The Midwest Universities Collaborative Access Team (MUCAT) was organized in 1990 for the purpose of developing and operating a sector (insertion device and bending magnet lines) at the Advanced Photon Source. MUCAT secured funding for the construction of the undulator line in FY96 and saw first light in the First Optics Enclosure in February 1998. Commissioning experiments on the main undulator line began late in FY99. The main undulator line presently consists of two experimental stations in tandem for x-ray studies in the 3-40 keV energy range. The optics for the main line consists of flat perfect Silicon (111) monochromator crystals and an (optional) focusing mirror. A high-energy branch line station (30-130keV), funded by FZ Jülich, is in the latter stages of commissioning. The branch station optics are flat controlled-mosaic (annealed) Silicon (111), (311), and (331) Bragg-geometry crystals. Simultaneous operation of the main line and high-energy station is possible; maximum flux for the high-energy station is available when the shared source, an APS undulator A, is operated at or near closed gap. In FY2000, the Department of Energy funded our proposal to construct a bending magnet beam line in Sector Six. The construction of this facility is in progress.
This consortium brings together scientists from several universities and national and international laboratories with common interests in the use of synchrotron radiation for materials science. The scientific program of our CAT is varied, but generally characterized by an emphasis on materials research. The magnetic scattering and spectroscopy portion of the scientific program concentrates on resonant and non-resonant scattering studies of magnetic materials. Research efforts in the surface scattering program are centered on the study of the kinetics and growth of 2-dimensional systems, the role of defects in epitaxy, ordered non-epitaxial overlayers, phase transitions and investigations of liquid surfaces. The brilliance of undulator radiation obviates the need for strong focusing of the beam, resulting in a small footprint on the sample with low divergence. A liquid surface diffractometer is used to probe the chemistry and physics of monolayer films at liquid surfaces as well as realistic models of biological membranes and their reaction to various stimuli and environments. High energy x-rays are used for in situ studies of materials processing using a new high temperature furnace constructed at the Ames Laboratory and studies of pair distribution functions of poorly or partially ordered structures.
Presently, the member institutions of MUCAT include: Ames Laboratory - USDOE, Iowa State University, Forschungszentrum Jülich GmbH, University of Missouri - Columbia, Georgia Institute of Technology, Washington University - Saint Louis, University of Wisconsin - Madison, Kent State University, State University of New York - Stony Brook, and Michigan State University.
There are currently three experimental stations in the sector. The four-circle diffractometer and liquid surface diffractometer in 6-ID-B have been declared operational (November 2000 and November 2001 respectively) while the commissioning phases for the high-energy station (6-ID-D) and surface science station (6-ID-C) are nearly complete. At the same time, we are developing the bending magnet beam line (6-BM-A,B) in the sector. Further details of the status of each station are given below.
This station was declared operational in November 2000 and is currently accepting general users as well as the members of the CAT. In addition to general scattering experiments, the four-circle diffractometer serves as the principle facility for magnetic scattering experiments, the small surface science chamber, and for lower energy runs (up to 40keV) of the high temperature furnace. A variety of detectors are available for use including the standard complement of scintillation counters, solid state detectors and a MAR345 image plate system. The diffractometer itself will be replaced within the next year with a Huber Psi diffractometer (ordered from Blake Industries) so that a horizontal as well as vertical scattering geometry can be employed. This station, then, will be fully compatible with the diffractometer in 6-ID-D.
This station was declared operational in November 2001 and is currently accepting general users as well as members of the CAT. The diffractometer is capable of in-plane scattering measurements of liquid surfaces as well as the standard reflectivity measurements. Currently, measurements are being done with single point detectors. However, an instrumentation proposal is being written by D. Vaknin for the purchase of a position sensitive detector for this station.
The surface chamber was physically brought to the APS in December 2000 from the University of Missouri. The instrument is essentially a MBE machine on a Psi diffractometer. In addition to LEED, Auger, and ion sputtering, there are two evaporation sources that can deposit while the sample is in the x-ray beam and the sample temperature is designed to be variable from 20 K to 1600 K. A particularly useful capability of the beam line is that it can cover a large range in reciprocal space. This is accomplished through an internal arc that permits precise sample motion over 100 degrees and a large Be window that yields a large angular range.
The high-energy station has been engaged in parallel operation with the main undulator line for about six months and several experiments have been successfully performed. The diffractometer has been operated in both horizontal and vertical scattering geometries. Three types of detectors have been implemented: NaI scintillation detectors, an image plate area detector and a solid state Ge detector for low background measurements. A Liquid He cryostat, complete with regulation and a pumping station for stable low temperature operation, (1.2K - 300K) has been successfully tested in horizontal scattering geometry. A closed cycle cryostat (6K-300K) is available for operation with the Eulerian cradle.
The first generation of high-energy monochromators proved to be slightly strained leading to a loss in flux. A new mounting concept for the first monochromator crystals was implemented in July 2002. While two out of the three sets of crystals performed nicely for 6-ID-D, we discovered that the new arrangement partly blocks the beam of 6-ID-B. The monochromator cooling for the high energy station has been re-engineered to avoid this problem. Drawings and crystals have been prepared and the mechanical work has been done by the Ames Laboratory shop. Tests at FZ Jülich on the gamma-ray diffractometer indicate that with the new mounting concept and special annealing process, we can achieve unstrained Si mosaic crystals with a rocking curve width of 10''. With these revisions, 6-ID-D will shortly resume commissioning operation and should attain operational status in six months.
The design of the beam line will allow a wide variety of standard scattering and spectroscopy techniques to be used. We have, however, optimized the design for experiments using two important techniques: atomic pair distribution function analysis and in situ high temperature powder diffraction studies of materials processing. Both of these techniques make use, primarily, of high energy (>40keV) beams with stringent shielding requirements in the experimental enclosure. The development of a station focused on using high energy x-rays for local structure determination via atomic pair distribution function (PDF) analysis is particularly exciting. This use of this technique will allow determinations of local and intermediate range structure in many disordered, or partially ordered, systems of technological importance including semiconductor alloys, ferroelectric materials, polymers, nanoporous or microporous materials such as zeolites, and aperiodically ordered systems such as quasicrystals.
The preliminary design report for the bending magnet line was submitted to the APS September 2001 and approved in December 2001. A "request for quotes" for the enclosures (6-BM-A,B) was submitted to the APS in September 2001. It was sent to vendors in January 2002 and ultimately let to Teknit. Construction of the enclosures was substantially completed during the summer shutdown and PSS validation and initial shielding verification were completed in October. The specifications and drawings for beam line components have been completed and are being sent to vendors for quotes. Specifications for the monochromator are being completed at this time.
Use of the Advanced Photon Source is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-Eng-38.
The Midwest Universities Collaborative Access Team (MUCAT) sector at the APS is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through the Ames Laboratory under Contract No. W-7405-Eng-82.