Detectors

Detectors suitable for high pressure diffraction experiments are dictated by the working X-ray energy, cell geometry (limited 2 theta angle) and cell composition (diamonds for the DACs, mainly boron gaskets for the Paris-Edinburgh press). Ideally the detector should therefore have :

• a large field of view of more than 150 mm diameter,
• a high spatial resolution (point spread function of 150mm FWHM or less),
• low noise and high dynamic range (14 bits or more),
• a good sensitivity at high X-ray energies above 30 keV,
• stable and easily tractable spatial distortions,
• and fast reading and erasing times (a few seconds or less).

ID27 is currently equipped with three large area X-ray detectors that can be choosen to fulfil the critical criteria for each experiment, as detailed below.

1. MAR345 image plate

The MAR345 image plate detector fulfils most of the above criteria but has very long readout time of 1 minute or more depending on the choice of the pixel size. This becomes problematic when compared to exposure times of the order of 1 second or less due to the high photon flux available at beamline ID27. The detector consists of large (345 mm diameter) image-p late, of which the scanned size can be configured. Possible resolutions are 1200, 2400, 3450 pixels. Pixel size can be chosen, minimum 100 µm.

2. MAR165 CCD

The large area MAR165 CCD detector has a much reduced readout time to 5 seconds in full binning mode. However, this detector has a much smaller input surface of only 165 mm diameter which is not sufficient to access low d-spacing at high resolution.

3. Perkin Elmer Flat Panel

The Perkin-Elmer large area detector fulfills most of the criteria (high efficiency, large area and fast readout), but has a limited dynamic range (12 bits). This means that measurement of, for e.g. diffuse scattering in the presence of strong Bragg scattering, is impossible without double exposures.