High frame rate RIXS spectroscopy using a JUNGFRAU detector with an iLGAD sensor

TL;DR

This study demonstrates time-resolved RIXS in the soft X-ray range at the EuXFEL hRIXS spectrometer using a JUNGFRAU detector equipped with an inverse LGAD sensor. The detector achieves high spatial sampling along the dispersive axis, low noise, and high frame rates (up to $\sim47\,\mathrm{kHz}$ per train), yielding a resolving power $R>10^4$ at around $928.5\ \mathrm{eV}$. The work also investigates intra-train FEL effects on the measured RIXS signal and demonstrates pump-probe RIXS on CuO, providing a proof of concept for time-resolved studies at multi-kHz frame rates. These results establish the JUNGFRAU-iLGAD as a strong candidate for maximizing the hRIXS spectrometer’s potential, while outlining practical considerations for FEL-induced sample effects and future sensor optimizations to push soft X-ray performance further.

Abstract

Resonant inelastic X-ray scattering (RIXS) is a powerful photon-in, photon-out spectroscopy technique for probing electronic, magnetic, and lattice excitations in matter. Time-resolved RIXS extends this capability through a stroboscopic optical pump-probe scheme to characterize the time evolution of the photoexcitation and subsequent relaxation dynamics of a sample. This technique is, however, extremely photon-hungry, requiring high-repetition-rate and intense X-ray facilities. The Heisenberg RIXS (hRIXS) spectrometer at the Spectroscopy and Coherent Scattering (SCS) instrument of the European X-ray Free-Electron Laser (EuXFEL) is designed to exploit high-repetition-rates, while maintaining optimal time and energy resolution. In this work, we demonstrate the successful deployment of a JUNGFRAU detector equipped with an inverse Low Gain Avalanche Diode (iLGAD) sensor for time-resolved RIXS studies in the soft X-ray range, using the hRIXS spectrometer. A spatial resolution of $19.71 \pm 0.7~μ\mathrm{m}$ and a resolving power exceeding 10,000 were achieved at an unprecedented frame rate of 47 kHz. Intra-train resolved data measured with a high FEL peak fluence of $1.8~\mathrm{mJ/cm^{2}}$ for a 928.5 eV ph photon energy and 1.1 MHz repetition rate from cupric oxide (CuO) revealed a decrease in the emitted signal by ~10% over a time interval of $340~μ\mathrm{s}$, indicating FEL-induced effects that require monitoring when conducting high-repetition-rate experiments. These results establish the JUNGFRAU-iLGAD as a promising detector to harvest the full potential of the hRIXS spectrometer, and validate its suitability for soft X-ray applications.

Figures (13)

• Figure 1: Strixel sensor geometry with the elongated $25\!\times\!225$ µm pixel design and corresponding mapping between the multiplication layer implants and the original JUNGFRAU ASIC pixel.
• Figure 2: JUNGFRAU-iLGAD detector used in this work, connected to a 160 CF flange for vacuum integration. Key components are the front-end module with the iLGAD sensor and ASICs, the readout board, and coolant inlet and outlet (not visible).
• Figure 3: Left: Average noise per memory cell for different sensor bias voltages, measured from dark frames with 5µs integration time. Cell 15 exhibits lower noise due to larger physical capacitance. Right: Pixel-wise noise map for memory cell 15 at 300V.
• Figure 4: Left: Energy spectrum of a single pixel under uniform illumination of 1.49keV Al K-shell fluorescence photons, at 300V sensor bias. Right: Summed energy spectra from all pixels, grouped by memory cell, after gain calibration, indicating a uniform energy response.
• Figure 5: Left: Average gain constant per memory cell at 200V, 250V, and 300V bias voltage. Right: Pixel-wise gain map for memory cell 15 at 300V.