Track electrons

Movement of information in a computer could be likened to a railway system. Carriers of information (bits or bytes) move together (like a train and wagons) from one location to another along electronic tracks. It is important that no two bits of information are mixed up, and therefore all the moves must be carefully synchronized with a clock. This situation resembles the movement of trains on a railway many trains use the same track but are not all in the same place at the same time. The railways run to a timetable. Similarly, information is moved around the computer under the control of the central processor unit (CPU). 

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.

FIGURE 3.3 Section of a low-LET track selected to show a blob. On high-energy electron tracks, spurs outnumber blobs by about 50 1. 

FIGURE 3.9 A typical Q8+ ion in water. Secondary electron tracks (without scattering) are shaded the core region is dotted. Figures in parentheses denote ejected electron energy, classical ejection angle, and estimated range (qualitative). Reproduced from Mozumder (1969), by permission of John Wiley Sons, Inc. ... 

Another procedure for calculating the W value has been developed by La Verne and Mozumder (1992) and applied to electron and proton irradiation of gaseous water. Considering a small section Ax of an electron track, the energy loss of the primary electron is S(E) Ax, where S(E) is the stopping power at electron energy E. The average number of primary ionizations produced over Ax is No. Ax where o. is the total ionization cross section and N is the number density of molecules. Thus, the W value for primary ionization is 0)p = S(E)/No.(E). If the differential ionization cross section for the production... 

Zaider and Brenner (1984) have developed computer code for fast chemical reactions on electron tracks Zaider et ah (1983) have performed MC simulation of... 

Bartczak et al. (1991 Bartczak and Hummel, 1986, 1987,1993, 1997) have used random flight MC simulation of ion recombination kinetics for an isolated pair, groups of ion-pairs, and entire electron tracks. The methodology is similar... 

Before a clinical trial starts, the use of technical aids such as IVRT, remote data entry and electronic diaries has to be considered. In Section 7.5.3.3, mention will be made of the use of electronic tracking system that provide status and monitoring reports. All these systems utilise computer systems that must be validated. Double and McKendry described computer validation as the process which documents that a computer system reproducibly performs the functions it was designed to do. The document Guidance for Industry - Computerised Systems used in Clinical Trials published by the FDA in 1999 gives clear recommendations of what is required (also see Section 7.5.4.1). 

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... 

However, this information is absolutely insufficient for explaining the changes that occur in a molecular medium exposed to ionizing radiation. The final chemical transformations are determined by the microstructure of the short-lived primary excitation and ionization regions of the track and by the spatial distribution of the radicals produced. For instance, in order to make the theoretical model of water radiolysis agree with experimental data, it was necessary to give the nonhomogeneous distribution of radicals in electron tracks as initial conditions.7... 

Such a presentation of a fast electron track as a set of spurs, blobs, and short tracks is widely used in radiation chemistry for describing the processes that occur in a condensed medium exposed to electron or gamma radiation.7 However, this presentation is not the only one there is. Other possible approaches are discussed in Ref. 305, where, in particular, the authors note that the most general description of track structures is the one using correlation functions. 

Studying the electron tracks with the Monte Carlo method, the authors of Refs. 302 and 303 have used the so-called stochastic approach, within which one fixes a simultaneous picture of the spatial distribution of excitation and ionization events. The tracks found this way are sets of spatial points where the inelastic scattering events took place. With this at hand it proves to be possible to calculate the energy absorption spectrum in sensitive volumes of the irradiated medium303 and to calculate the shape of the line and the slope of electronic spin echo signals.302 Such a... 

Fig. 16 is a sketch of a photograph of electron tracks produced by passing a narrow beam of X-rays through a moist gas and cooling the gas by a sudden expansion so as to condense moisture on the electrons and positively charged molecules. The direction of the narrow beam of X-rays is indicated by the arrows. It will be seen that the tracks nearly all start from the beam and are all about the same length. The... 

The length of an electron track is found to be proportional to the square of the initial energy of motion of the electron. The greater this energy, the more electrons it can knock out of atoms before it is stopped. 

If a narrow ray of X-rays is passed through the chamber along the dotted line PQ, just before an expansion, then the water vapor condenses on the ions produced by the electrons knocked out of atoms by the X-ray photons. Each ion gives a small droplet so that the electron tracks are made visible and can be photographed. The tracks only last for a very short time and so must be photographed immediately after the expansion. 

When passed through moist air in a C.T.R. Wilson expansion chamber it produces no visible tracks itself and no electron tracks, but it does produce some short thick tracks believed to be due to nitrogen atoms with which the neutrons have collided. 

The density of excitation and ionization is not necessarily the same for all radiation qualities. For example, it is greater along the track of an a-par-ticle than for an electron track. For a primary-recoil electron produced by Co 7-rays in water, the distance between successive ionizations is about 1000 A. TTie ionized track is, therefore, sparse. At each point of ionization, secondary electrons give rise to further ionizations, forming a group of ion-pairs. In contrast, a-particles form a continuous track as a result of overlapping between the spheres of ionization. 

Consider the above example of 100 pieces of information, but instead of being processed serially, the information is carried over a network of ten electronic tracks. Now the time taken to move this information is only 1 msec viz., the system is ten times faster than the serial one. However, simply providing ten new tracks is not an answer in itself. Each track must be provided with its own microprocessor to deal with the information, and the processors must be able to communicate with each other so the information flows in an orderly fashion. (It is no use having two different train tracks if each train arrives at the same platform at exactly the same time ) Thus, the transputer is a microprocessor that has its own memory bank and input and output lines enabling... 

Electrons tracks are less dense than the tracks of heavy charged particles and the spurs are more widely spaced (Chapter 4). The induced physical processes are comparable in many regards to the effect of exposure to heavy particles or ions. X- and gamma rays actually are indirect methods of producing fast electrons in matter. As a consequence, chemical effects can be considered very similar in nature. Some specific features may nevertheless arise from the significantly different dose rates i.e. amount of absorbed energy per unit time) as a consequence of different LET values. 

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