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 by ICRU in the report 47 are overestimated in the energy range 4-10 MeV, since delivered from calculations carried out with the ICRU sphere in air. Such a procedure is clearly contraddictory with the definition of the ambient dose equivalent given by ICRU itself. In this paper, the values of the convention coefficients consistently evaluated by Monte Carlo calculations using the FLUKA code, are presented. The resulting ambient dose equivalent is shown not to be conservative with respect to the effective dose equivalent when the photon energy exceeds about 3 MeV.