Ionization spurs

It is also probable that free radicals are formed from excited water-molecules outside the ionization spurs. These would contribute to the net formation of hydrogen and hydroxyl free radicals. Therefore, although important refinements have been added, the fundamental description of the action of ionizing radiations on water remains as summarized by Allen. ... 

Figure 7 Picosecond kinetics of hydrated electron recombination in the ionization spurs as a function ofthe temperature [7], Hydrated electron Is produced In 15 ps pulse of 8 MeV electrons delivered by the electron accelerator ELYSE. The absorbance of 2 cm of pure water Is analyzed by a laser at 790 nm. Due to the red-shift of the hydrated electron spectrum with Increasing temperature, the absorbance maximum value at790nm decreases from 23°C to350 °C. In the time range of 3 ns, the recombination of hydrated electron appears less and less efficient with Increasing temperature.

It is clear that even radiations of lowest LET value, most penetrating, must deposits concentrations of energy in narrow zones on the molecular level. From experiments on aqueous solutions and later on solids, one can assume, that ca 80% of energy is deposited in single ionizations. That proportion turns into increase of participation of multi-ionization spurs with the increase of the LET value of the applied radiation. Effects of proton beam of comparatively low energy of 10 MeV are already easy to recognize. 

Fig. 1. Origin of multi-ionization spurs electrons of last generation deposit the energy in a small region (ca 20% of total absorbed energy of low LET radiation). 

The next example of comparatively simplicity, this time nonaqueous, is the crystalline alanine. There are several products of irradiation of that solid crystalline amino acid. In this state it occurs as zwitterion as NMR shows, i.e. the amine group is protonated -N+H3. Single ionization spurs, of a low energy, cause deamination which leads to detachment of ammonia and formation of a free radical. Pulse radiolysis of single crystals of L-alanine shows, that the alanine derived radical CH3-C H-C02-, which shows the spectrum with maximum at 348 nm [9], stabilizes during 5 milliseconds [10], It is usually observed not spectroscopically but by the EPR method [11] it shows extreme stability, being applied as reference dosimeter. 

Fig. 2. Multi-ionization spur in C,H polymers chain scission, and if spur energy > 100 eV, also debris present, e.g. acetylene, methane and increased yield of hydrogen.

It is astonishing to realize, that these authors did not make a conclusion, that low molecular weight products can form only as the result of dramatic disruption of the chain. Subtle changes caused by single ionization spurs are not able to produce these compounds (see infra). 

Low molecular products of multi-ionization spurs could be considered as indicators of the radiation yield of multi-ionization spurs, as these products cannot be formed by singleionization spurs. However, they are only a part of large spurs products, which include cleaved chains. The difficulty withmulti-ionization spurs is, that they are of very different size, connected with their long spectrum of deposited energy of 30-500 eV. Very different kinds and amounts of small debris reported in the literature are caused by that variety of energies. 

Fig. 3. Reactive end of interrupted polyethylene chain (from multi-ionization spur) reacts with another chain in the neighbourhood, forming Y-type crosslink. 

Fig 4. X-type crosslinking ofpolyethylene from a single ionization spur. 

The rather non-conventional approach to radiation chemistry of polymers leads to conclusions which indicate that the role of multi-ionization spurs in radiation chemistry of polymers cannot be neglected. In spite of low participation of these spurs in radiolysis of low Z materials (ca 20% of total deposited energy), these spurs can explain formation of two basic, different types of crosslinks. Formation of low molecular weight products of radiolysis is also explained, as well as other phenomena. Application of spurs philosophy to polymers is also advantageous in explanation of energy transfer from single ionization spurs and lack of transfer from multi-ionization spurs. 

Different paths of chemical changes from both types of spurs can express the hope that new effects will be found, connected with the identification of multi-ionization spurs, as the interpretation of hydrogen yield in irradiated HNBR at starting doses shows. More running... 

In Sect. 4.9.1, experimental rationalization was provided for the W value of ionization in gaseous and liquid water, giving respectively 30.0 and 20.8 eV. The corresponding ionization potentials are respectively 12.6 and 8.3 eV. For the purpose of diffusion and stochastic kinetics, one often requires the statistical distribution P(i,j) of the number of ionizations i and excitations j, conditioned on i ionizations, for a spur of energy . Pimblott and Mozumder (1991) write P(i, j) = r(i) 2(j i), where F(i) is the probability of having i ionizations and 2(j i) is the probability of having j excitations conditioned on i ionizations. These probabilities are separately normalized to unity. 

Integral W values oj ionization for incident electron energies E, as measured in Combecher s (1980) experiments on gaseous water, can be well fitted by the equation W(E) = W(°°)(l - I/E)-1, where W(°°) = 30.0 eV is the value in the high-energy limit. A similar equation is assumed for liquid water. In contrast, the entity-specific Wi value of ionization, defined for a certain energy deposition in a spur, shows a minimum at 20 eV in... 

When averaged over the distribution of energy loss for a low-LET radiation (e.g., a 1-MeV electron), the most probable event in liquid water radiolysis generates one ionization, two ionizations, or one ionization and excitation, whereas in water vapor it would generate either one ionization or an excitation. In liquid water, the most probable outcomes for most probable spur energy (22 eV) are one ionization and either zero (6%) or one excitation (94%) for the mean energy loss (38 eV), the most probable outcomes are two ionizations and one excitation (78%), or one ionization and three excitations (19%). Thus, it is clear that a typical spur in water radiolysis contains only a few ionizations and/or excitations. 

These results imply that the use of the representative single ion-pair distribution in the ionization produced by low-LET irradiation in liquid hydrocarbons can be approximately justified even though the track itself has considerable contribution from multiple-ion-pair spurs and short tracks. It also means that even in the case of an isolated ion-pair, the long-time limit of the existence probability is perturbed by the long-range coulombic field. 

In conclusion we may state that there is evidence for multiple ion-pair recombination in spurs yet a theoretical analysis of free-ion yield and scavenging at low-LET based on the geminate ion-pair picture is meaningful in view of the similarity of the recombination process in the geminate and multiple ion-pair cases. However, if this analogy holds, the geminate ionization yield has to be somewhat less than the true ionization yield. 

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