X-ray spectrometer with an analyzer crystal having a partly variable and

Abstract

An imaging optical system having a Rowland geometry can be used in a spectrometer for X-ray fluorescence. For the focusing of the X-ray beam emanating from the specimen to be analyzed use is made of a curved analyzer crystal 28 whose radius of curvature may be variable, as in the case of a crystal surface 29 in the form of a logarithmic spiral 40. If such an analyzer crystal is to be made sufficiently large so as to achieve adequate intensity in the X-ray detector, a part of the crystal would have to be given a radius of curvature which is smaller than permissible so as to avoid fracturing of the crystal. In accordance with the invention, a first part 40 of the reflective surface 29 has a radius of curvature which is dependent on the location on the crystal whereas another part 42 of the reflective surface has a constant radius of curvature 44. A crystal part having a constant radius of curvature exhibits angular deviations, but for as long as these angular deviations are smaller than a given (not very low) limit value, they can be ignored in relation to other, larger deviations of the log spiral part. Such larger deviations occur notably when a multilayer mirror is chosen for the analyzer crystal 28.