November 2012 Archives

A recent IDAT publication from James, Chris, and Ken investigating the source of the point spread function in detectors using CCDs coupled fiber optic tapers.

The point-spread function (PSF) of a fiber-optic taper-coupled CCD area detector was measured over five decades of intensity using a 20 µm X-ray beam and 2000-fold averaging. The “tails” of the PSF clearly revealed that it is neither Gaussian nor Lorentzian, but instead resembles the solid angle subtended by a pixel at a point source of light held a small distance (27 µm) above the pixel plane. This converges to an inverse cube law far from the beam impact point. Further analysis revealed that the tails are dominated by the fiber-optic taper, with negligible contribution from the phosphor, suggesting that the PSF of all fiber-coupled CCD-type detectors is best described as a Moffat function.

the authors go on to suggest that:

…we expect that by fitting an expression for the spot-PSF convolution as described here directly to pixel values will result in more accurate spot intensity integrals than those currently being obtained using conventional profile-fitting methods (which assume that the intensity of a pixel is due exclusively to X-ray photons falling directly upon it). A “fitting approach” would eliminate systematic errors in background estimation arising from the tails and also suppress the influence of shot noise from X-ray photons falling on pixels outside the “true” spot area.

Holton, J. M., Nielsen, C. and Frankel, K. A.”The point-spread function of fiber-coupled area detectors” Journal of Synchrotron Radiation, 19, 6, (Nov, 2012)

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