Abstract
The performance and parameters of the online photon single-shot spectrometer (PSSS) at the Aramis beamline of the SwissFEL free-electron laser are presented. The device operates between the photon energies 4 and 13 keV and uses diamond transmission gratings and bent Si crystals for spectral measurements on the first diffraction order of the beam. The device has an energy window of 0.7% of the median photon energy of the free-electron laser pulses and a spectral resolution (full width at half-maximum) ΔE/E on the order of 10-5. The device was characterized by comparing its performance with reference data from synchrotron sources, and a parametric study investigated other effects that could affect the reliability of the spectral information.
Highlights
The pulse-to-pulse measurement of spectra of self-amplified spontaneous emission (SASE) X-ray free-electron lasers (FELs) (Bergmann et al, 2017) is of fundamental importance for several experimental techniques ranging from resonant inelastic X-ray scattering (RIXS) (Lemke et al, 2013; Chergui, 2016; Obara et al, 2017; Park et al, 2019; Kimberg & Rohringer, 2016; Błachucki et al, 2014; Kayser et al, 2019) to protein crystallography (Tono et al, 2015; Moreno-Chicano et al, 2019)
The SASE process changes the spectral properties of the X-ray pulse on a pulse-to-pulse basis, which requires a device for online, non-invasive measurements of the X-ray spectra for both experimental spectral normalization and performance optimization (Rehanek et al, 2017)
A good example of the use of an online spectrometer is in X-ray absorption spectroscopy (XAS), where measurements have been reported using both SASE and sample-transmitted spectra (Boutet & Hunter, 2018; Katayama et al, 2013; Brenner et al, 2019)
Summary
The pulse-to-pulse measurement of spectra of self-amplified spontaneous emission (SASE) X-ray free-electron lasers (FELs) (Bergmann et al, 2017) is of fundamental importance for several experimental techniques ranging from resonant inelastic X-ray scattering (RIXS) (Lemke et al, 2013; Chergui, 2016; Obara et al, 2017; Park et al, 2019; Kimberg & Rohringer, 2016; Błachucki et al, 2014; Kayser et al, 2019) to protein crystallography (Tono et al, 2015; Moreno-Chicano et al, 2019). A heliumfilled flight tube between the crystal and the camera ensures sufficient transmission even at lower online spectra with tunability in photon energies Both resolution and spectral range (Katayama et al, 2016). The grating pitches are spectrometer (PSSS) (Rehanek et al, 2017) combines a chosen such that the first-order diffracted beam is always far transmission grating with bent Si crystals to create a spectro- enough from the zeroth order so that the crystals can be put meter that has a good resolution, large spectral range, and safely into it and do not block or affect the propagation of good transmission for online spectral measurements of photon the main beam to the experimental station downstream. The diamond grating diffracts the incoming FEL beam in the horizontal plane, sending the first order to the bent crystal spectrometer while the zeroth order continues further downstream with 80% or more of the incoming flux. The field of view of the detector is about 4 mm, so the spectra becomes cut off as one approaches that limit, and the integrated intensity drops off as a part of the spectrum is cut out, as shown in Figs. 3(c) and 3(d)
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