Radiation chemistry is a subdivision of nuclear chemistry which studies the chemical effects of ionizing radiation on matter. This is quite different from radiochemistry, as no radioactivity needs to be present in the material which is being chemically changed by the radiation. An example is the conversion of water into hydrogen gas and hydrogen peroxide.
Radiation interactions with matter As ionizing radiation moves through matter its energy is deposited through interactions with the electrons of the absorber.J. W. T. Spinks. R. J. Woods: An Introduction to Radiation Chemistry, Third Edition, John-Wiley and Sons, Inc., New York, Toronto 1990. The result of an interaction between the radiation and the absorbing species is removal of an electron from an atom or molecular bond to form radicals and excited species. The radical species then proceed to react with each other or with other molecules in their vicinity. It is the reactions of the radical species that are responsible for the changes observed following irradiation of a chemical system.Turner, J.E. Atoms, Radiation, and Radiation Protection. United States: Pergamon Books Inc., Elmsford, NY, 1986. Print
Charged radiation species (α and β particles) interact through Coulombic forces between the charges of the electrons in the absorbing medium and the charged radiation particle. These interactions occur continuously along the path of the incident particle until the kinetic energy of the particle is sufficiently depleted. Uncharged species (γ photons, x-rays) undergo a single event per photon, totally consuming the energy of the photon and leading to the ejection of an electron from a single atom.Bigelow, R. A. Radiation Interactions in Matter. Electrons with sufficient energy proceed to interact with the absorbing medium identically to β radiation.
An important factor that distinguishes different radiation types from one another is the linear energy transfer (LET), which is the rate at which the radiation loses energy with distance traveled through the absorber. Low LET species are usually low mass, either photons or electron mass species (β particles, positrons) and interact sparsely along their path through the absorber, leading to isolated regions of reactive radical species. High LET species are usually greater in mass than one electron,Essentials of radiation, biology and protection, S. Forshier, Cengage Learning, Jul 22, 2008, p46 for example α particles, and lose energy rapidly resulting in a cluster of ionization events in close proximity to one another. Consequently, the heavy particle travels a relatively short distance from its origin.
Areas containing a high concentration of reactive species following absorption of energy from radiation are referred to as spurs. In a medium irradiated with low LET radiation, the spurs are sparsely distributed across the track and are unable to interact. For high LET radiation, the spurs can overlap, allowing for inter-spur reactions, leading to different yields of products when compared to the same medium irradiated with the same energy of low LET radiation.Simon M. Pimblott, Jay A. LaVerne, J. Phys. Chem., 1994, 98 (24), pp 6136–6143, , Publication Date: June 1994
Reduction of organics by solvated electrons A recent area of work has been the destruction of toxic organic compounds by irradiation;Zhao C et al. (2007) Radiation Physics and Chemistry, 76:37-45 after irradiation, "dioxins" (polychlorodibenzo-p-dioxins) are dechlorinated in the same way as PCBs can be converted to biphenyl and inorganic chloride. This is because the solvated electrons react with the organic compound to form a radical anion, which decomposes by the loss of a chloride anion. If a deoxygenated mixture of PCBs in isopropanol or mineral oil is irradiated with gamma rays, then the PCBs will be dechlorinated to form inorganic chloride and biphenyl. The reaction works best in isopropanol if potassium hydroxide (caustic potash) is added. The base deprotonates the hydroxydimethylmethyl radical to be converted into acetone and a solvated electron, as the result the G value (yield for a given energy due to radiation deposited in the system) of chloride can be increased because the radiation now starts a chain reaction, each solvated electron formed by the action of the gamma rays can now convert more than one PCB molecule.Ajit Singh and Walter Kremers, Radiation Physics and Chemistry, 2002, 65(4-5), 467-472Bruce J. Mincher, Richard R. Brey, René G. Rodriguez, Scott Pristupa and Aaron Ruhter, Radiation Physics and Chemistry, 2002, 65(4-5), 461-465 If oxygen, acetone, nitrous oxide, sulfur hexafluoride or nitrobenzeneA. G. Bedekar, Z. Czerwik and J. Kroh, "Pulse radiolysis of ethylene glycol and 1,3-propanediol glasses—II. Kinetics of trapped electron decay", 1990, 36, 739-742 is present in the mixture, then the reaction rate is reduced. This work has been done recently in the US, often with used nuclear fuel as the radiation source.Energy Citations Database (ECD) - - Document #10116942Process for the solvent extraction for the radiolysis and dehalogenation of halogenated organic compounds in soils, sludges, sediments and slurries - US Patent 6132561
In addition to the work on the destruction of aryl chlorides, it has been shown that aliphatic chlorine and bromine compounds such as perchloroethylene,V. Múka, *, R. Silber, M. Pospíil, V. Kliský and B. Bartoníek, Radiation Physics and Chemistry, 1999, 55(1), 93-97 Freon (1,1,2-trichloro-1,2,2-trifluoroethane) and halon-2402 (1,2-dibromo-1,1,2,2-tetrafluoroethane) can be dehalogenated by the action of radiation on alkaline isopropanol solutions. Again a chain reaction has been reported.Seiko Nakagawa and Toshinari Shimokawa, Radiation Physics and Chemistry, 2002, 63(2), 151-156
