A. 0. Allen
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Radiation Chemistry Today
Radiation chemistry is in some ways quite a different game from those discussed in the other contributions to this symposium. For one thing the viewpoint toward radiation is quite different. In most applications, radiation is used as an indicator or tool to study changes occurring in matter. Radiation chemistry deals with the effects of radiation on material itself. In background and goals, radiation chemistry and radiochemistry are quite different. The impact of radiation upon any system usually leads to a complex of vaguely defined chemical and physical alterations, and it is the business of radiation chemistry to sort out the chemical changes occurring in a system, to determine how they vary with the irradiation conditions and to attempt to explain why the radiations produce these particular changes rather than others. Radiation chemistry today, despite recent advances, is much closer to the pioneering stage than most other fields of chemistry. Radiation chemistry may be defined as the chemical effects of electromagnetic radiatiou or charged particles having energies of 50 ev and above. The effectsof ultraviolet light of lower energy are usually called photochemistry, though there is no real dividing line between the two branches. The subject of radio-biology is also very closely related to radiation chemistry. The primary effects of radiation on living bodies are no doubt chemical in nature, although the chemistry involved is not yet understood in detail. When high-energy electromagnetic radiations (Xrays, gamma rays) are absorbed in matter, their energy is converted to the energy of electrons which start out at high speeds in the material but lose their energy by interaction with the atoms and molecules through which they pass. Fast neutrons undergo collisions with atoms in the material which cause the atoms to take off and move very rapidly through the matter. As they move, collisions rapidly strip the outer electrons from the moving atom and one again has a charged particle. Other radiations such as beta and alpha rays, and fast particles from machines, originally consist of charged particles. So in all cases it is a charged particle which actually gives up energy to the material being irradiated. Charged particles lose their energy by electrical interaction with electrons in the atoms through or by which they pass. As the fast particle whizzes by, its charge exerts a force on the electrons in the atom and may leave some of them in different orbits of higher energy than their original state. Thus the charged particle leaves a train of excited molecules or atoms in its wake. If the energy transferred is greater then the ionization potential of the molecule an electron may escape the molResearch performed under the auspices of the U.S. Atomic Energy Commission.
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ecule altogether leaving a positively charged ion behind, and if this secondary electron has sufficient energy it may in turn induce further excitations and tertiary ionizations. The ionization process thus usually r e sults in a group of ionized and excited molecules located close together. Such a group is sometimes called a "spur" on the track of the primary particle, although the group is usually closer to spherical than linear in shape and might be better thought of as a cluster than a spur. The probability that a passing fast charged particle will interact with a given molecule is greater the slower the speed of the particle, since the slow particle hasmore time to interact with theklectron system of the molecule. Consequently the track of avery fast charged particle consists of a series of spurs located very far apart, while with a slower particle such as an alpha ray the spurs may coalesce into a continuous track (Fig. 1). The excited molecules and ions formed by the radiation are in most systems chemically unstable and are rapidly converted to neutral free radicals and neutral molecules, which in their turn undergo further reactions. Historical
My own introduction to radiation chemistry occurred after the feasibility of the nuclear chain reaction had been established, and plans were underway for building a full-scale plutonium production reactor. An urgent problem was to assess the probable extent of corrosion in the projected reactor. Physicists had decided that the best scheme for cooling the reactor would be by water rushing through aluminum pipes. The difficulty
Figure 1. A, Typical distribution of free radicolr in the trock of o 40-kov electron. 8, Typical distribution of free radicoh in the track of an 18-Mov deuteron. C, Typical distribution of froo radicals in the track of a 5.5-Mev d p h a ray,
