3 Accelerators and Other Sources for the Study of Radiation Chemistry
Downloaded by UNIV OF PITTSBURGH on March 2, 2016 | http://pubs.acs.org Publication Date: April 17, 1998 | doi: 10.1021/ba-1998-0254.ch003
James F. Wishart Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973-5000
This chapter is intended as a guide to aid in the design of radiolysis experiments for researchers who are new to the field. It summarizes the features and limitations of the several kinds of particle accelerators and X-ray and gamma radiation sources used in the study of radiation chemistry. The chapter describes (1) the types of ionizing radiation and their interactions with matter, (2) the architecture and use of X-ray and gamma radiation sources, and (3) the electrostatic and radio-frequency methods of particle acceleration. The advantages and disadvantages of four types of accelerators are compared. A description of proton and heavy-ion accelerators completes the chapter.
Jtiadiolysis, the initiation of reactions by ionizing radiation, is a powerful tech nique for studying chemical systems. In radiolysis, energetic charged particles or photons transfer their energy in varied increments to the molecules of the medium, inducing ionizations and molecular excitations. The rate of energy loss depends on the type of particle, its energy, and the density of the medium. These interactions occur in a confined region around the path of the particles (the "track"), wherein the concentration of molecular fragments is high. Out side the track the solution is unperturbed. Ionization events frequently result in secondary electrons with enough energy to induce further ionizations and excitations within their own tracks or "spurs". At subpicosecond time scales, radiolysis results in an inhomogeneous distribution of ionized and excited sol vent molecules, which cross-react to yield solvated electrons, ions, radicals, and molecular products. These primary products can then react with solutes to form transient species of interest. Photolysis, on the other hand, deposits excitation in specific molecules through light absorption. The spatial pattern of energy deposition mirrors that of the chromophore distribution. In some cases excitation directly generates ©1998 American Chemical Society
In Photochemistry and Radiation Chemistry; Wishart, James F., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1998.
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Downloaded by UNIV OF PITTSBURGH on March 2, 2016 | http://pubs.acs.org Publication Date: April 17, 1998 | doi: 10.1021/ba-1998-0254.ch003
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PHOTOCHEMISTRY AND RADIATION CHEMISTRY
the species of interest, whereas in others, reaction with a precursor is required. While direct excitation offers advantages in time resolution over schemes involv ing second-order reactions, in some cases high-background absorption, low quantum yield, or the lack of a suitable photosensitizer system precludes the application of photolytic techniques to study certain chemical systems. Many such systems are amenable to study by radiolysis. Without the need for a strongly absorbing species in solution, a wider range of the spectrum may be available for detection. This chapter is meant to serve as a guide to the type of accelerators and other radiation sources available for the study of chemical reactions. Radiolysis techniques are in widespread commercial use in applications such as the prepa ration of materials, sterilization of medical supplies, preservation of foodstuffs, as well as medical therapy. The devices used for these applications will not be covered here, although some of them are also used in chemical kinetics research (i). When designing photolysis experiments, parameters such as wavelength, pulse width, and pulse energy are critical factors. In much the same way, operat ing parameters of accelerators are critical to the design of radiolysis experi ments. The particle type and energy determine the spatial distribution of energy deposition within the sample and the yields of primary radiolysis products. The particle-beam pulse width and pulse structure define the available time resolution. The charge-per-pulse is analogous to laser pulse energy; at lower doses, signal intensity varies with dose, whereas at higher levels, higher-order effects appear (significant bimolecular reactions between radiolysis products, for example).
Types of Ionizing Radiation and Their Means of Interaction with Matter There are two types of ionizing radiation: energetic photons such as X-rays and gamma rays, and energetic charged particles such as electrons, protons, neutrons, alpha particles, and other atomic nuclei. Low-energy X-rays (
