Patent Number: 051577020
Section: summary

FIELD OF THE INVENTION The present invention relates to a double crystal monochromator for use in synchrotron radiation studies and more particularly to a mechanically actuated double crystal monochromator which is operative in an ultrahigh vacuum in the order of 10.sup.-10 Torr. BACKGROUND OF THE INVENTION Monochromators are generally well known for use in the energy range from a few kilovolts and up. These monochromators generally use single crystals of silicon or germanium as energy dispersive elements. An article by J. A. Golovchenko of Bell Laboratories printed in the Review of Scientific Instruments, Vol. 52, No. 4, April 1981, describes a double crystal monochromator wherein each crystal element is changed by the experimenter to obtain a constant output direction as well as a constant beam position as energy is varied and to make the central ray of the selected beam impinge at the same point on all monochromator crystals independently of the chosen energy. This was achieved by sampling the output beam intensity for angular drifts out of parallelism between two crystals with an electronic drive to correct the beam. SUMMARY OF THE PRESENT INVENTION The double crystal monochromator according to the present invention is provided with a second crystal which is maintained in a parallel relation to a first crystal within a few seconds of arc and can be traversed linearally in a straight line to capture the diffractive rays from the first crystal as it rotates through an angle of up to 60.degree.. The higher energy levels impinging on the first crystal create a need for cooling of that crystal in an ultrahigh vacuum in a range of 10.sup.-10 Torr. The monochromator advantageously provides for crystal rotation from 12.degree. to 72.degree. from the incoming horizontal beam, crystal angular accuracy of .+-.5 seconds of arc for total beam error through the instrument of .+-.20 seconds of arc (.+-.0.97 mm beam movement at a distance of 10 meters downstream of the second crystal). In another aspect of the invention the beam of the second crystal travels parallel to the incoming beam within 0.0005 inch. In a further aspect of the invention the first crystal which is exposed directly to the synchrotron radiation must be cooled to a temperature less than 150.degree. C. since the first crystal cannot exceed 150.degree. C. maximum temperature. In this regard the second crystal is heated to a temperature within .+-.5.degree. C. of the first crystal in order to maintain a crystalline plane structure spacing the same as the first crystal. In another aspect of the invention a piezoelectric actuator is provided to control the pitch rotation of one crystal relative to the other within a range of 40 seconds of arc. The actuator also provides a dither of up to 2 seconds of arc at frequencies up to 30 Hz. One of the primary advantages of the present invention is the ability to adjust the pitch and roll of each crystal to a registration better than 0.01.degree. while under vacuum. The crystals must also be adjustable in the vertical direction to an accuracy of 0.001 mm. The crystal subassemblies advantageously are also easily removeable from the monochromator. A further advantage of the present invention is the development of an all mechanical system that uses computer control only for scan position, scan rate, piezoelectric inputs and temperature control, thus eliminating the requirement for computer program software control so that all interlocks are mechanically built into the monochromator. Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims.