Theoretical Investigation Of The Biological Effects Of Ionizing Radiation

The 3-dimensional track structure(3DTS) is the main starting point of theoretical work in the study of radiation biological effect. In recent years, the new features and trends in this field of research include: 1) electrons with energy of several electron-volts(ESEV) were confirmed as a new mechanism inducing DNA strand breaks, and the track simulation of ESEV became a new direction of 3DTS; 2) to establish mechanism-oriented model on the basis of 3DTS; 3) the 3DTS generation in tissue-equivalent (TE) materials; 4) the bystander-effect and the relevant studies. Works reported in this dissertation were carried out around the points stated above.Some 3DTS-related concepts and theories were retrospected in chapter 1.Based on the cross section data reported in literature, we generalized the 3DTS model to 0~10 eV energy region and implemented the simulation code in chapter 2. With this code we studied the spatial distribution of low-energy electron (LEE) in track structure and the range distribution of LEE. Further theoretical work can be done with the tool.Chapter 3 proposed the concept of clustering clusters, which is a more meaningful description of quality factor. Using this concept and the concept of nanometer ionization spectrum, we proposed the Ionization Clustering Cluster model (ICC). The calibrated ICC model can give better description for the experimental data of Gl/S and Late-S V79 cell irradiated by protons and α-particles of different energies. Furthermore, we verified the approximate equivalence of the dynamics model, the linear quardratic (LQ) model and the ICC model. Then the physical meanings of the model parameters were analyzed in terms of dynamics.In chapter 4, the applicability of the condensed-history (CH) algorithm in problems of microscopic scale was explored. A criterion was proposed to determine whether the CH algorithm can be used: the penetration range R is much smaller than the characteristic length / of the problem. We found that the CH method can be used in the problem of characteristic length larger than 100 nm. Because the CH algorithm is less dependent on cross section data, it can simulate the experimental environment of actual materials. And the calculating time is reduced by more than an order of magnitude.In chapter 5 the dose distribution of Electron Microbeam (EMB) device was calculated with the 3DTS code of CH type. The single event size distribution agrees well with the results of event-by-event code reported in the literature. The calculated neighbor-to-target ratio, Ryvr. attains a maximum value around 60-70 keV , which indicates that electrons of these energies should be avoided in the experiments. Furthermore, the effect of finite beam-width were studied and it was found thatthe single event size distribution in HeLa cell nucleus, in cytoplasm and in neighbors shift continuously with the beam-width. And Rnt increases with the beam-width. The results are useful for the design and calibration of EMB.