Patent Number: 051739305
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring now in detail to the drawing, there is shown a monochromator which includes a housing 10 having therein a plurality of mirrors 11, 12, 13 and 14 having opposed pairs of mirror faces 18, 19 and 20, with the housing being provided with inlet and outlet windows 24 and 25, respectively for each of the mirror pairs. Filters 26 mounted on the windows 24 reject visible and ultraviolet light. The mirrors 11-14 are mounted in channels 28 which are provided with pivot pins 30 which extend through apertures (not shown) in the housing to allow the mirrors to pivot in the housing. The channels 28 are also provided with other pivot pins 33 which extend through apertures in a pair of links 35 which serve to hold the mirrors in a parallel relationship. The housing 11, the pivot pins 30 and 33 and the links 35 are precisely made to accurately hold the opposed mirror faces 18, 19 and 20 to within less than 1 arc second from absolute parallel. Also, the opposite surfaces of each of the mirrors 11-14 are parallel to each other to within less than 1 arc second. All of the mirror surfaces should be superpolished to a smoothness of less than about 3 Angstroms RMS. When tested with visible light, the mirror surfaces should be flat to better than 1/20th wave. Materials which can be used for the mirrors and which can be polished to this degree are known. Each pair of opposed or facing mirror faces 18, 19 and 20 is coated with a multilayer coating such that the pairs 18, 19 and 20 each have different coatings but the coatings on any given pair are identical. These coatings are made up of alternating layers of high-Z diffractor material of a thickness d1 separated by layers of low-Z spacer material of thickness d2. Such coatings and the methods of making them are known. The multilayer coatings constitute synthetic Bragg crystals, with x-ray reflection taking place by Bragg diffraction. The wavelength at which the peak of the reflected flux occurs is given by the Bragg relation: n(.lambda.)=2DSin.theta., where n is the order of the diffraction (usually taken to be unity), D is the multilayer diffractor and spacer thickness parameter and .theta. is the angle at which the radiation strikes the mirror surface. One skilled in the art will be aware of which high Z materials will give the best diffraction performance at any desired wave length and which low Z spacer materials should be used with the selected high Z diffractor layers. He will also be aware of how to select the desired thicknesses d1 and d2 to obtain the desired diffractor and spacer thickness parameter. Inasmuch as each pair of opposed mirror faces has its own multilayer coating, different from the coatings on the other pairs of faces, it can readily be seen that each pair of opposed faces will have a peak reflection of x-rays at some wavelength which is different from the peak reflection of the other pairs of faces. This allows one to select the pair of mirror faces which provides the greatest reflection of x-rays at the wavelength desired. The housing is mounted for sliding movement between a plurality of guides 40, as best shown in FIG. 2. The guides 40 are mounted on a base 41 (FIG. 1) and are precisely made and positioned to very accurately position the housing as it is moved from one position to another in the guides 40. A driving mechanism of a known type 44 mounted on the base 41 is connected to the housing by a rod 45 for moving the housing 11 within the guides 40. From the above, it will readily be apparent that the wavelength of peak reflectivity of x-rays by a multilayer mirror is dependent on the angle at which the x-ray beam strikes the mirror, so that one can vary the wavelength at which maximum reflectivity occurs by adjusting the angle of incidence. This is accomplished in this apparatus by pivoting the mirrors on the pivot pins 30. A traversing mechanism 50 of a known type is mounted on the top of the housing and is adapted to traverse a rod 51 which is secured to one of the links 35 to pivot the mirrors to adjust the angle at which a polychromatic x-ray beam 54 strikes the mirror. It can be seen that upward movement of the rod 51 increases the angle of incidence, while downward movement of the rod 51 lowers the angle of incidence. In operation, the driving mechanism 44 is actuated to move the housing 10 in the guides 40 to bring into the path of the polychromatic x-ray beam 54 that pair of mirror faces, 18, 19 or 20 which offer the best reflection over the desired wavelength range. The traversing mechanism 50 is then actuated to pivot the mirrors to change the angle of incidence to select the desired wavelength in the output beam 55. The polychromatic beam 54 reflects off the surface 20 of the mirror 14 (when the housing is positioned as shown in FIG. 1) to strike the surface 20 of the mirror 13 at the same angle of incidence. A monochromatic beam 55 is reflected out of the housing 10 through the outlet window 25 associated with the mirror pair 13 and 14.