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Radiation Chemistry: Background, Current Status and Outlook
E
arly Years. It can be said that, along with X-rays, Roentgen (1896) also discovered the chemical action of ionizing radiation.1 However, the first publication is attributed to Marie and Pierre Curie (1899) in Comptes Rendus.2 Within a few years, various chemical effects of ionizing radiations were observed, ranging from ozone production in oxygen to discoloration of paper. The early history of this pioneering research in France has been adequately summarized by Ferradini and Bensasson.3 M. Curie also described the radiolysis of aqueous solutions of radioactive compounds. The word radiolysis was coined by a somewhat mislaid analogy with electrolysis. Much later, Lind (1961) defined radiation chemistry as the science of chemical effects brought about by the absorption of ionizing radiation in matter (mainly due to electronic processes), thus separating it from radiochemistry (arising from nuclear transformations).4 In 1907, W. H. Bragg came to the far-reaching conclusion that the number of decomposed water molecules was almost equal to the number of ionizations produced in air by radon.5 In 1942, M. Burton christened the field radiation chemistry, realizing that it remained nameless for 47 years. He introduced the unit G for radiation chemical action, defining it to be the number of molecules formed or destroyed by the absorption of 100 eV of ionizing radiation.6 It replaced the earlier notation M/N, where M is the number of molecules transformed and N is the number of ion pairs formed. This unit G has been intentionally made purely experimental, free from hypothesis or theory. Currently, G is used for experimental measurement, while g refers to primary yields (sometimes hypothetical). He also invented the , to indicate “under the action of ionizing symbol radiation”, without implying any mechanism.7 For all of these and other sterling contributions, he was named by M. Magat as the godfather of radiation chemistry at a symposium banquet in Moscow in 1965. M. Curie’s conjecture that ionic species bring about chemical transformations in aqueous solutions was largely superseded by the radical reaction mechanism of Weiss (1944), involving H and OH radicals.8 This was superseded again, at least for the major reducing species, by Hart and Boag’s discovery of the hydrated electron (1962) through its absorption spectrum,9 which was theoretically predicted by Platzman by analogy with ammoniated electron.10 Current Status. During and after the Second World War, there was greatly increased activity in the study of radiation effects in water and aqueous solutions, arising from the necessity of assessing biological effects of radiation on one hand and from understanding the performance of reactors under operating conditions on the other. This surge is still going on, both in theoretical and experimental aspects. At about the same time, it became apparent to the practicing chemists the contrasts of this newly found discipline from two other related fields of photochemistry and hot atom chemistry. In photochemistry, reactants are usually activated to a well-defined excited state by the action of nearly monochromatic light. A corollary of this phenomenon is that only one molecule is activated at a time (Einstein principle); therefore, the subsequent reactions are homogeneous throughout. r 2011 American Chemical Society
In radiation chemistry, on the other hand, the activation is largely unselective, giving rise to many excited and ionized molecules in close proximity by the action of a single high-energy charged particle or photon. The subsequent reactions have a higher probability between neighbors on the same track than with reactive species on other tracks. This situation gave rise to the concept of spurs, which are small spherical regions containing the activated reactants, widely separated from similar entities on the particle track. The spur theory was particularly successful at low incident LET (e.g., energetic electrons), later augmented by blobs and short tracks of slightly elongated geometry. High LET particles (e.g., protons, α-particles, etc.) produce continuous cylindrical tracks by the coalescence of adjacent spurs. The products are therefore quantitatively different, giving rise to the concept of track effects. Hot atom chemistry deals with reactions between atoms or radicals produced with energies much in excess of that of surrounding atoms or molecules. Such nonthermal reactions are expected to be relatively unaffected by ambient temperature. By contrast, the prevailing hypothesis is that the reactants in radiation chemical processes are first thermalized and then undergo reactions. Such reactions are expected to be influenced by ambient temperature. There is abundant experimental evidence supporting this hypothesis. Hot atoms are usually, but not exclusively, produced by nuclear transformations. Other methods include UV photons, high-energy charged particles, electrical discharges, and so forth. Tremendous advances are being made in the areas of timedependent yields and absorption spectra of eaq. The initial ionization yield in liquid water is much greater than that in water vapor, although the process by which it occurs is not yet fully understood theoretically. One possibility is concerted protoncoupled electron transfer with adjacent water molecules. Another is optical charge transfer for far-UV photolysis. In any case, the mechanism for excess ionization is due neither to direct nor to autoionization, and it may depend on available energy. The yield of eaq at room temperature is now believed to be (G value) 4.2 at ∼1 ps and 2.7 at the microsecond time scale, with an uncertainty of (0.2 G unit. Factors contributing to the uncertainty are dosimetry, differences in experimental setups in different laboratories, and so forth. The gradual fall of the yield of eaq from the picosecond to microsecond time scale is due to various reactions of the hydrated electron, which are fairly well accounted for by diffusion or stochastic methodology, but there may still be some unaccounted difference at intermediate time scales. At the microsecond time scale, the extant reactants are well homogenized, and ordinary chemical kinetics may be confidently used. Reaction rates of a vast number of transient species are now available, starting from the early 1960s. Recently, radiation chemical yields in liquid water have been investigated at higher temperatures going up to and beyond the Received: September 20, 2011 Accepted: November 4, 2011 Published: December 01, 2011 2994
