The complete understanding of radiation chemical reactions is very difficult, because above reactions are controlled by the many parameters such as physical constants and different LETs of quantum beams. The radiation induced reaction will start at the excitation and ionization of solvent or bulk material and then the energy should be transported to the solutes or the objected sites. The reaction will be changed by the LET of the beam, solvents (or bulk material) and the dielectric constants of the system, etc. In such a way, even for the experts in this field, complete understanding of reactions has been seriously difficult.

Thus, sometimes, radiation chemistry was pointed as “dirty science” by other researchers, who are in the closed field such as photochemistry. However, the commercial, medical and industrial applications of ionizing radiation are widely progressed in the society. Under the condition, the basic and complete understandings of radiation induced reaction are highly appreciated. In this manuscript, the author intended to describe the outlook of the radiation chemical reaction as clear as possible.

The direct observation of short-lived intermediate species would be very significant for understanding of primary reactions or processes in radiation chemistry. An ultrafast pulse radiolysis is a powerful technique that has the potential to probe the kinetics and reactions of the intermediate species with femtosecond/pico-second temporal resolution. In this report, we also presented some demonstrations of observation of hydrated and pre-hydrated electrons in water using pulse radiolysis technique.

In quantitative studies on radiation chemistry, or various responses of materials to radiation, knowledge of amount of energy absorbed in the material from the radiation should be required, if possible, knowledge of spatial distribution of the energy, as well. Radiation dosimetry which includes some methods such as calorimetry, ionization chambers, chemical dosimetry etc. can serve to give these quantities. Recently, numerical calculation of transport of radiation based on the Monte Carlo method is increasingly adopted to estimate the radiation doses.

Basic knowledges on photon and charged particle interactions with matters have been summarized in terms of cross sections for partial processes.

Radiation chemistry of water and aqueous solutions has been intensively studied for many years, over the past century since the discovery of radiation. Relevant to the various application fields of engineering and radiology etc., plenty of fundamental processes have been revealed. Nevertheless, there still exist many controversial issues, such as ultrafast processes and reaction kinetics in different phase (supercritical state), to which much attention is still attracted even now. In this chapter, essences of radiation chemical work of water and aqueous solutions, including historical activities, fundamental processes, and approaching methods etc., will be reviewed.

Radiation chemical methods have been applied to various studies on chemical reactions for years, because redox active species generated by radiolysis can efficiently generate reaction intermediates of target chemical reactions. In this article, basic aspects of radiation chemical methods are summarized first. Then, our experimental results on reaction mechanistic studies are summarized as examples for wide applicability of radiation chemical methods.

Radiation induced ionization and thermalization, and geminate ion recombination in the condensed matter was described. Whether the geminate ion recombines each other or escapes to free ions is determined by the relation between the thermalization distance and the Onsager distance indicating the effective distance of the Coulomb interaction. This kinetic are represented by the Smoluchowski equation that means the Brownian motion of electrons and radical cations with the Coulomb interaction. This is determined by the material properties such as the diffusion constant of the electron and radical cation, the dielectric constant, the temperature and the thermalized distance distribution function. By analyzing the experimental data of geminate ion recombination, it is possible to obtain the diffusion constant and the thermalized distance distribution function in the condensed matter.

Understandings of radiation-induced chemical reactions in a solid phase have been advanced by stationary spectroscopic measurements of unstable intermediates using a low-temperature matrix-isolation (MI) technique. In this article, the low temperature MI technique is briefly explained, showing high-resolution ESR spectra of paramagnetic intermediates (H atom, CH₃^· radicals, CH₄^+· radical cations, H…CH₃ radical pairs) generated by radiolysis of methane in a solid argon matrix at 4 K, and elementary processes of those radical formation reactions are discussed.

Effect of ionizing radiation for polymers has been paid attention to from 1940’s as newly polymer modification method. The elucidation of phenomena in polymers induced by ionizing radiation, the establishment of evaluation method of radiation effects, and the industrial application have been developed remarkably and today these achievements have led to many practical applications. In this chapter, we took a brief look at historical story about ionizing radiation effects for polymers, and overviewed characteristic phenomena induced by ionizing radiation and the analysis method.

Among the pathways of radiation-induced damage to DNA, the formation of H₂O^·+ and an electron by direct energy deposition in the hydration layer of DNA plays a crucial role in the initial stage. The formation of neutral sugar-phosphate radicals originating from the deoxyribose-phosphate backbone contributes a large fraction of strand breakage. In addition, the interaction of low-energy electrons with the DNA molecule, and the formation of clustered DNA damage are discussed here.

Monte carlo simulations of water radiolysis chemistry including solvation of electrons and quantitative study of DNA double strand breaks is introduced as an interface of radiation chemistry and biology. In addition experimental study for formation of cloud condensation nuclei by ionizing radiation is introduced as incredible important study for the demonstration of cloud formation by cosmic-ray, which may govern climate change of the earth.

Practical uses of ion beams have been being widely spread especially in medical and industrial fields. Ion beams show a wide variety of irradiation effects depending on ion’s mass (atom) and energy (velocity). Two of the greatest characteristics of ion beams are briefly summarized in this article: one is high density of ionization events near radiation trajectory (track) and the other is nuclear collisions between ions in beam and atomic nucleus in matter. Specifically, correlation between track structure in water and yields of water decomposition products as well as typical mechanism of damage formation in crystalline materials are shown. In addition, future perspectives related to some open questions are also mentioned. Figures 1 and 3 are used with permission of Radiation Research.