Automatic steering of a high velocity beam of charged particles

In a radiation generating machine:

• means for producing and accelerating a beam of charged particles;
• an X-ray target positioned in the path of said beam to generate X-rays for irradiation of a subject;
• means for supporting said particle beam producing and accelerating means and for moving said beam producing and accelerating means around a subject to be irradiated and thereby changing the position of said beam producing and accelerating means relative to the direction of a force field of the earth;
• means for steering the accelerated beam over a certain predetermined beam path relative to said accelerating means;
• secondary emission forming means disposed around the predetermined beam path upstream of the beam relative to said X-ray target for generating a field of ionizing radiation outwardly of the beam when the beam impinges upon said secondary radiation forming means;
• ionization chamber detector means disposed around the predetermined beam path for detecting the intensity of the ionizing radiation field generated by said secondary emission forming means, said ionization chamber detector means including chamber means filled with an ionizable medium, and electrode means operatively associated with said chamber means for detecting the ionization current signal flowing within said chamber means as a function of position around said beam due to ionization of the medium fill therein caused by ionizing radiation incident thereon;
• means for comparing the detected ionization current signals as a function of position of the beam and for deriving an error signal determinative of the deviation of the beam, if any, from the intended beam path;
• said beam steering means being responsive to said error signal for steering the beam along a predetermined beam path;
• and
• sample and hold means for sampling and holding the error signal to which the beam steering means is responsive, whereby upon initiation of the beam after interruption of the beam the beam steering means is responsive to the same error signal upon initiation of the beam as it was at the time the beam was interrupted.