The PERCIVAL detector: first user experiments

Abstract

The PERCIVAL detector is a CMOS imager designed for the soft X-ray regime at photon sources. Although still in its final development phase, it has recently seen its first user experiments: ptychography at a free-electron laser, holographic imaging at a storage ring and preliminary tests on X-ray photon correlation spectroscopy. The detector performed remarkably well in terms of spatial resolution achievable in the sample plane, owing to its small pixel size, large active area and very large dynamic range; but also in terms of its frame rate, which is significantly faster than traditional CCDs. In particular, it is the combination of these features which makes PERCIVAL an attractive option for soft X-ray science.

Keywords: X-ray detectors; XPCS; detectors; holographic imaging; ptychography; soft X-rays.

Figure 1

Results of FTH and phase retrieval with the PERCIVAL detector. (a) Map of fixed gains used for the composed final hologram (central part only). For color definition see the legend in (c). (b) Hologram [same crop as in (a)] assembled from all data recorded with different gain settings and left-handed polarised soft X-rays. Contour lines mark transitions from one fixed gain region to the next. (c) Line scan of the hologram in (b) through its center. Pixel values using different gains are depicted using different colors. (d) Difference hologram corresponding to (b). (e) FTH reconstruction of (d) revealing magnetic domains inside the field of view and (f) corresponding phase-retrieved reconstruction. Scale bar is 300 nm. (g) Phase-retrieval transfer function and Fourier ring correlation of the reconstruction in (f). Gray dashed lines indicate the 0.5 and half-bit thresholds.

Figure 2

(a) Schematic outline of the experimental setup used for the experiment described by Kharitonov et al. (2021 ▸). The FEL beam was focused using KB mirrors, thus creating a divergent probe geometry. A restricting aperture was used to limit spatial fluctuations. The sample was placed after the focus. Values in brackets correspond to the setup used with the PERCIVAL detector in 2020. (b) Optical microscopy image of the sample. Two scanned areas, one highlighted in a magenta square and one in green, were used. The magenta area indicates non-symmetric structure made up of letters and the green area bright and dark lines that radiate from a common center. Comparison of typical diffraction patterns measured with (c) the PERCIVAL detector and (d) the CCD camera reported by Kharitonov et al. (2021 ▸). Photons with approximately two orders of magnitude intensity and higher diffraction angles were captured with the PERCIVAL detector compared with the typical measurements in (d). Achieving the same dynamical range in (d) demands an increase in intensity and, consequently, a beamstop. The green scales in (c) and (d) show the maximum measured angle of diffraction in Fourier (q) space.

Figure 3

The sample reconstruction transparency of the sample from (a) the data measured with the PERCIVAL detector and (b) during the experiment described by Kharitonov et al. (2021 ▸).

Figure 4

FRC curves calculated from the independent reconstructions shown in Figs. 3 ▸(a) in orange and 3(b) in blue. The FRC shows a 1.3× improvement of the image obtained from the PERCIVAL detector data, reaching 250 nm.

Figure 5

(a) Series of speckle patterns measured from a static sample at P04 (PETRA III). The speckle forming the structure factor peak exhibits a single-frame speckle contrast of about 30%. The inset shows a microscope image of the static sample. (b) Waterfall plot of intensity over 1000 speckle patterns with a 0.12 s single frame exposure. The data were taken from the structure factor ring in (a). (c) Intermediate scattering function from the data shown in (b). For comparison, correlation functions with relaxation times τ = 1000 s and τ = 100 s are shown, highlighting the high stability during the experiment.

Figure 6

Timescales versus photon energy and/or photon wavelength in the soft and hard X-ray domains. The accessible range for common XPCS detectors are marked. The combination of the PERCIVAL sensor postprocessing (and thus its high sensitivity of low energy photons), together with its high frame rate (up to 300 Hz), will allow the scientific community to access a virtually unknown timescale. The star marks a typical timescale of skyrmion diffusion (size 1 µm) at room temperature (Zázvorka et al., 2019 ▸). The dynamic range is also compared for the detector systems analyzed at the photon energy E _ph = 700 eV.