Beam position correction device

A method of positioning an electron beam upon a given reference point on a detector comprising a 270.degree. sector and a 90.degree. sector of a cylinder separated by a fixed gap between the straight portions of the sectors towards which the beam is directed, comprising the steps of:

• generating an electron beam of a desired current intensity;
• directing the said beam towards and focusing the said beam upon the surface of the said sectors, the radial portions of which are aligned along coordinate axes which are mutually perpendicular;
• causing the beam to impinge upon a series of closely spaced points upon a heat sink proximate to said sectors along a line parallel to one of said axes;
• causing the beam to be directed at right angles intermittently along the second of said coordinate axes and onto the said sectors so as to sample the currents collected by each of said sectors;
• determining the difference in the electron beam current reaching each sector;
• causing the electron beam to be deflected in a direction towards the sector which has collected the lesser current;
• repeating the above series of steps until the said difference in current reaches zero;
• and
• repeating the process of sampling the current reaching the two sectors along the line parallel to the second axis until the currents reaching the sectors are equal along both axes.