Development of Pulse Width Measurement Techniques in a Picosecond Range of Ultra-short Gamma Ray Pulses

Ultra-short gamma ray pulses of the picosecond and femtosecond range can be generated using laser Compton scattering with 90-degree collisions at the UVSOR-II electron storage ring. Measurement techniques for a gamma ray pulse width in the sub-picosecond to picosecond range are being developed. As the first stage of the development, we tested a pulse width measuring method for the gamma ray pulses with pulse width of 4.8 ps (FWHM) consisted of a multi-pixel photon counter (MPPC) and a digital oscilloscope. The time resolution of the MPPC was measured as 477 ps (FWHM) by using a single photon counting technique. It was indicated the shortest pulse width that the MPPC could evaluate was 82 ps under ideal conditions. However, the measured gamma ray pulse width was 540 ps. The main reason for the large discrepancy is considered to be the noise of the trigger signal synchronized with the gamma rays. The measured gamma ray pulse width can be lowered by increasing the signal-to-noise ratio of the trigger signal and using a fast photodetector, a microchannel plate photomultiplier tube (MCP-PMT) with a time resolution of few tens of picoseconds. As the next stage, we will develop a pulse width measurement technique in the femtosecond range by using a pump-probe technique with a femtosecond laser and ultra-short gamma ray pulses.