UConn IMS Xray Facilities

R.T. Jones
June 2, 2006

In February, 2006 I was taken on a guided tour of the Institute for Materials Science (IMS) X-ray Diffraction and Spectroscopy Laboratory at the University of Connecticut. The tour was led by Jack Gromek. He showed me several instruments that are used by UConn researchers to study things like protein structure and polymers. Jack is an expert on how these instruments work and how they are used for biological and materials science applications, but he was hesitant to recommend them for our application. He answered my questions as well as he could, but for most things he simply pointed to the manuals. He also said that most of the information I was asked for could be found in references posted on the manufacturer's web site.

The place to begin in this search is the IMS X-ray Diffraction and Spectroscopy web site, where a list of the X-ray diffraction instruments operated by the IMS is found. This list seems to be somewhat out of date, because the newer Oxford Diffraction instrument does not appear in the list. There were two instruments that seemed most interesting for the study of large diamond monocrystals.

1. Oxford Diffraction Xcalibur PX Ultra This instrument is described on the Oxford Diffraction web site. The documentation on the web site separates the instrument into the X-ray source, the detector, and the platform. The source is the Enhance Ultra Cu which features a 3kW water-cooled copper target. Emerging from the collimator is a Cu Kα X-ray beam that focuses down to a diameter 300μm a few cm from the collimator exit. The brochure does not say what the power of the X-ray beam that exits from the collimator. P. Best estimates that about 1% of the source power appears in the form of X-rays, and only 10-15% of these exit the collimator. From these numbers one can estimate the X-ray power on target to be of order 5W. The brochure does not say what the divergence angle of the beam is, but one can guess from the dimensions of the instrument that it is less than 1mr. The documentation lists this source as a "single wavelength source". It also describes it as having "high brilliance" and "multi-layer optics." Multi-layer optics is presumably used to focus the Cu Kα X-rays through the collimator aperture. The emission spectrum of Cu Kα radiation is described in [1]. The detector for this instrument is the Onyx CCD detector which is a 2k x 2k pixel circular imager of diameter 16.5cm. Each pixel is read out with 17 bits of precision / dynamic range. The entire image takes 2 seconds to read out when the resolution is reduced to 512 x 512 pixels and 3 seconds at half-resolution. The diameter of an individual pixel is 60μm. The CCD chip is cooled to -45° by a Peltier cooler so that a dark current less than 0.06 e-/pixel/s is achieved. Source, detector and sample are all mounted on the Kappa goniometer which provides a 4-circle single-axis positioning post. From the picture on the web site, it appears that the main source is fixed and the different rotation axes on the central post are used to independently rotate the sample, the detector, and up to two other instruments such as secondary sources and detectors. It seems that for diamond studies this base would have to be rigged with a x-y translation stage for the target.
2. Bruker AXS D8 Advance
I have fewer details regarding this instrument. The manufacturer's web site has some details regarding this device. There is some choice of sources, including a 2kW copper X-ray tube and a 18kW rotating-anode copper source, both producing copper Kα radiation that is filtered using a graphite monochromator. The collimator slits produce a beam of diameter 0.5mm. The divergence angle is determined by the intrinsic width of the Cu Kα lines assuming that the the graphite monochromator is ideal, leading to a rms divergence angle of 250μr. The appearance of the Oxford Diffraction source collimator is similar to the Bruker Advance, suggesting that a similar scheme is used by both to monochromate the source, using a highly-oriented pyrolytic graphite (HOPG) crystal followed by a long needle-shaped capillary that uses total external reflection of X-rays at low-angle incidence to focus the beam to a small spot. The web site for IFG-Adlershof GMBH [2] has a nice description of how this works. The primary detector for the Bruker diffractometer is the Hi-Star multi-wire imaging detector with 1024 x 1024 pixels over an area of diameter 11.5cm.