was that no one knew whether under the influence of the unprecedentedly intense radiation in these reactors the water might react with the aluminum as if it were calcium or sodium, as indeed thermodynamics suggested. To test this possibility, Milton Burton, who was head of the radiation chemistry section of the plutonium project, rounded up all the available radiation sources in the country at the time, which consisted of a few cyclotrons and one of the first Van de G r a d machines ever built. Fortunately, it turned out that radiation does not increase corrosion of aluminum. This, however, was not the beginning of radiation chemistry. The subject in its modern form goes back to the work of Mme. Curie, who noticed that the alpha rays from radium preparations would decompose water and cause chemical changes in other substances. She was followed by a number of workers who made quantitative observations of the effects of alpha rays on various materials, mainly gases. Most prominent among the students of the subject was S. C. Lind, formerly president of the American Chemical Society, whose extensive work on alpha rays was mainly concerned with their effects on gases. He emphasized the importance of ionization in inducing these reactions (I). I n general his finding was that everything seemed to decompose under the influence of alpha rays. Simple gases under sufficiently long continued irradiation would be converted to a mixture of all possible compounds that might be formed by combination of the atoms present. Of course in the case of hydrocarbons the number of pop sible products is so large that not all would be formed; but a hydrocarbon is converted by long-continued alpba bombardment to a mixture of hydrocarbons of smaller and greater molecular weight than the original, resembling petroleum in its genera1 characteristics. In fact a theory was seriously proposed (2) that petroleum originated by the action of radiation, from radioactive m a terials contained in sedimentary rocks, on the organic plant material which these rocks contained. During the 1920's X-ray machines were developed for medical purposes which were sufficiently powerful to produce noticeable chemical effects. The study of these effects was pioneered by Hugo Fricke, who began by developing the ferrous sulfate dosimeter (5), which is still used as the best standard reaction by most radiation chemists. Fricke's work with E. J. Hart on effects of X-rays on aqueous solutions is a classic in the field (4). Astonishingly enough, the effect of X-rays on water was found to be quite different from that of alpba rays. X-rays apparently did not decompose pure water at all, although they did produce reactions on substances dissolved in the water; by contrast, alpha rays produced extensive decomposition of pure water into hydrogen, oxygen, and hydrogen peroxide. This exemplifies the kind of mystery that it is the business of radiation chemistry to solve. Since the war, the problem of water decomposition has been extensively worked on and is now fairly well understood, although of course many problems remain to be cleared up. I n general the advances in radiation chemistry in the last 20 years have been very gratifying. From empirical groping, radiation chemistry has grown into a sophisticated body of science, which a t least in some areas is capable of precise predictions. These advances are due to continuing financial support by the
United States Atomic Energy Commission in this country and by the United Kingdom Atomic Energy Authority in Great Britain. I n recent years extensive support of this subject has also been forthcoming in the U.S.S.R., Japan, and other countries. This support is of course based primarily on potential uses of the subject. On the negative side these applications involve understanding the destructive effects of radiation, for instance in the interior of nuclear reactors and in living bodies; and on the positive side, the possibility of useful material syntheses by radiation. Rodiotion Sources for Experimental Work
The most convenient sources for experimental studies of irradiation of materials are gamma ray sources, with cobalt-60 probably the best. For work with gases, beta and alpha ray sources have also been used but are somewhat more difficultto handle. Machine sources include X-ray machines and particle accelerators such as the Van de G r a d , the cyclotron, the linear accelerator, and various machines based on the principle of the condenser bank, or group of condensers which is charged in parallel and then discharged in series, thereby attaining a flash of current at high voltage. Selling high voltage machines to laboratories for radiation chemical use is quite a lively business today. The advantage of the m a chine is that it is flexible and controllable in energy and intensity, and above all can be used to obtain pulses in which a great deal of radiation is produced in a very short controlled time. Advantages of the radioactive source are tbat it requires no maintenance, no operation, and no moving parts except what may be required to insert the sample and remove it. An advantage of gamma ray sources is that they can be arranged to supply a uniform field of radiation over a large volume, while the machines always put out a beam the intensity of which varies rapidly with position in the field. Both types of source have their place, depending on the kind of information desired. Rodiotion Chemistry of Water and Aqueous Solutions
