Radionuclides electron capture

The emission of y rays follows, in the majority of cases, what is known as P decay. In the P-decay process, a radionuclide undergoes transmutation and ejects an electron from inside the nucleus (i.e., not an orbital electron). For the purpose of simplicity, positron and electron capture modes are neglected. The resulting transmutated nucleus ends up in an excited nuclear state, which prompdy relaxes by giving offy rays. This is illustrated in Figure 2. 

Cyclotrons and accelerators are sources of charged particles (i.e., protons, deuterons, a particles, etc.), and the radionuclides produced are generally proton rich and decay by positron emission and/or electron capture. A positive ion beam is eventually extracted from the cyclotron, regardless of whether positive or negative ions were accelerated. The isotope of interest is separated from the target for use in chemical syntheses. Accelerator- or cyclotron-produced radioisotopes tend to be the most expensive as only one radionuclide is produced at a time. 

All astatine isotopes, with the exception of At, produce other radionuclides by their decay, consequently complicated decay curves can arise. In astatine isotopes, electron capture (EC) always produces -radiation, h. Hours min, minutes s, seconds. 

Fluorine-18 is an artificial radionuclide, discovered in 1937. It decays with a half-life of 109.8 min for 97% by positron emission and for 3% by electron capture to the stable isotope oxygen-18. The maximum jS+-particle energy is 0.635 MeV [4],... 

The stabilization process can proceed by several different processes, such as spontaneous fission, a-particle emission, -particle emission, positron emission, y-ray emission, or electron capture. In all decay processes the mass, energy, and charge of radionuclides must be conserved, and many nuclides can decay by a combination of any of the above-mentioned processes. 

The radioiodine 123I, on the other hand, is very useful in nuclear medicine because it has good radiation characteristics for scintigraphy, such as decay by electron capture, a half-life of 13 h, and y emmision of 159 keV. However, the much shorter half-life, together with the more complex radionuclide production, makes this radionuclide less available and more expensive compared to 131I. 

In modem machines, protons, deuterons and a particles with energies of several 100 MeV up to about 1 GeV are available. Proton linacs serve frequently as injectors of 50 to 200 MeV protons into proton synchrotrons. For the production of radionuclides, relatively small cyclotrons are used by which particle energies of the order of 10 to 30 MeV and ion currents of the order of 100 pA are available. Radionuclides obtained by reactions with protons exhibit decay or electron capture (s). 

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. 

A range of radionuclides was chosen including pure beta emitters, beta/gamma emitters and electron capture radionuclides. These radionuclides were used to prepare single nuclide sets of calibration standards with a matrix typical of that routinely encountered (Table 1). The quench level was varied by altering the ratio of aqueous fraction to cocktail whilst maintaining the total volume of liquid in the vial. The set of calibration standards were dark-adapted and then counted to determine the measurement efficiency over a range of quench levels. 

Figure 1 Measured predicted efficiencies for radionuclides decaying by electron capture...

Radionuclides are unstable due to the unsuitable composition of neutrons and protons or excess energy and, therefore, decay by emission of radiations such as a particles, > particles, / + particles, electron capture, and isomeric transition. 

When a radionuclide is proton rich, but has energy less than 1.022 MeV, then it decays by electron capture. In the process, an electron from the nearest... 

Note that when the transition energy is less than 1.022 MeV, the radionuclide definitely decays by electron capture. However, when the transition energy is more than 1.022 MeV, the radionuclide can decay by positron emission and/or electron capture. The greater the transition energy above 1.022 MeV, the more likely the radionuclide will decay by positron emission. Some examples of radionuclides decaying by electron capture are ... 

A radionuclide may decay by a, (3(3+ emissions, or electron capture to different isomeric states of the product nucleus, if allowed by the rules of quantum physics. Naturally, these isomeric states decay to lower isomeric states and finally to the ground states of the product nucleus, and the energy differences appear as 7-ray photons. 

What types of radionuclides would decay by f3 and / + emission and electron capture ... 

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. 

Electron capture (EC). A mode of decay of a proton-rich radionuclide in which an orbital electron is captured by the nucleus, accompanied by emission of a neutrino and characteristic X-rays. 

Neutrino (v). A particle of no charge and mass emitted with variable energy during / + and electron capture decays of radionuclides. 

Besides the generators described above there are X-ray sources based on radioactive materials to provide the excitation of the sample. The advantage of using these materials is that an isotope can be selected to provide a mono-energetic beam of radiation that is optimized for the specific application. One method consists to select a radionuclide that is transformed by internal electron capture (lEC). This mode of decomposition corresponds to the transition of one level-K electron into the nucleus of the atom. For a nuclide X, the phenomenon is summarized as follows ... 

Some nuclei undergo radioactive decay by capturing an electron from the A or L shell of the atomic electron orbits. This results in the transformation of a proton to a neutron, the ejection of an unobservable neutrino of definite energy, and the emission of an x-ray where the electron vacancy of the or L shell is filled by an atomic electron from an outer orbit. Because the net change in the radionuclide species is from atomic number Z to Z — 1, similar to the nuclide change from positron emission, electron capture generally competes with all cases of positron beta decay. 

Several radionuclides of copper are suitable for labeling mAbs, their fragments, or peptides for PET (e.g., Cu, Cu, or " Cu Table 6.7-2) [216]. " Cu decays with a half-life of 12.7 hours by three different pathways (1) electron capture (41%) emitting Auger and conversion electrons, (2) positron (P+) emission (19%), and (3) P emission (40%). The P emissions make the radionuclide useful not only for PET but also for radiotherapy of tumors. Indeed, in one study, " Cu-labeled octreotide inhibited the growth of CA20948 rat pancreatic tumors in Lewis rats [217]. In another study, Cu-labeled 1A3 mAbs were used to treat hamsters implanted s.c. 

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