Preliminary Result of Photon Counting Acquisition Scheme for Laser Pump/ X-ray Probe Experiments*

R&D project has been initiated for a proposed ultralow emittance (~50pm.rad) synchrotron light source built in Beijing. The R&D includes the development of high repetition rate laser pump/X-ray probe for ultrafast dynamics detection in future source. In a typical laser pump/X-ray probe measurement, the X-ray pulse follows a laser pulse in adjustable delay. We are interested in the difference between laser on and laser off at different delay, which will snapshot dynamic process. To capture this trivial difference, it requires the acquisition system to single out the signal from this special X-ray pulse at adequate S/N ratio. For the R&D of high repetition rate pump-probe, we have set up a prototype counting acquisition system based on NIM modular electronics, which was tested in Beijing Synchrotron Radiation Facility (BSRF). The laser will be synchronized with a camshaft bunch at 124 kHz, a tenth of the revolution frequency. Avalanche Photo Diode (APD) was used to detect the X-ray pulse from this camshaft bunch due to its nanosecond response. Before the laser is delivered, we mimic the 124 kHz laseron signal. The signals from APD are separated by power dividers into two Constant Fraction Discriminator (CFD) input channels. The signal in laser-on/off channel is gated out using the 1.24MHz timing signal divided from 499.8 MHz RF signal, while the mimic laser-on signal gated out at 124 kHz. Multiplied by ten times, the mimic laser-on signal counts should be consistent with the laser-on+off counts, if our counting modular works well. We carried out this test at 1W1B wiggler beam line to measure the Fe fluorescence signal. The performance of our system is demonstrated in the good consistency between mimic laser on and laser on+off signals.