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Radiation chemistry

Belongs to subject Radiation chemistry

Radiation chemistry is a subdivision of nuclear chemistry which is the study of the chemical effects of radiation on matter; this is very 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.

As ionizing radiation moves through matter its energy is deposited through interactions with the electrons of the absorber. 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. It is the reactions of the radical species that are responsible for the changes observed following irradiation of a chemical system. 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. 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. 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. 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.

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. 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. This work has been done recently in the USA, often with used nuclear fuel as the radiation source. 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 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 2.7 forms a mixture of the colloid and the soluble Tc(IV) compounds. 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. 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 HO, the HO cation can react with water to form a hydrated proton (HO) and a hydroxyl radical (HO). Furthermore, the solvated electron can recombine with the HO 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 respectively hydrogen atoms and dioxygen radical anions. Some substances can protect against radiation-induced damage by reacting with the reactive species generated by the irradiation of the water. For example, the SF radical formed by the reaction of solvated electrons and SF undergo further reactions which lead to the formation of hydrogen fluoride and sulfuric acid. In water the dimerisation reaction of hydroxyl radicals can form hydrogen peroxide, in saline systems the reaction of the hydroxyl radicals with chloride anions form hypochlorite anions. To process materials, either a gamma source or an electron beam can be used. After the pulse of radiation, the concentration of different substances within the material are measured by emission spectroscopy or Absorption spectroscopy, hence the rates of reactions can be determined. This allows the relative abilities of substances to react with the reactive species generated by the action of radiation on the solvent (commonly water) to be measured.

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