Radiation chemistry product yields

Radiolytic ethylene destruction occurs with a yield of ca. 20 molecules consumed/100 e.v. (36, 48). Products containing up to six carbons account for ca. 60% of that amount, and can be ascribed to free radical reactions, molecular detachments, and low order ion-molecule reactions (32). This leaves only eight molecules/100 e.v. which may have formed ethylene polymer, corresponding to a chain length of only 2.1 molecules/ ion. Even if we assumed that ethylene destruction were entirely the result of ionic polymerization, only about five ethylene molecules would be involved per ion pair. The absence of ionic polymerization can also be demonstrated by the results of the gamma ray initiated polymerization of ethylene, whose kinetics can be completely explained on the basis of conventional free radical reactions and known rate constants for these processes (32). An increase above the expected rates occurs only at pressures in excess of ca. 20 atmospheres (10). The virtual absence of ionic polymerization can be regarded as one of the most surprising aspects of the radiation chemistry of ethylene. 

The areas where the use of the track model has been found particularly expedient are (1) LET variation of product yields in the radiation chemistry of liquids (2) the yield of escaped ions and its variation with particle LET (3) energy loss in primary excitations and ionizations (4) radiation-induced luminescence and (5) particle identification. 

In radiation chemistry, the track effect is synonymous with LET variation of product yield. Usually, the product measured is a new molecule or a quasi-stable radical, but it can also be an electron that has escaped recombination or a photon emitted in a luminescent process. Here LET implies, by convention, the initial LET, although the actual LET varies along the particle track also, the secondary electrons frequently represent regions of heterogeneous LET against the background of the main particle. 

Operationally, a procedure may be based on measuring the yield of a reaction traceable to ionization, usually giving a lower limit to the ionization yield. Thus, in the radiation chemistry of hydrocarbon liquids, the product of an electron scavenging reaction (for example, C2H3- radical from the scavenger C2H5Br)... 

In contrast to liquid water, a detailed mechanistic understanding of the physical and chemical processes occurring in the evolution of the radiation chemical track in hydrocarbons is not available except on the most empirical level. Stochastic diffusion-kinetic calculations for low permittivity media have been limited to simple studies of cation-electron recombination in aliphatic hydrocarbons employing idealized track structures [56-58], and simplistic deterministic calculations have been used to model the radical and excited state chemistry [102]. While these calculations have been able to reproduce measured free ion yields and end product yields, respectively, the lack of a detailed mechanistic model makes it very difficult... 

Radiation chemistry highlights the importance of the role of the solvent in chemical reactions. When one radiolyzes water in the gas phase, the primary products are H atoms and OH radicals, whereas in solution, the primary species are eaq , OH, and H" [1]. One can vary the temperature and pressure of water so that it is possible to go continuously from the liquid to the gas phase (with supercritical water as a bridge). In such experiments, it was found that the ratio of the yield of the H atom to the hydrated electron (H/eaq ) does indeed go from that in the liquid phase to the gas phase [2]. Similarly, when one photoionizes water, the threshold energy for the ejection of an electron is much lower in the liquid phase than it is in the gas phase. One might suspect that a major difference is that the electron can be transferred to a trap in the solution so that the full ionization energy is not required to transfer the electron from the molecule to the solvent. 

Understandably, most workers who use radiolysis, photoionization, CTFS, or CTTS as the means for generation of (secondary) radical ions pay little attention to the nature of short-lived precursors of these ions. After all, the subject of interest is a secondary rather than a primary ion. This ad hoc approach is justifiable because radiolytic production is just another means of obtaining a sufficient yield of the radical ion. Quite often in such studies, the radiolysis is complemented by other techniques for radical ion generation, such as plasma oxidation, electron bombardment-matrix deposition, and chemical and electrochemical reduction or oxidation. While the data obtained in these studies are useful, there is little radiation chemistry in such—nominally, radiation chemistry—studies. 

