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ESRF interest & commitment to (ultra-) fast science
with X-rays
Kees ScheidtDiagnostics group, Machine division
ESRF interest & commitment to (ultra-) fast science with X-rays
1) Thorough tests of the quasi-isochronous Storage Ring in 1995 :inadequate since incompatible with high-intensity demands
so : typical bunch length remains ~100ps (fwhm) at 5mA single bunch
2) Operation of various filling patterns to satisfy the whole usercommunity, including specialized time-resolved beamlines
3) Development of X-ray time resolved detector forsub-pico second time resolved diffraction experiments
pump
streak- tube
output : ultra-fastmodulation
jitter-freesynchronisation
X-ray Probe100 picosec
laser
100 femtosec
sample
rampHV
ultra-fast X-ray diffraction experimentsthat use the pump-probe technique with :
a 100femtoSec Laser light pump pulse and
a 100picoSec X-ray probe pulse
the ultimate time resolution depends on :
1) intrinsic time resolution of the Streak Camera for X-rays 2) the quality of synchronization
Objective : 1) sub-picoSec time resolution 2) high quality data by accumulation over many shots
the Streak-Tube :improved version ofcommercial model (Photonis)
10 picosec
5mm
460ps fwhm result when accumulating 900 shots
Critical laser issues :
1) Amplitude stability
2) Contrast (or pre-pulse energy variations)
DQE of Streak Camera
1,E-04
1,E-03
1,E-02
1,E-01
1,E+00
5 10 15 20 25 30
Energy in KeV
'Csi-500' 'Ki-1000 'Ki-500'
'KBr-1000' 'Kbr-500' 'Au-300'
Sensitivity for hard X-rays is low due to poor DQEof photo-cathodes
decreasing DQE forincreasing Energy
best : Cesium-Iodide(100)
low : Potassium-Iodide (10) Potassium-Bromide
poor : Gold(1)
Conclusion (1):
Optical synchronization by a GaAs photo-switch between afemtosec-laser and a Streak-Camera attains values <100fs when :
-Laser energy >25uJ per pulse
-Laser stability 0.25% rms
-Laser contrast >104
In these conditions (obtained after delicate adjustments) the time resolution of the system in accumulation mode is determined bythe tube’s intrinsic time resolution, at ~500fs fwhm for UV light
The exact time resolution for hard X-rays was never assessed since various attempts to produce a sub-pico sec event in an appropriatePump-Probe experiments were without success.
Conclusion (2):
The accumulating Jitter-Free X-ray streak camera for sub-pico sec pump-probe experiments was Not a scientific successdue to :
1) The difficulty of conceiving an appropriate experiment
2) The required delicate adjustments on laser, photo-switchand streak-camera making it not user-friendly
3) The particular limitations of a Streak-Tube detector:a) No real space-dimensionb) Low sensitive photo-cathode material