The main events in the ten years following the discovery of the emission of radiation from uranium at the very end of the nineteenth century are listed at right.
Henri Becquerel inherited a collection of uranium salts from his father who had conducted studies of fluorescence and phosphorescence. He discovered by accident that a photographic plate became blackened when stored in a drawer with one of his samples. It became apparent that the uranium was emitting penetrating radiation; achieving the same effect as exposing the plates to visible light; turning the photographic emulsion into silver grains.
Much of Becquerel's radiation was deflected by a magnetic field. It gradually became evident that the new radiation was at least partly made up of a hail of charged particles, both positive and negative.
Stable nuclei lie along a line on a mass versus atomic number plot. All these nuclei along the line on the graph are stable.
Unstable nuclei that have too few neutrons are alpha emitters (fast moving 4He nuclei). The energy of the alpha particle is gained by Coulomb repulsion. All alpha particles, from a given nucleus, have the same energy.
238U (atomic number 92) decays via alpha emission to the nucleus of an isotope of thorium.
Note: alpha decay is usually accompanied by a gamma ray emission.
The alpha particle work function (the energy required to remove an alpha particle from the nucleus) is very large, (about 28 MeV). Alpha decay would be impossible if it were not for the wave nature of matter. The alpha particle, acting as a wave, may occasionally tunnel out of the nucleus, appearing outside with very little residual energy (as if by magic). When this happens coulomb repulsion drives the alpha particle away at speed. Alpha particle energy varies from 4 MeV, for long half-life elements (10 million years) to 9 MeV, for elements with very short half-live (microseconds). The half-life range is 20 orders of magnitude, but alpha particle energy varies only by a factor of 2.
| Note: alpha particles with average energies of 15 MeV and occasional energies up to 28 MeV, have been observed in three body fission events that occur with a frequency of ~1 in 400. |
Nuclei that have too many neutrons emit a beta particle (electron) and a neutrino. Tritium, for example, has an extra neutron. Tritium decays by beta emission ,with a neutrino, but without a gamma ray.
The neutrino (v) carries away energy, so the measured energies of all beta particles from a given nucleus are not the same. The "missing" energy was a puzzle that stumped physicists for 30 years, until the discovery of the neutrino.
Unstable nuclei may emit gamma rays (g) which are high energy photons. An excited nucleus may emit just a gamma ray, and most alpha decay is accompanied by gamma emission (equation above). Many beta emitters also emit gamma rays but there are some exceptions, for instance, the tritium decay above has no gamma emission.
Note: there are two other less common possibilities. A nucleus may emit a positive electron (positron) or just one single neutrino.
PenetrationAlpha particles have a short range (~1-2 cm) in air. Alpha particles will penetrate thin aluminum foil, but not lead foil, or human skin. Radon, an alpha emitter, is dangerous when breathed in on dust particles, because the dust lodges in the lungs. Polonium 210 is a pure alpha emitter that is harmless, unless ingested and distrubuted around the body, where it destroys the internal organs. Beta particles are more penetrating - they will pass through paper and very thin heavy metal foils. Beta particles will penetrate about a cm into the body. Gamma rays will pass through the human body, and pass through a few centimeters of lead, and many meters of concrete. Neutrinos will pass through the Earth! Note: each type of radiation ionizes air, except a neutrino flux. Neutrinos pass through not only air, but the Earth itself, with little disturbance. |
In a fission reaction, uranium (or a similar heavy nucleus, typically plutonium) splits into two (occasionally three) smaller neutron-rich unstable nuclei, and a few free neutrons. The most celebrated product of fission bombs in the atmosphere is strontium 90. Strontium is chemically similar to calcium. A small amount of strontium 90 in rainwater finds its way into cow's milk and from there into the human food supply, babies first!
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Chain reaction Fission in a large mass of uranium (or similar material) proceeds by a chain reaction. A spontaneous fission releases two or more free neutrons. When a neutron is absorbed by another heavy nucleus it induces another fission. Slowing the free neutrons by collision with light nuclei (by adding a moderator such as deuterium oxide) increases the chance of an induced fission before the free neutron escapes from the mass. A critical mass of fissionable material must be assembled in a compact spherical lump to establish an uncontrolled chain reaction (a bomb). |
A fission reaction releases energy, because the combined binding energies of the smaller nuclei, are less than the binding energy of the original heavy nucleus. The mass difference between the starting material and the products is converted to heat, according to the well known relationship....
This principle: the energy equivalence of matter, was put forward by Einstein and demonstrated by the US on the Japanese in 1945 with a uranium bomb over Hiroshima, and three days later, with a plutonium equivalent over Nagasaki. They called the first Little Boy and the Secind Fat Man. The Hiroshima bomb contained about 50 kg of highly enriched uranium, the nagasaki bomb had ony about 7 kg of weapons grade plutonium.
 Fat Man |
The bombs were dropped and the war with Japan ended almost at once, but historians will continue to argue the rights and wrongs of the issue, and to ask quetions about the real motives of the politicians and the US military, for centuries to come.
Converting just one gram of matter to energy in this low-yield fission explosion at Bikini Atoll was a demonstration to end all demonstrations. Note the expanding shock wave in air which reaches the ships in a matter of seconds. A similar shock wave is shown on the sea from the explosions of heavy weapons.
Nuclear power stations
Fission can be controlled to produce heat which in turn can be used to run steam engines to produce electric power. Crude but effective, given a steady supply of uranium, and a place to dump the waste.
Fusion is the opposite of fission. In a fusion reaction two light nuclei combine to form one heavier nucleus with an atomic mass less than that of iron 56. The binding energy of the product nucleus is less than the binding energy of the original lighter nuclei and again there is a mass loss in the reaction which appears as heat. The most common fusion reactions are the series of two-body reactions in the Sun that convert hydrogen to helium. The simplified overall reaction is....
A temperature of about 106 K is required to initiate the fusion of tritium and deuterium which releases 17.6 MeV of energy per helium 4 nucleus. The reaction is used in a so called Hydrogen bomb.