7. Impact of Recommendations

Abstract

This Report has reviewed relevant data and has recommended values of quantities that play an important role in radiation dosimetry, especially those needed in measurement standards. This Section considers some of the implications of the recommended changes for dosimetric measurements and on calculations made in the fields of radiation medicine, industry, and other applications, including radiation research. Recommended values and uncertainties are given for Wair, the average energy required to produce an ion pair, the heat defect of liquid water, hW, and the radiation chemical yield for the Fricke dosimeter, G(Fe3þ). A new value is also recommended for the product, Wair sg,air, for Co g rays. The humidity correction, kh, for air-filled ionization chambers is reviewed, but no changes are recommended. However, it is noted that, for high precision, the variation of kh with relative humidity or, more properly, with the partial pressure of water vapor, should be considered. Data for the heat defect of graphite are reviewed, but no definitive conclusions could be reached and more study is recommended. The value of Wair for electrons is left unchanged at 33.97 eV, but its standard uncertainty has been increased from 0.05 eV (0.15 %) to 0.12 eV (0.35 %). This will have an impact on the uncertainty of airkerma standards based on free-air chambers and will for many standards become the dominant component. The available data for Wair indicate that it can be considered constant at high energies. However, for electron energies below about 10 keV, Wair cannot be considered constant. Furthermore, as was pointed out in Section 5.5, when the air kerma is obtained from a charge measurement, a correction should be applied for the charge of the initial electrons set in motion by the photons. The combined correction for these last two effects (see Table 5.7) can be significant for low-energy photons (up to 0.7 %) and could give rise to changes in primary standards. Recommendations have been made for the mean excitation energies for air, graphite, and liquid water as well as for the graphite density to use when evaluating the density effect (2.265 g cm). From these recommendations, tables of the stopping powers for electrons, protons, alpha particles, and carbon ions have been generated (see the Appendix). For air, no change in the value of the mean excitation energy is recommended, i.e., Iair 1⁄4 85.7 eV but now with an uncertainty of 1.2 eV (1.4 %); stopping power values for all particles thus remain unaltered, except for carbon ions, for which an Iair value of 82.8 eV was implicitly used in ICRU Report 73 (2005). For electrons in graphite, the change in the electronic stopping powers relative to those given in ICRU Report 37 (1987a) is shown in Fig. 7.1. The value of Ig has increased from 78 eV to 81 eV, and the standard uncertainty has decreased from 4 eV to 1.8 eV. The increase in the mean excitation energy and the change in the density used to evaluate the density-effect correction both result in a decrease in the electronic stopping power. For the secondary electrons produced by Co g rays, the electronic stopping power in graphite decreases by about 0.7 %, while for high-energy electrons, the decrease is more than 1 %. For liquid water, there is a 4 % relative increase in Iw, from 75 eV, as used in ICRU Report 37, to 78 eV, with a relative standard uncertainty of 2.6 %, which also results in a decrease in the electronic-stopping-power values. For protons and carbon ions, the change in electronic stopping powers relative to the values given in previous ICRU Reports are shown in Figs. 7.2 and 7.3, respectively, which, in addition to the changes in I values and densities mentioned above, are based on improved calculations using the Bethe–Bloch expression for Sel/r, see Eqs. (4.17) and (4.18), and are complemented with experimental data at low energies as described in the Appendix. For photons, following the analysis of photoeffect cross sections with regard to the use of renormalized values and of the two options for determining Compton cross sections (impulse approximation versus Waller– Hartree theory), tables of mass energy-absorption coefficients for air, graphite, and water have been given. No recommendations on the choice of these options are given in this Report, but some discussion is included on the effects of considering them. Ratios of the renormalized-to-unrenormalized mass energyJournal of the ICRU Vol 14 No 1 (2014) Report 90 doi:10.1093/jicru/ndw033 Oxford University Press