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.<ref>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</ref> Gamma irradiation has been used in the synthesis of nanoparticles of gold on iron oxide (Fe2O3). 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. 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.IAEA 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 carbohydrates) 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