Pulse width measurement of laser Compton scattered gamma rays in picosecond range

Ultra-short gamma ray pulses of the picosecond and femtosecond ranges can be generated using laser Compton scattering with 90° collisions at the UVSOR-II electron storage ring. Measurement techniques for a gamma ray pulse width in the femtosecond range are being developed. As the first stage of pulse width measurement, we tested a pulse width measuring method for the gamma rays 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. The results indicated that the shortest pulse width that an MPPC can evaluate is 82 ps under ideal conditions. However, the experimental data were affected by a time jitter. The measured gamma ray pulse width including time jitter was 540 ps. The main reason for the large discrepancy was considered to be the noise of the trigger signal. We successfully reduced the time jitter to 77 ps after an improvement. 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.