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The Journal of Physical Chemistry Letters critical temperature. Such data are urgently required for operations of nuclear reactors using water as a moderator. The higher temperature is desirable for thermodynamic efficiency. Work done in different laboratories has certain differences regarding yields and mechanisms, but it is generally agreed that (i) water density is the main determinant that continually decreases with temperature and (ii) all primary yields seem to increase with temperature. The measured homogeneous yield of eaq, under a fixed pressure of 25 MPa, increases from 2.7 at room temperature to ∼3.7 at 300 °C, beyond which it undergoes a minimum and a maximum in a complicated way. The peak of the absorbance shifts gradually to lower energy up to 390 °C. Theory, hand-in-hand with experiment, has been quite successful in explaining reactions and yields using the track concept and track entities. Stochastic calculations have shown promise in calculating reactant and product yields in aqueous solutions, under various conditions of LET and scavenger concentrations, by replacing homogeneous rate constants with reaction radii and probabilities of reaction. A particular form of stochastic calculation, called the Independent Reaction Time Model (IRT), has been developed in the Physical Chemistry Laboratory at the University of Oxford, which classifies the bimolecular reactions sequentially and generates a table of first reaction times using random numbers and is consistent with the relative diffusion coefficients of pairs of reactants and with their reaction radii. The shortest reaction time is registered, and the relevant pair is removed from further consideration. Next, the IRT is generated for the remaining reactants with the next reaction registered and the corresponding reactants removed. The computation is continued until all desired reactions are over and the entire process is repeated a large number of times, typically several thousand, for good statistical average or until the computer attains its preassigned limit. In principle, these methods should apply to any number of reactants, but in practice, it has been limited to a few reactants at present due to limitation of computer memory. Electron mobility and its temperature dependence have been measured accurately in many hydrocarbon liquids, along with the lowest energy of the conducting state (V0). In some experiments, mixtures of hydrocarbon liquids have been used. Theoretical models have been developed in terms of quasi-free and trapped states of the electron or by using an activated hopping model, with reasonable but not complete success in both cases. Electron mobilities can be classified as low or high depending on whether μ is ,10 or .10 cm2 v 1 s 1. Low electron mobilities are accompanied by positive activation energy, easily explained by a trapping model, or by activated hopping. High electron mobility is indicative of motion in the quasi-free state, also known as the conducting band. Such mobilities decrease with temperature, and several theoretical models are available for its description. Theoretical models are being refined for electron solvation in various condensed media. At present, the absorption spectrum of the hydrated electron has been completely understood theoretically, at least at room temperature. Outlook. Radiation chemistry stands intermediate between radiation physics and radiation biology, both in terms of concepts and relevant time scales. Increased overlap with photochemistry (especially solar photochemistry) can be predicted. Currently, one journal, Radiation Physics and Chemistry (Elsevier), deals exclusively with radiation effects. A number of articles published there stress applied radiation chemistry. Many of fundamental papers are still being published in J. Chem. Phys.,
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J. Phys. Chem. A/B/C, J. Chem. Phys. Lett., and the like. Recently, two edited books have appeared that aim at a coherent account of high-energy charged particle and photon interactions with matter in vitro and in vivo with industrial applications. These are (1) Charged Particle and Photon Interactions with Matter: Chemical, Physicochemical and Biological Consequences with Applications; Mozumder, A., Hatano, Y., Eds.; Marcel Dekker: New York, 2004 and (2) Charged Particle and Photon Interactions with Matter: Recent Advances, Applications and Interfaces; Hatano, Y., Katsumura, Y., Mozumder, A., Eds.; CRC Press: Boca Raton, FL, 2011. The interested reader will find much useful and up-to-date information in these books. In the future, most attention will probably be focused to industrial and experimental applications. In theory, there is a need to explain the remaining discrepancy between experiment and calculation for the yield of eaq at short times. It could be due, inter alia, to fluctuations, which presently calculations do not take care of adequately, but it is not known definitely as of yet. There is a need to rationalize the mobility dependence of the free-ion yield in liquid hydrocarbons on a firm theoretical basis. Similarly, there is also a need to understand the dependency of mobility on the density and composition of mixtures. Note: Only a very brief account is presented because of space limitation. Many important topics and contributors have been left out or severely condensed. References are provided only for important early work. A. Mozumder Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, United States
’ REFERENCES (1) (a) Roentgen, W. C. On a New Kind of Rays. Nature 1896, 53, 274–276. (b) Roentgen, W. C. Uber eine Neue Art von Strahlen. Ann. Physik (Leipzig, Ger.) 1898, 64, 1, 12, 18. (2) Curie, P.; Curie, M. Effets Chimiques Produits par les Rayons de Becquerel. Comptes Rendus 1899, 129, 823–825. (3) Ferradini, C. P.; Bensasson, R. V. Glimpses of Ninety Years of Radiation Chemistry in France. In Early Developments in Radiation Chemistry; Kroh, J., Ed.: Royal Society of Chemistry: Cambridge, U.K. 1989; Chapter 8. (4) Lind, S. C. Radiation Chemistry of Gases; ACS Monograph No. 2 (revised), Reinhold Publishing Corporation: New York. 1961. (5) Bragg, W. H. On the Ionizations of Various Gases by the α-Particles of Radium-No.2. Philos. Mag. 1907, 13, 333–357. (6) Burton, M. Radiation Chemistry. J. Phys. Colloid Chem. 1947, 51, 611–625. (7) Burton, M. Radiation Chemistry: A God Fatherly Look at its History and its Relation to Liquids. Chem. Eng. News 1969, 47, 86–98. (8) Weiss, J. Radiochemistry of Aqueous Solutions. Nature 1944, 153, 748–750. (9) Hart, E. J.; Boag, J. W. Absorption Spectrum of the Hydrated Electron in Water and in Aqueous Solutions. J. Am. Chem. Soc. 1962, 84, 4090–4095. (10) Platzman, R. L. Energy Transfer from Secondary Electrons to Matter. In Basic Mechanisms in Radiobiology, NAS-NRC Pub. 305; Magee, J. L., Kamen, M. D., Platzman, R. L., Eds.; National Research Council: Washington, D.C., 1953; p 22.
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