Radioactive decay Subject

In NAA the sample is made radioactive by subjecting it to a high dose (days) of thermal neutrons in a reactor. The process is effective for about two-thirds of the elements in the periodic table. The sample is then removed in a lead-shielded container. The radioisotopes formed decay by B emission, y-ray emission, or X-ray emission. The y-ray or X-ray energies are measured by EDS (see Chapter 3) in spe-... 

In conclusion, the agreement of all of these methods based upon radioactive decay furnishes a strong clue that the earth s crust as we know it today was formed about five billion years ago. What preceded is a subject of intense interest and monumental disagreement. 

Fig. 17. Biological model recommended for describing the uptake and retention of cerium by humans after inhalation or ingestion. Numbers in parentheses give the fractions of the material in the originating compartments which are cleared to the indicated sites of deposition. Clearance from the pulmonary region results from competition between mechanical clearances to the lymph nodes and gastrointestinal tract and absorption of soluble material into the systemic circulation. The fractions included in parentheses by the pulmonary compartment indicate the distribution of material subject to the two clearance rates however, these amounts will not be cleared in this manner if the material is previously absorbed into blood. Transfer rate constants or functions, S(t), are given in fractions per unit time. Dashed lines indicate clearance pathways which exist but occur at such slow rates as to be considered insignificant compared to radioactive decay of the cerium isotopes.

Once the radionuclides reach the sediments they are subject to several processes, prime among them being sedimentation, mixing, radioactive decay and production, and chemical diagenesis. This makes the distribution profiles of radionuclides observed in the sediment column a residuum of these multiple processes, rather than a reflection of their delivery pattern to the ocean floor. Therefore, the application of these nuclides as chrono-metric tracers of sedimentary processes requires a knowledge of the processes affecting their distribution and their relationship with time. Mathematical models describing some of these processes and their effects on the radionuclide profiles have been reviewed recently [8,9,10] and hence are not discussed in detail here. However, for the sake of completeness they are presented briefly below. 

The first four modes of radioactive decay can be plotted on a single diagram (Fig. 10.3), which allows for a prediction of the nature of the daughter nucleus from a parent subject to any one of the above processes. 

Nuclei can emit radiation spontaneously. The general process is called radioactive decay. While this subject will be discussed in detail in Chapters 3, 7, 8, and 9, we need to know a few general ideas about these processes right away (which we can summarize below). 

Measurements of activity in blood of seven subjects were continued for 14 d after inhalation and showed that lead was lost from blood with a biological half-life of 18.0 (s.e.) 0.9 d. Because of radioactive decay, it was not possible to continue measurements for a longer period. Rabinowitz et al. (1976) gave oral doses of the stable lead isotope 204Pb to four subjects daily for several months, and used mass spectrometry to measure 204Pb in blood. The decline in 204Pb in blood was found to be exponential over periods of about 100 d from the end of the period of... 

The combination of radiolabeled sulfide and the bimane-HPLC method is particularly powerful because one of the main obstacles to the use of labeled sulfide is, that aside from radioactive decay, the compound is subject to rapid oxidation in the presence of air. The breakdown products of chemical sulfide oxidation are the same as those of biological oxidation. Previously it has been impossible to check routinely the purity of the purchased isotope and its subsequent purity during a series of experiments. It is our experience that newly purchased sodium sulfide sometimes contains up to 10% thiosulfate as well as traces of sulfite and sulfate (Figure 2), and that the sulfide once hydrated readily oxidizes if stored in a normal refrigerator. 

Then, having broached the subject of the relaxation of the ion s atmosphere—its taking up a dissymmetric shape when the ion moves—we went on to tackle the subject of relaxation quite generally. For example, if an electric field is suddenly applied to a solution, it would orient the solvent dipoles therein. A new equilibrium would then be set up. The relaxation time is a measure of the time it takes to set up this new equilibrium. At first it seems peculiar that one should call it a measure of and not the time itself. However, the situation is similar to that of radioactive decay because in changing from state 1 to state 2, the concentration of aradioactive nucleus decreases exponentially with time, taking an infinite time to disappear completely. Since this is not a practical measure, we agree to use another measure of the rate of decay—the time to decline by 63%. 