My own involvement with radiation chemistry began with this particular subject and still continues although I have also been interested in other fields. In the early days, after discovering that radiation did not in fact accelerate the corrosion of aluminum by water, we turned our attention to trying to understand the decomposition of water itself, and particularly the baffling difference between the effects of X-rays and of alpha rays on the pure substance. The reactor contains both gamma rays and fast neutrons, the latter reacting chiefly with the water to produce fast protons which should p r e sumably act like alpha rays in their chemical effects, so that we expected a mixture of the two types. Experiments in the reactor with pure water proved discouraging because the amount of decomposition turned out to be extremely sensitive to impurities. Progress was made when we gave up trying to study pure water and turned to the study of solutions, using the classical work of Fricke as a taking-off place. Along with products from chemical reactions of the dissolved molecules, we always found hydrogen gas arising from the water. Soon we put the key together-the fact tbat the radiation produces from water not only free radicals, Volume 45, Number 5, May 1968
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which diffuse through the solution and react with solutes causing them to change their chemical form, hut also hydrogen molecules and hydrogen peroxide molecules (5). The latter are formed by the interaction of free radicals with one another in the spurs where the radicals are very close together, by such reactions as H H = Hz, OH OH = H202. Many of the radicals however diffuse out of the spur and become widely separated so that they have little probability of finding another radical but prefer to react with anything which is dissolved in the water. In very pure water, the radicals have nothing to react with initially hut each other, and hydrogen and hydrogen peroxide are formed. However by the time these have built up to detectable concentrations further radicals have been generated, and react with these molecular products to cause them to revert to water by a chain of reactions such as H H202= Hz0 OH; OH Hz = Hz0 H. This concept explains the difference between the action of alpha rays and X-rays. Alpha rays form radicals in a dense track (Fig. 1C) and they practically all react with one another, with very few escaping to cause rewmhination of the products. From the little separated spurs formed by the X-rays, on the other hand, a large proportion of the radicals escape by diffusion and are available for producing back reaction, unless some reactive substance is present in the water which then can serve to protect the water decomposition products from rewmbination by radical reaction. Quantitatively the situation may be defined by specifying molecular and radical yields for water decomposition by a particular kind of radiation. The radical yield GI! is usually defined as the number of free radicals escapmg initial recombination and available for chemical reaction, per 100 ev of energy put into the solution by the radiation, while a molecular yield, GH, for instance, is defined as the number of hydrogen molecules formed per 100 ev energy input. To explain the observed values of these yields required the development of a new variety of chemical reaction kinetics, describing the competition between diffusion out of, and interaction within, a group of neutral species distributed within a certain volume. This theory has been developed mainly by J. L. Magee and his wworkers (6) and has been successfully applied to explain observations on a number of radiation systems. The theory is most simply applied to consideration of the effects on the molecular yields of hydrogen or of hydro-
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Figure 2. Lowering of molesubr yields by added radical ssavengors. Ordinote, rotio of molesvlor yields with ond without oddod scavenger; abrcinm, concentration of scavenger multiplied b y o sonstant, different for emch scavenger, which sewer to bring the date onto the same curve.
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gen peroxide when different concentrations of "radical scavengers" are added, i. e., substances which react with the radicals that serve in the spur as parents of the particular molecular product looked at. Empirically, it is found that the form of the function relating the molecular yield to the concentration of the reactive solute is the same for all solutes and for both molecular products (Fig. 2) (7). The curve shown in Figure 2 is obtained by a rough approximation in which various terms involved in the interaction between recombination and diffusion were neglected. When the higher order terms are taken into account by more complicated calculations made with a wmputer, much better agreement is obtained with experiment. A more complicated form of the theory is required to treat the problem of denser tracks, such as those formed by heavy charged particles; but here again good agreement has been obtained with experiment (Fig. 3).
