Track structure simulation electrons

Track structure simulation has found application in many areas of radiation research since the pioneering studies of Mozumder and Magee [35]. These studies all employ essentially the same type of approach, a collision-to-collision modeling of the trajectory of the primary radiation particle and of its daughter secondary electrons, with the most significant difference between different calculations being the interaction cross sections used to describe the... 

Stahin, M. G., Hamm, R. N., Tnmer, J. E., and Bolch, W. E. 1994. Track structure simulation and determination of prodnct yields in the electron radiolysis of water containing various solutes. Radiat. Prot. Dosim. 52 255-258. 

The simulation of a heavy ion track structure employs essentially the same methodology as described for energetic electrons except that... 

The IRT method was applied initially to the kinetics of isolated spurs. Such calculations were used to test the model and the validity of the independent pairs approximation upon which the technique is based. When applied to real radiation chemical systems, isolated spur calculations were found to predict physically unrealistic radii for the spurs, demonstrating that the concept of a distribution of isolated spurs is physically inappropriate [59]. Application of the IRT methodology to realistic electron radiation track structures has now been reported by several research groups [60-64], and the excellent agreement found between experimental data for scavenger and time-dependent yields and the predictions of IRT simulation shows that the important input parameter in determining the chemical kinetics is the initial configuration of the reactants, i.e., the use of a realistic radiation track structure. 

The Monte Carlo track structure code kurbuc simulates electron tracks in water vapor for initial electron energies 10 eV-10 MeV [174]. The code kurbuc provides all coordinates of... 

The computer simulation makes it possible to calculate the spatial and energy distribution of ejected electrons and the distribution of ions and excited molecules at different distances from the axis of the track.12"14. Knowing the spatial and energy structure of the track, one can determine the features of primary radiation-chemical reactions in tracks of particles of different nature,15 as well as to describe the evolution of the track and to calculate the yield of radiolysis products.16... 

Figure 1 Simulated ionization track due to an alpha ray of a few MeV of energy coming from the left side in the blue water continuum. Each red circle is an ionization event. This local distribution, in the nanometer range, is the beginning ofa complex chemistry. Ionizations occur mainly around the trajectory axis of this incident ion, and this area is named "core track". Some high energy electrons can be ejected and they can form their own track named "delta ray". When delta rays are sufficiently numerous (that depends on the inciden t ion energy and charge) a new area around the core can be named "penumbra". The penumbra has the characteristics structure ofa "low LET area" because the ionizations are produced by high enrgy electrons.

None of this can be detected by standard geometric criteria. First-principles simulations like CPMD allow for new wave-function-based descriptors [231] as the electronic structure is - in addition to the positions of all atomic nuclei involved -available on the fly . Of course, the above mentioned fundamental problem that the interaction energy is not an observable quantity is in first-principle simulations as apparent as in static calculations. However, the wavefunction naturally tracks all electronic changes in an aggregate. A wavefunction-based descriptor would also be helpful in traditional molecular dynamics because snapshots can be calculated with advanced static quantum chemical methods. 

Theory has played an essential role in the development of radiation chemistry, especially in its early days when experiment was unable to probe the early events that set the stage for the observable chemical reactions. Three themes that were established early on were scattering processes, electronic structure determinations, and track simulations. Work done in the 1950s through 1970s depended greatly on formal development, both because the basic possibilities needed to be identified and because the available computational tools could not tackle the complexity of the problems. The decades that followed... 

The disparate time and length scales that control heterogeneous catalytic processes make it essentially impossible to arrive at a single method to treat the complex structural behavior, reactivity and dynamics. Instead, a hierarchy of methods have been developed which can can be used to model different time and length scales. Molecular modeling of catalysis covers a broad spectrum of different methods but can be roughly categorized into either quantum-mechanical methods which track the electronic structure or molecular simulations which track the atomic structme (see the Appendix). 

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