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Yields of Ionization and Excitation in Irradiated Matter, Chapter 8 (CROSBI ID 758578)

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Srdoč, Dušan ; Inokuti, Mitio ; Krajcar Bronić, Ines Yields of Ionization and Excitation in Irradiated Matter, Chapter 8 // Atomic and molecular data for radiotherapy and radiation research, Final report of a co-ordinated research programme. 1995.

Podaci o odgovornosti

Srdoč, Dušan ; Inokuti, Mitio ; Krajcar Bronić, Ines

engleski

Yields of Ionization and Excitation in Irradiated Matter, Chapter 8

Chapter 8 deals with physical quantities that characterize stochastic processes induced in matter exposed to ionizing radiation and related mean values. The term ionizing radiation includes photons and charged particles (e.g., electrons, pions, protons, alpha particles, or heavy ions) with energy sufficient to ionize molecules. (The term molecules is used to indicate atoms and molecules in gases or appropriate structural units in condensed matter.) These particles collide with molecules many times and thus dissipate their energy in the absorber. Quantities treated below describe major consequences of a multitude of collisions. A review of recent progress in theoretical and experimental research on the ionization yield and its statistical fluctuations in gases, is presented. Excitation yields in gases are discussed in Section 8.5, and W G values in the condensed phase arc discussed in Section 8.6. The ionization and excitation of molecules are the dominant processes when particles traverse matter, although non-ionizing processes such as molecular dissociation into neutral fragments also take place. Many secondary electrons ejected in the primary ionizing events carry sufficient energy to cause further ionizations, and a substantial fraction of radiation energy is delivered to matter through secondary electrons. The molecular entities generated by the interactions of the initial particles and the secondary electrons are termed initial species. Subsequently, these initial species may form new species during thermalization and in thermal collisions with other molecules, as discussed in Chapter 6. Evaluation of the initial yield of ions and excited states is essential for dosimetry, radiation chemistry, and biology. A full description of ionization is given by the probability distribution P(T0, j), that is, the probability that a particle of initial energy T0 will give rise to precisely j ion pairs. Often the description is condensed into two parameters: the mean number of created ion pairs N ; and the variance VN. Statistical fluctuations in the ionization yield are usually characterized by the Fano factor F = VN/Nj. If particles lose all their energy in the absorber the average amount of energy used to create an ion pair is termed the W value, W = TQ/NJ. For particles losing a fraction A of their energy a differential value w = A/Ni is defined. A comprehensive review of W values in gases, including some data on W in rare gas liquids and solids, was published by the International Commission on Radiation Units and Measurements in 1979 [1]. Recent evelopments in experiment and theory presented in this Chapter have significantly enlarged our knowledge on the ionization yield for various kinds of ionizing radiation. In radiation chemistry, in which both ions and excited states need to be discussed, the yield Ns(Tg) of any initial species s in irradiated matter is expressed in terms of the G value, GS(T0) = (100/To)Ns(T0), i.e., the number of species produced per 100 eV of absorbed energy. The knowledge of W-values is crucial to radiation dosimetry using ionization chambers and to proportional counter spectroscopy. W values are the link between the energy deposited in the gas by ionizing radiation, and the resulting electric charge measured. More precisely, in radiation dosimetry using a gas chamber, one is determining the number of ion pairs produced by either primary particles or secondary particles released in the gas, some of which may result from cavity walls. The knowledge of the W value for the particles involved remains the most fundamental in the conversion of the ionization to the energy absorbed. Notice also that the knowledge of the stopping power (discussed extensively in Chapter 7) is also required in the analysis. For measurements in charged particle beams which traverse a chamber, the differential w value is relevant in calculating the absorbed dose, especially for parallel plate chambers, since only a portion of each track deposits energy in the chamber. lonization chamber measurements, like proportional counter measurements can make use of W values to correct the energy calibration obtained with a reference source to that for the radiation being measured. For electrons the degradation spectrum or the slowing-down spectrum y(T0, T ) is fundamental in theoretical calculation of all previously mentioned physical quantities. Its meaning is as follows: y(T0, T)dT is the summed pathlengths of all the electrons that are generated from a stationary source of electrons of energy TQ at unit intensity and have kinetic energies between T and + dT. In theoretical studies, a complete database is needed of al absolute cross-sections (for ionization, excitation, and dissociation) for the interactions of photons and charged particles, especially electrons, with molecules. Once the degradation spectrum has been determined, the yield of ionization, and other initial species, can be calculated. Calculations may be tested by comparison with experimentally determined quantities such as w, W and F, and thus the accuracy and reliability of the basic atomic and molecular data can be inferred. At the same time, values such as w, W, F, and stopping powers, which are derived from the more fundamental atomic and molecular data, are also directly pertinent to radiation dosimetry, nuclear spectroscopy and radiation chemistry. In particular they determine the accuracy of absorbed dose determination and kerma measurements of direct relevance for radiation therapy using different kind of radiations.

W value; Fano factor; ionization yield; gases; gas mixtures

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Podaci o izdanju

Atomic and molecular data for radiotherapy and radiation research, Final report of a co-ordinated research programme

1995.

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