Techniques of Radiation Chemistry

The most commonly used sources of radiation are the 60Co gamma source for continuous irradiation and pulsed high-energy ( 1 MeV) electron beams for fast kinetic studies. Detailed descriptions of several such sources and accelerators are given in numerous books, as are the various methods used by radiation chemists for dosimetry, sample preparation and irradiation, and common product analysis. Several new developments in the analytical procedures, both in the determination of final products and in the direct observation of transient species, will be discussed below. 

Liquid chromatography can also be applied advantageously to the examination of reactant disappearance. For example, it has been used to determine the disappearance of 5-bromouracil in solutions which contain excess uracil (Bhatia and Schuler, 1973b). Such a determination is, of course, impossible by straight spectrophotometry. 

Another aspect of pulse radiolysis which has been improved is the pulse duration. For most experiments of interest to the physical organic chemist the common machines with pulse durations of 10 7-10 5 s are quite satisfactory, though for certain reactions, such as those involving protonation, examination on a shorter time scale can be of value. Several accelerators which supply nanosecond pulses are currently in use, but they are employed mostly with microsecond detection systems. Work in the 10-12-10-1° s region has recently become possible by the stroboscopic technique utilizing the fine structure pulses from a linear accelerator (Bronskill et al., 1970). More recently, a system which produces a single pulse of 40 picoseconds has been constructed (Ramler et al., 1975) and utilized for the observation of hydrated electrons at very short times (Jonah et al., 1973). 

Various detection methods can be used with pulse radiolysis, and the recent developments in these methods are discussed below. 

Electron spin resonance observation of organic radicals during in situ radio lysis of solutions was initiated by Fessenden and Schuler (1960) (their major work on hydrocarbon solutions was published in 1963). A review on the e.s.r. spectra of radiation-produced radicals summarizes the literature up to 1968 (Fessenden and Schuler, 1970). However, e.s.r. observation of radicals during irradiation of aqueous solutions was not achieved until 1968 (Eiben and Fessenden, 1968 Avery et al., 1968). 

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The study of radiation chemistry might be divided, from the experimental point of view, into two parts. The first is the study of unstable intermediates which have short lifetimes and thus cannot be studied by the usual methods of chemistry. The second part is the study of the final products of the radiolysis which are measured by common chemical techniques. 

The general subject of tracer techniques in radiation chemistry has been reviewed by Burr (29). 

Pulse radiolysis is a powerful tool for the creation and kinetic investigation of highly reactive species. It was introduced to the field of radiation chemistry at the end of the 1950s and became popular in the early 1960s. Although the objects of this modern technique were, at first, limited to solvated electrons and related intermediates, it was soon applied to a variety of organic and inorganic substances. As early as 1964, ionic intermediates produced by electron pulses in vinyl monomers were reported for the first time. Since then, the pulse radiolysis method has achieved considerable success in the field of polymer science. 

A very interesting technique has been used by Walker and Back124 in which the photolyses of methane, ethylene, and ethane have been carried out in a windowless system at 584 A., the helium resonance line. Since this is a study which has crossed the line into the realm of radiation chemistry, no discussion of the work will be presented. It does, however, represent a radiation chemical study using monochromatic ionizing radiation and it deserves, therefore, the attention of interested researchers. 

Radiation chemistry, like all fields of science, has been strongly influenced by the techniques that were available to make measurements. As more and more sophisticated techniques become available, more and more sophisticated questions were posed and answered. In this short review of the history of radiation chemistry, I will select the various eras of radiation chemistry, as defined by the techniques available, and discuss the concepts and conclusions of the time. At the end, I will summarize where radiation chemistry is and what are the unanswered questions and new techniques that are needed to answer these questions. Much more on such topics will be found in the rest of the volume. 

At this point, I would like to discuss two techniques that do not conveniently fit the technique ordering/timeline for the advances in radiation chemistry. Use of high-LET radiation has been common since the beginning of radiation chemistry. As was mentioned earlier, high-LET radiation studies were common in early experiments because sufficient energy could be deposited to make it possible to observe reaction products. If low-LET sources were used, so little energy was deposited that the yield of products was too low to measure. ... 

