Grant Bunker, Director
April 28, 2000
A lot has been happening in recent months - the big news is that we are now officially operational and accepting beam time applications. See "Access" on the main home page for details.
We have been making steady progress in our XAFS capabilities and monochromator thermal stability. At this point (at long last) I'm pleased with the XAFS data we are getting and the rate at which we can acquire it. We can now obtain good quality EXAFS data on sub-millimolar samples in a few minutes using the latest version of the multilayer array analyzer detector (MAAD) that we have developed. The performance (solid angle, background rejection) is similar to that of conventional multielement Ge detectors, but without the dead time losses and saturation. It effectively has no count rate limits. I'm also convinced at this point that it is a practical device in terms of alignment and setup.
Ke Zhang has been making good progress on the XAFS stopped flow system, which should be compleeerhnext couple of months.
Considerable progress has been made on the beamline in recent months, and the XAFS capabilities are getting close to where we want them to be. We have implemented continuous scanning in hardware, with a minimum (1 keV) scan time of about two seconds, while accurately keeping track of the energy at each sampling interval. This gives us very high duty cycle, with very little time is spent not taking data. We envision most routine XAFS scans to be done on the time scale of a minute or so.
Beam stability has improved and position feedback is being experimented with. We are implementing several approaches to beam tracking, and the user will be able to choose the best one for his or her needs. The monochromators are nominally fixed exit, and with position feedback using the tune of the second crystal to throw the beam slightly I believe we should be able to get constant beam to within several microns. Alternatively, if the user's apparatus can fit on a table, the whole apparatus can be translated up and down in the traditional manner. This has been tested and works at the several micron level.
Good beam stability and tracking is needed for the multilayer analyzer. Our goal is to support routine fast scanning of multiple enzyme samples within the displex using the multilayer analyzer.
The displex works well now, very low vibration (because heat conduction is through and exchange gas and the expander is decoupled mechanically from the sample), and rapid cool-down. Problems encountered in early use of the unit were identified and fixed.
Control software for continuous scanning and a simplified graphical user interface have been written. The interface is in Java and this will ultimately allow remote control of the beamline through a browser interface. At present remote control is offered by four alternative means: VNC (virtual network computing), Timbuktu, X-windows, and EPICS Channel Access. In most cases the main data acquisition program runs on one of two beamline control computers; a Sun workstation running Solaris, and a Compaq running NT. Other options are possible however.
On the scattering front, improvements have been made to the CCD detector control software to improve its user friendliness. Still plenty to do in that area however. We have procured solution scattering analysis package written by P. Thiyagarajan (running on top of Igor Pro). As always FIT2D is available for scattering analysis.
Stay tuned... Grant Bunker, Director