The Decay Process

The annual input rate of dead plant matter to soils in temperate regions, the net primary productivity of plants, is about 1 kg m 2 of carbon or 20 Mg ha-1 of dry matter. The annual input rate is perhaps 2 kg C m-2 or 40 Mg ha-1 of dry matter in humid tropical forests, and decreases to virtually zero to 0.1 kg C m-2 in deserts and arctic tundra. This input is part of the nonhumus organic matter, which includes original plant and microbial tissue and partially decomposed material. These nonhumus substances contain carbohydrates and related compounds, proteins and their derivatives, fats, lignins, tannins, and various partially decomposed products in roots and plant tops. The portion contributed by dead animal matter is insignificant because the 

FIGURE 6.1. Organic matter decomposition and formation of humic substances. (From F. E. Bear, ed. 1964. Chemistry of the Soil, ACS Monograph Series No. 160, p. 258.) 

FIGURE 6.2. Idealized diagram for the decay of crop residues in soil under conditions that are optimal for microbial activity. (From E J. Stevenson. 1982. Humus Chemistry. Wiley, New York.) 

Results of field experiments confirm that carbon becomes increasingly resistant to decomposition with time. This has led some investigators to conclude that the organic component exists in three major fractions when considered on a dynamic basis (1) decomposing plant residues and the associated biomass, which turn over every few years (2) microbial metabolites and cell wall constituents that become stabilized in soil and have a half-life of 5 to 25 years and (3) the smallest fraction, resistant organic matter, ranging in age from 250 to 2500 years or more. 

FIGURE 6.3. Decomposition rates of fresh organic matter in England and in Nigeria. (From D. S, Jenkinson and A. Ayanaba. 1970. Soil Scl. Soc. Am. Proc. 43 912.) 

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K, L, M,. ..), 5 is the energy shift caused by relaxation efiects and cp is the work fimction of tlie spectrometer. The 5 tenn accounts for the relaxation effect involved in the decay process, which leads to a final state consisting of a heavily excited, doubly ionized atom. 

Since the half-life is independent of the number of radioactive atoms, it remains constant throughout the decay process. Thus, 50% of the radioactive atoms disintegrate in one half-life, 75% in two half-lives, and 87.5% in three half-lives. 

The study of the chemical behavior of concentrated preparations of short-Hved isotopes is compHcated by the rapid production of hydrogen peroxide ia aqueous solutions and the destmction of crystal lattices ia soHd compounds. These effects are brought about by heavy recoils of high energy alpha particles released ia the decay process. 

Decay products of the principal radionuclides used in tracer technology (see Table 1) are not themselves radioactive. Therefore, the primary decomposition events of isotopes in molecules labeled with only one radionuclide / molecule result in unlabeled impurities at a rate proportional to the half-life of the isotope. Eor and H, impurities arising from the decay process are in relatively small amounts. Eor the shorter half-life isotopes the relative amounts of these impurities caused by primary decomposition are larger, but usually not problematic because they are not radioactive and do not interfere with the application of the tracer compounds. Eor multilabeled tritiated compounds the rate of accumulation of labeled impurities owing to tritium decay can be significant. This increases with the number of radioactive atoms per molecule. 

There are five fundamental t T)es of nuclear decay process, as listed in Table 22-3. Figure 22-5 on the next page diagrams how nuclear decays affect N and Z. As the figure suggests, the decay process of any particular unstable nuclide depends on the reason for its instability. 

Fig. 3 Formation and decay of the transient absorption of Py + monitored at 470 nm. Time profiles for a PyODNn (n=l 5), b PyODNn (n=l 4) after subtraction of the time profile of PyODN5. The transient at 470 nm mainly from Py + from PyODNn decayed by two processes. The decay process with a time range of 100 /is and the longer component with a decay time > 1 ms were observed... 

Thus, by measuring activity as function of time yields a curve as shown in Figure 1. However, the pattern of activity is more complicated if the product of the decay process is also radioactive. 

Methods using excited states to measure the dynamics of supramolecular systems have the additional dimension that the excited state has a lifetime and this decay has to be considered in the kinetic treatment of the data. The decay processes are parallel... 

The SE term accounts for the relaxation effects involved in the decay process, which leads to a final state consisting of a heavily excited, doubly ionized atom. 

