Data Collection Sequence for the X-ray Image Intensifier/CCD Detector System

This is an overview of a full calibration/ data collection strategy necessary to obtain quantitative results from the X-ray Image Intensifier/CCD read-out detector system (XRII/CCD) [21,22] or similar detector system. It is intended as a check list for anyone performing a crystallography experiment. Aspects which require further consideration are discussed in Section 16.5.

The following list is for measurements which may be performed separately from the data collection, before or after.

1a: Calibration measurements to determine intensity non-linearity. At present the ESRF XRII/CCD system appears to be sufficiently linear to avoid the need for correction.

1b: Collect response of CCD camera to uniform light source (illuminated piece of paper). ``Correct for dark count'' to get CCD uniformity of response function. (Averaging images will improve accuracy.) (Probably performed before the experiment, at the normal operating temperature.)

1c: Place diffuse scatterer/fluorescent cell at the sample position.

1d: Take 1-D 2$\theta$ scan of diffuse source/fluorescent cell distribution. At each angular position the counter must be left long enough to reduce counting statistics to the required level. (Scans at other angles than the plane of the synchrotron may be useful if it is possible to obtain these.)

1e: Recording the 2-D source distribution on X-ray film or image plates may be useful to verify the extent to which the intensity distribution is circular symmetric.

1f: Collect reference 2$\theta$ scan without diffuse scatterer/fluorescent cell, or with cell containing only solvent.

This is a proposed sequence for efficient collection crystallography and calibration data. This pre-supposes that the detector linearity is known, that the CCD pixel to pixel flat-field response is available, and that a diffuse scatterer with a known angular distribution of intensity for the required wavelength is available. (The order is the result of practical experience.)

2a: Place calibration grid on the detector.

2b: Place diffuse scatterer/fluorescent cell at sample position.

2c: Measure sample to mask distance.

2d: Collect spatial distortion calibration grid image.

2e: Collect flood-field image with diffuse scatterer/fluorescent cell.

2f: Remove diffuse scatterer/fluorescent cell.

2g: Collect reference image for flood-field exposure integration time without fluorescent cell, or with cell containing only solvent.

2h: Collect beam centre image or ``wax'' image, unless a semi-transparent beam-stop is being used.

2i: Place sample at sample position.

2j: Collect diffraction data images (without changing the magnetic environment e.g. Do not change the position of large metallic objects such as the diffractometer cradle).

2k: Collect ``dark count'' (empty capillary) images for standard data exposure integration time(s). (Averaging images will improve accuracy.)

2l: If data must be collected in a different magnetic configuration the spatial distortion and ideally the flood-field response should be re-calibrated. (Repeat steps 2a-2h.)

2m: Remove sample

2n: Place diffuse scatterer/fluorescent cell at sample position.

2o: Collect flood-field image.

2p: Collect spatial distortion calibration grid image.

2q: Collect reference image for flood-field exposure integration time without fluorescent cell, or with cell containing only solvent.

(Steps 2n-2q are included to verify if that the spatial distortion has remained constant since it was last calibrated.)