CURVES FROM GANGULY Bi MAGEE, USING q = 0.56 [ N O i l
0 33 MeV HELIUM ION O 18 MeV DEUTERONS
A CO~~~-RAYS
Figure 3. Lowering of molecular hydrogen yield by added nitrite for different qualities of radiotion.
According to these theories the behavior of any aqueous solution under irradiation should he determined entirely by the relative reactivity towards the free radicals of any substances present in the solution. These relative reactivities can be determined by competition of two solutes put in simultaneously toward reaction with one of the radicals. With normal sources of radiation the free radicals of water are formed a t such low concentrations and have such short lifetimes that direct determination of their nature is impractical. I n studying the behavior of water containing only the water radiolysis products hydrogen, oxygen, and hydrogen peroxide, N. Barr and I (8) decided that the results could he interpreted quantitatively only if two different kinds of reduciugradicals existed, one of which reacts slower with hydrogen peroxide than with oxygen, while the other reduces both approximately equally fast. At the same time, E. Hayon and J. Weiss (9) from a study of solutions of chloroacetic acid, concluded that two different products of reduction of this acid (HC1 and Ha) must be due to two different kinds of reducing radicals. The only reasonable proposal for the identity of the two species was that one of them was atomic H and that the other must be either an acidic form Hz+or the basic form which, having lost a proton, would merely he a hydrated electron, e,,-. Shortly afterwards Czapski and Schwarz (10) showed by a study of the effect of ionic
strength on competition for diierent solutes that the radical which reacts readily with hydrogen peroxide, and which is the major one formed in the radiolysis of neutral water, in fact carries a negative charge. This was shown by testing the wmpetition for this radical between a neutral species (peroxide) and an ion carrying a negative charge (nitrite). When a neutral salt was added the reaction of the radical with nitrite ion was speeded up compared with the reaction of the neutral species. When the relative reaction rates with peroxide and with a positive ion (H+) were tested the opposite was found; the added salt decreased the rate with the positive ion. This was precisely what would be expected if the radical bore a negative charge; and, as seen in Figure 4, the variation of the relative rates with concentration of added 7mr. =,,. -, inert salt agress quantitatively with that expected from t h e theoretical Bronsted equation for salt effects on reaction rates. I n fact this provides one of the best examples of quantitative agreement with the Bronsted equation, which is expected to hold exactly only when dealing with very dilute solutions, as we have here. ~ h u s Figure 4. Effect of ionic strength ( w r i e d by odding inert salts) MI the reoction rates of it was shown that the reducing rodicd formed in water the major reducing radiolyrir relotive t o i h reaction with H20n. points, reaction with nitrite ion; radical formed in Upper middle points, with oxygen; lower points, water radiolysis is with hydrogen ion. The liner are the retvlts in fact a hydrated expected for a reacting species of unit negoelectron, hi^ ti;yhorge. on the basis of the BrEnrted findine stimulnt,ed thesearch for its absorntion soectrum whichhasnowbeen thoroughly studied by the technique of pulse radiolysis. The hydrated electron can also be observed in the photolysis of certain aqueous solutions. 7,----.r
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Hydrocarbons
The radiolysis of hydrocarbons has been the subject of literally hundreds of research papers. Since it is found that all organic compounds radiolyze largely by breakage of the CH and CC bonds which they contain, the hydrocarbons provide a model system for studying major processes in the radiolysis of all organic and biological materials. The saturated hydrocarbons, paraffinic or naphthenic, have been found to break up largely to form free radicals under radiation, similar to the behavior of water. The nature and amounts of these free radicals have been determined by adding scavengers such as iodine, which capture them to form organic iodides whose nature can be determined. More certain identification is provided by the elegant method of Schuler and Fessenden (If), who determined the electron spin resonance spectrum of radicals present at steady state concentrations in a hydrocarbon or fluorocarbon which was simultaneously
