Atoms electron capture

The presence of heteroatoms usually provides a convenient feature for improving selectivity by employing selective detection mechanisms. GC may then use flame photometric detection (FPD) for S and P atoms and to a certain extent for N, Se, Si etc. thermoselective detection (TSD) and nitrogen-phosphorus detection (NPD) for N and P atoms electron capture detection (ECD) for halogen atoms (E, Cl, Br, and 1) and for systems with conjugated double bonds and electron-drawing groups or atomic emission detection (AED) for many heteroatoms. 

In the laboratory capture process, any of the various electron shells contribute to the capture rate however the K-shell gives the dominant contribution. At temperatures inside the sun, e.g. 1, = 15, nuclei such as 7Be are largely ionized. The nuclei however are immersed in a sea of free electrons resulting from the ionization process and therefore electron capture from continuum states is possible (see e.g., [45], [46]). Since all factors in the capture of continuum electrons in the sun are approximately the same as those in the case of atomic electron capture, except for the respective electron densities, the 7Be lifetime in a star, ts is related to the terrestrial lifetime rt by ... 

A Dalton is defined as l/12tli of the mass of a C atom. It differs from an atomic mass unit (amu), which is defined as l/16th of the mass of a atom. Electron capture dissociation Electron detachment dissociation Electron-induced dissociation... 

As a common consequence of any interaction of nuclear radiation with matter, electron vacancies are created in the K, L, M shells of the atoms. Radioactive decay can also create vacancies in the daughter atoms (electron capture, internal conversion). Electron vacancies can cause X-ray transitions or - as shown by Auger (1925) - the vacancy is filled at the expense of a shell electron emission with the energy... 

Paradoxically, the gas-density detector, which can supply the molecular mass of an unknown, can be considered to be a selective detector in gas chromatography (58). This comment effectively illustrates the dearth of convenient selective detectors. At best such detectors help to identify compounds that contain halogen atoms (electron-capture or thermoionic detectors), sulfur, nitrogen, or phosphorus (flame-photometric or thermoionic detectors). Physiological detectors have also been used in certain rare cases (insects that react to sexual pheromones, for example, or the chemist s nose, a dangerous and hazardous application). 

In almost all cases X is unaffected by any changes in the physical and chemical conditions of the radionucHde. However, there are special conditions that can influence X. An example is the decay of Be that occurs by the capture of an atomic electron by the nucleus. Chemical compounds are formed by interactions between the outer electrons of the atoms in the compound, and different compounds have different electron wave functions for these outer electrons. Because Be has only four electrons, the wave functions of the electrons involved in the electron-capture process are influenced by the chemical bonding. The change in the Be decay constant for different compounds has been measured, and the maximum observed change is about 0.2%. 

For any nucHde that decays only by this electron capture process, if one were to produce an atom in which all of the electrons were removed, the effective X would become infinite. An interesting example of this involves the decay of Mn in interstellar space. For its normal electron cloud, Mn decays with a half-life of 312 d and this decay is by electron capture over 99.99% of the time. The remaining decays are less than 0.0000006% by j3 -decay and a possible branch of less than 0.0003% by /5 -decay. In interstellar space some Mn atoms have all of their electrons stripped off so they can only decay by these particle emissions, and therefore their effective half-life is greater than 3 x 10 yr. 

There are four modes of radioactive decay that are common and that are exhibited by the decay of naturally occurring radionucHdes. These four are a-decay, j3 -decay, electron capture and j3 -decay, and isomeric or y-decay. In the first three of these, the atom is changed from one chemical element to another in the fourth, the atom is unchanged. In addition, there are three modes of decay that occur almost exclusively in synthetic radionucHdes. These are spontaneous fission, delayed-proton emission, and delayed-neutron emission. Lasdy, there are two exotic, and very long-Hved, decay modes. These are cluster emission and double P-decay. In all of these processes, the energy, spin and parity, nucleon number, and lepton number are conserved. Methods of measuring the associated radiations are discussed in Reference 2 specific methods for y-rays are discussed in Reference 1. 

Electron Capture and /5" "-Decay. These processes are essentially the inverse of the j3 -decay in that the parent atom of Z andM transmutes into one of Z — 1 andM. This mode of decay can occur by the capture of an atomic electron by the nucleus, thereby converting a proton into a neutron. The loss of one lepton (the electron) requires the creation of another lepton (a neutrino) that carries off the excess energy, namely Q — — Z(e ), where the last term is the energy by which the electron was bound to the atom before it was captured. So the process is equivalent to... 

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. 

FIGURE 17.9 In electron capture, a nucleus captures one of the surrounding electrons. The effect is to convert a proton (outlined in red) into a neutron (outlined in blue). As a result, the atomic number decreases by 1 but the mass number remains the same. 

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... 

Janak K, G Becker, A Colmsjo, C Ostman, M Athanasiadou, K Valters, A Bergman (1998) Methyl sulfonyl polychlorinated biphenyls and 2,2-bis(4-chlorophenyl)-l,l-dichloroethene in gray seal tissues determinated by gas chromatography with electron capture detection and atomic emission detection. Environ Toxicol Chem 17 1046-1055. 

The nuclear decay of radioactive atoms embedded in a host is known to lead to various chemical and physical after effects such as redox processes, bond rupture, and the formation of metastable states [46], A very successful way of investigating such after effects in solid material exploits the Mossbauer effect and has been termed Mossbauer Emission Spectroscopy (MES) or Mossbauer source experiments [47, 48]. For instance, the electron capture (EC) decay of Co to Fe, denoted Co(EC) Fe, in cobalt- or iron-containing compormds has been widely explored. In such MES experiments, the compormd tmder study is usually labeled with Co and then used as the Mossbauer source versus a single-line absorber material such as K4[Fe(CN)6]. The recorded spectrum yields information on the chemical state of the nucleogenic Fe at ca. 10 s, which is approximately the lifetime of the 14.4 keV metastable nuclear state of Fe after nuclear decay. 

The rate of electron accumulation at ionized traps in the depletion zone of the Schottky barrier in the Au/ZnO contact is in proportion to the concentration of unoccupied traps, frequency of metal parti-cle/metastable atom interaction events, and to the probability of electron capture per a trap in a single event of interaction between metastable atoms and metal particle. 

Capture, K-Electron—Electron capture from the K shell by the nucleus of the atom. Also loosely used to designate any orbital electron capture process. 

Perhaps the most fruitful of these studies was the radiolysis of HCo(C0)4 in a Kr matrix (61,62). Free radicals detected in the irradiated material corresponded to processes of H-Co fission, electron capture, H-atom additions and clustering. Initial examination at 77 K or lower temperatures revealed the presence of two radicals, Co(C0)4 and HCo(C0)4 , having similar geometries (IV and V) and electronic structures. Both have practically all of the unpaired spin-density confined to nuclei located on the three-fold axis, in Co 3dz2, C 2s or H Is orbitals. Under certain conditions, a radical product of hydrogen-atom addition, H2Co(C0)3, was observed this species is believed to have a distorted trigonal bipyramidal structure in which the H-atoms occupy apical positions. 

HC1 is formed by a chain mechanism with a very high yield (M /N value —103), presumably because of the high reactivity of the Cl atom. The chain is initiated by the production of H and Cl atoms. Dissociative electron capture by Cl2 requires about 1.6 eV energy. Therefore, presumably both kinds of ions are initially produced by excitation and ionization. The chain is propagated simply as follows ... 

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