W, F, and I: Three quantities basic to radiation physics.

摘要

The W value is an index of the mean number of ions produced in a gas subjected to ionizing radiation. Formally, it is defined as the radiation energy absorbed (usually expressed in units of eV) 'per ion pair of either sign produced', or, in a simpler language, 'per electron liberated'. The basic knowledge up to 1961 is eloquently articulated in a classic essay by Platzman (1), which Professor Doke loves to cite. The theme of Platzman was to explain from the point of view of basic physics the magnitude and characteristics of the ratio W/I, where I is the (first) ionization threshold energy. In summary, major characteristics are as follows. (1) The W value for a given gas depends weakly on the properties of the radiation such as the mass and charge of particles or initial energies (provided they are sufficiently high). This makes the ionization measurement useful as a method of dosimetry, viz., the determination of the absorbed energy. (2) The ratio W/I is always greater than unity because a part of the absorbed energy must be used in nonionizing events such as discrete excitation or molecular dissociation into neutral fragments and also in producing subexcitation electrons, viz., electrons with kinetic energies too low to cause electronic excitation or ionization (2). (3) The ratio W/I is 1.7-1.8 for rare gases, and 2.1-2.6 for gases of common molecules (depending on the electronic structure, going from 'hard' to 'soft'). Calculation of the W value is possible from three approaches: (i) the energy balance of Platzman, heuristic for general understanding and appropriate for an estimate; (ii) the Fowler equation (3) for the direct evaluation of the mean number of ions produced; and (iii) the method of Spencer and Fano (4) through the degradation spectra (or the track length distributions) of charged particles, most importantly of electrons, present in the medium. The Fowler method is good for obtaining the mean number of ions or excited states resulting from the incidence of particles of relatively low energies, while the Spencer-Fano method is good for the incidence of high-energy particles.