While γ-rays and X-rays of various energies are all sparsely ionizing, in the body they generate electrons with somewhat different spectra of LET values (ICRU 1970). To quantify the differences, reference is usually made to the dose average LET or to the mean values of the related microdosimetric parameter dose-averaged linear energy, y.
Figure 1-4 gives the dose average LET values for the electrons released by monoenergetic photons (solid curves) and compares these values to the averages for 29 kV mammography X-rays and 200 kV X-rays (solid circles and squares, respectively; ICRP 2003). In addition to the dose average, LD, of the unrestricted LET, the diagram contains the dose averages, LD,Δ, of the restricted LET, LΔ. The restricted LET treats the Δ-rays beyond the specified cutoff energy Δ as separate tracks. This accounts in an approximate way for the increased local energies due to Δ-rays and therefore provides larger values that are more meaningful than those of unrestricted LET.
High-energy photons (e.g., 60Co γ-rays) release Compton electrons of comparatively high energy and correspondingly low LET. Photons of less energy (e.g., conventional 200 kV X-rays) produce less energetic Compton electrons with higher LET. This explains the substantial difference between the mean LET of high-energy γ-rays and conventional X-rays. For lower-energy X-rays the photon energy is further reduced, and the photo effect (i.e., the total transfer of photon energy to electrons) begins to dominate. Accordingly, the average energy of the electrons begins to increase again, which explains the relatively small difference in average LET between 200 kV X-rays and soft X-rays. At very low photon energies (i.e., less than about 20 keV) the LET values increase strongly, but these ultrasoft X-rays are of little concern in radiation protection because of their very limited penetration depth.
The dose average, LD,Δ, of the restricted LET is a parameter that correlates with the low dose effectiveness of photon or electron radiation. With a cutoff value Δ=1keV, the numerical values of LD,Δ are consistent with a low-dose RBE of about 2 for conventional X-rays versus γ-rays. A similar dependence on photon energy is seen in the related microdosimetric parameter dose lineal energy, y, which has been used as reference parameter by the liaison committee of the International Commission on Radiological Protection (ICRP) and the International Commission on Radiation Units and Measurements (ICRU) in The Quality Factor in Radiation Protection (ICRU 1986). Figure 1-5 gives values of its