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Nuclear physics

Nuclear physics is the branch of physics concerned with the nucleus of the atom.


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 it. 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 that radioactivity was significantly more complicated.

For instance, it was found that an electric or magnetic field could split such emissions into three beams. For want of better terms, the rays were given the alphabetic monickers alpha, beta, and gamma, names they still hold today. It was immediately obvious from the direction of electromagnetic forces that alpha rays carried a positive charge, beta rays carried a negative charge, and gamma rays were neutral. From the magnitude of deflection, it was also clear that alpha particles were much more massive than beta particles. Passing alpha rays through a thin glass membrane and trapping them in a discharge tube allowed researchers to study the emission spectrum of the resulting gas, and ultimately prove that alpha particles are in fact helium nuclei. Other experiments showed the similarity between beta radiation and cathode rays, and between gamma radiation and x-rays.

These researchers also discovered that many other chemical elements have radioactive isotopes. Radioactivity also guided Marie Curie to isolate radium from barium; the two elements' chemical similarity would have otherwise made them difficult to distinguish.

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. Two of the first three weapons it produced were dropped on Japan; Production was then intended to gear up to about one bomb per week but Japan surrendered before any more nuclear bombs were dropped.

During the Second World War and the early Cold War, development of nuclear technology proceeded with only minimal attention paid to the long-term dangers of radiation and radioactive contamination. High-level waste from plutonium production was stored in large tanks with a design life of only a few decades and no plans for longer term storage, lower level waste was allowed to soak into the ground with little idea of its long-term movement. Many nuclear weapons were tested in the atmosphere (that is, above the surface of the earth), releasing enough radioactive material to raise the world's level of background radiation very significantly. Eventually the Limited Test Ban Treaty put an end to these tests by the United States and the USSR (though underground testing continued in both countries, and France and China continued atmospheric testing for a while afterwards).

Nuclear power reactors were subsequently developed for use in submarines, ships and for commercial power generation. Since the 1960s it has been claimed by opponents of nuclear power 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. These claims remain controversial, see linear no threshold model, radiation hormesis. In the light of these claims, public concern rose drastically, safety measures were tightened, and use of radioactive materials such as thorium in gas mantles 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.

Radioactive isotopes continue to have many important applications, including tracing biological processes in the human body for diagnosis; preserving foods in jars by killing bacteria, seeds, and bulbs; and dating of geological deposits based on assumptions of decay rates and isotope ratios at the time of deposit. Between these applications and the continued use of nuclear power, nuclear technology is still in wide use despite public concern. Construction of new power reactors continues particularly in Asia, and research on new reactor types both using nuclear fission and nuclear fusion continues.

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Last updated: 08-17-2005 13:53:31
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