being irradiated with a beam of electrons. This kind of spectrum, obtained under these conditions, not only tells us the identity of the radical but also a great deal about its internal molecular structure. It is found that nearly all the radicals present in an irradiated hydrocarbon are alkyl radicals which may be formed by simple breakage of a C--C or C-H bond from the hydrocarbon without skeletal rearrangements. Thus normal pentane yields only straight chain butyl or pentyl radicals; no isobutyl or isopentyl radicals are found. The products from radiolysis of a pure, saturated hydrocarbon include saturated hydrocarbons formed by combination of all the possible radicals. Unsaturated hydrocarbons are also produced which are believed to result in part from disproportionation of free radicals (e. g., c8H1 CzH7.= CaHs CaHd and in part from splitting out of these unsaturated molecules directly in the primary chemical step. A great deal of effort has been put on the problem of discovering how these neutral radicals arise from the excited and ionized species primarily reduced by the radiation. The results have been worked out in greatest detail for gaseous hydrocarbons. Comparison with photochemical results and with mass spectroscopy are especially valuable in interpreting the radiolysis mechanisms (19). The free radicals and other products are found to arise from three sources: split-up of neutral excited molecules, reaction of ions with molecules, and neutralization of ions by electrons. Liquids differfrom gases with respect to ionization because of the process of initial recombination. As pointed out by Jaff6 in 1908 (IS), the electron resulting from ionization in a liquid will be slowed down by impact with the surrounding molecules and brought to thermal energies while still within a short distance of the parent ion from which it came. It is then drawn back by the electric field of this ion and recombines with it, forming a neutral excited molecule, within an extremely short time. Thus only a small fraction of the ions and electrons produced ever become free and available to react with dilute solutes or to be measured by conductometric determination. Only recently have we been able to measure the absolute values of ion yields in liquid hydrocarbons ( I d ) , and they were found to be very small-only about 4% of the number of free ions formed when the same materials are irradiated in gaseous form. The aromatic hydrocarbons show chemical decomposition yields smaller by a factor of 10 or more than the aliphatic hydrocarbons. Their stability seems to be due to a tendency to pass over to a stateof excitation in which the shielded pi electron system on the aromatic ring has taken the excitation energy. These states appear to go over to triplet states of somewhat lower energy than the corresponding singlet and then lose energy by quenching rather than by dissociation. Neutralization of the ions of aromatic hydrocarbons seems to have a high probability of leading to triplet states rather than to states of higher energy which might tend to dissociate. If an aromatic hydrocarbon is mixed with an aliphatic hydrocarbon, the aliphatic component is greatly stabilized against radiation decomposition; the energy originally given up to the aliphatic molecules by the radiation seems to be transferred to the aromatic component. There has been considerable discussion as
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to the mechanism of this transfer; it now seems probable that. transfer of positive charge from aliphatic hydrocarbon ions to the aromatic hydrocarbon is responsible for a major part of the observed effect. Unsaturated hydrocarbons, and many unsaturated organic wmpounds generally, undergo polymerization as a major reaction under irradiation. This is hardly unexpected, since polymerization in many systems is brought about by reagents that produce free radicals, and radiation is certainly such a reagent. However, another kind of polymerization is known to organic chemists, the "ionic polymerization" which results from the formation of carbonium ions. Now it has been found that under radiation certain hydrocarbons can polymerize either by an ionic mechanism, which is apparently induced by the radiation-produced ions, or a free radical mechanism. The ionic mechanism is however inhibited by the presence of traces of water in the organic substrate, and in preparations dried only with ordinary care, the free radical mechanism will generally predominate (16). Radiation polymerization can be made to give products of physical properties entirely comparable to those formed by other more usual processes. For instance, ethylene under radiation can be made to give polyethylenes with a range of physical properties wmparable to those of commercial products, depending upon the temperatures and pressures of the material during irradiation. Since polymerization is a chain reaction, the yields can be very high, with G values typically around 10,000, as compared to 1-10 for ordinary radiolytic reactions.
