Radiation chemistry sources

Flowever, in order to deliver on its promise and maximize its impact on the broader field of chemistry, the methodology of reaction dynamics must be extended toward more complex reactions involving polyatomic molecules and radicals for which even the primary products may not be known. There certainly have been examples of this notably the crossed molecular beams work by Lee [59] on the reactions of O atoms with a series of hydrocarbons. In such cases the spectroscopy of the products is often too complicated to investigate using laser-based techniques, but the recent marriage of intense syncluotron radiation light sources with state-of-the-art scattering instruments holds considerable promise for the elucidation of the bimolecular and photodissociation dynamics of these more complex species. 

The electron itself is frequently used as a primary source of radiation, various kinds of accelerators being available for that purpose. Particularly important are pulsed electron sources, such as the nanosecond and picosecond pulse radiolysis machines, which allow very fast radiation-induced reactions to be studied (Tabata et al, 1991). Note that secondary electron radiation always constitutes a significant part of energy transferred by heavy charged particles. For these reasons, the electron occupies a central role in radiation chemistry. 

Poly(methylmethacrylate), PMMA, Is a well-known degradable polymer in the radiation chemistry of macromolecule (1). Hatzkis reported that PMMA is an excellent resist material usable in the microfabrication technology for manufacturing the microelectronic devices where X-rays and electron beams are used as radiation sources (2). 

Also noted in Figure 7A are three small ion mobility peaks at drift times of about 28,30 and 33 ms. These unwanted ions are formed in the ion source by the clustering of the Cr ion to HCl, HCOOH, and CHjCOOH. These impurities are not introduced with the buffer gas, but are formed by the Ni-induced radiation chemistry that is continuously occurring in the ion source. Because these ions do not interfere with the IM reaction of interest in Figure 7, their presence can be ignored as long as their... 

There have been remarkable advances in synchrotron radiation research and related experimental techniques in the range from the vacuum ultraviolet radiation to soft X-ray, where the most important part of the magnitudes of these cross-section values is observed, as shown below. Therefore, it is also concluded that synchrotron radiation can bridge a wide gap in the energy scale between photochemistry and radiation chemistry. Such a situation of synchrotron radiation as a photon source is summarized in Fig. 1 [5,6]. 

The ionization of a molecule and the rupture of a chemical bond by ionizing radiation necessarily result in the pairwise formation of radical species. The pairwise correlation of radical species will be more or less retained in solid polymers where the radical migration is restricted. This heterogeneity of spatial distribution of radical species affects the radiation chemistry of polymers. Another source of spatial heterogeneity is the heterogeneous deposition of radiation energy [6, 7]. Low LET radiations such as y-rays produce an ensemble of isolated spurs. Each spur is composed of a few ion-pairs and/or radical... 

It is difficult to span the intervening energy gap between photo- and radiation chemistry, however, high powered pulsed lasers, utilising multiphoton absorption by the medium, do much to remedy this situation. For the most part, the work described falls into two categories, data with steady state irradiation i.e. light sources and Co-y rays, and pulsed experiments as with lasers and pulsed electron accelerators such as Van de Graaffs and Linacs. 

Early work in this field was conducted prior to the availability of powerful radiation sources. In 1929, E. B. Newton "vulcanized" rubber sheets with cathode-rays (16). Several studies were carried out during and immediately after world war II in order to determine the damage caused by radiation to insulators and other plastic materials intended for use in radiation fields (17, 18, 19). M. Dole reported research carried out by Rose on the effect of reactor radiation on thin films of polyethylene irradiated either in air or under vacuum (20). However, worldwide interest in the radiation chemistry of polymers arose after Arthur Charlesby showed in 1952 that polyethylene was converted by irradiation into a non-soluble and non-melting cross-linked material (21). It should be emphasized, that in 1952, the only cross-linking process practiced in industry was the "vulcanization" of rubber. The fact that polyethylene, a paraffinic (and therefore by definition a chemically "inert") polymer could react under simple irradiation and become converted into a new material with improved properties looked like a "miracle" to many outsiders and even to experts in the art. More miracles were therefore expected from radiation sources which were hastily acquired by industry in the 1950 s. 

The discussion in this section is primarily based on Lind s book. Much of the early work in radiation chemistry was done either with radium sources and/or radon sources. These sources produced primarily alpha rays and weak beta rays. The lack of penetrating power of these particles made early experiments very difficult. 

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. 

The development of the sources led to studies in polymers, solids, organic systems, which were too numerous to mention. One only needs to look at the chapters by Dole, Willard and others in the book on the history of radiation chemistry to find the wide range of chemical systems that could be studied. ... 

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. ... 

Early on, the differences in the products from radiolysis of water were noticed. It was found, as mentioned above, that the radiolysis of pure water seemed to lead to almost no damage. If there were impurities in the water, radiation damage would occur. However, irradiation by high LET radiation would clearly lead to the formation of hydrogen. Further experiments showed that if one irradiated a sealed sample with high-LET radiation, and then the sample was irradiated with a low-LET source, the gas formed by the radiation would then disappear and it would appear as if there were no long-term decomposition. These data were part of the reason that A. O. Allen proposed the theory of radiation chemistry where back reactions occurred. ... 

Wishart JE. (1998) Accelerators and other sources for the study of radiation chemistry. In Photochemistry and Radiation Chemistry, Wishart JE, Nocera DG (eds.) Adv Chem Ser, Vol. 254, pp. 35-50. American Chemical Society, Washington, DC. 

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. 

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]

Finally, it is noteworthy that in the particular field of radiation chemistry, radiation catalysis may permit a better utilization of the radiation energy, and considered from this standpoint, might constitute a source of practical applications of the radiations. 

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. 

Because of their high intensity. X-ray tubes were commonly used as laboratory radiation sources for radiation chemistry experiments until they were superceded by particle accelerators during the middle part ofthe 20th Century. They still retain specialized uses in research applications such as being used as the radiation source for MARY (MAgnetic field effect on Reaction Yield) spectroscopy studies of radical cation lifetimes and reactivity in alkane solvents [14,18]. MARY spectroscopy uses fluorescence to detect variations in singlet-triplet dynamics in radical ion pairs as a function of magnetic field. It is particularly useful for short-lived transients that are difficult to study by ESR. 

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