Electron capture, chemical effects

Chemical effects of nuclear decay have been studied in Germanium through the use of Ge and Ge. Ge decays to Ga with a 275 day half-life by 100% electron capture with no y quanta emitted. Ge is a P emitter which decays to As with a 11.3 h half-life, by three jS transitions having maximum energies of 710 keV (23%), 1379 keV (35%) and 2196 keV (42%). From this are calculated maximum recoil energies of 1.7 eV, 4.5 eV and 10.2 eV, respectively. 

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. 

Various mechanisms for electret effect formation in anodic oxides have been proposed. Lobushkin and co-workers241,242 assumed that it is caused by electrons captured at deep trap levels in oxides. This point of view was supported by Zudov and Zudova.244,250 Mikho and Koleboshin272 postulated that the surface charge of anodic oxides is caused by dissociation of water molecules at the oxide-electrolyte interface and absorption of OH groups. This mechanism was put forward to explain the restoration of the electret effect by UV irradiation of depolarized samples. Parkhutik and Shershulskii62 assumed that the electret effect is caused by the accumulation of incorporated anions into the growing oxide. They based their conclusions on measurements of the kinetics of Us accumulation in anodic oxides and comparative analyses of the kinetics of chemical composition variation of growing oxides. 

The second result relates to an electronation procedure in which field emission into liquids provides a source of low energy electrons which are capable of effecting selective chemical changes (Noda et al., 1979). Thus dissociative electron capture by alkyl halides, and electron capture followed by protonation, afford alternative routes to specific spin adducts, as exemplified in Scheme 8. 

The Acid Effect. The possible mechanistic role of hydrogen atoms in the current radiation grafting work becomes even more significant when acid is used as an additive to enhance the copolymerisation. At the concentrations utilised, acid should not affect essentially the physical properties of the system such as precipitation of the polystyrene grafted chains or the swelling of the polyethylene. Instead the acid effect may be attributed to the radiation chemical properties of the system. Thus Baxendale and Mellows (15) showed that the addition of acid to methanol increased G(H2) considerably. The precursors of this additional hydrogen were considered to be H atoms from thermalised electron capture reactions, typified in Equation 5. 

The most suitable and effective method to date that is capable of analyzing several neurosteroids simultaneously is gas chromatography/electron capture-negative chemical ionization/mass spectrometry (GC/EC-NCI/MS) with selected ion monitoring (SIM). This technique allows a focus on a few specific ions bearing structural information. 

For use in geochronology, the decay constant of a radioactive nuclide must be constant and must be accurately known. For a-decay and most (3-decays, the decay constant does not depend on the chemical environment, temperature, or pressure. However, for one mode of 3-decay, the electron capture (capture of K-shell electrons), the decay "constant" may vary slightly from compound to compound, or with temperature and pressure. This is because the K-shell (the innermost shell) electrons may be affected by the local chemical environment, leading to variation in the rate of electron capture into the nucleus. The effect is typically small. For example, for Be, which has a small number of electrons and hence the K-shell is easily affected by chemical environments, Huh (1999) showed that the decay constant may vary by about 1.5% relative (Figure l-4b). Among decay systems with geochronological applications, the branch decay constant of °K to °Ar may vary very slightly (<1% relative). 

Secondary electrons, i.e. those that have been ejected from atoms by incident radiation, will cause further ionisations or excitations until their energy is reduced to — kT, when they are said to be thermalised. They may then be captured by positive ions or neutral molecules. Since all ionising radiations then basically give rise to these secondary electrons, it is to be expected that their chemical effects will be essentially similar. 

On the other hand, Hansen et al. [28] measured A -x-ray intensity ratios for various elements following A -capture decay of radioactive nuclides and pointed out that the KP/Ka ratios by electron capture (EC) decay are considerably different from those by photon and electron impact ionization. Paic and Pecar [29] found that the Kp/Ka ratios for Ti, V, Cr, and Fe by EC are smaller by almost 10% than those by photoionization (PI), but no appreciable difference was observed for Cu and Zn. A similar excitation mode dependence was measured for Mn by Arndt et al. [30]. They stated that the reason for the difference is due to the excess 3d electron in EC and the large shakeoff probability in PI. Rao et al. [31] also observed smaller KP/Ka intensity ratios by EC for Mn and Fe. Since no appreciable difference was found for high-Z elements, they concluded that the difference observed for 3d elements can be ascribed to the chemical effect. It is usual that the chemical forms of the samples for EC measurements are different from those for PI. In order to elucidate the excitation mode dependence on the Kp/Ka ratios in 3d elements, it is necessary to perform theoretical calculations which takes into account the chemical effect as well as the difference in the electron configurations. 