Whatever the mechanism of cross-linking may be, the result is of commercial value (16). Thus the Raychem Co., Redwood City, Calif., has developed a method of cross-linking polyethylene wire insulation by irradiation with electron beam. During the year 1966 their total sales of this product amounted to $18,000,000. Also the Cryovac division of W. R. Grace and Co. is producing several million pounds per year of irradiated polyethylene film, which is tougher and more heat resistant than the unirradiated film. Aside from Dow Chemical Co.'s small plant for the manufacture of ethyl bromide by gamma rays, polymer cross-linking seems to be the only commercial use of radiation chemistry. This is a disappointingly small practical result to come out of all the research effort which has been put into radiation chemistry in recent years. There is no doubt that other useful applications will be found. One of these certainly is the manufacture of polymers. The difficulty here seems to lie in the engineering design of appropriate reactors rather than in any inherent high costs of the required radiation. Polymerization is a highly exothermic process, and the engineering design of a polymerization reactor depends critically upon the way the heat generated by the r e action must be removed from the system. Geometry of heat production and flow is quite different if the r e action is induced homogeneously by irradiation rather than by mixing of a catalyst with the monomer. With engineering development work in this field actively proceeding a t the present time, we may soon hope to see a breakthrough in the use of radiation in polymer manufacture. Pulse Radiolysis
Effects of Radiation on Polymers
Radiation exerts two opposing effects on polymers. On the one hand, it breaks up the polymer molecules into smaller pieces (degredation). On the other, it causes liberation of a hydrogen atom from each of two adjoining molecules with formation of a link between the two molecules (cross-linking). The existence of cross-links in a polymer makes the material tougher and higher melting and is very desirable for certain applications. The cross-linking of polymers by radiation has been much studied. The irradiation of any organic compound results in breaking of CH bonds, leaving free bonds on the carbon atoms while the hydrogen atoms go off together in pairs to form hydrogen gas. In a liquid the resulting free radicals can diffuse as a whole through the solution and eventually meet together and combine. In a solid polymer it is not clear how these centers get together. One proposed mechanism is that a hydrogen atom from a neighboring carbon will pop into the vacated hydrogen space, producing a new free bond on the atom adjacent to the original free bond position. This process will continue, with the free bond flowing up and down the chain, until the free bond happens to find itself next to a free bond formed on the adjacent molecule which is likewise traveling up and down. Another mechanism, possible perhaps only with amorphous polymer, is that the long-chain molecules as a whole may move with respect to one another (like worms in a can) until the free bonds find themselves in proximity. 294
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If a material is irradiated with an intense burst of high energy electrons over a very short time (typically a few microsewnds), free radicals or ions formed in this material will remain at detectably high concentrations after the pulse for times ranging from microseconds up to millisewnds or longer. During this short time they may be detected by passing a beam of light of the proper frequency through the material and noting how the absorption of light due to the transient intermediate decreases with time as the intermediate decays away. In this way the absorption spectra of the intermediates may be determined and the kinetics of their decays studied. By adding various reagents, the absolute r e action rate of the intermediate with the added reagent can be found. Machines yielding sufficientlyhigh radiation intensities for this purpose have hewme available only in recent years. By this scheme, which is known as pulse radiolysis, the existence of many intermediates in radiation chemistry which has been postulated on the basis of chemical-kinetic schemes has been directly demonstrated. I n particular, the solvated electron in water has been shown to have a strong ahsorption in the red similar to that shown by the solvated electron in ammonia and amines, and reaction rates of the hydrated electron with a large number of added materials have been determined. The properties of a variety of organic free radicals and ions have also been studied by this useful technique. Figure 5 shows the oscilloscope trace of the absorption due to triphenylmethyl ion, (CsH6)3C+,formed 6y the oxidation of triphenyl carbinol dissolved in cyclohexaue. It disap-
