History
Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel while working on phosphorescent materials. These materials glow in the dark after exposure to light, and he thought that the glow produced in cathode ray tubes by x-rays might somehow be connected with phosphorescence. So he tried wrapping a photographic plate in black paper and placing various phosphorescent minerals on them. All results were negative until he tried using uranium salts. The result with these compounds was a deep blackening of the plate.
However, it soon became clear that the blackening of the plate had nothing to do with phosphorescence because the plate blackened when the mineral was kept in the dark. Also non-phosphorescent salts of uranium and even metallic uranium blackened the plate. Clearly there was some new form of radiation that could pass through paper that was causing the plate to blacken. (Many books state that Becquerel accidentally discovered radioactivity.)
At first it seemed that the new radiation was similar to then recently discovered x-rays. However further research by Becquerel, Pierre Curie, Marie Curie, Ernest Rutherford and others discovered three of the several different types of radioactivity, namely alpha decay, beta decay, and gamma decay. These researchers also discovered that many other chemical elements have radioactive isotopes.
The dangers of radioactivity and of radiation were not immediately recognized. Acute radiation poisoning was observed early on, but it was initially assumed that, like fire, if no immediate effect was observed there was no danger. Moreover, it was not realized that if radioactive material was taken into the body, it would continue to radiate while inside, often causing cancer or other severe problems. Many physicians and corporations began marketing radioactive substances as patent medicine; one particularly alarming example was radium enema treatments. Marie Curie, before her death, spoke out against this sort of treatment, warning that the effects of radiation on the human body were not well understood.
During the Second World War, it was realized that the energy released by radioactivity could possibly be used to wreak massive destruction. Both the Axis and the Allied forces began projects to develop such weapons; the Manhattan Project in the United States ultimately succeeded. The weapons it produced were dropped on Japan.
During the Second World War and the early Cold War, development of nuclear technology proceeded with only minimal awareness of the long-term dangers of radiation and radioactive contamination. Many nuclear weapons were tested in the air, releasing enough radioactive material to raise the world's level of background radiation very significantly. Eventually the Nuclear Test Ban Treaty put an end to these tests.
Nuclear power was also used in submarines, ships and for commercial power generation. Only in the 1960s did it begin to be realized that longterm exposure to low levels of radiation could lead to serious health problems, and that radioactive contamination of the environment could be taken up by humans, leading to just such longterm exposure. Since this realization, public concern rose drastically, and safety measures were tightened. Use of radioactive isotopes was curtailed.
Public concern was greatly increased by nuclear accidents, particularly those at Three Mile Island and Chernobyl. This concern is not very discriminating, in many cases consisting of a blanket fear of anything labelled "nuclear". For example, nuclear magnetic resonance imaging (NMRI) spectroscopy, which has nothing whatsoever to do with radioactivity, was renamed magnetic resonance imaging (MRI) to quell public fear.
Nevertheless, radioactive isotopes have many important applications, including tracing biological processes in the human body for diagnosis, preserving foods in jars by killing bacteria, and dating of geological deposits based on assumptions of decay rates and isotope ratios at the time of deposit. Between these applications and the need for nuclear power, nuclear technology is still in wide use.
Measurement
The SI unit for measuring radioactivity is the Becquerel. One Becquerel is the amount of radioactive material that produces a single decay per second. Thus Becquerel of an element with a short half-life is much less material than a Becquerel of an element with a long half-life, but one can expect the amount of energy emitted to be comparable.
As one might expect, a Becquerel is actually a tiny amount of radioactive material, so in practice one usually sees numbers of gigaBecquerels.
The amount of radioactivity is normally obtained by measuring the radiation produced or by measuring the amount of radioactive material (in grams, say) and using its known properties.
See also
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