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. 

By electron capture (e) or internal conversion, electrons are taken away from inner orbitals and the vacancies are filled with electrons from outer orbitals with resulting emission of characteristic X rays. Electrons may also be emitted by an internal photoeffect. Finally, at least one electron is missing, and this may also cause breaking of the chemical bond. As electron capture leads also to a change of the atomic number (Z = Z — 1), it is not possible to distinguish the effects due to the capture of an electron from those that are caused by the change of Z. In internal conversion, however, the atomic number is not changed, and the chemical effects observed in this... 

The K0 IKa x-ray intensity ratios by photoionization and electron-capture decay have been calculated for several chemical compounds of 3d elements by the use of the discrete-variational Xo (DV-Xa) molecular orbital method. The calculated results indicate that the K/3lKa ratios depend on the excitation mode as well as the chemical effect. For the similar chemical environments the K0 /Ka ratio by photo ionization is larger than that by electron-capture decay, due to the excess 3d electron in the latter case. However, the difference is small, sometimes negligible in comparison with the chemical effect. Possible reasons for large difference in earlier experiments are discussed and future experiments are suggested. 

Clonazepam, proprietary name Clonopin, is a benzodiazepine with chemical structure closely related to diazepam. The mechanism of action is the same as described for diazepam, but tolerance does not develop as rapidly as with diazepam. Clonazepam is currently approved for use in absence seizures, infantile spasms, akinetic seizures, and Lennox-Gastaut syndrome. Plasma concentrations associated with maximal effectiveness of the drug range from 15 to 60 ng/mL. At concentrations higher than 80 ng/mL, no additional seizure protection is observed, and toxicity (drowsiness and ataxia) ensues. The most suitable methods adaptable to routine analysis are based on GLC with electron capture detection, although HPLC methods also are effective. ... 

Radioactive decay is a statistical process, there being nothing in any nucleus that allows us to predict when it will decay. The probability of decay in a given time interval is the only thing that can be determined, and this appears to be entirely constant in time and (except in the case of electron capture) unaffected by temperature, pressure or the chemical state of an atom. The probability is normally expressed as a half-life, the time taken for half of a sample to decay. Half-lives can vary from a fraction of a second to billions of years. Some naturally occurring radioactive elements on Earth have very long half-lives and are effectively left over from the synthesis of the elements before... 

Tladiation chemists have been aware for about 15 years that the presence of dilute solutes in liquid hydrocarbons can change the course of radiation chemical reactions by other than the normally expected secondary radical reactions. For example, Manion and Burton (40) in early work on the radiolysis of benzene-cyclohexane solutions, drew attention to the possibility of energy transfer from solvent to solute. Furthermore, it is known that in hydrocarbon solvents certain solutes are capable of capturing electrons, thus interfering with the normal ion-recombination process (14, 20, 65, 72). Though ionic products can be observed readily in hydrocarbon glasses [e.g., (19, 21)] demonstration of effects which can be specifically ascribed to electron capture in the liquid state has been elusive until recently. Reaction of positive ions prior to neutralization can play an important role as demonstrated recently by studies on... 

Brookhaven National Laboratories (BNL) has constructed an electron-capture detector capable of direct air sampling. The electron capture detector follows a catalytic reactor that contains H2 and Pd/Molecular Sieve 5A at 140°. This reactor effectively removes oxygen from the air sample stream by forming water. Additionally, chlorofluorocarbons are combusted and reduced to HCl and HF. These acids, along with HjO, are removed from the sample stream via a desiccant. However, such gases as SFg and per-fluorinated cyclic aliphatics [perfluorodimethyl-cyclobutane (PDCB), perfluoromethylcy do hexane (PMCH), and perfluorodimethylcyclohexane (PDCH)] survive the reactor and are detected via an electron capture device. These chemical compds survive due to their unusual thermal stability. BNL has demonstrated a lower limit of detection of these compds with their breadboard instrument (on a continuous sampling mode) of 0.1 parts per trillion (Ref 12)... 

Careful selection of the derivatization technique may also be employed to enhance the detection limit of an analyte. The conversion of amphetamines to tetrafluorophthaloyl derivatives yields compounds possessing high electron capture cross-sections and the sensitivity toward such compounds, using negative ion chemical ionization (NICI) can be better than that achieved using positive ion formation by two orders of magnitude. Employing NICI in conjunction with SIM, the detection of amphetamine derivatives in quantities as small as 10 fg (SOamol) may be effected. 

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