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Radioactivity
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Radioactivity

Radioactivity is the process by which unstable atomic nuclei decay. This process normally produces ionizing radiation with a relatively large amount of energy. This energy can be harnessed in the form of nuclear power, or it can be very dangerous if released by radioactive contamination in the environment.

Overview

Atomic nuclei are bound together by the strong nuclear force, which is much stronger than the forces encountered in everyday life, electromagnetism and gravity. It is, however, a very short-range force. As a result, atomic nuclei are held together very strongly in spite of the very strong electromagnetic repulsion of the protons for one another. If an atomic nucleus changes state in some way, a large amount of energy will be released. That is, the energy released will be much greater than that released in a chemical reaction involving a single molecule; chemical reactions rely on the electromagnetic force.

Unstable atomic nuclei can undergo a number of different reactions:

Beta decay: In simple beta decay, a neutron becomes a proton, emitting a beta particle (a high-speed electron). This converts the nucleus to another element, raising its atomic number by one. Alternatively, a proton can become a neutron, emitting a beta-plus particle (a high-speed positron). This reaction decreases the atomic number by one. This second reaction is very rare among naturally occurring isotopes, but happens for many artificial isotopes. Both reactions normally leave the nucleus in an excited state.

Alpha emission: In alpha emission, a nucleus emits an alpha particle (a Helium-4 nucleus). This changes both the atomic number and the number of neutrons in the nucleus, and it normally leaves the nucleus in an excited state.

Gamma emission: a nucleus transitions from an excited state to a lower-energy state, emitting a gamma ray (a high-energy photon).

Fission: a nucleus breaks into two smaller nuclei, and possibly some fast neutrons, beta particles, alpha particles, and gamma rays. Fission is normally quite rare unless the nucleus has absorbed a neutron. Since fisson releases neutrons itself, this allows (under suitable conditions) a nuclear chain reaction which leads to a very large number of fission events in a short period. This is the process used in nuclear weapons and nuclear reactors.

Many of the atomic nuclei that arise from these processes are themselves unstable, so in large samples of radioactive material, many decay chains are going on simultaneously.

An unstable nucleus waits a random amount of time before it decays; this is a quantum mechanical process. This time is not, however, completely unpredictable. The decay time follows an exponential distribution, and the most useful way to characterize the process is to give the half-life of a nucleus: the time after which there is a 50% chance that it will have decayed. When dealing with a macroscopic amount of radioactive material, the half-life is the time after which half of the material remains in its original form. Half-lives vary from effectively infinite, for stable elements, to hundreds of thousands of years for Uranium, to microseconds and less for artificial isotopes.

In the natural world, radioactivity comes from naturally-occurring radioactive isotopes. Like all heavy elements, these come originally from the interiors of stars. Some, such as Uranium, were formed directly in stars, and are still present only because their half-lives are so long that they have not yet completely decayed. Radiogenic isotopes, such as Carbon-14, are present because they are formed by the decay of longer-lived elements (this is how all the Helium currently available was formed: although it is not radioactive, it escapes from the Earth easily, so Helium is obtained from underground reservoirs).

Human technology has found uses for many radioactive isotopes. These uses have ranged from nuclear weapons to nuclear power to glowing watch dials to smoke detectors. Most of these isotopes are manufactured in nuclear reactors, which allow the creation of unstable or unusual isotopes through the exposure of stable isotopes to neutrons.

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