Short-lived daughter nuclide

Very good agreement was obtained between theoretical values and data from experimental models using different short-lived daughter nuclides. 

A selective elution technique from the lon-exchange column Is often successful for a very rapid Isolation of a short-lived daughter nuclide from relatively long-lived parent activities which are strongly held to the Ion-exchange column. 

Figure lj. Polarographic cell used for rapid radiochemical detection of short-lived daughter nuclide (163). 

Radionuchdes Any kind of unstable (radioactive) atoms that may differ in atomic number Z and/or neutron number N from other atoms Radionuclide generators Set-up to separate repeatedly short-lived daughter nuclides from longer-lived mother nuclides by chemical methods Radiotoxicity Toxicity of radionuclides... 

Application of short-lived radionuclides has the advantage that the activity vanishes after relatively short periods of time. This aspect is of special importance in nuclear medicine. Short-hved radionuclides may be produced by irradiation in nuclear reactors or by accelerators, but their supply from in-adiation facilities requires matching of production and demand, and fast transport. These problems are avoided by application of radionuclide generators containing a longer-lived mother nuclide from which the short-hved daughter nuclide can be separated. 

Studies of (half-life = 19.8 min) activity relative to " Pb (half-life = 26.8 min) in boundary layer air indicate that these are in equilibrium, indicating that the particles are not removed more rapidly than a couple of hours (Turekian et ai, 1999). Attaching of these short-lived daughters on aerosols, and the mean life of the aerosols being much longer than their half-lives means that the nuclide to be tracked on most atmospherically interesting timescales is Pb with its half-life of 22 yr. 

Generator, radionuclide. A device in which a short-lived daughter nuchde is separated chemically from a long-lived parent nuclide adsorbed on an adsorbent material. For example, "mTc is separated from "Mo from the "Mo-99mTc generator with saline. 

Radioactive decay of and Pu form Am and Am, which are also important and persistent sources of alpha radioactivity in discharge fuel. Another persistent americium radioisotope is 152-year Am, formed by neutron capture in Am. Its isomeric decay and the beta decay of its short-lived daughter result in 163-day Cm, which is the most intense source of alpha activity in discharged uranium fuel. Successive neutron captures lead to em, em, and Cm. Higher-mass curium nuclides are usually not important in power reactor fuel. 

Several techniques to measure air concentrations are outlined by Breslin (1980). Most of the techniques for measuring radon use the fact that both radon-222 and the short-lived daughters are alpha- emitting nuclides. The sample is collected and taken back to the laboratory for "alpha-counting" or an alpha-detector is taken to the field for on-site measurement. There are several ways to measure alpha decay. A scintillation flask is one of the oldest and most commonly used methods. The flask is equipped with valves which are lined with a phosphor (silver-activated zinc sulfide) and emit light flashes when bombarded with alpha particles. Other methods draw the air through a filter (or filters) for a variety of time intervals and then count the number of alpha-decays occurring on the filter. EPA (1986) and NCRP (1988) reports provide more in-depth discussions of these methods. 

Allow a decay of no more that 24 h to allow short-lived natural nuclides (radon daughters - see Appendix D) to decay. 

Routine estimation of Ra is readily achieved by gamma spectrometry measurement of its short-lived daughter, Ac, which can be verified by values for other Th chain nuclides ( Pb, Bi and T1) obtained at the same time. Deriving values for Ra should be similarly straightforward however, there are several potential sources of error that can make accurate determination problematic. 

A number of now extinct radioactive isotopes have existed in the early solar system. This is shown by the variations that they induce in the abundances in their daughter nuclides. Their main use is in establishing a chronology between their parental presolar stellar sources, and the formation of the solar system and the planets. An active debate is presently going on whether some of these short-lived nuclides could have been made within the early solar system by an intense flux of energetic protons from the young sun. 

Let us consider now species 1 and 2 linked in a decay chain with the parent nuclide 1 being longer-lived than the daughter nuclide 2 (i.e.. A, < A2). After a relatively short time, the terms exp(-A2f) and exp(—A2O become negligible with respect to exp(—Ajf). As a result, equation 11.28 is reduced to... 

Figure 11,8 Composite decay curves for (A) mixtures of independently decaying species, (B) transient equilibrium, (C) secular equilibrium, and (D) nonequilibrium, a composite decay curve b decay curve of longer-lived component (A) and parent radio nuclide (B, C, D) c decay curve of short-lived radionuclide (A) and daughter radionuclide (B, C, D) d daughter radioativity in a pure parent fraction (B, C, D) e total daughter radioactivity in a parent-plus-daughter fraction (B). In all cases, the detection coefficients of the various species are assumed to be identical. From Nuclear and Radiochemistry, G. Friedlander and J. W. Kennedy, Copyright 1956 by John Wiley and Sons. Reprinted by permission of John Wiley and Sons Ltd.

In addition to the processes of stellar nucleosynthesis, there are two other ways in which isotopes are produced. One is radioactive decay. Many of the nuclides produced by explosive nucleosynthesis are unstable and decay to stable nuclei with timescales ranging from a fraction of a second to billions of years. Those with very short half-lives decayed completely into their stable daughter isotopes before any evidence of their existence was recorded in objects from our solar system. However, radioactive nuclei from stellar nucleosynthesis that have half-lives of >100 000 years left a record in solar system materials. For those with half-lives of more than 50 million years some of the original nuclei from the earliest epoch are still present in the solar system today. The ultimate fate of all radioactive nuclides is to decay to their stable daughter nuclides. Thus, the only real distinction between isotopes produced by stellar nucleosynthesis and those produced by decay of radioactive nuclides produced by stellar nucleosynthesis is the time scale of their decay. We choose to make a distinction, however, because radioactive nuclides are extremely useful to cosmo-chemists. They provide us with chronometers with which to constmct the sequence of events that led to the solar system we live in, and they provide us with probes of stellar nucleosynthesis and the environment in which our solar system formed. These topics appear throughout this book and will be discussed in detail in Chapters 8, 9, and 14. 

Because the short-lived nuclides are extinct, a different approach must be taken to use them as chronometers. Equation (8.9) cannot be used to calculate a date because the number of parent nuclides, N, is zero and the equation is undefined. However, if a short-lived nuclide was homogeneously distributed throughout a system, then one can determine the order in which objects formed within that system based on the amount of radionuclide that was present when each object formed. The oldest object would form with the highest amount of the radionuclide relative to a stable isotope of the same element, and the youngest will have the lowest amount. Obviously, no chronological information can be obtained about objects that formed after the radionuclide has reached a level too small to detect the radiogenic daughter isotope. 

Consider a hypothetical system that has an initial abundance of a short-lived radionuclide Nr. This nuclide is present as a fixed fraction of the parent element and its abundance can be written as the ratio of the radionuclide to a stable reference isotope of the same element, Ns. When the short-lived nuclide has completely decayed to its daughter nuclide, D, we have ... 

Studies of short-lived radionuclide generators (4-6) do not adequately treat the quantitative problems of the daughter nuclide elution or those specific to their optimal clinical use. Two essential physical characteristics of a generator are the yield of the daughter nuclide and its radiochemical and radionuclidic purity. To realize the full potential of a short-lived radionuclide generator for medical studies requires that these two characteristics are optimized and are compatible with parameters important to clinical use such as total perfused volume and duration of the scintigraphic examination. 

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