Radiation absorption reactions

In the preceding ten chapters of this book, we have described various important chemical and physical changes brought about by the absorption of ionizing radiation in gaseous and condensed media. Wherever possible, we have tried to elucidate the underlying mechanism with a discussion of the properties and reactivities of the intermediate species. However, the book would remain incomplete without discussion of some of the various uses that have been found for radiation-induced reactions in science and industry. 

RADIATION-SENSITIVE GROUPS. Although the absorption of radiation energy is dependent only on the electron density of the substrate and therefore occurs spatially at random on a molecular scale, the subsequent chemical changes are not random. Some chemical bonds and groups are particularly sensitive to radiation-induced reactions. They include COOH, C-Hal, -SO2-, NHz, C=C. Spatial specificity of chemical reaction may result from intramolecular or intermolecular migration of energy or of reactive species -free radicals or ions. 

Lewis wrote to Paul Ehrenfest that he thought he had "hit upon something pretty fundamental." But Lewis had difficulty publishing his paper, "On the Theory of Reaction Rate," which the editor and referees of the Journal of the American Chemical Society worried was too speculative and insufficiently oriented toward experimental verification.96 Lewis ended by rejecting the radiation hypothesis, even issuing a press release to announce that a decision now had been made between the two rival theories of violent collision and radiation absorption in favor of the former. 97... 

The two preceding methods have been combined to determine the tritium isotope effect. In a tritium-labeled substrate in D20, the change in infrared radiation absorption arises almost entirely from release of protons because the concentration of the tritium species in the reactant is small. Thus, the rate constant kH (determined by the change in the DOH absorption) represents release of protons. The constant kT for release of tritium to the solvent is determined from radioactivity measurements of water from the same reaction mixture. In the enolization of... 

The FDS5 pyrolysis model is used here to qualitatively illustrate the complexity associated with material property estimation. Each condensed-phase species (i.e., virgin wood, char, ash, etc.) must be characterized in terms of its bulk density, thermal properties (thermal conductivity and specific heat capacity, both of which are usually temperature-dependent), emissivity, and in-depth radiation absorption coefficient. Similarly, each condensed-phase reaction must be quantified through specification of its kinetic triplet (preexponential factor, activation energy, reaction order), heat of reaction, and the reactant/product species. For a simple charring material with temperature-invariant thermal properties that degrades by a single-step first order reaction, this amounts to -11 parameters that must be specified (two kinetic parameters, one heat of reaction, two thermal conductivities, two specific heat capacities, two emissivities, and two in-depth radiation absorption coefficients). 

The observation that the yields of the major products are linear and less than linear shows that they are initial, not secondary, products, in the sense that they are not formed by radiolysis of products or by reaction with chemicals formed by the radiolysis. There is no proof of their direct formation in the initial event of radiation absorption, but they were the major products in the lower dose that made analysis possible. To prove further that the trimers and tetramers are initial products, the radiolysis of 10 M DFBP solution in C6F6 was studied. The yields of trimers and tetramers were the same without DFBP, indicating that they are not formed by the reaction with DFBP. 

It is rather atypical that a photochemical reaction will proceed in a single molecular pathway. Thus, several elementary steps are involved. Normally, the majority of them are dark (thermal) reactions while, ordinarily, one activation step is produced by radiation absorption by a reactant molecule or a catalyst. From the kinetics point of view, dark reactions do not require a different methodological approach than conventional thermal or thermal-catalytic reactions. Conversely, the activation step constitutes the main distinctive aspect between thermal and radiation activated reactions. The rate of the radiation activated step is proportional to the absorbed, useful energy through a property that has been defined as the local volumetric rate of photon absorption, LVRPA (Cassano et ak, 1995 Irazoqui et al., 1976) or the local superficial rate of photon absorption, LSRPA (Imoberdorf et al., 2005). The LVRPA represents the amount of photons that are absorbed per unit time and unit reaction volume and the LSRPA the amount of photons that are absorbed per unit time and unit reaction surface. The LVRPA is a property that must be used when radiation absorption strictly occurs in a well-defined three-dimensional (volumetrical) space. On the other hand, to... 

Indirect Photoreactions. Enhancements of halocarbon photoreaction rates are not limited to compounds such as DDE and DCB, which directly absorb solar radiation. Such effects have also been observed with hah ocarbons that have little or no absorption of solar radiation. These reactions most likely occur through indirect mechanisms, in which the radiation is absorbed by natural chromophores. In this section, we provide evidence for such indirect photoreactions. Then, in subsequent sections, we discuss possibk mechanisms for indirect photoreactions of halocarbons. 

The third strategy for minimizing NOx, known as posttreatment, involves removing NOx from the exhaust gases after the NOx has already been formed in the combustion chamber. Two of the most common methods of posttreatment are selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR).7 Wet techniques for posttreatment include oxidation/absorption, oxidation/absorption/reduc-tion, absorption/oxidation, and absorption/reduction. Dry techniques for posttreatment, besides SCR and SNCR, include activated carbon beds, electron beam radiation, and reaction with hydrocarbons. 

Alessi et al. (2005) examined the y-irradiation and electron-beam processing of an epoxy-resin system in the presence of a photoinitiator. They showed that increasing the irradiation dose frequency and photoinitiator concentration greatly increases the temperature reached by the samples. The increase in temperature of the system during irradiation is related to the balance between the heat-evolution rate, due to both polymerization reaction and radiation absorption, and the rate of heat release towards the environment. High dose rates and high photoinitiator concentmtions increase the reaction rate and the difference between the heat produced and the heat released to the environment (Alessi et al. (2005). 

Apart for exothermic curing reactions, another thermal effect has to be considered, due to the interaction of ionizing radiation with matter. The temperature profile depends on the balance among (on one hand) the rate of heat production, due both to curing reactions and radiation absorption, and (on the other hand) the heat released, in unit of time, by the system toward the environment. Taking constant the geometry of the reacting system, the heat released toward the environment is constant, while the heat production increases with the pulse frequency. 

The condensed phase density p, specific heat C, thermal conductivity A c, and radiation absorption coefficient Ka are assumed to be constant. The species-A equation includes only advective transport and depletion of species-A (generation of species-B) by chemical reaction. The species-B balance equation is redundant in this binary system since the total mass equation, m = constant, has been included the mass fraction of B is 1-T. The energy equation includes advective transport, thermal diffusion, chemical reaction, and in-depth absorption of radiation. Species diffusion d Y/cbfl term) and mass/energy transport by turbulence or multi-phase advection (bubbling) which might potentially be important in a sufficiently thick liquid layer are neglected. The radiant flux term qr... 

Fig. 27 Intrinsic stability map for homogeneous solid with quasi-steady gas and surface reaction, negligible radiation absorption = 0) or surface absorption of radiation (fr = 0).

Absorptive reactions are (n, a), (n, p), in,y), or (n, fission). In the case of an (n, y) reaction, the neutron may be detected through the interactions of the gamma emitted at the time of the capture, or it may be detected through the radiation emitted by the radioisotope produced after the neutron is captured. The radioisotope may emit /3 or or y or a combination of them. By counting the activity of the isotope, information is obtained about the neutron flux that produced it. This is called the activation method. If the reaction is fission, two fission fragments are emitted being heavy charged particles, these are detected easily. 

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