In addition to the work on the reduction of organic compounds by irradiation, some work on the radiation induced oxidation of organic compounds has been reported. For instance, the use of radiogenic hydrogen peroxide (formed by irradiation) to remove sulfur from coal has been reported. In this study it was found that the addition of manganese dioxide to the coal increased the rate of sulfur removal.P. S. M. Tripathi, K. K. Mishra, R. R. P. Roy and D. N. Tewari, "γ-Radiolytic desulphurisation of some high-sulphur Indian coals catalytically accelerated by MnO2", Fuel Processing Technology, 2001, 70, 77-96 The degradation of nitrobenzene under both reducing and oxidizing conditions in water has been reported.Shao-Hong Feng, Shu-Juan Zhang, Han-Qing Yu, and Qian-Rong Li, "Radiation-induced Degradation of Nitrobenzene in Aqueous Solutions", Chemistry Letters, 2003, 32(8), 718
Reduction of metal compounds
In addition to the reduction of organic compounds by the solvated electrons it has been reported that upon irradiation a pertechnetate solution at pH 4.1 is converted to a colloid of technetium dioxide. Irradiation of a solution at pH 1.8 soluble Tc(IV) complexes are formed. Irradiation of a solution at pH 2.7 forms a mixture of the colloid and the soluble Tc(IV) compounds.T. Sekine, H. Narushima, T. Suzuki, T. Takayama, H. Kudo, M. Lin and Y. Katsumura, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004, 249(1-3), 105-109 Gamma irradiation has been used in the synthesis of nanoparticles of gold on iron oxide (Fe2O3).Satoshi Seino, Takuya Kinoshita, Yohei Otome, Kenji Okitsu, Takashi Nakagawa, and Takao A. Yamamoto, "Magnetic Composite Nanoparticle of Au/γ-Fe2O3 Synthesized by Gamma-Ray Irradiation", Chemistry Letters, 2003, 32(8), 690
It has been shown that the irradiation of aqueous solutions of lead compounds leads to the formation of elemental lead. When an inorganic solid such as bentonite and sodium formate are present then the lead is removed from the aqueous solution.M. Pospίšil, V. Čuba, V. Múčka and B. Drtinová, "Radiation removal of lead from aqueous solutions- effects of various sorbants and nitrous oxide", Radiation Physics and Chemistry, 2006, 75, 403-407
Polymer modification Another key area uses radiation chemistry to modify polymers. Using radiation, it is possible to convert monomers to polymers, to crosslink polymers, and to break polymer chains.Energy Citations Database (ECD) - - Document #7313004IAEA report - Radiation Formation of Hydrogels for Biomedical Applications; the use of radiation technique - Mechanism of the radiation-induced crosslinking of polymers in aqueous solution Both man-made and natural polymers (such as carbohydratesIAEA-TECDOC-1422) can be processed in this way.
Water chemistry Both the harmful effects of radiation upon biological systems (induction of cancer and acute radiation injuries) and the useful effects of radiotherapy involve the radiation chemistry of water. The vast majority of biological molecules are present in an aqueous medium; when water is exposed to radiation, the water absorbs energy, and as a result forms chemically reactive species that can interact with dissolved substances (solutes). Water is ionized to form a solvated electron and H2O+, the H2O+ cation can react with water to form a hydrated proton (H3O+) and a hydroxyl radical (HO.). Furthermore, the solvated electron can recombine with the H2O+ cation to form an excited state of the water. This excited state then decomposes to species such as hydroxyl radicals (HO.), hydrogen atoms (H.) and oxygen atoms (O.). Finally, the solvated electron can react with solutes such as solvated protons or oxygen molecules to form hydrogen atoms and dioxygen radical anions, respectively. The fact that oxygen changes the radiation chemistry might be one reason why oxygenated tissues are more sensitive to irradiation than the deoxygenated tissue at the center of a tumor. The free radicals, such as the hydroxyl radical, chemically modify biomolecules such as DNA, leading to damage such as breaks in the DNA strands. Some substances can protect against radiation-induced damage by reacting with the reactive species generated by the irradiation of the water.
The reactive species generated by the radiation can take part in following reactions; this is similar to the idea of the non-electrochemical reactions which follow the electrochemical event which is observed in cyclic voltammetry when a non-reversible event occurs. For example, the SF5 radical formed by the reaction of solvated electrons and SF6 undergo further reactions which lead to the formation of hydrogen fluoride and sulfuric acid.K.-D. Asmus and J.H. Fendler, "The reaction of sulfur hexafluoride with solvated electrons", The Journal of Physical Chemistry, 1968, 72, 4285-4289
In water, the dimerization reaction of hydroxyl radicals can form hydrogen peroxide, while in saline systems the reaction of the hydroxyl radicals with chloride anions forms hypochlorite anions.
The action of radiation upon underground water is responsible for the formation of hydrogen which is converted by bacteria into methane.
Equipment
Radiation chemistry applied in industrial processing equipment