Radiation chemistry irradiation

The radiation chemistry has been mainly discussed in terms of degradation reactions (as above) involving the loss of gaseous products and the irreversible change of the stoichiometry [203]. However, more recent results showed that polymers irradiated with radiation deposit-... 

Detailed studies on radiation chemistry of PEO have been performed [74-77]. Upon y-irradiation, the gel-dose drops abruptly along with an increase in the concentration and molecular weight of the polymer, thus reaching values of 0.15-0.25 Mrad in the range of practical interest [75]. Oxygen is a strong inhibitor and when it is carefully removed from the solution, crosslinking of PEO occurs at doses as low as 0.01 Mrad [76]. 

Evidence indicates [28,29] that in most cases, for organic materials, the predominant intermediate in radiation chemistry is the free radical. It is only the highly localized concentrations of radicals formed by radiation, compared to those formed by other means, that can make recombination more favored compared with other possible radical reactions involving other species present in the polymer [30]. Also, the mobility of the radicals in solid polymers is much less than that of radicals in the liquid or gas phase with the result that the radical lifetimes in polymers can be very long (i.e., minutes, days, weeks, or longer at room temperature). The fate of long-lived radicals in irradiated polymers has been extensively studied by electron-spin resonance and UV spectroscopy, especially in the case of allyl or polyene radicals [30-32]. 

Radiation cross-linking of PVC in the absence of additives has been reported by several authors [20,21,267,268]. PVC by itself is not readily cross-linkable by EB radiation. In the study of radiation chemistry of PVC, it is known that the oxidation takes place in the presence of atmospheric oxygen during irradiation [269] and hydroperoxide groups are produced on oxidation. The formation of carbonyl group along with the elimination of HCl from PVC on irradiation in air has also been reported [270,271]. 

This definition of electrochemistry disregards systems in which nonequilibrium charged species are produced by external action in insulators for example, by electric discharge in the gas phase (electrochemistry of gases) or upon irradiation of liquid and sohd dielectrics (radiation chemistry). At the same time, electrochemistry deals with certain problems often associated with other fields of science, such as the structure and properties of sohd electrolytes and the kinetics of ioific reactions in solutions. 

Chemical effects from the absorption of charged-particle irradiation were observed almost immediately following the discoveries of X-rays and the electron in the last decade of the nineteenth century. The field, though, remained unnamed until 1942, when Milton Burton christened it radiation chemistry. At present, it has developed into a vigorous discipline embracing radiation physics on one hand and radiation biology on the other. The purpose of this book is to give a coherent account of the development of this field with stress on the fundamental aspects. 

The interaction of neutrons with organic molecules occurs mainly through knock-on of protons. Thus, the radiation chemistry is similar to proton irradiation. Radiation chemistry by positive ions is of increasing importance on account of ion implantation technology, plasma development and deposition processes, and cosmic irradiation. 

The U.S. - Australia Symposium on Radiation Effects on Polymeric Materials contained research presentations on fundamental radiation chemistry and physics as well as on technological applications of polymer irradiation. This paper represents a hybrid contribution of these two areas, examining a field of extensive technological importance. Spin casting of radiation sensitive polymer resists for microelectronic fabrication was studied using photophysical techniques that are sensitive to the fundamental radiation response in the ultraviolet range. 

X-ray diffraction, 39 160-161, 164-165 Radial factor, 22 214-218 Radiation, see also Irradiation cosmic, see Cosmic radiation terrestial origin, 3 288-293 Radiation chemistry, heterogeneous kinetics of, 3 198-203... 

Lind [2] has defined radiation chemistry as the science of the chemical effects brought about by the absorption of ionizing radiation in matter. It should be distinguished from radiation damage which refers to structural transformation induced by irradiation, particularly in the solid state. The distinction is not always maintained, perhaps unconsciously, and sometimes both effects may be present simultaneously. Following a suggestion of M. Curie around 1910, that ions were responsible for the chemical effects of radioactive radiations, the symbol MjN was introduced to quantify the radiation chemical effect, where M is the number of molecules transformed (created or destroyed) and N is the number of ion pairs formed. Later, Burton [3] and others advocated the notation G for the number of species produced or destroyed per 100 eV (= 1.602 x 10 J) absorption of ionizing radiation. It was purposely defined as a purely experimental quantity independent of implied mechanism or assumed theory. 

The irradiation of water is immediately followed by a period of fast chemistry, whose short-time kinetics reflects the competition between the relaxation of the nonhomogeneous spatial distributions of the radiation-induced reactants and their reactions. A variety of gamma and energetic electron experiments are available in the literature. Stochastic simulation methods have been used to model the observed short-time radiation chemical kinetics of water and the radiation chemistry of aqueous solutions of scavengers for the hydrated electron and the hydroxyl radical to provide fundamental information for use in the elucidation of more complex, complicated chemical, and biological systems found in real-world scenarios. 

Some time ago, an allyl-like radical was observed in irradiated crystals of 5 -dCMP [26]. This radical was thought to be a sugar radical, although no likely scheme was proposed for its formation. It now appears that this radical is formed on 5-methyl cytosine impurities in these crystals [27]. This radical forms by deprotonation at the methyl group of the cytosine cation, 5meCyt(Me—H) , and may have important consequences in the radiation chemistry of DNA since the ionization potential of 5-methyl cytosine is lower than that of either cytosine or thymine. 

