Primary radiation-induced reactions

The electron itself is frequently used as a primary source of radiation, various kinds of accelerators being available for that purpose. Particularly important are pulsed electron sources, such as the nanosecond and picosecond pulse radiolysis machines, which allow very fast radiation-induced reactions to be studied (Tabata et al, 1991). Note that secondary electron radiation always constitutes a significant part of energy transferred by heavy charged particles. For these reasons, the electron occupies a central role in radiation chemistry. 

Very primary events in the chemical effect of radiations on matter are excitation and ionization of molecules, which result in the formation of neutral free radicals and radical ions. These reactive species play vital roles in the radiation-induced chemical reactions. As they are paramagnetic with an unpaired electron, electron spin resonance (ESR) spectroscopy has been a useful method for elucidating the mechanism of radiation-induced reactions in solid matter where radical species can be trapped temporarily. Since the early days of the chemical application of ESR, this method has been applied very often to the identification and quantification of free radicals in polymers irradiated by radiation [1]. This is probably because, from the view-point of fundamental research, a variety of free radicals are readily trapped in solid polymers and, from the view-point of applied research, these free radicals have close correlation with radiation-induced crosslinking and degradation of polymers. 

The spatial distribution of the energy loss events of a eharged partiele is usually referred to as a traek. This eoneeptual pieture of a traek is the baekbone of the theoretical description of radiation chemistry. Tracks are considered to have a transitory existence and exist so long as permitted by the dilfusion and fast reactions of radiation-produced intermediates (ions, electrons, and radicals). A large body of radiation-physical and radiation-chemical phenomena requires track models for their elucidation, including (1) LET variation of product yields (2) energy loss in primary excitations and ionizations (3) yield of escaped ions (4) radiation-induced luminescence and (5) particle identification. 

Ionization of atoms or molecules is the main primary event induced by the interaction of radiations with condensed matter. The charged species produced by ionization, if not removed from the irradiated system, will naturally tend to recombine. The conventional theories of recombination treat the transport and reactions of charged species only after the electrons ejected from atoms or molecules become thermalized by dissipating their initially high kinetic energies to the surrounding medium and form a spatial distribution around their parent cations. The thermalization in condensed phases is fast and is usually... 

So far as vinyl monomers are concerned, ionic propagation proceeds with carbonium ions (cationic polymerization) or carbanions (anionic polymerization) at the chain ends. The study of the initiation process of radiation-induced ionic polymerization seeks to elucidate how these ions are formed from the primary ionic intermediates. Possible reactions... 

Hankiewicz E (1995) Hydroxyl radical-induced reactions in polyadenylic acid as studied by pulse radiolysis II. Reactions of primary radicals with oxidants. Bull Pol Acad Sci Chem 43 41-49 Hankiewicz E (1996) Hydroxyl radical-induced reactions in polyadenylic acid as studied by pulse radiolysis - Part III. Consecutive reactions. Radiat Phys Chem 47 61-65 Hankiewicz E (1998) Hydroxyl radical-induced reactions in polyadenylic acid as studied by pulse radiolysis. IV. Reactions of primary radicals with reductants. Bull Pol Acad Sci Chem 46 455-464 Hankiewicz E, Bothe E, Schulte-Frohlinde D (1992) Hydroxyl radical-induced reactions in polyadenylic acid as studied by pulse radiolysis, part. I. Transformation reactions of two isomeric OH-adducts. Free Rad Res Commun 16 391-400... 

In principal, the primary processes induced by ionizing radiation in rigid polymers are the same as those in fluid solutions ionization and excitation are followed by the formation of new reaction intermediates such as ions, excited states, and radicals. However, the reactions of these intermediates may be much affected by the chemical and physical structure of the medium polymer. Besides, a large variety of chemical additives in the polymers make it difficult to understand the early reactions induced by ionizing radiation. 

The intense primary y radiation due to nuclear fission, the secondary y radiation emitted by the fission and activation products and the radiation from the fission products give rise to radiation-induced chemical reactions. The most important reaction is the radiation decomposition of water in water-cooled reactors, leading to the formation of H2, H2O2 and O2. Many substances dissolved in the water influence the formation of H2 (Fig. 11.18). In most closed coolant systems equipment for... 

In the present volume, our purpose is to familiarise the larger communities of students and chemists in other specialities with this relatively little-known but essential domain of chemistry. The covered topics range from the basics (primary phenomena and mechanisms) to the broad fields of their application. Understanding radiation-induced chemical and biochemical reactions is essential for improving existing processes and developing new ones. 

Origin of different nature of both kinds of primary products of radiolysis was found in different size of spurs, as the centers of radiation induced chemical reactions were called. The... 

Complex mechanism of class (ii) and (iii) reactions may account for the puzzling result in the studies on radiation-induced fluorescence in cis- and trans-decalins containing 3-100 mM of benzene [54], where it was concluded that on the time scale of geminate recombination of primary pairs in trans-decalin (< 1 ns), the hole is scavenged by benzene with rate constant of 7.7x1010 g-l (vs. (5-5.5)xl09 M l s-1 observed in the transient conductivity experiments [7,8,12,14]). This was taken as evidence for the involvement of short-lived, reactive excited solvent holes. 

Superoxide dismutases are able to protect cells from damaging effects caused by radiation 3i8-32o> reaction was proposed, in which SOD removes reactive oxygen species, generated by the irradiation and thus preventing the organisms from an attack by primary radicals or their reaction products, respectively Other reports on the radioprotective effect include the induction of superoxide dismutase in irradiated whole organs However, a radiation-induced SOD synthesis was not... 

The magnetic resonance methods are applied to investigate triplet states [1]. To obtain these triplet states in reaction centers of photosynthetic bacteria, the electron transfer has to be blocked by reducing the primary acceptor, an ubiquinone, or by removing it [2]. Microwave radiation induces the spin transitions and the resulting absorption changes of the molecules involved can be detected. 

The radiation-induced degradation mechanism of chitosan in the aqueous HjOj solution is more complicated than that in solid state. Besides the reactions in solid state, the primary reactions might occur as follows (reactions (31.11) through (31.14)) ... 

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