Dose mean lineal energy for neutrons.

Several proposals have been presented recently (ICRU 40, Zaider and Brenner, Kellerer) on the definition of the quality factor for charged particles on lineal energy density rather than on LET. The models of ICRU 40 and Kellerer result in a direct proportionality of the quality factor to y in the lower range of y up to to about 100 keV.μm-1 with q(y)=0.3 y. From this the neutron quality factor is derived to be Q(n)=0.3 ȳ(D). The version of Zaider and Brenner results in a expression Q(n)=0.03 + 0.24 ȳ(D). A similar expression was used by Bengtson in 1969. Without the knowledge of the f(y) distribution, Y(D) can be calculated using the proximity function approach by Kellerer applied on calculated ion event tracks. Such calculations were performed for protons, α particles and heavy recoils in the energy range of 0.2 MeV.amu-1 up to 15 MeV.amu-1 in water vapour. From the charged particle data ȳ(D) is then calculated as a function of neutron energy in soft tissue from thermal energy up to 14.1 MeV. The new proposals are related to the f(y) distribution in a sphere of 1 μm diameter. Smaller target sizes have, however, been proven to be relevant in quantitative radiation biology. The variation of ȳ(D) with target size (1 nm up to 1μm) is shown as a function of neutron energy and the deviation from the LET approximation is discussed, which is caused by straggling effects and δ ray escape and is most pronounced at small target sizes. Results of the several new concepts are compared with each other and with old ones, based on q(L), and the impact on neutron radiation protection is discussed