PHOTON SCIENCE

Some small considerations for your coherent scattering experiments:

1) Most coherent scattering experiments make use of 2D detectors. The chosen detector system places restrictions on your experiment, since the detector has to be able to resolve coherent scattering features ('speckles'). The 'speckle' size can be estimated as a function of wavelength, sample-to-detector distance and the defining slit size:
d_speckle ≈ lambda x R/s
'R' is the sample-to-detector distance, 's' is the beam size on the sample (or the defining slit size) and 'lambda' is the X-ray wavelength. The value of d_speckle should be equal or greater than the detector pixel size. Using 8 keV X-rays and the standard detector distance of 5 m at P10, it is possible to calculate maximum beam sizes for the standard 2D detectors at P10. This results to the following estimates:
a) Pilatus detector (172x172µm²) : ~4.5µm (i.e. impossible to use without focusing!)
b) Eiger detector (75x75µm²): ~10µm (i.e. still not really reachable without focusing)
c) PI-LCX detector (20x20µm²): ~40µm
For XPCS experiments it has been shown that one can relax this criteria a bit, but for CDI experiments it might be necessary to restrict these values even further to reach the right oversampling ratios.