Ionizing track

As a high-energy particle passes through matter, it creates an ionization track that contains positive ions. These ions are chemically reactive because their bonds are weakened by the loss of bonding electrons. Even though each cation eventually recaptures an electron to return to electrical neutrality, many ions first undergo chemical reactions that are the source of the damage done by nuclear radiation. 

Living cells are delicately balanced chemical machines. The ionization track generated by a nuclear particle upsets this balance, almost always destroying the cell in the process. Although the body has a remarkable ability to repair and replace damaged cells, exposure to radiation can overload these control mechanisms, causing weakness, illness, and even death. 

The hydrated electron is commonly detected by pulse radiolysis with electron beams. In the case of highly-structured track, this very reducing species reacts easily with oxidant like OH radical in its vicinity at earliest time after the ionization track is formed. That is the reason why it is a real challenge to detect this species yet with heavy ion irradiation. Giving a G-value remains delicate because the concentrations are lower than 10 M and dose must be measured with a high accuracy. As it is shown in Fig. 5, the time dependence of hydrated electron is typical of a track structure in space and time in the first 100 ns the concentration of initial hydrated electron is at least divided... 

The superoxide radical (HOj/Oj") is a peculiar case in water radiolysis. Its yield of production increases with LET which is completely contrary to the recombination rule in dense ionization tracks. Actually, the general trend is that these radical recombinations increase the production of molecular species (H2 and H2O2). The low reactivity of this radical in pure water essentially due to its... 

Several methods have been published to simulate the time-evolution of an ionization track in water. Monte Carlo (with the IRT method or step-by-step) and deterministic programs including spur diffusion are the main approaches. With the large memory and powerful computer now available, simulation has become more efficient. The modeling of a track structure and reactivity is more and more precise and concepts can now be embedded in complex simulation programs. Therefore corrections of rate constants with high concentrations of solutes in the tracks and the concept of multiple ionizations have improved the calculation of G-values and their dependence on time. 

The relatively low overall yields of radicals were attributed to the high recombination rate of closely spaced base ion radicals in the densely ionized track core. The proximity of these radicals coupled with Coulomb attractions facilitates fast core ion radical-ion radical recombination. However, neutral sugar radicals in the core are not affected by Coulomb attractions, thus they do not recombine as readily. Therefore, most of the neutral sugar radicals stabilized at 77 K are presumed to form in the core. On the other hand, most of the base radicals that are stabilized at 77 K are assumed to form in the isolated, low LET-like spurs formed by delta-rays. The similarity in the behavior of the base radicals in argon ion-beam irradiated samples and in y irradiated samples lends support to this picture.In this model C(N3)H is in equilibrium with C and is found to act as an ion-radical. 

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. 

At the chemical level, a solute molecule (DNA, RNA, and protein) in a biological system can be affected by radiation in two different ways. When an ionization track passes either directly through a molecule or close enough so that the created ions can drift to and interact chemically with the molecule before they recombine and neutralize in solution, the phenomenon is called a direct radiation effect. On the other hand, since the largest fraction of almost any biological system consists of water (e.g., 70-80% of a typical cell), the most frequent initial radiation interactions will be with water molecules. When this occurs, ion radicals and free radicals are created. 

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.

The particular case of superoxide radical (HO 2/Op in water radiolysis comes from the property that its formation yield increases with LET, a behavior contrary to fast and natural recombination of other radicals in dense ionization tracks [4]. Logically, this increased... 

Hurst, G.S., "One-atom detection in individual ionization tracks," Opt. Letters, 1978, 16-18. 

Although the effects of spin coherence have been mainly studied using radiation-chemical processes as an example, published are the first works on the MARY spectra of radical ion pairs produced in solutions by photoionization. Probably, there are no principle obstacles to the application of the method of quantum beats to these systems. Interpretation of results is expected to be more simple, in this case, because of the use of monochromatic sources of ionization and the absence of cross recombination effects typical of the ionization track. Another manifestation of spin coherence, observed experimentally but omitted in this review, is the beats induced by resonance microwave pumping [36-38]. The range of applications of this phenomenon for studying spin-correlated radical ion pairs has yet to be outlined. 

Figure 32 Two cases for the orientation of ionizing track and field.

Fission fragments produce dense ionization tracks from which electron escape is very low. Even at a field strength of 38 kV/cm, only 3% of the initial ionization is collected in liquid argon (Hitachi et al., 1987). 

Electron transfer through the liquid/vapor interface can be studied by various methods which differ only in detail. A diode test cell is used which is filled with liquid to some level between the electrodes (see Figure 14). Electrons are injected into the liquid at the cathode either by the photoelectric effect or from the ionization tracks produced by radioactive sources deposited at the electrode surface. Irradiation of liquid and vapor space by a burst of high energy X-rays can also be used. Under an applied electric field the electrons move toward the liquid/vapor interface. At the interface, they encounter a barrier where they will be trapped for a certain time x. 

In another technique, the liquid/vapor interface is filled continuously with electrons from ionization tracks at the cathode (Schoepe and Rayfield, 1971). A steady-state current is measured at the anode. The electron supply is then interrupted electrically by application of a suitable voltage to a grid adjacent to the cathode. The current at the anode decreases in time as described by Equation 36 (see Figure 16). 

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