Operating the CCD Detector System

The Camera

The camera consists of a slit/shutter assembly, Phi rotation axis and ADSC CCD detector which is mounted on a translation base. The camera itself is mounted on a motorised table which is driven by the beam alignment software, the operation of which is described here. The detector system is controlled by an alpha workstation, known as bl96alpha1.

• The Slits

  There are two sets of slits that define the size of the beam hitting the sample. Each set comprises two pairs of slits, one for the horizontal and the other for the vertical dimension. Set 1 is on the input side, nearer the beam pipe; Set 2 is on the output side, nearer the sample position. The dimensions of the slits may be adjusted with a knob on the top of the assembly. There are two dials on either side of a central rail. The outer dial reads in full mm and the inner dial reads in fractions of a mm.

• The Ion Chambers

  There is an ion chamber after each set of slits, so that if there is any ionising radiation coming through the slits, a voltage should be readable by the ion chamber and displayed on the black terminal outside the hutch in the User area. This display terminal can toggle between Ion Chamber 1 and Ion Chamber 2. For either one, 3 choices of gain (x1, x10 or x100) may be selected. The maximum reading that can be displayed is 199.99. When it goes above that value, the display will show a steady line (hyphen), and a lower gain setting should be chosen.

• The Shutter

  The camera shutter is situated between Ion Chamber 2 and the sample position, so that an ion chamber reading is still obtainable although the shutter may be closed. This is particularly useful for tracking the beam in between exposures.

• The Phi axis

  Standard goniometer heads may be mounted here. An adequate translation range is possible. The original design of the camera had the phi axis mounted on the far side of the base. Because of the geometry of the beam on Station 9.6, the Phi axis was rotated to the near side. This required the reversal of the sense of rotation of the motor, to maintain the convention that a clockwise rotation of Phi would move the diffraction pattern up on the plate. The result is that the dial on the Phi motor is now of the wrong sense, i.e., a clockwise rotation of 90° starting from 0, would take the dial to 270, but in fact it is truly at 90°.

  The design allows the user to disengage the motor by hand and rotate it in increments of 90°, for aligning the sample. At the end the gear should be re-engaged, by leaving the Phi axis in the 'click' position. If the axis is left out of this position the motor will not be able to rotate it.

  N.B. At phi = 0, the goniometer-head-locking-pip on the phi axis should be pointing upwards.

  A goniometer head in place on the phi axis. Also shown are the beam stop (left), cryocooler nozzle (middle), fibre optic light source and slit assembly (on the right).

• The Crystal Viewing System

  The sample is viewed via a CCD camera, placed underneath the detector translation track. Light reflected from the sample travels through the little glass window at the base of the beamstop holder and is reflected by a plane mirror onto the CCD camera. The same display is relayed to another monitor outside the hutch. For more information please see the Crystal Viewing System pages

• The Beam Stop

  This is mounted on two sliding bars, to ensure translation parallel to the detector track. Its distance from the sample is adjustable over a reasonable range. At long crystal-to-detector separations, it should be fully extended to allow the low resolution orders to be visible on the plate. If it is not close enough to the sample, at shorter crystal-to-detector separations, the yoke holding it will obscure part of the pattern. Thus it might appear that the crystal is not diffracting to the resolution you anticipated. When adjusting the beam stop position, it should be pushed/pulled so that the pressure is applied evenly on both slides to allow a smooth translation.

The CCD Detector

The ADSC CCD detector has

• a 2 x 2 array of CCD modules with 400 micron dead strip between them
• an active area of 188 x 188 mm (2300 x 2300 pixels, each pixel being ~0.0816mm)
• a 20 bit digitization range
• fast readout time ranging from 1 second (for 2 x 2 binning) to 9 seconds

	The detector in place on the camera.
	The crate which controls the operation of the detector (left), the small chiller unit which provides the cooling (next to crate) and the detector itself (right).