Patent Number: 041538447
Section: description

The apparatus shown in FIG. 1 is designed to operate on an incident electron beam 10 of electrons moving in parallel paths and having their spin longitudinally aligned. The electron beam passes into a cylindrical electron lens 1 consisting of four elements, in which the electrons are accelerated or decelerated from an initial energy E.sub.0 to an energy E' (the electron velocity being expressed in terms of energy in the usual way). The internal diameter of the cylindrical lens 1 is 30 mm. The lens is designed for a beam diameter of about 5 mm. The axial electric fields produced between the successive elements of cylindrical lens are so dimensioned that the value of the energy E' corresponds to the relation EQU E'=R.times..DELTA.E in which R is the relative resolution of the energy filter and .DELTA.E is the energy range of the electrons that is acceptable for the subsequent scattering and scattering measurements. Following the cylindrical lens system 1 is a spherical plate capacitor 2, having a mean radius of 10 cm. and a resolution capability of 5%, operating as an energy filter, in the electric field of which the electron beam is at the same time bent by 90.degree., so that the electrons leaving the spherical capacitor 2 have a spin that is aligned transversally. The spherical capacitor 2 is equipped with an input diaphragm 12 and an output diaphragm 13 necessary to produce the high resolution. The beam leaving the output diaphragm 13 of the spherical capacitor 2 then passes through a second cylindrical lens system consisting of four elements, in this case having an internal diameter of 15 mm, in which the electrons are accelerated to the energy level previously determined to be suitable for scattering of the electrons by the particular crystal selected as the monocrystal of the device. The beam is at the same time focussed by the lens system 3 on the crystal, the crystal in the illustrated case being a tungsten monocrystal 4 presenting a (100) crystal plane in which the incident beam is focussed. Among the several beams of the pattern produced at the monocrystal 4 by scattering, beams reflected at conjugated angles are picked up by secondary electron multipliers 5,5' having a diameter of 1.5 cm and a length of 5 cm and the currents or pulses obtained in consequence are supplied to a measuring circuit not shown in the drawing. The two reflected rays reach the electron multiplier detectors 5 and 5' in each case through a device consisting of two parallel grids 6 between which an electric field for slowing down or repelling electrons is applied, this device therefore operating as a high-pass electron velocity or electron energy filter. The grids are produced from metallic foils by photochemical treatment and have a transparency of more than 80%. By means of this device, the electrons coming out of the monocrystal surface as the result of inelastic interaction with the scattering centers on the surface are separated from the scattered beam. A fine mesh tungsten wire grid 7 having a transparency of 80% is provided for electrostatic shielding of the scattering region. The components 1 to 7 of the embodiment of the invention illustrated in FIG. 1 are enclosed in a cylindrical vacuum vessel having a diameter of 25 cm and a length of 40 cm. The pressure in the vacuum vessel is in the range of 10.sup.-11 mbar., in order that the crystal surface can be maintained in a clean condition over a long period. The degree of polarization P of the incident beam is calculated by the following relation EQU P=f.times.(I.sub.1 -I.sub.2)/(I.sub.1 +I.sub.2) in which I.sub.1 and I.sub.2 are the measurement magnitudes respectively determined by the detectors 5 and 5' that can be count rates or current magnitudes, and in which f is a calibration factor. FIG. 2 illustrates another embodiment mentioned above, in which a multiplicity of collector plates 8, having read-out wires 9, are arranged in the hemispherical space above the monocrystal 4, so that the measurement can be made with the particular pair of pairs of plates 8 that pick up the scattered beams chosen for measurement. In the hemispherical space above the monocrystal there are provided hemispherical concentric grids 10 for energy selection of scattered electrons, followed by a curved channel electron multiplier array 11. FIG. 3 is a view of the portion of the apparatus of FIG. 1 including the diaphragm 13, the velocity-adjusting lens system 3, and the crystal 4 at right angles to the plane of FIG. 1 and through the axis of the lens system 3, showing the presence of an additional pair 5" and 5'" of detectors having their axes in a plane at right angles to the plane of the axes of the dectors 5 and 5'. Although the invention has been described with reference to particular specific illustrative embodiments, variations are possible within the inventive concept. Thus, equivalent magnetic field producing means may be used instead of an electrostatic lens system for focusing the electrons in the components 1 and 3 of the apparatus and likewise a magnetic field can be used instead of the spherical capacitor 2. A magnetic field can also be used for trapping or deviating the slow inelastically backscattered electrons before they reach the detectors.