Seminars

A sub-picosecond hard X-ray streak camera using single-photon counting

by Prof. Jorgen Larsson (Atomic Physics Division, Lund University, Sweden)

Thursday, January 20, 2011 from to (Europe/Berlin)
at Room 3.11
Description
We have developed a hard X-ray accumulating streak camera that achieves sub picosecond time resolution by using single photon counting. A high repetition rate of 2 kHz was achieved by use of a readout camera with built in image processing capabilities. The effects of sweep jitter were removed by using a UV timing reference. The use of single photon counting allows the camera to reach high quantum efficiency by not limiting the divergence for the photoelectrons.
Presently the fastest kind of X-ray detector is the streak camera. It has been extensively used in time resolved measurements and a time resolution down to 233 fs [1] has been demonstrated using UV, and 350 fs using 1.5 keV X-rays [2].  This time resolution is however not yet reached for hard X-rays. Due to the dispersion of the photoelectrons and imperfect electron optics the fastest streak cameras rely on limiting the divergence of the photoelectrons to compensate. This implies a severe reduction in quantum efficiency. 
We have developed a streak camera that uses single-photon counting to reduce the effects of dispersion and imperfect imaging. Images are analyzed in real time, and a UV timing reference is used to track and compensate the sweep jitter. The use of a readout camera with built-in image processing capabilities reduces the bandwidth required to transfer data to the host computer. Thus a frame rate of 2 kHz could be reached.
To test the system a second UV pulse was generated by splitting off part of the power in the reference beam. This second beam was then used as a simulated X-ray signal at a fixed delay of 8 ps. We have also demonstrated sub-ps resolution for X-rays by repeating a series of ultrafast X-ray diffraction experiments.