Design of a fast rotational chopper for X-ray pump–probe experiments at Diamond Light Source

New trends in X-ray crystallography are concerned with the study of transient conditions of atomic structures, which take place after an energy activation agent is introduced. These time-resolved experiments require a fast mechanical shutter to interrupt the X-ray beam in a pump–probe cycle, with the aim to generate a stroboscopic effect. Thus, only diffraction data that are representative of the activated structure are actually collected. A rotating type of shutter, also known as chopper, is presented with the purpose to enable time-resolved experiments to be performed at I19 small-molecule single-crystal diffraction beamline. Exceptional stability in the rotational speed is critical to achieve the desired stroboscopic effect with minimum jitter. This requirement can be addressed only through design by the specification of suitable components and implementation of high-precision methods in manufacturing. The proposed equipment is comprised of a spindle supported on air bearings coupled to a slotted disc rotating inside a vacuum enclosure and driven by a brushless servo motor. Advanced control features are proposed to ensure that speed stability is achieved. Preliminary tests produced very encouraging results, giving strong indication that the chopper satisfies the specifications required for time-resolved experiments.