CHARACTERIZATION OF CCD DETECTOR ELECTRONICS FOR RECORDING WEAK X-RAY SCATTERING

点击翻译按钮获取中文摘要

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We addressed the detector offset (baseline) fluctuation issue with our MarCCD165 detector as described last year. The improved baseline stability of ~0.2 ADU was verified earlier this year immediately following the upgrade by the detector vendor through our service contract. In parallel, we modified our data collection protocol for additional compensation of the baseline fluctuation, making this detector highly effective in x-ray scattering studies of weakly scattering samples. The two major changes to data collection protocol to compensate for this small level of electronic drift are 1) each set of a series of data collection (typically 20 images of 10s exposure each) is preceded by a recording of ¿dark image¿ which is subtracted automatically from each data frame in the data collection software. Each dark image is also stored as a file series number ¿zero¿ for later use; 2) the detector received a mask which covers small areas on the detector active area at 3 and 9 o¿clock positions so that each data frame has an internal measure of true baseline level in all four detector quadrants, which can have different baseline levels due to independent readout circuits. The SSRL-developed data processing program MarParse has an option of using either a user-defined global baseline value or the actual offset values measured individually for four quadrants in the first step of data processing. These developments allow us to provide users with much more reliable solution scattering data in combination with the larger detection area for azimuth averaging, compared to the much smaller area covered by our linear detector. The MarCCD165 detector, which was originally purchased for single crystal diffraction studies and other studies involving strong x-ray scatterers, has been in routine use in protein solution x-ray scattering for the entire duration of 2006. The test results we obtained in the process of these improvements have also provided a bench mark for specifying desired detector characteristics for our new CCD detector