Radiative capture

VOI83] J. Voignier, S. Joly, and G. Grenier, "Mesure De La Section Efficace De Capture Radiative Du Lanthane, Du Bismuth, Du Cuivre Naturel et De Ses Isotopes Pour Des Neutrons D Energie Comprise Entre 0,5 et 3 MeV," Proc. Int. Conf. Nuclear Data for Science and Technology, Antwerp, Belgium (1983), p. 759. 

In this reaction scheme, CH4 is produced by two steps of radiative association with slow rate constants. Because the destruction of C by electron capture (radiative) is four orders of magnitude slower than the destruction of a molecular ion by dissociative recombination, there is not a rapid loss of C , allowing production of a saturated hydrocarbon. We recall that chemical equilibrium arguments predict preponderant conversion of carbon monoxide to methane and water. There is little evidence for this, as stated earlier. The gas-phase production of CH4 from CO and H2 then proceeds by a very high-energy kinetic path, namely He + CO = C +... 

Stable particles in sufficient number, all the oligo-radi-cals and nuclei generated in the continuous phase are captured by the mature particles, no more particles form, and the particle formation stage is completed. The primary particles formed by the nucleation process are swollen by the unconverted monomer and/or polymerization medium. The polymerization taking place within the individual particles leads to resultant uniform microspheres in the size range of 0.1-10 jjLvn. Various dispersion polymerization systems are summarized in Table 4. 

In this group the most commonly used reaction is that of radiative neutron capture. Also here are to be found (n, 2 n) and (y, n) reactions, although very few studies have been done with these reactions, and isomeric transitions (although these may often be more profitably discussed along with electron capture reactions). 

Photoionization refers to the ionization of a neutral entity. Its inverse is the radiative capture of a free electron. By virtue of detailed balancing, the cross section of radiative capture,cr, and that of photoionization, <7., involving the same initial and final states, i and f, are related by... 

Neutrons may collide with nuclei causing one of the following reactions inelastic scattering, elastic scattering, radiative capture, or fission. 

Radiative capture (n, y) takes place when a neutron is absorbed to produce an excited nucleus. The excited nucleus regains stability by emitting a gamma ray. 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

The excess free carriers (and excitons) do not represent stable excited states of the solids. A fraction of them recombine directly after thermahzation either radiatively or by multiphonon emission. In most materials, nonradiative transitions to defect states in the gap are the dominant mode of decay. The lifetime of free carriers T = 1/avS is determined by the density a of recombination centers, their thermal velocity v, and the capture cross section S, and may span 10-10 s. Electrons, captured by states above the demarcation level, and holes, captured by states below the hole demarcation level, may get trapped. The condition for trapping is given when the occupied electron trap has a very small cross section for recombining with a free hole. The trapping process has, until recently, not been well understood. 

It appears at this time that one of the most important mechanisms involved in the luminescence of rare earth ions is energy exchange between them. One may clearly differentiate between two distinct mechanisms (a) radiative exchange and (b) nonradiative exchange. In the radiative mechanism, a photon emitted by ion A is captured by ion B. Since the photon has left the A system, the capture of it by B cannot decrease the lifetime of A. However, f the photon is shuttled back and forth between similar or dissimilar ions, the fluorescent lifetime could well be increased by radiation trapping. This is an interesting phenomenon and warrants further discussion. 

Cabezas and DeShazer (43) studied terbium lifetimes in Hughes borosili-cate glass. Their work was concerned with radiative transfer of energy between rare earth ions. That is, they were interested in finding photon-capture systems that would be useful for possible laser applications. They... 

These primordial nucleosynthesis reactions began with the production of deuterium by the simple radiative capture process ... 

Radiative capture, 4(n, y)A + 1. As discussed earlier, this cross section shows a 1/v energy dependence, and this process is important for low-energy neutrons. 

The measurement of neutron fluxes by foil activation is more complicated because the neutrons are not monoenergetic and the monitor cross sections are energy dependent. The simplest case is monitoring slow neutron fluxes. Radiative capture (ivy) reactions have their largest cross sections at thermal energies and are thus used in slow neutron monitors. Typical slow neutron activation detectors are Mn, Co, Cu, Ag, In, Dy, and Au. Each of these elements has one or more odd A isotopes with a large thermal (n,y) cross section, 1-2000 barns. The (n,y)... 

When A and B are identical atomic species (e.g., H + -H259 or He+-He260), radiative emissions may result from neutral target excitation as well as from electron capture by the ionic projectile into the same excited state. Both processes yield Lyman a radiation in the case of H+-H collisions,259... 

The basic spontaneous radiative capture reaction (8.11) was first suggested as a source of antihydrogen by Budker and Skrinsky (1978) in it the photon carries away the excess energy. The matter equivalent of... 

Fig. 8.9. Schematic energy diagrams illustrating recombination mechanisms. The ionization continuum is shown shaded, (a) Spontaneous radiative capture, reaction (8.11) (b) stimulated radiative capture by irradiation with laser light, reaction (8.12) (c) three-body recombination in which the excess energy is removed by an extra positron, reaction (8.14).

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