SETTING UP A MOUNTED CRYSTAL FOR DIFFRACTION: PART 1 MAR IMAGE-PLATE AND RIGAKU R-AXIS/ IMAGE-PLATE

DETECTORS

An image-plate is a thin layer of photosensitive material (very fine BaFBriEu^* crystals) which is oxidised to an excited state (Eu^^-^Eu^^) when incident X-ray photons strike i t The latent images thus formed have a half-Ufe measurable in hours. However, the decay in the excited regions has been shown to be measurable after about 5 minutes After exposure to X-rays, the plate is automatically illuminated with visible laser radiation which causes an electronic transition (Eu^*—>Eu^*) yielding luminescence from the stimulated regions. This luminescence is proportional to the intensity of the X-ray photons that caused the stimulation. The luminescence is measured by a scanning photomultiplier and stored as a digital signal. The image is then erased by a hydrogen erase-lamp illuminating the surface of the image-plate and causing the entire surface to assume the unexposed (Eu^*) state.

The (j)-oscillation geometry/ image-plate detector system is most useful for straightforward collection of data. That is, the crystal is placed in the X-ray beam path, and a series of angular oscillation images about <(> are collected by the detector, scanned, and written to a computer file. The images can then be processed (Section 3.3). For collections of particular desired sets of reflections (spots), the FAST area-detector system is of particular use (Section 3.2.4).

The mounted crystal/ goniometer assembly was connected to the spindle, which ensured that the crystal in the capillary was close to the X-ray beam path, and could be easily manoeuvred into this path. The centring of the crystal in the beam path was achieved by viewing the crystal through a microscope, or on a TV monitor. The view was perpendicular to the beam path (Figure 3.2.3). Firstly, the crystal was brought into view by translating the capillary lengthwise. Once in view, a cross-hair scale was used to first align the crystal to a common point at (|)=0° and <|)=180°, and then at (t>=90° and (|)=270°. A further check was made at this point that the crystal was still in the beam at the first two positions, and any adjustments were then be made by repeating the procedure for aU four positions as described. The final check was that the crystal was suitably in the beam

(b) DIFFRACTED X-RAYS I \ i Backstop \ Mylar Film X-RAY BEAM FROM GENERATOR 6-oscillation ^ Beamstop X-rav Beam Sealed ■... Capillary Detector plate (In protective Case)

(a) Detector plate Closer to beam

Lens of TV Camera or o f Microscope.

View through microscope or TV monitor while centring a crystal

Oo

Figure 3.2.3

Typical set-up for X-ray diffraction. The system illustrated is a MAR-Image Plate type system, other systems are principally similar. a) The detector captures more high-angle reflections at closer detector to beam distances (Section 3.1.2, Section 3.2.3).

through the whole 360°of (j).

Once the crystal was centred, the data collection hardware was set up. This usually took the form of a computer windows display which allowed remote handling of the various parts inside the X-ray enclosure. The enclosure is a lead impregnated perspex unit which contains the X-ray diffraction hardware, no X-rays can enter the enclosure until it is properly closed. The spindle was turned by defining values on the (|> axis, and an arbitrary (|)=0® setting was set at the start of the data collection.

Likewise, the distance between the detector and the X-ray beam source could be varied. This is useful for various reasons: firstly the strength of diffracted X-rays that meet the detector is greater, the closer in to the beam the detector is. This is useful if a weakly diffracting crystal is being studied. Secondly, the larger the unit cell of the crystal being studied, the closer the spots recorded on the detector will tend to be (Section 3.1.1). This may present a spot overlap problem during processing (Section 3.3), thus greater distances may be required to solve this problem. Thirdly, the resolution of the reflections recorded toward the edge of the detector is higher, the closer in the detector is to the beam. This is because the higher resolution reflections are diffracted at higher angles, and will only impinge on the detector if it is close enough to be in their path. These reflections are also much weaker, thus the closer in the detector is, the more likely they are to be significantly recorded above the background (Figure 3.2.3).

Initially, a 1° test oscillation was usually taken to check that the crystal was in the beam, and to give an indication of how strong the diffraction of the crystal was. The response of the image-plate detector to incoming X-rays is linear to a certain point, after which the pixels are registered as overloaded. The software reported back on the maximum pixel intensity in the image. The strength of diffraction was gauged by the relative intensity of pixels in spots to pixels in the background, and a reasonable exposure time per degree was estimated from this. Utility programs which allow profiles of pixel intensity values to be sampled across a spot gave a good indication on how far above the general background the peaks of the spots were.

Other considerations were how long the crystal will be exposed to the X-rays since radiation damage occurs at a detectable rate. Some crystals are very resistant to damage, others very prone to damage. The final consideration was in determining the number of degrees per image taken. The unit cell and symmetry will dictate how crowded the spots

will appear to be, a very sparse distribution of spots means that more than one degree of data may be recorded in one image. The advantage of this is that the overall time for data collection is reduced because a plate-scan will happen less than once per degree.

Finally, the collection strategy was programmed. The number of degrees of data required was programmed, with the number of degrees per oscillation image and an exposure time per degree. More than one pass of the spindle over the same oscillation image was programmed. That is, a 15 minute pass between 1° and 2® was performed as three 5 minute passes over the same angle. This should have resulted in more consistent spot decay on the image plate.

When satisfied that the data would be of suitable quality, the collection was left to proceed. Occasional checks on the images and to see if the hardware was operating normally were carried out. Processing of image-plate data is described in section 3.3.

3.2.4 SETTING UP A MOUNTED CRYSTAL FOR DIFFRACTION: PART 2 -