On the conversion coefficients from fluence to ambient dose equivalent

The conversion coefficients from photon fluence (or other physical quantities) to ambient dose equivalent recommended in ICRU Report 47 are overestimated in the energy range 4-10 MeV, since they are derived from calculations carried out with the ICRU sphere in air. Such a procedure is clearly in contradiction with the definition of the ambient dose equivalent given by ICRU itself. The values of the conversion consistently evaluated by Monte Carlo calculations using the FLUKA code, are presented. The resulting ambient dose equivalent is shown not be be conservative with respect to the effective dose equivalent when the photon energy exceeds about 3 MeV. (Author)