Dependence of Ionization Processes on the Type of Charged Particles

Various types of ionizing radiation are used to act on the gas medium—uranium and transuranium fission fragments, a particles, у quanta, fast electrons and protons. In the case of у quanta, the gas ionization is provoked by fast electrons arising as a result of Compton scattering, the photoeffect, and effect of electron-positron pair formation. In the initial stage of ionization processes, primary ionization during the immediate interaction of the charged particle with the atoms is distinct from secondary ionization due to the interaction of atoms with electrons formed as a result of primary ionization.

The process of ionization of an atom may be viewed as a binary collision of an oncoming charged particle with one of the electrons of the atom’s shell [3]. Because

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S. P. Melnikov et al., Lasers with Nuclear Pumping,

DOI 10.1007/978-3-319-08882-2_4

Fig. 4.1 Normalized spectra of primary electrons in neon during ionization by uranium fission fragments (1) and by fast electrons with an initial energy of 1 MeV (2) [4]

of the great difference in the masses of heavy charged particles (for example, a fission fragment) and the electron, only a comparatively small percentage of the fragment energy can be transferred to the orbital electron. Thus the electron spectrum arising due to primary ionization by heavy particles is softer than the analogous spectrum formed during ionization of a gas by fast electrons [4, 5]. For example, Fig. 4.1 shows the spectra of primary electrons during the ionization of neon by uranium fission fragments and fast electrons with an initial energy of 1 MeV [4]. Although both spectra have maxima in the low-energy region, the spectrum formed by fast electrons has a significant percentage of electrons with energies up to ~10 keV. The average energy of the primary electrons formed in neon as a result of ionization by fast electrons and fission fragments is equal to 150 eV and 40 eV, respectively. In the case of fission fragments, these electrons may provide an additional one or two acts of ionization (on average), while in the case of the fast electrons, it is from five to ten [5]. Information about the average energy of primary electrons in ionization by fission fragments for other rare gases agrees with the above data for neon. It follows from the calculations in [6] that the average energies of primary electrons in the ionization of He, Ar, Kr, and Xe by fission fragments are 60 eV, 44 eV, 41 eV, and 35 eV, respectively.

Despite the specifics of primary ionization by heavy and light charged particles discussed previously, differences in the radiation effects on the gas media of different types of ionizing particles are not substantial, because the ultimate result is a combined effect of primary and secondary ionization. This is confirmed by the data cited below, from which it follows that the final electron energy distribution and the energy cost of forming an ion-electron pair in different gases are virtually independent of the type of charged particles. The same conclusion can be drawn from analysis of the luminescence spectra of gas media and the characteristics of gas NPLs, which depend not on the type of the charged particles, but on the specific power deposition. In this regard, the same kinetic models are used to calculate the characteristics of NPLs excited by heavy charged particles and those excited by fast electrons.