Shortly after the discovery of the hydrated electron. Hart and Boag [7] developed the method of pulse radiolysis, which enabled them to make the first direct observation of this species by optical spectroscopy. In the 1960s, pulse radiolysis facilities became quite widely available and attention was focussed on the measurement of the rate constants of reactions that were expected to take place in the spurs. Armed with this information, Schwarz [8] reported in 1969 the first detailed spur-diffusion model for water to make the link between the yields of the products in reaction (7) at ca. 10 sec and those present initially in the spurs at ca. 10 sec. This time scale was then only partially accessible experimentally, down to ca. 10 ° sec, by using high concentrations of scavengers (up to ca. 1 mol dm ) to capture the radicals in the spurs. From then on, advancements were made in the time resolution of pulse radiolysis equipment from microseconds (10 sec) to picoseconds (10 sec), which permitted spur processes to be measured by direct observation. Simultaneously, the increase in computational power has enabled more sophisticated models of the radiation chemistry of water to be developed and tested against the experimental data. 

In recent years, two different approaches, deterministic [9,19] and stochastic [10,20], have been used with a good level of success to model the radiation chemistry of water. Each approach leads to reasonable agreement between calculated results and experimental data obtained for a wide range of LET from room temperature up toca. 300°C [9,10]. There are, however, fundamental differences between the two models. The deterministic model is based on the concept of an average spur [8,9,19,23] at the end of the physicochemical stage (ca. 10 sec), which contains the products of processes (I), (II), (III), (IV), and (V) in certain yields and spatial distributions, and in thermal equilibrium with the liquid. For low LET... 

Product yields in the radiolysis of water are required for a number of practical and fundamental reasons. Model calculations require consistent sets of data to use as benchmarks in their accuracy. These models essentially trace the chemistry from the passage of the incident heavy ion to a specified point in time. Engineering and other applications often need product yields to predict radiation damage at long times. Consistent sets of both the oxidizing and reducing species produced in water are especially important to have in order to maintain material balance. Finally, it is impossible to measure the yields of all water... 

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. 

Reactivities of a variety of alcohols, ethers, and amides toward hydroxy radicals derived from Fenton s reagent have been compared with those obtained from radiation chemistry in the absence of iron species.728 b Reactivities of different C—H bonds indicate that hydroxyl radical is a strongly electrophilic radical so that electron supply is more important than C—H bond strength in determining its reactivity. Fenton s reagent thus serves as a useful means for studying the one-electron oxidation and reduction of the resulting carbon-centered radicals with iron(II, III) species.73a,b Furthermore, in these systems the addition of copper(II) complexes that can intercept free radicals effectively often leads to enhanced yields of oxidation products.72 73... 

Radiation chemistry of aqueous solutions has also been applied to the study of micellar systems. Considerable micellar effects on the yield of radiolytic products and on rates of radical reactions have been observed by several authors (Gebicki and Allen, 1969 Fendler and Patterson, 1970 Bansal et al., 1971 Patterson et al., 1971, 1972 Fendler et al., 1972 Wallace and Thomas, 1974 Gratzel et al., 1974). These observations led to conclusions on the permeability of micelles to various radicals and on the location of substrates in micelles. Recent experiments have also demonstrated a very efficient trapping of e4q by positively charged micelles even when chemical reaction between them did not take place (L. K. Patterson, personal communication). 

One of the main problems in investigation of mechanochemical transformations consists in the relations between product yield and mechanical energy consumed by the process. Butyagin and Pavlychev [8] proposed to characterize mechanochemical yield by the ratio of the moles of product to the amount of the energy consumed (mol/MJ), similarly as in radiation chemistry. In reality, the researches most often record the dependence of the transformation degree a versus the time of mechanical treatment of powder mixture in mills. If the power of apparatus is known,... 