The use of radiation chemistry to study proteins in water can be accomplished through slow or fast techniques gamma radiolysis and pulse radiolysis, respectively. The difference between these two applications of radiation chemistry is that, in the former case, a continual irradiation of the water produces a steady-state flux of radicals and usually involves a gamma-ray generator such as a °Co source to produce the radicals. In the latter case, an electron accelerator is used to deliver short bursts of electrons to water in the nanosecond (10 s) to picosecond (10 s) time scale. 

Detailed accounts of the development of radiation chemistry and its tools can be found elsewhere. The purpose of this chapter is to describe the basic characteristics of continuous and pulsed sources of ionizing radiation for radiolysis studies, and to provide a broad overview of the present and near-future status of radiolysis instrumentation worldwide, for the benefit of readers who would like to use these powerful techniques to advance their own research. It is inevitable under the circumstances that some facilities may be missed and that future developments will soon render this overview out-of-date, however the substantial progress that has been made in the years since the previous reviews appeared [14-16] merits description here. 

With the use of the recent investigation techniques, the new materials can be well characterized and the processes induced by the radiation can be well understood on the basis of the knowledge already obtained and with the help of fundamental studies which are going on in the field of radiation chemistry of polymers. 

The history of radiation chemistry of polymers started in the early 1950s (Chapiro 1962 Charlesby and Alexander 1955). Poly(Ai-vinyl pyrrolidone) (PVP) was and still is an often applied polymer, also as hydrogel, in medicine and pharmacy. In 1955 Charlesby and Alexander first reported on cross-linking of PVP (Charlesby and Alexander 1955). Since then various other water soluble polymers have been radiochemically cross-linked, even for creating new biomaterials (Hoffman 1981). Hydrogels can be synthesized by radiation techniques in different ways ... 

TJrimary processes in radiation chemistry are largely caused by sec-ondary electrons whose energies are less than 100 e.v. This is why direct study of the transient species produced by low energy electrons is so important for understanding the results of radiation chemistry. Most investigations have been carried out by mass spectrometry techniques (e.g., by Derwish et al. (3) and Melton (9) for NH3) and a few by optical spectroscopy (6). Mass spectrometry gives much information about ions and some of their reactions and neutral species (radicals) but in an indirect way. 

Whereas much of the underlying mechanisms for the effects of radiation on materials were outlined using steady state radiation sources, the advent of pulse radiolysis on the heels of flash photolysis opened a window into direct observation of the intermediates. One of the early discoveries utilizing pulse radiolysis was the spectrophotometric detection of the hydrated electron by Boag and Hart (35,36). Since then thousands of rate constants, absorption spectra, one-electron redox potentials and radical yields have been collected using the pulse radiolysis technique. The Radiation Chemistry Data Center at the University of Notre Dame accumulates this information and posts it (at www.rcdc.nd.edu/) for the scientific community to use. They cover the reactions of the primary radicals of water and many organic radicals and inorganic intermediates. 

Despite the growth of the possible applications of radiation chemistry, the limited knowledge of radiation chemical techniques and the large facilities necessary to employ these techniques have limited its use. However, new accelerator facilities, with faster time resolution and greater ease of use, will lead to both a quantum jump in research capabilities and research opportunities in radiation chemistry. In the present book, we endeavor to provide an overall view of the different aspects of the subject in its present status. We also want to show chemists in general and chemical kineticists, photochemists, physical-organic chemists and spectroscopists in particular the opportunities in utilizing radiation chemical techniques to study their chemical problems... 

Because of the diverse nature of the field, the chapters of this book are authored by several experts in their particular areas of research. The introductory chapter highlights the accomplishments of radiation chemistry during the last century and set out some possible future developments. This is followed by chapters on techniques in ultra-fast radiation chemistry techniques, on techniques using heavy ions, the observation of chemistry using spin and the chemistry evolving from the use of muons. These provide an experimental foundation for the science. After discussion of the techniques, fundamental radiation-chemical... 

Radionuclides with extremely long half-lives or in very large amounts will not be at such extremely low concentrations, as indicated by Eq. (2.7). Radionuclides with half-lives in excess of 10 years, such as and Th, can be measured by some of the conventional techniques of analytical chemistry as alternatives to radiation measurement. Radionuclides at concentrations much above lO Bq/1 usually are not submitted to a low-level radioanalytical chemistry laboratory because of the threat of contaminating other samples. 

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