Reactions positron emissions

The first two reactions are found for many of the heavy radioactive nucleides. Alpha emission has been observed also for a number of the neutron-rich nucleides in the rare-earth region. The third reaction, positron emission, occurs for most neutron-rich nucleides, many of which also decompose by electron capture (the fourth reaction). (Electron capture is classed as a spontaneous decomposition because the electrons are always available in the atom for capture it is the s electrons, principally l5, that are captured they are the only electrons with finite probability at the nucleus.) The last two reactions, proton and neutron emission, occur only rarely. 

Applications in agrochemicals [42, 43], pharmaceuticals [44,45], and positron emission tomography (PET) [46, 47, 48 49] have resulted in the resuscitation of the Wallach reaction The Wallach technique provides high-specific-activity F-radiolabeled aromatic fluoride for PET studies, in contrast to the low-specific-ac-tivity product by the Balz-Schiemann route... 

In 2003 LJngstrOm and Samuelsson described the synthesis of a radiopharmaceutical for PET (positron emission tomography) using a microwave-assisted Stille reaction [25]. l-(2 -Deoxy-2 -fluoro- 6-D-arabinofuranosyl)-... 

C22-0094. Two isotopes used in positron-emission imaging are C and O. On which side of the belt of stability are these nuclides located Write the nuclear reactions for their disintegrations. 

C22-0099. Complete the following nuclear reactions (a) positron emission from Si (b) electron capture by... 

Beta (+) or positron emission ( J>+) This type of decay occurs when a nucleus has a greater number of protons than neutrons. In this process, a proton is converted into a neutron by emitting a positive particle known as a (11 particle or positron. The positron is a particle having the mass of an electron but carrying a positive charge. It is sometimes called the antielectron and shown as e+. The reaction can be shown as... 

Make sure that in alpha, beta, gamma, and positron emission the particle being emitted is on the right-hand side of the reaction arrow. In electron capture, the electron should be on the left side of the arrow. 

Electron capture is a particular form of )3 decay, described for the first time by Alvarez (1938) and practically equivalent to positron emission. When the decay energy of the nuclide is not sufficient to attain the reaction... 

At very high temperatures, above 3 or 4 billion k, silicon is consumed so quickly that positron emission and electron capture reactions which might modify the n/p ratio are largely short-circuited. The weak interaction does not have time to convert any appreciable fraction of protons into neutrons during the brief period of thermonuclear combustion. It follows that, starting with matter that is initially dominated by nuclei containing equal numbers of neutrons and protons, such as oxygen-16 and silicon-28, the final products must conserve Z = N, unless they move away from nuclear stability beyond calcium-40, the last stable a element. 

A lunar expedition brought back some moon rocks. Analysis of the rocks showed them to contain 17% potassium-40 and 83% argon by mass. The half-life of K-40 is 1.2 x 10 years. K-40 decays through positron emission. Ar-40 is the decay product of the reaction. How old was the rock sample (in years) ... 

Another important characteristic is that ion beams can produce a variety of the secondary particles/photons such as secondary ions/atoms, electrons, positrons. X-rays, gamma rays, and so on, which enable us to use ion beams as analytical probes. Ion beam analyses are characterized by the respectively detected secondary species, such as secondary ion mass spectrometry (SIMS), sputtered neutral mass spectrometry (SNMS), electron spectroscopy, particle-induced X-ray emission (PIXE), nuclear reaction analyses (NRA), positron emission tomography (PET), and so on. 

Aryltriazenes can also be decomposed by hydrogen fluoride in organic solution after extraction from their aqueous mother phase. In this case, hydrogen fluoride can be used in small excess but the nature of the solvent is crucial for example, tetrahydrofuran gives complex mixtures, dichloromethane promotes radical reactions (dimerizations, reductions) and acetic acid favors triazene decomposition before fluorination. Aromatic and haloaromatic compounds seem to be the best solvents.283 Such a technique, especially suited for the rapid introduction of an 18F atom, has been employed to produce [ 8F]haloperidol (3), the specific receptors of which have been localized in the brain by positron emission transaxial tomography.298... 

Positrons can be produced by either nuclear decay or the transformation of rhe energy of a gamma ray into an electron-positron pair. In nuclei that are proton-rich, a mode of decay that permits a reduction in the number of protons with a small expenditure of energy is positron emission. The reaction taking place during decay is... 

The very short reaction times required for the alkylation of substrate 11a with benzylic bromides using Nobin as an asymmetric phase-transfer catalyst are important for the synthesis of 18F-fluorinated amino adds for use in positron-emission tomography (PET)-imaging studies. Thus, Krasikova and Belokon have developed a synthesis of 2-[18F]fluoro-L-tyrosine and 6-[18F]fluoro-L-Dopa employing a (S)-Nobin-catalyzed asymmetric alkylation of glycine derivative 11a as the key step, as shown in Scheme 8.14 [29]. The entire synthesis (induding semi-preparative HPLC purification) could be completed in 110 to 120 min, which corresponds to one half-life of18 F. Both the chemical and enantiomeric purity of the final amino acids were found to be suitable for clinical use. 

It was shown in Problem 21.7 (third line of mass difference equation) that a positron emission can occur (Q is + and the reaction is spontaneous) only if the nuclidic mass of the parent species exceeds the nuclidic mass of the daughter by at least twice the rest mass of the electron, 2(0.00055) = 0.00110 u. In the present case, the actual mass difference between parent and daughter nuclides is 7.01693 — 7.01600 = 0.00093 u. We can see that positron emission in this case is not possible. This means that 7Be must undergo electron capture. 

Numerous other amino acid decarboxylases have been isolated and characterized, and much interest has been shown as a result of the irreversible nature of the reaction with the release of C02 as the thermodynamic driving force. Although these enzymes have narrow substrate-specificity profiles, their utility has been widely demonstrated. Additional industrial processes will continue to be developed once other decarboxylases become available. Such biocatalysts would include the aromatic amino acid (E.C. 4.1.1.28), phenylalanine (E.C. 4.1.1.53) and tyrosine (E.C. 4.1.1.25) decarboxylases, which likely could be used to produce derivatives of their respective substrates. These derivatives are finding increased use in the development of peptidomimetic drugs and as possible positron emission tomography imaging agents.267-268... 

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