A double-focusing soft X-ray spectrometer with independent adjustment of Bragg and observation angles

A double-focusing spectrometer for use in the soft X-ray region (10-120 AA) is described, which uses an organometallic pseudocrystal coated on a concave spherical glass lens. Source, crystal, and channeltron detector lie on a Rowland circle which rotates about the source as the Bragg angle changes, while the take-off angle is constant. Alternatively the take-off angle can be varied without changing the Bragg angle. Focusing properties, approximate expressions for solid angles, and formulae suitable for use in absolute calibration are derived. Geometrical and diffraction components of the resolution are treated. Double focusing improves transmission by a factor of about 6 relative to a spectrometer utilizing a cylindrical crystal of equal area. An efficiency of 5*10 -6 (which can probably be improved) within a solid angle of 0.041 sr and a resolution of about 1% (in agreement with the diffraction limit) were obtained in measurements with carbon K X-rays (44.6 AA).