Radiolysis of alkane liquids fits the classical picture of radiation chemistry, namely, large yields of ions are initially produced which on recombination give rise to excited states and other products. Much radiation chemistry of alkanes is also interpreted in terms of free radicals. The exact connection between the three reactive regimes of excited states, ions, and free radicals is not always clearly established. 

In summary, in this first era of radiation chemistry it was discovered that the medium absorbs the energy and the result of this energy absorption leads to the initiation of the chemical reactions. The role of radium in these systems was not as a reactant or as a catalyst, but instead as a source of radiation. Most quantitative work was done with gases. It was learned that there was a close correspondence between the amount of ionization measured in a gas and the yield of chemical products. Solid and liquid-phase radiolysis studies were primarily qualitative. 

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. ... 

Very few investigations of the radiolysis of nitroalkanes have been reported, and no systematic study of their radiation chemistry has been made. Low molecular weight nitroparaffins were irradiated with y-rays from a cobalt-60 source using dose rates between 0.5 and 2.5 x 10" rad.h and the products analyzed by gas chromatography and mass spectrometry . The yield of gaseous products from irradiated nitromethane was drastically reduced if after a short irradiation, such as is obtained with a linear accelerator, the samples were immediately quenched in liquid nitrogen. Inder these conditions [Pg.668]

Rzad and Schuler" studied the radiation chemistry of a solution of " C-cyclopropane in hexane over the concentration range 10 " to 10 M. The main radioactive products, which appear to result from ion molecule reactions, are propane formed by H2 transfer (50 %) and by H transfer (20 %) and mixed nonanes (30 %) formed by the addition of CaHg unit to a hexyl ion. At the lower concentrations, very pronounced dose dependence of the yields was observed. This was ascribed to a competitive formation of olefins in the radiolysis. For cyclopropane-cyclohexane solutions the chemical processes seem to be considerably more complicated. The observed yield of total radioactive products extrapolated to zero concentration of cyclopropane are 0.05 and 0.11 G units for hexane and cyclohexane, respectively. These limiting yields are of the order of magnitude of and appear to be related to, the free ion yields in these systems. Since cyclopropane was found to react with hydrocarbon ions" it is used quite often as a scavenger for positive ions, as in the work of Davids and coworkers . 

Radiation chemistry and hot atom chemistry of cyclopropane 905 TABLE 7. The yields of the main products in the system T2 +cydopropane ... 

An important difference between the radiation chemistry of water and of organic liquids is that the concept of the spur (a reasonably well-defined volume in which the formation of the reactive species occurs along the track of the ionizing particle) becomes hazy. The radicals formed in water tend to recombine rather than react with the environment immediately after formation. The volume in which recombination is likely defines the spur. The radical products of irradiated organic liquids, however, are more likely to interact with their immediate environment than to undergo recombination. This is evidenced by the low molecular yields of hydrogen from irradiated organic systems. 

Accurate measurement of free-radical and molecular-product yields is important in radiation-chemistry studies on aqueous solutions, for these measurements enable quantitative predictions to be made regarding the extent of chemical changes during irradiations, and lead to an understanding of reaction mechanisms. Therefore, recent research has been directed toward the measurement of these yields, which are generally expressed as G values. An excellent account of the chemical methods used for determining G values... 

The radiation chemistry of cyclic oligoenes was studied, and the radiolytic yields of the final products are summarized in Table 3, which shows that l, 4-cyclohexadiene differs from all others in its high yield of hydrogen, both in the gas phase and in the liquid phase. Cserep and Foldiak attributed it to the presence of two doubly allylic CH2 groups. In addition, the geometric orientation of the allylic hydrogens is favourable for hydrogen... 

The resistance of benzene to radiation damage is reflected in G(H2) 0.004 pmol J" it is more than 100-fold less than the yield in cyclohexane. Other decomposition products are formed in similarly low yields and the radiation chemistry of benzene is dominated by its lowest singlet ( B ) and triplet ( B ) excited-state molecules. By adding suitable solutes the... 

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