Radioactive decay substance

The nature of the radioactive decay is characteristic of the element it can be used to fingerprint die substance. Decay continues until bodi die original element and its daughter isotopes are non-radioactive. The half-life, i.e. die time taken for half of an element s atoms to become non-radioactive, varies from millions of years for some elements to fractions of a second for odiers. 

As early as 1902, Rutherford and his colleague, the chemist Frederick Soddy, realized that emissions of alpha and beta rays changed the nature of the emitting substance. One example of such a change is the spontaneous radioactive decay of the uranium-238 isotope, which emits an alpha particle and produces thorium ... 

The science of kinetics deals with the mathematical description of the rate of the appearance or disappearance of a substance. One of the most common types of rate processes observed in nature is the first-order process in which the rate is dependent upon the concentration or amount of only one component. An example of such a process is radioactive decay in which the rate of decay (i.e., the number of radioactive decompositions per minute) is directly proportional to the amount of undecayed substance remaining. This may be written mathematically as follows ... 

In the previous discussion of radioactive decay, it was noted that the rate of decay is directly proportional to the amount of undecayed substance remaining. In a solution of a radioactive substance, a similar relationship would hold for the concentration of undecayed substance remaining. If a solution of a radioactive substance were allowed to decay and a plot were constructed of the concentration remaining versus time, the plot would be a curve such as that shown in Fig. 1. 

The concentration of small ions in the atmosphere is determined by 1) the rate of ion-pair production by the cosmic rays and radioactive decay due to natural radioactive substances, 2) recombination with negative ions, 3) attachment to condensation nuclei, 4) precipitation scavenging, and 5) transport processes including convection, advection, eddy diffusion, sedimentation, and ion migration under the influence of electric fields. A detailed differential equation for the concentration of short-lived Rn-222 daughter ions including these terms as well as those pertaining to the rate of formation of the... 

Half-life Time required for half of a radioactive substance to disintegrate by radioactive decay. 

It was a speculative proposal. Szilard supposed that neutrons might be better at triggering radioactive decay than alpha particles - but no one had yet shown this. And it required the identification of a substance that both captured and emitted neutrons. Moreover, to set off a chain reaction, the number of neutrons emitted would have to exceed the number captured. All the same, the possibility pointed to a dramatic, even terrifying conclusion, and it chilled Szilard to... 

A value of a = 1 may be obtained in Equation 15-5 under two different situations, as we have noted. First, there is the special circumstance where A is the only reactant (that is, A is unstable and decomposes without any reaction with other substances) and where B and C do not exist (thus, k = k). A common example of this situation is radioactive decay, in which a given radioactive isotope spontaneously decomposes into the isotope of another element at a rate characterized by a rate constant k. 

This is the same model process that we described above for radioactive decay of if we substitute decay constants by rate constants, and amount of substance by concentration, and assume that [A]0 = 1 mol dm-3, we can adapt equation (7.40) derived in Problem 7.5(c) to describe how [B] varies with time ... 

The changing of radioactive elements into other elements through radioactive emissions is called either radioactive decay or radioactive disintegration. By using radioactive rays, it is possible to detect whether a substance is radioactive or not. There are several methods to detect the types of radiations and their intensities. The most commonly used device to check the intensity of radioactivity is the Geiger-Mtiller counter. 

The rate of radioactive decay of an element is the number of atoms emitting a radioactive ray per a unit time. The rate of decay is directly proportional to the initial amount of substance and the structure of the nuclei. On the other hand, the rate of decay is independent of the physical and chemical properties of a radioactive atom. Temperature does not affect the rate of decay. The rate of... 

The half-life of a radioactive decay is the period of time required for half of the initial amount of the substance to disintegrate. The shorter the half-life of a radioactive decay, the higher the rate of radioactive decay and the more radioactivity. The half-life is the characteristic property of each element. 

Notice that with radioactive decay, the radioactive substance never totally disappears. The amount gets smaller and smaller but never becomes zero. This is called exponential decay, and a typical graph of it is shown in Figure 14.3. 

As already explained in chapter 1.1.4.2.3, the Kd concept must be rejected in most cases, because of its oversimplification and its low suitability for application to natural systems. For example, in degradation only the degrading substance is considered. This concept might be applicable for radioactive decay, yet if the decomposition of organic matter is considered, it is crucial to consider decomposition products (metabolites) that form and play an important role in transport themselves. 

Scintillation counter an instrument that measures radioactive decay by sensing the flashes of light produced in a substance by the radiation. (21.4)... 

Radioactive substances are widely distributed on the earth. Some are found in the atmosphere, but the major part is present in the lithosphere. The most important ones are the ores of uranium and thorium, and potassium salts, including the radioactive decay products of uranium and thorium. Uranium and thorium are common elements in nature. Their concentrations in granite are about 4 and 13mg/kg, respectively, and the concentration of uranium in seawater is about 3 pg/l. Some uranium and thorium minerals are listed in Table 1.1. The most unportant uranium mineral is pitchblende (UsOs). Uranium is also found in mica. The most important thorium mineral is monazite, which contains between about 0.1 and 15% Th. 

The decay of a radionuclide is a statistical process in the sense that it is not possible to predict exactly when a particular nucleus will disintegrate. One may, however, ascribe a probability that a nucleus will decay in unit time. This probability is known as the radioactive decay constant (transformation constant), X, of the radionuclide. The number of atoms of a radioactive substance disintegrating per unit time, 6N/dt, which is referred to as the activity of the substance, is proportional to the total number, N, of radioactive atoms present at time t, the constant of proportionality being X. 

The curie, abbreviated Ci, is a standard unit of radioactive decay. It was originally defined as the rate at which 1 g of radium decays. Because of the relatively long half-life of Ra, the isotope served as a convenient standard. The curie is now defined as the quantity of any radioactive substance in which the decay rate is 3.700 X 10 disintegrations per second (2.22 X 10 DPM). Because the efficiency of most radiation detection devices is less than 100%, a given number of curies almost always yields a lower than theoretical count rate. Hence, there is the distinction between DPM and CPM. For example, a sample containing 1 /rCi of radioactive material has a decay rate of 2.22 x lO DPM. If only 30% of the disintegrations are detected, the observed count rate is 6.66 X 10 CPM. 

Rutherford s work has made him known as the father of nuclear physics with his research on radioactivity (alpha and beta particles and protons, which he named), and he was the first to describe the concepts of half-life and decay constant. He showed that elements such as uranium transmute (become different elements) through radioactive decay, and he was the first to observe nuclear reactions (split the atom in 1917). In 1908 he received the Nobel Prize in chemistry for his investigations into the disintegration of the elements, and the chemistry of radioactive substances. He was president of the Royal Society (1926-30) and of the Institute of Physics (1931-33) and was decorated with the Order of Merit (1925). He became Lord Rutherford in 1931. 

Radionuclides have different stabilities and decay at different rates. Some decay nearly completely in a fraction of a second and others only after millions of years. The rates of all radioactive decays are independent of temperature and obey first-order kinetics. In Section 16-3 we saw that the rate of a first-order process is proportional only to the concentration of one substance. The rate law and the integrated rate equation for a first-order process (Section 16-4) are... 

Radioactive Strontium. 90Sr is not a naturally occurring substance its presence in the environment is a result of human activities, such as the prior testing of nuclear bombs in the air and leaks from radioactive storage and waste sites. Radioactive decay is the only way for decreasing the concentration of 90Sr. Eventually, all 90Sr will be converted to stable zirconium. 

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