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Project ID: HPRI-CT-1999-50004
Finanziato nell'ambito di: FP5-HUMAN POTENTIAL
Paese: Sweden

Ultrafast x-ray streak camera

A readout technique for X-ray streak cameras analogous to the well-known time-correlated single photon counting technique for time-resolved spectroscopy on atoms has been developed, tested and implemented at the LLC/MAXLAB D611 x-ray beam line. The time resolution of this x-ray detector has been boosted from few ps to 500fs and the time repetition rate from few Hz to 7 kHz.

The two major bottlenecks (temporal jitter between the light source and the detector and spread in kinetic energy of the secondary photoelectrons) have been addressed and eliminated thanks to the development of this readout technique. We have demonstrated the possibility to completely eliminate trigger jitter and to increase the temporal resolution beyond the single-shot temporal resolution. Single-photon counting is based on the implementation of two distinct elements: Firstly, timing fiducials are used to correct for timing jitter. Secondly, single X-ray photons are imaged on the readout camera and their centers of gravity are accurately determined. Hence the arrival of an X-ray photon can be accurately determined with respect to the timing fiducial.

The development of this technique makes this tool suitable for use at synchrotrons and laser facilities with very cost effective properties. It will be particularly powerful in the synchrotron case for which this fast x-ray detector will provide, at first, a unique opportunity to increase the time resolution from the 100ps time scale to few hundred femtoseconds; and second, a repetition rate of the data acquisition higher than any other comparable facility at this time scale.

Number of considerations have been taken into account in order to ensure the optimum temporal response of the x-ray streak camera. Only some of the mechanisms for temporal smearing are compensated for by the single-photon counting readout presented here. Collective effects due to spreading of the ensemble of secondary electrons such as imperfect focusing and smearing due to the velocity spread of secondary electrons are cancelled. Statistical processes involving single X-ray photons or primary electrons cannot be cancelled. Such an effect is the broadening due to the finite width of the slit. The position where a single photon hits the photocathode will influence at which position the center of gravity of the ensemble is imaged. A mechanism that may ultimately be the limit of the temporal resolution of streak cameras has so far not been an issue. It will take a finite time for the primary electrons with a kinetic energy corresponding to the X-ray photons to be converted to secondary electrons, which then will escape from the cathode in a diffusive process. Since this stage involves statistics of the primary as well of the secondary electrons, this effect will only partially be improved upon by the here described single-photon counting technique, described here.

In order to obtain a sub-ps streak camera capable of averaging at 5-10kHz (tested at 7kHz), the streak camera was built with a CsI photocathode, an acceleration slit which is imaged, a sweep plates in a meander geometry (transmission line), a magnetic lens focusing and a MCP-based back-end suitable for discriminating close-lying pulses at a synchrotron radiation source with high rep-rate. Initial test give a temporal resolution of 5ps. This is limited by the slit width. In order to decrease the pulse duration we have used a smaller slit down to 50-200 micrometers to have sub-ps resolution.

The streak camera has now reached its specifications in high-repetition rate mode 5-10kHz. 800fs. In addition, the single-photon counting has been demonstrated to work at higher repetition rates than 1kHz. The time resolution which is anticipated to be below 500fs has not been possible to demonstrate yet. However, the algorithms have been tested in situ at the beam line looking at a real signal with the ability of finding centers of gravities of single photons with 100fs accuracy.

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Lund University
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