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Modern Developments in X-Ray and Neutron Optics Episode 7

Tham khảo tài liệu 'modern developments in x-ray and neutron optics episode 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 230 P. Mercere et al. Wave-front measurement Closed loop Command device Visible CCD camera Hartmann wave-frontsensor Imaging system Flat deflection Focal spot imaging Imaging optic mirror Visible beam Removable YAG Ce crystal active optic 36 X - ray Ị CCD camera Fig. 15.9. Beamline LUCIA end station with the KB active optical system and the soft X-ray HWS 0- Illi 0 2 4 6 8 X dimension mm 10 rms 7.660 nm PV - 30.878 nm 0- . 0 2 4 6 8 10 X dimension mrn rms 0.828 nm PV 4 617 nm a b Fig. 15.10. Absolute residual wavefront measurements single CCD image treatment a before and b after closed-loop correction of the photon beam was tuned down to 700 eV to ensure a large illumination of the sensor with the central Airy disk. A closed-loop correction was then performed at E 3.64 keV A 0.34 nm the spatial filter pinhole having been removed. In a single iteration we succeeded in correcting the phase distortions from 7.7 nm rms and 30.9nm PV down to 0.8nm rms and 4.6nm PV Fig. 15.10 . With the KB system correctly aligned we performed knife-edge scans in both dimensions to characterize the beam. At the focal spot position the beam sections were measured at 2.4 X 2.86pm2 FWHM Fig. 15.11 . These dimensions are close to the theoretical limit given by the source size the geometry of the beamline and the slope errors of the KB mirrors measured about 1.1 prad . The performance of HWS at these high energies in particular the signal-to-noise ratio and the accuracy of the sensor is strongly limited by shot noise 15 Hartmann and Shack-Hartmann Wavefront Sensors 231 Fig. 15.11. Beam knife-edge measurements at the focal spot position after closed-loop correction with HWS from the photon-to-electron conversion process in CCDs. The residual wavefront that can be observed after correction in Fig. 15.10b is for example only the result of shot noise. To overcome this problem accumulation of several images is required. The signal-to-noise ratio and the repeatability of wavefront measurements