Picosecond Rate X-Ray Photon Counting Detector
Pumpprobe experiments are a powerful tool for directly studying the molecular dynamics of chemical reactions and energy transfer processes as they unfold on the femtosecond or picosecond time scale. Pumpprobe experiments with xray probes, where a chemical reaction is initiated by a first optical pulse (the pump) and the energy levels or molecular conformation of the activated complex is measured a short time later by a second optical pulse (the probe), have been enabled by new sources of bright, shortduration xray pulses such as the Linac Coherent Light Source (LCLS) and the Advanced Photon Source (APS after planned modifications). However, development of highspeed, highresolution imaging xray detectors has not kept pace, and the current state of the art is a time resolution of about 100 ps, 3 orders of magnitude longer than the duration of the xray pulses emitted by LCLS. The proposed pixelated waveform recorder will achieve picosecond xray timing resolution by combining a thin InSb photodiode detector array with a fast waveform sampling application specific integrated circuit (ASIC). In Phase I, the performance benefits of a custom fabricated InSb detector will be demonstrated. In parallel, the design of an ASIC capable of capturing pulse waveforms and reconstructing the time of arrival to the picoseconds scale will be developed. Subpicosecond xrays can be used to resolve motion on the time scale of molecular dynamics. The atomic motion on the fundamental time scale of a vibrational period (about 100 fs), via the making and breaking of chemical bonds and the rearrangement of atoms, ultimately determines the course of phase transitions in solids, the kinetic pathways of chemical reactions, and even the efficiency and function of biological processes. A thorough understanding of such dynamic behavior is a first step to being able to control structural evolution, and it is expected to have important scientific applications in solidstate physics, chemistry, materials science, and biology.
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