Radioactive decay


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Radioactive decay

  1. 1. Radioactive decay is the process by which an atomic nucleus of an unstableatom loses energy by emitting ionizing particles (ionizing radiation). Thereare many different types of radioactive decay table. A decay, or loss ofenergy, results when an atom with one type of nucleus, called the parentradionuclide, transforms to an atom with a nucleus in a different state, orto a different nucleus containing different numbers of nucleons. Either ofthese products is named the daughter nuclide. In some decays the parentand daughter are different chemical elements, and thus the decayprocess results in nuclear transmutation (creation of an atom of a newelement).The first decay processes to be discovered were alpha decay, beta decay,and gamma decay. Alpha decay occurs when the nucleus ejects an alphaparticle (helium nucleus). This is the most common process ofemitting nucleons, but in rarer types of decays, nuclei can eject protons,or specific nuclei of other elements (in the process called clusterdecay). Beta decay occurs when the nucleus emitsan electron or positron and a type of neutrino, in a process that changes aproton to a neutron or vice versa. The nucleus may capture an orbitingelectron, converting a proton into an neutron (electron capture). All ofthese processes result in nuclear transmutation.By contrast, there exist radioactive decay processes that do not result intransmutation. The energy of an excited nucleus may be emitted as agamma ray in gamma decay, or used to eject an orbital electron byinteraction with the excited nucleus in a process called internalconversion. Radioisotopes occasionally emit neutrons, and this results in achange in an element from one isotope to another.
  2. 2. Radioactivity was discovered in 1896 by the French scientist HenriBecquerel, while working on phosphorescent materials. These materialsglow in the dark after exposure to light, and he suspected that the glowproduced in cathode ray tubes by X-rays might be associated withphosphorescence. He wrapped a photographic plate in black paper andplaced various phosphorescent salts on it. All results were negative untilhe used uranium salts. The result with these compounds was a blackeningof the plate. These radiations were called Becquerel Rays.At first it seemed that the new radiation was similar to the thenrecently-discovered X-rays. Further research by Becquerel, ErnestRutherford, Paul Villard, Pierre Curie, Marie Curie, and others discoveredthat this form of radioactivity was significantly more complicated.Different types of decay can occur, producing very different types ofradiation. Rutherford was the first to realize that they all occur with thesame mathematical exponential formula (see below), and Rutherford andhis student Frederick Soddy were first to realize that many decayprocesses resulted in the transmutation of one element to another.Subsequently, the radioactive displacement law of Fajans and Soddy wasformulated to describe the products of alpha and beta decay.The early researchers also discovered that many other chemicalelements besides uranium have radioactive isotopes. A systematic searchfor the total radioactivity in uranium ores also guided Marie Curie toisolate a new element polonium and to separate a newelement radium from barium. The two elements chemical similarity wouldotherwise have made them difficult to distinguish.There are three types of radioactivity:Alpha DecayThe reason alpha decay occurs is because the nucleus has too many protons whichcause excessive repulsion. In an attempt to reduce the repulsion, a Helium nucleus isemitted.
  3. 3. Beta DecayBeta decay occurs when the neutron to proton ratio is too greatin the nucleus and causes instability. In basic beta decay, a neutron isturned into a proton and an electron. The electron is then emitted. Heresa diagram of beta decay with hydrogen-3.Gamma decayGamma decay occurs because the nucleus is at too high an energy. Thenucleus falls down to a lower energy state and, in the process, emits a highenergy photon known as a gamma particle. Heres a diagram of gammadecay with helium-3.The differences between artificialradioactivity and natural radioactivity are:Natural radioactivity:Nuclear reactions which occur spontaneously are said to be an example ofnatural radioactivity. There are three naturally occurring radioactiveseries among the elements in the periodic table. These are known as theuranium series, the actinium series and the thorium series, each namedafter the element at which the series start (except the actinium serieswhich starts with a different uranium isotope). Each series decaysthrough a number of unstable nuclei by means of alpha and betaemmission, until each series end on a different stable istope of lead.Artificial radioactivity:Not all nuclear reactions are spontaneous. These reactions occur whenstable isotopes are bombarded with particles such as neutrons. This
  4. 4. method of inducing a nuclear reaction to proceed is termed artificialradioactivity. This meant new nuclear reactions, which wouldnt have beenviewed spontaneously, could now be observed. Since about 1940, a set ofnew elements with atomic numbers over 92 (the atomic number of theheaviest naturally occurring element, Uranium) have been artificiallymade. They are called the transuranium elements.Uses of radioactivity: 1.preservation of food grains and seeds 2. some of the isotopes are used in the treatment of cancer. 3. some of the isotopes are used to study the proper functioning of internal organs. 4. Gamma radiations are used to sterilize the surgical instruments. 5. radio phosphorous is used for studying the rate of phosphorous assimilation by the plant. is used for finding out the faults in metal structures. is used for preparing synthetic elements (artificial transmutation) 8. isotopes are used in elucidation of reaction mechanism by using isotopic effects. breeder reactors radiations are used to prepare the fuel / fissile material. 10.trace concentrations of metals can be estimated by isotopic dilution analysis or neutron activation analysis.Bibliography