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

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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. The BL4-2 MarCCD165 detector displays a relatively large A/D converter offset fluctuation, making it highly challenging to record very weak signal levels accurately, despite the significant improvements in data precision due to radial integration of scattering signals. This is in fact a universal problem with almost all of optic fiber coupled CCD x-ray detectors. A project was initiated to determine the exact level of electronic stability required for actual solution x-ray scattering experiments. We measured offset level fluctuation as a function of time and assessed the exact degree of signal fluctuation with respect to the level of scattering signals from dilute protein and buffer solutions. We observed offset fluctuation of as much as 2-4 AD converter unit per pixel on average, corresponding approximately to 2-4 12 keV x-ray photons. We implemented a blank offset level measurement immediately prior to each x-ray exposure in Blu-ICE, and we have as a result been able to record weak signal levels more accurately on many occasions. We communicated our results to the detector manufacturer, who as a result implemented two major improvements within our annual maintenance contract: replacing some of key electronic components in the read out electronic circuit and implementing a compensation algorithm in the programmable field gate array in the circuit. Furthermore, measures were taken to stabilize the temperature of the detector electronics inside the BL4-2 experimental hutch in order to further stabilize the performance of the temperature-sensitive electronic components primarily responsible for the offset fluctuation. The detector now performs with improved stability and gives much more reliable buffer subtraction in most solution scattering studies.