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

本子项目是利用由NIH/NCRR资助的中心赠款提供的资源的众多研究子项目之一。子项目和研究者（PI）可能已经从另一个NIH来源获得了主要资金，因此可以在其他CRISP条目中表示。列出的机构是中心的，不一定是研究者的机构。BL4-2 MarCCD165探测器显示出相对较大的a /D转换器偏置波动，这使得准确记录非常微弱的信号电平非常具有挑战性，尽管由于散射信号的径向积分，数据精度得到了显着提高。这实际上是几乎所有光纤耦合CCD x射线探测器普遍存在的问题。开始了一个项目，以确定实际溶液x射线散射实验所需的电子稳定性的确切水平。我们测量了作为时间函数的偏移电平波动，并评估了相对于稀蛋白和缓冲溶液散射信号水平的信号波动的确切程度。我们观察到每像素平均有2-4个AD转换器单位的偏移波动，大约对应于2-4个12 keV的x射线光子。在Blu-ICE中，我们在每次x射线曝光之前立即实施了空白偏移电平测量，因此我们能够在许多场合更准确地记录弱信号电平。我们将我们的结果传达给探测器制造商，他们因此在我们的年度维护合同中实施了两项主要改进：更换读出电子电路中的一些关键电子元件，并在电路中的可编程场门阵列中实现补偿算法。此外，为了进一步稳定主要造成偏置波动的温度敏感电子元件的性能，我们还对BL4-2实验舱内的探测器电子元件采取了温度稳定措施。该探测器现在具有更好的稳定性，并在大多数溶液散射研究中提供更可靠的缓冲减法。

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.