Soft X-rays (in the range of 200-2000 eV) are of great interest for imaging applications for a large variety of sectors. These include: imaging of organic samples, e.g., in the “water window” (between C and O, K-edges: 277 eV to 525 eV), anomalous scattering experiments around many K-edges of light elements, and L-edges of 3d transition metals (relevant to magnetic and superconducting materials). Many of the techniques performed at synchrotrons in the tender and hard X-ray regimes are currently hindered in the soft X-ray range by the lack of fast, large area, high dynamic-range detectors, sensitive to photons <2 keV .
To extend the application of the state-of-the art hybrid X-ray detectors to a lower energy range, PSI and FBK are developing silicon sensors based on Low Gain Avalanche Diodes (LGADs). These LGAD sensors are optimized for soft X-ray detection: they include a charge multiplication layer with a gain of ~10 in order to amplify the signal and improve the signal-to-noise ratio, combined with a thin entrance window in order to increase the quantum efficiency at lower photon energies . This contribution will present tests performed in the soft X-ray regime using the developed pixelated LGAD sensors with a pixel pitch of 75 μm, bump-bonded to the EIGER single photon counting read-out chip . LGAD sensors with different multiplication layer designs and a large detection area of 4×4 cm2 have been characterized at the SIM beamline at SLS (PSI, Switzerland) between 200 eV and 900 eV. Additionally, as a proof-of-principle, ptychographic scans in the soft X-ray energy range have been performed. The high dynamic range, large area, fast frame rate, and low noise of the detector outdo the results previously obtained with hybrid charge-integrating detectors (such as Mönch), and allow to achieve a high resolution, mainly limited by the size of the detector . Characterization results of the sensor performances will be presented, as well as preliminary ptychographic reconstructions obtained with 500-700 eV photons.
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