A high speed chopper (X-ray pulse selector, XPS)

A high speed chopper  (X-ray pulse selector, XPS) which is synchronized at 946 Hz to a subharmonic (1/537600) of the radio frequency and made by Forschungszentrum Jülich is installed. The physical opening window of the chopper corresponds to 1.2 msec but when it is phased to select the single bunch in the PF-AR single bunch mode, the exposure time becomes the 50 psec duration of the X-ray pulse. The novel feature of the chopper is its continuous phase locking with a timing jitter of less than 2 nanoseconds. One can thus produce a 946 Hz pulse train of 50 psec pulses from the X-ray pulse trains at 794 kHz emitted from PF-AR ring. If a detector with gating capabilities such as an avalanche photodiode is used, the XPS is not needed, and a gated integrator system can be used instead. If the repetition rate of the reversible reaction is slower than 946 Hz, a millisecond single-shot shutter will be available to lower the repetition rate.