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Mean energy required to form an ion pair (W value) for various ionizing particles in air

The mean energy required to form an ion pair (W value) is defined as the mean energy spent by the incident particle of energy E for the formation of an electron - ion pairs after complete dissipation of the initial energy, W = E/N, where N is the total number of produced electron-ion pairs. For high-energy particles in thin media, when only a fraction of the particle energy (dE) is deposited in a medium, it is necessary to consider the differential w value, w = dE/dN. For sufficiently high incident energy, W value is approximately constant, and w = W is a good approximation. Available data on W or w are often fragmented, dispersed, and missing systematic. Several compilations and reviews, such as ICRU Report 31 and IAEA TECDOC 799, provide a basis for assessing the present knowledge of W values for different charged particles in various gases. Most available data exist for electrons. The W value for high-energy electrons (>10 keV) in dry air is well defined, 33.97 ± 0.05 eV, and also a dependence on air humidity has been studied. W and w values for high-energy protons (>20 MeV) have been also carefully analyzed. The value of 34.23 eV, with ±0.4%, has been recommended. The existing W or w data for heavy ions used in radiotherapy (He, C, Ne, Si, Ar) are fragmentary and most of them are measured for relatively low energies (<1 MeV/amu) where major variations in energy dependence are observed. There is a lack of experimental data at energies >1 MeV/amu of interest for radiotherapy. For carbon ions a few sets of experimental data on W exist at low energies and have been recently extended to higher energies. The average W value for C ions at energies >10 MeV/amu is 34.7 ± 0.9 eV, which is slightly higher than the recommended value (34.5 ± 0.5 eV) for all ions at energies >1 MeV/amu. There is a need for new W value measurements for heavy-ions in air at energies >10 keV/amu, and especially for C ions of several hundreds of MeV/amu that are of interest in carbon-ion therapy.

W value; radiotherapy; electrons; protons; heavy ions

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