Abstract

We measure the spatial coherence function of a quasi-monochromatic 1.1 keV X-ray beam from an undulator at a third-generation synchrotron. We use a Young's slit apparatus to measure the coherence function and find that the coherence measured is poorer than expected. We show that this difference may be attributed to the effects of speckle due to the beamline optics. The conditions for successful coherence transport are considered.

Author Keywords: Undulator radiation; X-ray region; Spatial coherence measurement; Young's experiment; Speckle size

PACS classification codes: 41.50.+h; 42.25.Kb; 42.30.Rx; 41.60.−m

Article Outline

1. Introduction

2. Experiment

3. Analysis and results

3.1. Speckle size

4. Conclusion

Acknowledgements

References

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Fig. 1. The 2-ID-B beamline at the Advanced Photon Source showing beamline optics depicted as thin lenses in the horizontal planes. Distances shown are in metres. Distance from the exit slit to the experiment was 8.0 m and from the distance from the undulator source to the exit slit was 52.3 m. The Young's slits were placed 10 mm downstream of the beamline exit window which is 700 m×700 m. The detector was an APD with 5 m slit placed directly in front of the detector window.

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Fig. 2. Young's double slit interference pattern at an X-ray energy of 1.1 keV, as measured by a scanning 5 m slit and APD detector. The monochromator entrance slit was 50 m and exit slit was 200 m. The data points are represented with crosses, whose size represents one standard deviation uncertainty. The solid line is a theoretical fit to the data used in the determination of the spatial coherence. (a) Young's slit separation of 20 m and (b) Young's slit separation of 50 m.

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Fig. 3. Measured and theoretical degree of coherence for an entrance slit width of 50 m with two exit slit settings. The theoretical profile is for an incoherent top-hat shaped source located at the exit slit. Gaussian fits to the data are shown in both plots. (a) Measured with exit slit at 200 m (), theoretical profile for 220 m and (b) measured with exit slit at 100 m (), theoretical profile for 120 m.

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Fig. 4. Two-dimensional CCD camera image of Young's interference pattern with a slit separation of 10 m at an X-ray energy of 1.5 keV. Image size is 0.82 mm by 0.42 mm and the fringe separation is 90 m. The shift in the fringes which occurs between upper and lower regions of the image is evidence of the possible effect of speckle. The secondary fringes are due to interference between the diffracted beam and the weak beam transmitted through the incompletely opaque slits at this energy.

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Corresponding author. Fax: +61-3-9347-4783; email: paterson@optics.ph.unimelb.edu.au