The present invention relates to a pulsed beam particle accelerator which can be used for particle radiation therapy. More particular, a device and method are provided to control the number of particles within a beam pulse. The particle accelerator comprises means for varying the number of particles within each beam pulse of said pulsed ion beam from a minimum value to a maximum value as function of the value of a beam control parameter. For each particle irradiation the required number of particles for each beam pulse is controlled by defining a value for said beam control parameter based on calibration data.
