A dosimetry device for an energy beam of particles coming from a source and comprising at least first and second ionization chambers (IC1 and IC2) each comprising a collector electrode and a polarization electrode, said electrodes of each considered ionization chamber being separated by a gap or space comprising a fluid, said two ionization chambers being configured to be traversed by a beam of particles coming from a same source, said two ionization chambers having different charge collection efficiency factors, said device being characterized in that it comprises an acquisition device connected to a calculator carrying out a first algorithm for calculating - a "gain" factor relative to a first ionization chamber, given by the equation: G = 1 − i 1 i 2 norm 1 − f 1 where - G is the "gain" factor relative to the first ionization chamber, - i 1 i 2 norm is the ratio of theoretical output signal values (i1 and i2) in said two ionization chambers (IC1 and IC2) normalized by the ratio of their respective amplification factors (R1/R2), each amplification factor depending on said fluid, the width of said gap or space and the penetrating power of the beam of particles in said fluid for the first ionization chamber, - f1 is the theoretical value of the charge collection efficiency factor in the first ionization chamber, said theoretical output signal values (i1, i2) and f1 being calculated as a function of intrinsic and/or extrinsic parameters of said considered two chambers (IC1 and IC2), as well as based on the value of the current intensity of the particle beam, said gain factor being independent of the value of said current of the particle beam, said calculator carrying out a second algorithm to calculate the dose flow rate deposited by the particle beam comprising the following steps: - measuring the output signals (i1 and i2) in said two ionization chambers and establishing the normalized ratio i 1 i 2 norm between said output signal values (i1 and i2); - calculating the charge recombination rate (1-f1) and deducting the charge collection efficiency factor in the first ionization chamber based on the results of the first algorithm and based on the knowledge of the "gain" factor of the first ionization chamber; - calculating the dose flow rate deposited by the beam based on the charge collection efficiency factor in the first ionization chamber. - a "gain" factor relative to a first ionization chamber, given by the equation: G = 1 − i 1 i 2 norm 1 − f 1 where - G is the "gain" factor relative to the first ionization chamber, - i 1 i 2 norm is the ratio of theoretical output signal values (i1 and i2) in said two ionization chambers (IC1 and IC2) normalized by the ratio of their respective amplification factors (R1/R2), each amplification factor depending on said fluid, the width of said gap or space and the penetrating power of the beam of particles in said fluid for the first ionization chamber, - f1 is the theoretical value of the charge collection efficiency factor in the first ionization chamber, - measuring the output signals (i1 and i2) in said two ionization chambers and establishing the normalized ratio i 1 i 2 norm between said output signal values (i1 and i2); - calculating the charge recombination rate (1-f1) and deducting the charge collection efficiency factor in the first ionization chamber based on the results of the first algorithm and based on the knowledge of the "gain" factor of the first ionization chamber; - calculating the dose flow rate deposited by the beam based on the charge collection efficiency factor in the first ionization chamber.