User_Manual-PILATUS-V1_2.doc 49/57
12 Factory Calibration and
Correction
The following calibrations are done at our premises:
1) Threshold calibration
The PILATUS detector systems come fully calibrated. See the system
information sheet in your user handbook for more information about the
calibrated energies and settings.
The discriminator thresholds in the individual pixels are set by an automated
procedure (described above). Ideally, the thresholds are set to 0.5 of the
beam energy, and the procedure to reset them is activated automatically by
the beamline software when the beam energy is changed. Special threshold
settings may be employed, for example, to discriminate against specific
fluorescence from the sample.
The threshold settings affect individual pixel sensitivity to some degree, so the
threshold settings and the flat-field settings should be coordinated.
2) Rate correction
The counter in the pixels is a classical paralyzable counter with a dead time
that depends on the voltage (Vrf) settings. The correction required is
negligible up to 10
4
counts/sec for standard settings, but becomes quite
significant approaching 10
6
/s; above ~2*10
6
/s (for standard settings) the
conversion is cut off at a "saturation" value. This value is printed in the header
and can be used as a flag in analysis software. Rate correction is optionally
turned on in the control software, usually at startup.
Threshold setting, flat-field correction, rate correction and bad pixel tagging
are all (optionally) handled by the detector software via a single command.
3) Distortion (only for multimodule systems)
A text file with a map of the offset in position and angle of each module with
respect to some common origin will be provided.
The processing program must incorporate this information, using it as a
lookup table to map sought reciprocal space positions onto detector positions.
4) Parallax
The silicon sensor is 0.320 mm thick. The parallax correction as a function of
energy and angle of incidence has been well modeled and is about 1 pixel
displacement at an angle of incidence of 45 deg. Parallax actually improves
spatial resolution because a spot that is spread over a few of pixels can be
localized better than a spot in just 1 pixel.
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