Atoms positron emission

There are other less common types of radioactive decay. Positron emission results in a decrease by one unit in the atomic number K capture involves the incorporation of one of the extranuclear electrons into the nucleus, the atomic number is again decreased by one unit. 

Notice that the result of K-electron capture is the same as positron emission mass number remains unchanged, whereas atomic number decreases by one unit Electron capture is more common with heavy nuclei, presumably because the n = 1 level is closer to the nucleus. 

Positron emission tomography (PET) makes use of a short-lived positron emitter such as fluorine-18 to image human tissue with a degree of detail not possible with x-rays. It has been used extensively to study brain function (see illustration) and in medical diagnosis. For example, when the hormone estrogen is labelled with fluorine-18 and injected into a cancer patient, the fluorine-bearing compound is preferentially absorbed by the tumor. The positrons given off by the fluorine atoms are quickly annihilated when they meet... 

Considerable interest has been focused on the efficient and rapid synthesis of 2-deoxy-2-[ F]fluoro-D-gIucose, a popular imaging agent for positron-emission tomography (see Section III, 1). However, introduction of a fluorine atom at C-2 by nucleophilic displacement is generally not easy on account of the weak nucleophilic character of the fluoride ion. One possible... 

Like positron emission, electron capture is never observed directly. However, after electron capture, the product atom is missing one of its 1 J electrons, as shown schematically in Figure 22-6b. When an electron from an outer orbital occupies this vacancy in the 1 orbital, a photon is emitted whose energy falls in the X-ray region of the... 

Mysteries such as this attract young people to science. Nuclear physics, however, tends to turn people off Nuclear power plant malfunctions and atomic bombs are frightening. Nevertheless, humankind has greatly benefited from scientific investigations of the nucleus. Science s hard-won knowledge of the atomic nucleus is used extensively in medicine, from imaging procedures such as positron emission tomography (PET) to radiation therapy, which has saved the lives of many cancer patients. 

The strategic placement of an 18F or 19F atom has also enabled positron emission tomography (PET) [7,8] and magnetic resonance imaging (MRI) [9,10] approaches to determine receptor occupancy and perform biodistribution studies, respectively. PET has also been used to study receptor expression patterns and aid in determining efficacious brain exposures and ultimately human efficacious doses. This topic will not be covered here, as it is covered in an earlier chapter in this volume. 

Atoms which are deficient in neutrons tend to decay via positron-emission, whereas those which have a neutron excess decay via -emission. 

Know that nuclear stability is best related to the neutron-to-proton ratio (n/p), which starts at about 1/1 for light isotopes and ends at about 1.5/1 for heavier isotopes with atomic numbers up to 83- All isotopes of atomic number greater than 84 are unstable and will commonly undergo alpha decay. Below atomic number 84, neutron-poor isotopes will probably undergo positron emission or electron capture, while neutron-rich isotopes will probably undergo beta emission. 

An alternative mechanism to positron emission, for the conversion of a proton to a neutron, involves a process known as electron capture (EC) in which the nucleus captures an orbital electron from an inner shell to restore the N P ratio. Subsequently, an electron from another orbital falls into the vacancy left in the inner shell and the energy released in the process is emitted as an X-ray, the atomic number again being reduced by one, e.g. 

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... 

Positron emission and electron capture both give a product nuclide whose atomic number is 1 less than the starting nuclide. Explain. 

Electron emission (ft -) A type of radioactive decay, where a neutron in the nucleus of an unstable atom converts into a proton and releases an electron and an antineutrino (compare with electron capture and positron emission). 

As a result of a positron emission the atomic number of an element decreases by one, whereas the atomic mass remains unchanged... 

Fluorine is a chemical element that in pure form occurs as a dimer of two fluorine atoms, F2. The fluorine atom has the ground state electron configuration ls22s22p5. There is only one stable, naturally occurring isotope of fluorine 19F. However, the radioactive isotopes 17F, 18F, and 20F are known. The inclusion of the isotope 18F (half-life 110 minutes) in bioor-ganic molecules is an important noninvasive technique used in the study of living tissue by positron emission tomography. 

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