Development of a Gas Monitor Detector for the PAL-XFEL

This paper presents the configuration and responses of a transmissive gas monitor detector (GMD) developed by the Pohang Accelerator Laboratory for its X-ray free-electron laser. The GMD is installed on the hard X-ray beamline to monitor the intensity of the X-ray pulses. The GMD exploits the photo-ionization of Xe at ~10 − 8 - 10 − 5 Torr by interactions with intense X-ray pulses and uses two-stage microchannel plates (MCPs) to detect the photo-ions and photo-electrons that are generated. The nominal gain of the MCP is 8.47 × 10 5 at an operation voltage of 1.5 kV. The results of the experiment show that the combined gain and detection efficiency of the MCP was approximately one-third of its reference value. However, readings obtained by using the developed GMD showed a strong correlation pulse by pulse with other diagnostic detectors, and the arrival time obtained from the time-of-flight spectrum of the photo-ions was consistent with our expectation and numerical results.