Nuclear Reactions
Nuclear decay, neutron capture, fission, and fusion are the four major nuclear reactions.
The understanding, use, and control of nuclear reactions is one of the most profound scientific accomplishments. Although chemical reactions of everyday experience generally take place between the electrons surrounding an atom there are important and fundamental processes that take place in the nucl eus of an atom. Nuclear reactions are different than chemical reactions. Nuclear energies are several orders of magnitude larger than the energies involved in chemical reactions.
In 1903, French physicist, Antoine Henri Becquerel won the Nobel Prize in physics for his work with spontaneous radioactivity. In 1898, French scientists, Marie and Pierre Curie, two-time Nobel Laureates, discovered the naturally occurring radioactive ele ments polonium and radium.
In the 1930s, French physicists Frédéric and Irène Joliot-Curie demonstrated artificial radioactivity by bombarding stable atoms with nuclear particles to create radioactive isotopes (radioisotopes). In 1932 British scientists, Sir John Cockcroft (1897-1967) an d Ernest Walton (1903- 1995), were the first to disintegrate the nucleus by bombarding it with high energy projectile-particles.
Before World War II, German chemists Otto Hahn and Fritz Strassmann discovered nuclear fission. During the war there was a frantic race to develop atomic weapons. The United States assembled a team of leading physicists and chemists directed by J. Robert Oppenheimer (1904-1967) at Los Alamos, New Mexico to take part in project code named Trinity. In 1945, they produced the world's first atomic explosion using a fission reaction. In August 1945, atomic bombs were dro pped on the Japanese cities of Hiroshima and Nagasaki. As a result the Japanese surrendered and World War II ended.
In elements heavier than hydrogen (hydrogen's nucleus has only a single proton), the nucleus is composed of protons and neutrons. The number of protons is described by the atomic number and is unique for each element. Along with the protons are neutrons, particles of mass similar to the p roton, but without any electrical charge. The sum of t he atomic mass of the protons and neutrons determines the atomic mass or atomic weight of the nucleus.
Elements can be found that do not have identical numbers of neutrons, these elements are isotopes of one another and they have nuclei with different atomic mass. Nuclei with differing weights, that is, unique combinations of protons and neutrons are called nuclides.
Not all atomic nuclides are stable. Radioactive nuclides are unstable and undergo various nuclear reactions that transform the particles within the nucleus and simultaneously release energy. It is important to remember that radioactivity is a process not a term to be applied to the energy or substance emitted as a result of the radioactive p rocess.
Nuclear reactions include alpha decay (the emission of a helium nucleus, He+ ), beta decay (a reaction where a neutron is transformed into a proton and a high energy electron is emitted), posit ron decay (a reaction where a nuclear proton converts into a neutron and a high-energy positron is emitted), and decay that produces gamma radiation (a high energy photon emission).
A common misconception is that x rays are a product of nuclear reactions. In fact, x rays are the result of high energy electron transitions (e.g., jumps between electron energy levels or orbits) in heavy elements.
Scientists study nuclear reactions by accelerating atomic particles (e.g., neutrons) that, upon collision with the target nucleus, transfer enough energy to stimulate nuclear reactions. When some elements are bombarded with protons, neutrons, or other accelerated (and therefore highly energetic) atomic particles, their nuclei may be transfor med to create unstable isotopes that are radioactive. These radioactive isotopes are created by nuclear reactions termed neutron capture.
In neutron capture reactions an element's atomic number (Z) remains unchanged but its atomic mass increases by one because a neutron is added to the nucleus.
Two other important nuclear reactions, fusion and fission, also start with the capture of a neutron. In these reactions, however, the energy levels are so high that the transformed nucleus is very unstable. At the lower energy levels involved in simple neutron capture the element simply becomes radioactive, that is, it undergoes decay reactions to form decay products.
The nuclear reactions of fission and fusion have profound scientific and controversial social consequences. Fission involves the splitting of the atomic nucleus. During fission reactions, a nucleus is split into two nuclei. When, under the right circumstances, uranium is bombarded with neutrons it can undergo nuclear fission to produce barium, krypton, and three neutrons (the basis of the chain reaction).
The use of fission reactions in nuclear reactors remains socially and politically controversial. Fission's most common use is as a trigger in fusion reaction bombs. Other debates usually concern the safe construction and operation of nuclear power facilities and the proper disposal of the dangerous radioactive products of termed nuclear waste.
Fusion is the nuclear reaction that fuels the Sun and stars, the reaction involves the combining of two nuclei into one nucleus. At stellar temperatures of 10-15 million degrees Celsius, hydrogen is converted to helium. The energy from these reactions provides the energy to sustain life on Earth. The basic reaction combines or fuses four hydrogen atoms into one helium atom with the emission of tremendous energy generated by converting mass into energy according to Einstein's equation E = mc2 .
The potential benefits to mankind for the proper development of fusion based technology are enormous. However, until methods are developed to control the reaction, its use will be limited to nuclear weapons.
As stars age the supply of hydrogen for fusion reactions is used up and stars must use increasingly heavier elements in their fusion reactions. The mass of the star determines what fuels it can use (up to iron). Except for the transformation of elements by nuclear reactions, all of the atoms in the universe heavier than helium are the products of the nuclear reactions that take place in dying stars.
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