Method for determining a water equivalent depth (1) between an entrance point (30) and a reference point (40). The method comprises the following steps: sending to a charged particle beam detector (50) placed at a reference point (40) within or beyond a body (20) a charged particle beam (10) whose energy is modulated between a minimum (70) and maximum (65) energy value acquiring the time dependent response (55) of said charged particle beam detector (50) determining from said time dependent response (55) a value of a statistical parameter (100) providing a calibration curve (95) expressing a relationship between values of said statistical parameter (100) and water equivalent depths (1) finally, extracting from this calibration curve (95) the water equivalent depth (1) corresponding to the value of the statistical parameter (100) determined from the time dependent response (55) of the charged particle beam detector (50) placed at the reference point (40).
