Device for automatic focusing of X-ray spectrometers for electron beam microprobes

摘要

1,236,987. X-ray analysis. G. DORFLER. Oct.24, 1968 [Oct.24, 1967], No.50619/68. Heading G1A. [Also in Division H5] An X-ray investigation device comprises a curved crystal focusing X-ray spectrometer having its detector D arranged on a Rowland circle R of the spectrometer crystal K to receive X-rays reflected from the crystal and also comprises an electron gun which produces a beam of electrons E so as to be incident at a point F on a specimen which point lies substantially on the Rowland circle and provides the source of X-rays for the spectrometer, the gun having means for deflecting the beam E so as to cause the point of incidence of the beam to scan the specimen in a manner that involves movement in a direction having a component parallel to the plane of the Rowland circle, the device including means, e.g. worm C and gear, operative in accordance with the movement of the point of incidence so as to control rotation of the crystal K about a substantially central axis of the working face of the crystal, which axis is perpendicular to the plane of the Rowland circle and intersects that circle at a point where the circle touches the crystal face, so that during the movement of the pt of incidence, the angle B between the normal to the working face at the said axis and a perpendicular from the point of incidence to the axis is maintained constant. The movement of the point of incidence may be effected in accordance with a first analogue voltage applied to one point of a difference amplifier, the other input of which has applied to it a second voltage developed as an anologue of the rotational position of the crystal K the output of the amplifier being connected to operate an electric motor so as maintain the value of the said angle. The linking between the motor and the crystal may include a coupling which is releasable to permit adjustment of the crystal so as to provide at the detector D a maximum intensity for a selected X-ray line when the beam E is in a given position on the specimen, e.g. an extreme scanning position. The motor may be mechanically coupled to drive not only the goniometer head supporting the crystal K but also the slide of a potentiometer, e.g. a ring potentiometer the stated second voltage being derived from this potentiometer. The goniometer head may be rotated by means of a thread spindle and a toothed wheel. A D.C. potential is preferably connected across the potentiometer, and this voltage may be variable to permit the setting of an intensity maximum for a given line at the detector. The first analogue voltage may be sawtooth in form and is preferably that voltage applied to the deflection means to achieve the deflection of the electron beam. In one embodiment, the same voltage generator produces both the analogue voltage and the voltage applied to the deflection means, but in a second embodiment separate generators are used. Preferably the direction of scanning movement is parallel to the plane of the Rowland circle, the raster being completed by movement of the beam perpendicular to the plane of the circle.