[en] An attempt was made to determine the microdosimetric parameters of a 650MeV α beam in the framework of the Dual Radiation Action Theory. Two methods were used: direct measurement by a classical proportional counter and calculations from the energy distributions. The event size distributions (Y) were experimentally measured using a spherical tissue-equivalent proportional counter simulating a site of 2μm diameter. The functions f₁(z) and particularly the mean value Zsub(D) were deduced from these distributions. Experimental measurements of energy losses in semiconductors of various thickness enabled the depth energy distributions of α particles to be evaluated and the mean value Zsub(D) to be calculated