ABSTRACT
Among all the radiation-chemical reactions that have been studied in solution, the most importantisthedecompositionofpurewater,the“lifesolvent”itself.Atthebeginningofthelastcentury,fruitfulobservationsweremadeontheradiationchemistryofwater(Giesel,1900;Curieand Debierne,1901;Kernbaum,1909;DuaneandScheuer,1913).Animportantbreakthroughisdueto Fricke’sstudiesonx-rayeffectsthatappearedtobeverysimilartothoseofγ-rays(Fricke,1934). Since that demonstration, a wealth of data has been obtained to understand the mechanisms of water decompositionunderirradiation,particularlyaftertheadventofnuclearenergyandtheextensive researchprogramaroundtheManhattanProject(Allen,1989).Oneofthesingularaspectsofradiationchemistryisenergydepositioninthemedia,characterizedbylinearenergytransfer(LET).In contrasttoUV-visibleandIRlightabsorptionthatishomogeneous,theenergyofionizingradiation is nonhomogeneously deposited in solutions. The direct visualization of α-particles branch tracks in theWilsoncloudchamberdemonstratedthenonhomogeneityoftheenergydeposition.Themacroscopicvisualizationofsuchaneffectcanalsobeperformedwiththeelectronbeamofanacceleratorfocusedonatransparentblockofpolymers(Figure12.1)(Gross,1957,1958).Thisimportant aspectinradiationphysicsandchemistryledtothedevelopmentofkineticsofnonhomogeneous processes at the beginning of the last century (Freeman, 1987). When an ionizing radiation passes throughasolution,itlosesenergyandgeneratesexcitedorionizedmolecules.Theejectedelectrons, calledsecondaryelectrons,possessenoughkineticenergytofurtherionizeandexcitethemolecules andtobreakandbuildchemicalbonds.Theinitialevent,energydeposition,classiŸedasradiation
FIGURE 12.1 This Captured Lightning® sculpture is created by irradiating a block of acrylic (Plexiglas) by a beam of electrons from a 3 MV particle accelerator. The Ÿrst Lichtenberg Ÿgures were actually twodimensional patterns formed in dust on the surface of charged insulating plates in the laboratory of their discoverer, German physicist Georg Christoph Lichtenberg (1742-1799). Pioneering research on the detailed behavior of charge storage and movement within dielectrics was performed by Bernhard Gross in the early 1950s. Gross conŸrmed that internal Lichtenberg Ÿgures could be created within a variety of polymers and glasses by injecting them with high energy electrons using a linear accelerator (LINAC).