If the abundances of nuclides present in a decay series are only subjected to the law of radioactive decay (no chemical or other physical processes are involved), the development in time to a state of quasi-equilibrium is governed by eq. (2.8), no matter how complicated the initial conditions are. If, for any reason, this state of equilibrium has not yet been reached and the initial abundances, given by eq. (2.9), of the various nuclides are known, the elapsed time can be deduced from the degree of disequilibrium. 

The half lifetime of a compound, subject to exponential decay, is the time required for the compound to decay to half of its initial value. Although this concept originated from the study of radioactive decay, it applies to many other fields as well, including phenomena that are described by nonexponential decays. It is mathematically defined as... 

Because strontium is an element, its atoms do not degrade by environmental processes such as hydrolysis or biodegradation. However, radioactive strontium will be subject to radioactive decay and transformation to other elements. Eventually, all of the radioactive strontium will be transformed into stable zirconium by the process of radioactive decay (see Section 4.2) ... 

The probability that a radioactive nucleus will decay in a given time is a constant, independent of temperature, pressure, or the decay of other neighboring nuclei. The disintegrations of individual nuclei are statistically independent events and are subject to random fluctuations. In a large number of nuclei, however, the fluctuations average out, and the fraction that decays in unit time is a constant and is numerically equal to the probability that a single nuclei will decay in that time. This rate of radioactive decay is known as the decay constant X, with dimensions of reciprocal time. 

The next subject we will address is that of scintillators. These are phosphors used to detect a, p, and y rays from incident sources. This application has become very important with the advent of CT-scanners and the PET-scanner, i.e.- CT = "computerized tomagraphy" and PET = "positron emission tomography". In all cases, radioactive Isotopes are used as the source of these "rays", including positrons, i.e.- positive electrons, p. Although scintillation, i.e.- detection of high energy radioactive-decay particles, may occur for nearly all phosphors, efficient scintillators must satisfy the following requirements for practical use. 

Molecules in electronic excited states are highly reactive. The presence of any dissolved oxygen or impurities makes it possible for the excited species to react with oxygen or the impurities, or to dimerize this decreases the probability of electronic transitions to the ground state. Note that fluorescence and phosphorescence are both radioactive decay processes, therefore they are both subject to the sarnie influences the differences that would exist would be the differences in reactions and reactivity of singlet vs. triplet states. 

In addition, consideration must be given to the fact that the decay products of radon are solids which are themselves again subject to radioactive decay. The vessels with radioactive water samples are therefore contaminated to a greater or lesser extent, so that before they are used again the radioactivity should have subsided and the vessels must be thoroughly cleaned. If the water contains dissolved radium 226, with the... 

Lutetium-176 is radioactive and subject to branched decay by P emission, mostly to Hf, though approximately 3 1% decays to Yb. However, the latter can be disregarded because of the long half-life of Lu. Thus, the decay scheme of interest is... 

For the treatment of the behavior of the radionuclides in the containment, it seems reasonable to divide them into three groups aerosols, tellurium, and iodine. The fission product noble gases as the fourth group of elements are distributed homogeneously over the entire free volume of the containment after a short time interval and they are only subject to radioactive decay thus, their behavior will not be discussed in detail in the following. In spite of the considerable differences between PWR and BWR plants, the environmental conditions within the containment are in most cases not so different as to influence fundamentally the chemical reactions of the fission products. Therefore, the discussions in the following sections will apply to both types of LWR plants where there are significant differences they will be mentioned in the respective section. 

Radionuclides are nuclides with an unstable nucleus, which are subject to radioactive decay. According to the type of radionuclide, this radioactive nuclei decay creates three main types of radiation (radioactive decay) ... 

When chemical is present in a compartment and is subject to loss processes, it is useful to calculate the time that is required for a defined fraction of the chemical mass in the compartment to be lost by that process. A half-life or half-time is commonly used to characterize rates of reaction or radioactive decay. More fundamental is the characteristic time, which is the reciprocal of the rate constant of loss. The half-time is... 

Errors Inherent to the Radiocarbon Dating Method. The decay of radiocarbon is radioactive, involving discrete nuclear disintegrations taking place at random dates derived from the measurement of radiocarbon levels are therefore subject to statistical errors intrinsic to the measurement, which cannot be ignored. It is because of these errors that radiocarbon dates are expressed as a time range, in the form... 

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