There has been continued interest in the radiation chemistry of the purines since early reports on oriented DNA by Graslund et al. [35] which suggest that the main trapping site of one-electron oxidation in DNA is the guanine base. It is remarkable that in aqueous solution, the electron adducts of the purine nucleosides and nucleotides undergo irreversible protonation at carbon with a rate constant 2 orders of magnitude higher than that for carbon protonation of the electron adduct in thymidine [36]. It is therefore important to know the properties of the various purine reduction products and to ask why they have not been observed in irradiated DNA. 

The previous section outlined the typical e loss and e gain products observed in the nucleic acid bases in the solid state. These studies can be applied to the study of the radiation chemistry of DNA. The relevance of the study of model systems is shown by considering the following remarkable observations. Years ago, Ehrenberg et al. showed the EPR spectra of the 5,6-dihydrothymine-5-yl radical observed in thymine, thymidine, and DNA. The spectra are nearly identical [46]. The reduction product observed in cytosine monohydrate is the N3 protonated anion. In solution, this reduction product gives rise to a 1.4-mT EPR doublet. The same feature is present in irradiated DNA at 77 K. Likewise, the result of e loss in guanine bases is characterized by a broad EPR singlet. The same feature is also evident in the EPR spectrum of DNA irradiated and observed at 77 K. 

Photochemistry and radiation chemistry of biomolecules in vacuo and in water. Radiolytic products from Auger cascade are interesting from the viewpoint of radiotherapy. The development of irradiation systems is required in which liquid samples can be irradiated with vacuum UV or ultrasoft x-rays of high intensity. 

Because of the multivalent nature of the actinide ions, understanding the radiation-induced change of the valence-state of the actinide in solutions under self-irradiation or external irradiation is a challenge in radiation chemistry. Some of the ions are strong a-emitters. It is also important from a practical viewpoint that the solution chemistry of actinide ions is closely related to the storage and the repository of the wastes. Much work combined with experiment and simulation has been conducted and reviews were summarized [136,140-144]. 

By its very nature this book is interdisciplinary. The first eleven chapters delineate the fundamentals of radiation physics and radiation chemistry that are common to all irradiation effects. Chapters 12 and 13 deal with specific liquid systems, while Chapter 14 is concerned with LET effects. Chapters 15 to 18 describe biological and medical consequences of photon and charged-particle irradiation. The rest of the book is much more applied in character, starting with irradiated polymers in Chapter 19 and ending with applications of heavy ion impact in Chapter 27. 

This book is not intended to give a complete survey of polymer irradiation. For this, we have to refer to such excellent texts as Atomic Radiation of Polymers by Charlesby and Radiation Chemistry of Polymeric Systems by Chapiro. Since these books were issued in 1961 and 1962, more experimental research has been done and more theories have been developed by these authors, who are also the authors of the first two chapters of this volume. New trends have been discovered, and more light has been shed on polymer irradiation by the authors of the following 18 chapters. Thanks to their work and efforts, polymer irradiation is making inroads into the plastic and related industries. 

Work along these lines could give a better understanding of the role played by ions in the radiation chemistry of polymers. The fact that ions can induce chemical reactions in irradiated organic solids is clearly demonstrated by the polymerization of certain crystalline monomers which are known to polymerize only when treated with ionic catalysts (3). 

One of the most pressing problems in the radiation chemistry of high polymers is to obtain definite proof of one or the other of the mechanisms proposed for the interoonversion of alkyl and allyl free radicals. Such information might also be of great help in understanding other effects produced by irradiating high polymers. 

T wo aspects of the radiation chemistry of polyethylene terephthalate (PET) are reviewed here the dependence of product yields on radiation dose and on dose rate. The review is limited to work with thin films from which air and water were pumped prior to irradiation. Moreover, it is judged that in the experiments described postirradiation effects were negligible. 

Poly (4-methyl-1-pentene). Poly(4-methyl-l-pentene) has not yet drawn much attention in radiation chemistry. As far as we know, only one study on high energy-irradiated poly(4-methyl-l-pentene) has been published (25), and this was in the form of a short communication. The ESR spectrum at liquid nitrogen temperature was a sextet with a hyper-fine splitting constant of 23 gauss. The radicals producing this spectrum were supposed to have structure XXI—i.e., radicals formed by side-chain scission. 

ESR Spectroscopy. Electron Spin Resonance spectroscopy is an important technique for investigating the role of radical intermediates in radiation chemistry. The technique has been used widely for many years in the study of radicals occurring in irradiated solid polymers (.6,7). However, by their very nature, such species are reactive and may only exist in low concentration. The identification of these species can also be a problem since in the majority of polymers the environment of the radicals leads to broad, unresolved ESR spectra, which makes detailed spectral analysis difficult. In recent years, many of these problems of sensitivity and resolution have been reduced by more sensitive and stable ESR spectrometers and by development of new methods of data handling and manipulation. 

Controlling the Crosslinking Density. The selective, controlled manipulation of polymers using radiation chemistry can be achieved by preferential irradiation shielding. By using a shield made of selected materials, of a certain shape, the overall properties of the irradiated polymer may be controlled and tailored. Thus, the cross-linking density can be locally controlled in a direction perpendicular to the direction of irradiation by (34) ... 

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