously emit relatively large amounts of energy, but it emitted the energy for long periods of time without changing in any perceptible way. Keep in mind that the principle of conservation of energy, which states that energy can neither be created nor destroyed, meant that energy was, in effect, draining away from her sample of radium. As Curie knew, this continual emission of energy must eventually leave a perceptible change in the physical properties of her radium sample. The fact that she could not quickly detect any change in the radium meant that the amount of energy contained in a small piece of radium must be very large. In 1904, Marie Curie expressed her insights in these words:
“Radium possesses the remarkable property of liberating heat spontaneously and continuously. A solid salt of radium develops a quantity of heat such that for each gram of radium contained in the salt there is an emission of one hundred calories per hour. Expressed differently, radium can melt in an hour its weight in ice. When we reflect that radium acts in this manner continuously, we are amazed at the amount of heat produced, for it can be explained by no known chemical reaction. The radium remains apparently unchanged. If, then, we assume that it undergoes a transformation, we must therefore conclude that the change is extremely slow, in an hour it is impossible to detect a change by any known methods.”
The ability of radium to emit large quantities of energy for prolonged times without perceptible change led the British physicist Ernest Rutherford (1871–1937) to speculate on the possibility of harnessing radioactive materials for warfare and for work. If one could release all of the energy at once, he wrote, one could create a tremendous bomb. This, after all, is what a bomb does: It releases a great deal of energy over a very short period of time. If, on the other hand, one could increase the rate of energy release in a gradual and controlled way, one would have a source of power sufficient to supply the largest of cities with its power needs. If only one could control the rate at which radium emitted its energy, Rutherford speculated, a small mass of radium could replace a very large mass of coal.
In principle, Rutherford was right. Nor was he alone. The British writer H. G. Wells (1866–1946) had the same sorts of insights. But while their instincts were correct, Rutherford and his contemporaries were wrong about the power source. It would not be radium that would power cities and serve as fuel for a new and more powerful type of bomb but rather the radioactive metals uranium and the yet-to-be-created plutonium.