Radiation fundamentals

Development of new applications of radiation modifications of the properties of polymers in high technology industries such as electronics and the exposure of polymer materials to radiation environments as diverse as medical sterilization and the Van Allen belts of space have resulted in a renewed interest in fundamental radiation chemistry of polymers. 

The U.S. - Australia Symposium on Radiation Effects on Polymeric Materials contained research presentations on fundamental radiation chemistry and physics as well as on technological applications of polymer irradiation. This paper represents a hybrid contribution of these two areas, examining a field of extensive technological importance. Spin casting of radiation sensitive polymer resists for microelectronic fabrication was studied using photophysical techniques that are sensitive to the fundamental radiation response in the ultraviolet range. 

This book emphasizes the technological significance of the effects of radiation on polymers and draws attention to the major interactions between fundamental science and advanced technology. Although the field of polymer radiation chemistry is not exhaustively covered, a sampling of the ongoing basic and applied research in this area is presented. Review chapters have been included that cover fundamental radiation chemistry, spectroscopic methods, materials for microlithography, and radiation-durable materials. 

Figure 2.20 The spectral range covered by the fundamental radiation of a Ti-sapphrre laser and the various harmonic generation processes second, third, and fourth harmonic generation (SHG, THG, and FHG, respectively) (courtesy of Quantronix).

Here, we focus our attention on the interplay that exists between solvation processes and ultrafast redox reaction in the vicinity of the strong oxidant hydroxyl radical (OH). This diatomic radical represents one of the most efficient oxidant of cellular components (proteins, lipids, DNA), contributes to Haber-Weiss reaction and plays some important role in fundamental radiation or stratospheric chemistry. Presently, we have investigated short-time water caging effect on transient electron delocalization-relocalization in the vicinity of nascent aqueous OH radicals. This specific electronic channel is represented by Eq.(l). 

Undulator radiation also has specific polarization properties, e.g., the fundamental radiation of a plane undulator as shown in Fig. 1.9 is completely linearly polarized, an undulator with helical magnetic structure produces circularly polarized light, and two crossed plane undulators with a dispersive section between them are capable of producing optional polarization which depends on the phase shift introduced by the dispersive element. 

SHG is a coherent process and in principle the experimental system needed to observe the response is very simple. The fundamental radiation from a laser source incident at an interface generates the harmonic beam via non-linear polarization of the medium. Typically, this beam is observed in reflection, but many studies have been undertaken in total internal reflection and transmission geometries. As the harmonic beam is well separated from the fundamental in frequency, it can be detected the difficulties arise due to the inherent inefficiency of the harmonic generation and the low intensities that need to be detected. The sensitivity and selectivity of SHG to the interfacial species in the presence of the same species in the bulk phase provides the driving force to overcome these experimental difficulties. 

The SHG signals arise from the second-order polarization induced in a non-centrosymmetric medium by the electric field E(a)) of the incident fundamental radiation given by the tensor equation... 

The intensity I(2co) of the SHG signal observed from an interface between two isotropic bulk phases illuminated with fundamental radiation of intensity /(w) is given by 17]... 

Such a one-photon ionization was performed with the use of photons of 10.5 eV/photon (=118 nm), that is, the ninth harmonics of the fundamental radiation of a Nd YAG laser [14]. At this photon energy a considerable number of ablated products can be one-photon ionized. 

Because of the diverse nature of the field, the chapters of this book are authored by several experts in their particular areas of research. The introductory chapter highlights the accomplishments of radiation chemistry during the last century and set out some possible future developments. This is followed by chapters on techniques in ultra-fast radiation chemistry techniques, on techniques using heavy ions, the observation of chemistry using spin and the chemistry evolving from the use of muons. These provide an experimental foundation for the science. After discussion of the techniques, fundamental radiation-chemical... 

The fluorescence lifetime measurements were performed with a streak camera (Agat SF 3M, VNIIOFI, Russia). A Nd YAG laser with excitation wavelengths of 532 and 266 nm (the second and fourth harmonic of fundamental radiation) was used as a light source. The laser radiation parameters of the fluorimeter were as follows (for 532 nm) pulse energy 160 (ij, duration 20 ps (fwhm), beam diameter 5 mm. The error in determining the fluorescence lifetimes in time intervals of several nanoseconds did not exceed 5 %. In addition to the laser equipment, the Cary 100 spectrophotometer (Varian, Inc., USA) and the Cary Eclipse spectrofluorimeter (Varian, Inc., USA slits width was 5 nm) were used for optical density measurements and fluorescence registration, respectively. 

The operating experience with the PAMELA vitrification plant at Belgoprocess has shown that this process has reached a fully developed and reliable industrial level. It has proven its ability to produce high quality final glass products within the narrow specifications as required for long-term storage and disposal. In particular, the outstanding records for occupational doses and radioactivity releases meet completely the fundamental radiation protection objectives for both workers and the public. 

This modification of the Ramsey technique was used by Hansch and coworkers for precision measurements on the hydrogen atom. The fundamental radiation of a cw dye laser at A. = 486 nm was frequency-doubled and through an acousto-optical modulator formed into an equidistant sequence of pulses. These pulses were sent into a high finesse resonator where they formed a standing wave with a corresponding pattern of spatially separated amplitude peaks [1270]. The achieved line width of the two-photon transition at X = 121 nm was below 5 kHz (Fig. 9.65). 

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