# Photon-In/Photon-Out X-ray Free-Electron Laser Studies of Radiolysis
## 3 Discussion and Applications
Radiolysis of water by gamma rays is indistinguishable from that by fast electrons. The initial energy loss by fast charged particles to a medium water molecule can vary widely according to kinematic limits, but the average energy loss is about 60 eV. This average is nearly constant and independent of the initial energy of electrons and heavy ions [64]. A 60 eV electron will scatter significantly along its path as it produces a series of lower energy ionization events. The result is a localized cluster or spur of ionized and excited water molecules that will decay as described above in the physico-chemical stage. Each initial event is a stochastic process so the number of ionized and excited water molecules in a spur varies from about one to six [65]. The true chemical stage starts at about 100 picoseconds when diffusion begins and lasts until about a microsecond. During this period the reactions of the sibling products within a spur will react and diffuse as the non-homogeneous spur decays. Heavy ions have a much higher rate of energy loss than electron so the spurs overlap to form a track. Track effects generally describe the chemical stage for fast electrons and heavy ions and the competition between reaction and diffusion is responsible for the observed difference in product yields. A variety of time resolved techniques are available for directly probing the temporal variation of radicals in the radiolysis of water during the chemical stage. Pulse radiolysis with electrons in the MeV energy range can use a variety of optical detection techniques to directly probe radical reactions. The single pulse dose from the sources is considerably greater than typically found with gamma rays but the results are expected to be the same. Long-term effects are considered to be due to reactions of radicals with solutes or at interfaces near to the water medium. Variations in radical yields and thereby long-term effects are dependent on the number of radicals produced in the physico-chemical stage as well as the subsequent track chemistry in the chemical stage. Examination of radiation effects on long time scales can utilize any number of standard spectroscopic or chromatographic techniques. Gamma rays are impractical for examination of real-time radical chemistry because they are continuous in nature. However, scavenging techniques are ideally suitable for use with continuous radiation sources. In addition, scavenging techniques can be used over an incredibly wide range of times. This technique makes use of a solute, scavenger, to react with a specific radical to give a measurable stable product. The scavenging capacity is defined as the product of the scavenger concentration and the rate coefficient for the scavenging reaction and represents the inverse of the lifetime of the radical. Observation of final product yields with variation of the scavenger concentration is a simple technique for estimating the radical kinetics. An even more sophisticated approach utilizes the relationship that the inverse Laplace transform of the scavenging capacity dependence directly gives the time dependence of the scavenged radical [70]. Scavenging techniques are commonly used where pulse radiolysis cannot be used as in gamma or heavy ion radiolysis. Several studies have shown that scavenger techniques are reliable to give real time information on radical chemistry [70]. As discussed above for pure water solutions, complex solutions containing solvated ions and ionic solids such as metal-oxides also undergo intra- and inter-atomic Auger decay and possibly ICD and ETMD involving the interfacial water molecules. A recent low-energy electron scattering study that simulates the secondary electron interactions  
Figure 5: **(a)** Composition estimate for Hanford tank waste, which shows relatively high proportions of Na and Al. Data adapted from Ref. [66]. The composition of the waste can promote crystallization and aggregation of Al hydroxides.