LET effects

The quantitative aspects of track reactions are involved some details will be presented in Chapter 7. The LET effect is known for H2 and H202 yields in aqueous radiation chemistry. The yields of secondary reactions that depend on either the molecular or the radical yield are affected similarly. Thus, the yield of Fe3+ ion in the Fricke dosimeter system and the initiation yield of radiation-induced polymerization decrease with LET. Numerous examples of LET effects are known in radiation chemistry (Allen, 1961 Falconer and Burton, 1963 Burns and Barker, 1965) and in radiation biology (Lamerton, 1963). 

Figures 3.3 and 3.4 show sections of low-LET tracks chosen to show a blob and a short track, respectively. Numerically, spurs dominate over blobs and short tracks. On the other hand, the fraction of energy held up in the extraspur entities is significant, and in a real sense these represent LET effect in electron tracks.

At low densities, the molecules are far apart. Therefore, there is little LET effect. 

Recent studies on LET effects In 2 mm thick samples of polymers have shown no difference between fast neutron and Co60 gamma irradiations for poly(olefIns) (32), a small difference for... 

Gervais B, Beuve M, Olivera GH, Galassi ME (2006) Numerical simulation of multiple ionization and high LET effects in liquid water radiolysis, Radiation Physics and Chemistry 75 493-513... 

For a given medium such as water, its density and the velocity of the incident heavy ion contribute to the formation of the columnar track structure. The spacing of the primary energy loss events must be within the delocalization of the secondary electrons for elfects attributed to high LET to occur. For this reason, typical LET effects will not be... 

A summary of the clinical results obtained with neon ions in Berkeley is presented in Table 7 [4,47]. Some fast neutron therapy results are also presented. Although the recruitments are not comparable, it should be pointed out that tumor types or sites for which an advantage was found with neon ions are those for which an advantage was also found with fast neutrons. This suggests a specific high-LET effect. 

By its very nature this book is interdisciplinary. The first eleven chapters delineate the fundamentals of radiation physics and radiation chemistry that are common to all irradiation effects. Chapters 12 and 13 deal with specific liquid systems, while Chapter 14 is concerned with LET effects. Chapters 15 to 18 describe biological and medical consequences of photon and charged-particle irradiation. The rest of the book is much more applied in character, starting with irradiated polymers in Chapter 19 and ending with applications of heavy ion impact in Chapter 27. 

Some new trends can be recognized in the points such as the interaction of short-lived active species in some spatial distributions measured by spin echo and pulse radiolysis methods. The application of polymers for drug-delivery systems is here discussed with reference to low temperature radiation polymerization techniques. Ion beam irradiation of polymers is also reviewed for which further research is becoming important and attractive for so-called LET effects and high density excitation problems. In the applied fields the durable polymers used in strong and dense irradiation environments at extremely low temperature are here surveyed in connection with their use in nuclear fusion facilities. 

Very recently Kouchi et al. constructed an ion beam pulse radiolysis system and use it for the study of the LET effect in irradiated polystyrene thin films [106]. The nanosecond pulsed MeV ion beam with the variable repetition rate was obtained by chopping ion beams from a Van de Graaff. Time profiles of the excimer fluorescence from polystyrene thin films, excited by He+ impact, were... 

LET Effects on Crosslinking, Main Chain Scission, Solubility Change,... 

For the investigation of the LET effect on the time profile, it is important to measure the time profiles in the following three energy regions for the same kind of ions (see the stopping power curves in Fig. 8). 

Very recently. LET effects on fluorescence lifetimes of low molecular polyethylene model compounds (n-alkane) have been studied by many kinds of pulse radiolysis - methods such as electron beam, ion beam and synchrotron radiation (SR) [40] pulse radiolysis techniques [41]. Figure 10 shows time profiles of the fluorescence from neat n-dodecane liquids irradiated many kinds of radiation with different LET. The fluorescence lifetimes from irradiated neat... 

Recently spin-coated PMMA thin films with a thickness of 0.45 pm on silicon wafer were irradiated with various ion beams (H+, He+, N+, Ni3+). Ion beam energy regions are from 300 keV to 4 MeV. Irradiated PMMA films were developed by isopropyl alcohol in these experiments. After the irradiation by ion beams on PMMA in a vacuum, the thickness of the films were measured both before and after development. Various radiation effects on PMMA films such as ablation (sputtering), main chain scission, and positive-negative inversion were observed as shown in Fig. 11. These phenomena are very different from those in 60 Co gamma-ray or electron beam irradiation. Large LET effects are considered to be due to high density excitation by ion beams. 

The application of ion beams to polymers has been worthy of remark in the fields of advanced science and technology since the radiation effects of ion beams on polymers are different from those of conventional radiation such as electron beams and gamma-rays. The effects of ion beams are called LET effects but the detailed mechanisms of these effects on polymers have not been elucidated so far. So-called high density excitation effects such as carbonization, blackening, ablation and formation of nuclear tracks, which only occur at high densities, have been studied by a number of advanced spectroscopic methods. 

Very recently LET effects of ion beams on both standard polymers such as polystyrene and low molecular polyethylene model compounds (n-alkanes) have been studied by time-resolved spectroscopic methods, that is, ion beam pulse radiolysis techniques. Further basic studies are necessary so that the detailed mechanisms of ion beams on polymers can be clarified, especially LET effects and high density excitation effects. 

A. Henglein and W. Schnabel, Radiation chemistry of synthetic macromolecules in solution, LET effects, North-Holland, Amsterdam, 1967. 

The peculiarity related to large and localized energy deposition is sometimes called as Linear Energy Transfer (LET) effects. LET effects on radiation... 

Stopping power can not be the universal parameter though it is still good one, that is, even at a fixed stopping power, the velocity of ions can severely affect the irradiation effects [41, 97]. The similar example was already found for liquid hydrocarbons as seen in the previous section [68], These findings have evoked the concept of radiation-quality effect, as a natural extension of LET effect. 

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