Target nuclides

In terms of atomic spectrometry, NAA is a method combining excitation by nuclear reaction with delayed de-excitation of the radioactive atoms produced by emission of ionising radiation (fi, y, X-ray). Measurement of delayed particles or radiations from the decay of a radioactive product of a neutron-induced nuclear reaction is known as simple or delayed-gamma NAA, and may be purely instrumental (INAA). The y-ray energies are characteristic of specific indicator radionuclides, and their intensities are proportional to the amounts of the various target nuclides in the sample. NAA can thus... 

Design and develop a novel molecule that provides satisfactory separation of a target nuclide. It should be easily synthesized. 

There are special extractants to extract each class of radionuclides crown ethers for cesium and strontium and phosphine oxides, carbamoylmethylphosphine oxides, and diamides for actinides, etc. It is unrealistic to have a single extractant that can extract all target nuclides with nearly the same effectiveness. So, a promising technical decision is to mix extractants for different radionuclides and extract them simultaneously. 

A = the activity of the product radionuclide (disintegrations per second). n = the number of atoms of the target nuclide in the sample. 

At. Wt. = the atomic weight of the element Iso. Abd. = the isotopic abundance of the target nuclide expressed as a fraction. 

Target Nuclide Nuclear Reaction Product Radionuclide... 

The prompt gamma-rays emitted following neutron or charged particle interactions with the target nuclide may be used as a basis for non-destructive analyses. The important advantage of this technique is that the determination does not depend in any manner on the half-life of a product radionuclide. In fact, using this technique, the product nuclide need not even be radioactive. Many conventional activation determinations are limited in their sensitivities by short half-life product radionuclides, or the fact that the most abundant or highest cross section isotope of the element to be determined leads to a stable product on irradiation. 

A shorthand notation has been developed for nuclear reactions such as the reaction discovered by Curie and Joliot. The parent (or target) nuclide and the daughter nuclide are separated by parentheses that contain the symbols for the particle that hits the target and the particle or particles released in this reaction. 

They contain a great number of nucleons which they can transmit to the target nuclide in one reaction. Thus, nuclides may be obtained that are far away from the target nuclide in the chart of the nuclides. 

Fig. 8.11 gives a survey of the location in the chart of the nuclides of the products obtained by various low-energy nuclear reactions. By reactions with neutrons an isotopic compound nucleus is formed which may emit particles or photons, depending on its structure and excitation. In (n, y) reactions the excitation energy of the compound nucleus is given off in the form of y-ray photons. In (n, p) reactions the compound nucleus emits a proton and the product is an isobar of the target nuclide. 

Nuclear reactions may lead to stable or unstable (radioactive) products. In general, (n, y), (n, p), and (d, p) reactions give radionuclides on the right-hand side of the line of p stability that exhibit decay, whereas (p, n), (d,2n), (n, 2n), (y, n), (d, n) and (p, y) reactions lead to radionuclides on the left-hand side of the line of p stability that exhibit p decay or electron capture (e). (n, y), (d, p), (n, 2n) and (y, n) reactions give isotopic nuclides, and these cannot be separated from the target nuclides by chemical methods, except for the application of the chemical effects of nuclear transformations which will be discussed in chapter 9. 

With respect to chemical separation of isotopic nuclides from target nuclides after (n, y) reactions, changes of the valence state and of complexation are of special interest. Some examples are listed in Table 9.3. All nuclides produced by (n, y) reactions are found in appreciable amounts in lower valence states and free from com-plexing ligands, respectively. 

Nuclear reactions are usually represented as eqs. (4.1) and (4.2), that is, on the left is the symbol for the target nuclides, the first symbol in the parenthesis indicates the bombarding particle (or projectile), the second the emitted particles, and the symbol of the product on the right. In the equation, the left side of the comma shows the system of the reactants, and the right the system of the products. Before and after the nuclear reaction, both the sum of mass number and the sum of atomic number remain unchanged. 

The spallation reaction is another example of special nuclear reactions, by which many nuclides relatively small mass number (about 10 to 20, the smaller the number, the higher the yield) in comparison with the target nuclide is produced simultaneously. The example is ... 

The target material for irradiation must be pure and preferably monoiso-topic or at least enriched isotopically to avoid the production of extraneous radionuclides. Radionuclides are separated from the target material by appropriate chemical methods such as solvent extraction, precipitation, chromatography, ion exchange, and distillation. Cyclotron-produced radionuclides are typically neutron deficient and, therefore, decay by / + emission or electron capture. Also, the radionuclides, which are different from the target nuclides, do not contain any stable (or cold ) atoms and are called carrier-free. Another term for these preparations is no-carrier-added (NCA), because no cold atoms have been intentionally added to the preparations. 

The tern N in the basic neutron activation analysis is equal to w0Na/AW, in which w is the weight of the element, 0 the percent isotopic abundance of the target nuclide, Na Avogadro s number, and AW the atomic weight of the element. Thus, the basic equation can be rewritten as follows ... 

It is generally agreed that neutron activation analysis has shown great sensitivity for many elements. Absolute sensitivities of detection depend on the atomic weight of the element, the fractional abundance of the target nuclide, and its cross section for thermal neutrons (which are fixed values) as well as on the available neutron flux, the irradiation time, the decay period, and the counting efficiency of the detector (which are variable parameters). The formulae described under Fundamentals (vide supra) will make it clear that, unless conditions are exactly specified, published values cannot easily be compared especially as the definitions for sensitivity chosen by the investigators may be different. Experimental sensitivities may be idealized because of matrix problems, difficulties in radiochemical separations, and other analytical problems associated with the analysis of complex, real samples. 

The area under the peak (shaded area in Fig. 8.5(b)) is proportional to the amount of the radioactive nuclide. If all other factors in (9.9) are known, the number of target nuclide atoms N can be calculated. 

The reaction path a indicates that the energy Q is emitted as a y-ray. In reaction path b the Q is retained as excitation energy of the target nuclide. The latter exists in an excited state and may transform to the ground state quite rapidly or may exist for a measurable time as an isomer. An example of inelastic scattering reaction (12.22b) is the formation of an... 

Although these considerations are inqmrtant in the choice of a radionuclide to use for a particular purpose, the possibilities of production may be limited either by the availability of facilities (reactors, different accelerator beams) or sinq>ly by the reaction cross-sections. Figure 4.7 (right side) shows the reaction paths needed to transform a given target nuclide into a product nuclide. If the target nuclide is denoted by 2X, a (n,y) reaction yields the same product as a (d,p) reaction, i.e. The choice of reaction dqjends on the... 

Notes The nuclear reactions for the production of Iodine Isotopes are listed. For i23Te(p, n) STe Is the target nuclide, (p, n) Is nuclear reaction which Indicates bombarding with proton, with a emitting of neutron, and 23 is produced. In the nuclear reactions, p Is proton n Is neutron 2n means two neutrons, d Is deuterium, a Is alpha particle, Is gamma rays, f Is fission products. EC means decay by electron capture, and (P ) means decay by beta emission. 

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