Design and performance of the ASAXS instrument at the Advanced Photon Source

The SAXS instrument on the high brilliance undulator beam line (ID-12, BESSRC-CAT) at APS has been designed to produce high-resolution scattering patterns in the millisecond time domain. This instrument is equipped with a 20 cm x 20 cm position sensitive gas detector and a 15 cm x 15 cm high-resolution position sensitive CCD mosaic detector. A photodiode detector mounted on a 3 mm diameter beam stop permits quick alignment of the instrument as well as precise measurement of the transmitted beam intensity. The ease of changing the sample to detector distance and tuning of x-ray energy enables easy access to different Q ranges. With this instrument we routinely measure data in a Q range of 0.001 to 1 angstrom-1. The exposure time with the CCD detector varies from 0.1 second to 10 sec depending on the scattering cross-section of the samples. Techniques to interface ancillary equipment for time-resolved studies and software for faster online analysis of the data have also been developed. We have obtained excellent data on the unfolding of proteins in the millisecond time domain, ASAXS of metallic alloys by using this instrument