A (3-particle is an electron. An unstable nuclide in (3-particle production creates an electron as it releases energy in the decay process. This electron is created from the decay process, rather than being present before the decay occurs. 

For many of the analytical techniques discussed below, it is necessary to have a source of X-rays. There are three ways in which X-rays can be produced in an X-ray tube, by using a radioactive source, or by the use of synchrotron radiation (see Section 12.6). Radioactive sources consist of a radioactive element or compound which spontaneously produces X-rays of fixed energy, depending on the decay process characteristic of the radioactive material (see Section 10.3). Nuclear processes such as electron capture can result in X-ray (or y ray) emission. Thus many radioactive isotopes produce electromagnetic radiation in the X-ray region of the spectrum, for example 3He, 241Am, and 57Co. These sources tend to produce pure X-ray spectra (without the continuous radiation), but are of low intensity. They can be used as a source in portable X-ray devices, but can be hazardous to handle because they cannot be switched off. In contrast, synchrotron radiation provides an... 

Reabsorption and reemission. In scattering media the effect of reabsorption plays a very important role (see Section 6.3). The consecutive reemission will slow down the decay process (see Section 6.4). The effect is not very big, but it can be... 

Two methods to secure very small samples of francium for examination use the decay processes of other radioactive elements. One is to bombard thorium with protons. The second is to start with radium in an accelerator, where, through a series of decay processes, the radium is converted to actinium, which in turn rapidly decays into thorium, and finally, thorium decays naturally into francium. Following is a schematic of the decay process used for the production of small amounts of Fr-223 which, in turn, after several more decay processes ends up as stable lead (Pb) ... 

Radium is extremely radioactive. It glows in the dark with a faint bluish light. Radiums radioisotopes undergo a series of four decay processes each decay process ends with a stable isotope of lead. Radium-223 decays to Pb-207 radium-224 and radium-228decay to Pb-208 radium-226 decays to Pb-206 and radium-225 decays to Pb-209. During the decay processes three types of radiation—alpha (a), beta ((5), and gamma (y)—are emitted. 

Element 116 is the result of the decay process of element 118. (See entry for element 118 for details.)... 

One conclusion that can be drawn from this is that even the best of scientific labs and investigators can be seduced into scientific misconduct. Fortunately, such misconduct is rare in the scientific community. Another conclusion is that because the decay chain of element 118 produced the following new elements—116, as it was the result of the decay process of the nonexistent element 118, 114, 112, 110, 108, and 106— it throws suspicion on the existence of element 116. Still, many of the elements identified by the suspect decay chain have also been independently produced synthetically. 

Let us consider, for instance, the decay process of the unstable oxygen isotope... 

Alpha particles are composed of two protons and two neutrons. Thus they have Z = 2, N = 2, and A = 4 and correspond to a helium nucleus He. The emission of a particles thus produces a decrease of 4 units in A. An unstable nuclide undergoing a decay may emit a particles of various energy and thus directly reach the ground level of the stable product. Alternatively, as in )3 emission, an intermediate excited state is reached, followed by y emission. Figure 11.7 shows, for example, the decay process of ioTh., which may directly attain the ground level of by emission of a particles of energy 5.421 MeV or intermediate excited states by emission of a particles of lower energy, followed by y emission. 

The low decay energy prevents accurate determinations of half-life by direct counting. Reported half-lives range from 3 to 6.6 X 10 ° a. The most recent direct determination (Lindner et al., 1989) assigns a half-life of (4.23 0.13)Xl0 °a to the decay process of equation 11.117, which is fairly consistent with the indirect estimates of Hirt et al. (1963) [(4.3 0.5) X 10 ° a]. The isochron equation is normalized to the °Os abundance ... 

While it is apparent that there are several pathways for relaxation of the excited state of iron porphyrins, the exact steps involved in these different pathways are still to a great extent unknown. It is the purpose of this study to explore the role of spin state and axial ligands in the decay process of the excited states of iron porphyrins. 

As the absorption spectra did not change after laser light irradiation, no appreciable reaction seems to occur through the state. Thus, the decay processes from the state can be assumed to be only fluorescence, intersystem crossing, and internal conversion. The quantum yields of internal conversion ( are, therefore, estimated by the following equation ... 

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