Towards hybrid pixel detectors for energy-dispersive or soft X-ray photon science

Abstract

JUNGFRAU (adJUstiNg Gain detector FoR the Aramis User station) is a two-dimensional hybrid pixel detector for photon science applications at free-electron lasers and synchrotron light sources. The JUNGFRAU 0.4 prototype presented here is specifically geared towards low-noise performance and hence soft X-ray detection. The design, geometry and readout architecture of JUNGFRAU 0.4 correspond to those of other JUNGFRAU pixel detectors, which are charge-integrating detectors with 75 µm × 75 µm pixels. Main characteristics of JUNGFRAU 0.4 are its fixed gain and r.m.s. noise of as low as 27 e(-) electronic noise charge (<100 eV) with no active cooling. The 48 × 48 pixels JUNGFRAU 0.4 prototype can be combined with a charge-sharing suppression mask directly placed on the sensor, which keeps photons from hitting the charge-sharing regions of the pixels. The mask consists of a 150 µm tungsten sheet, in which 28 µm-diameter holes are laser-drilled. The mask is aligned with the pixels. The noise and gain characterization, and single-photon detection as low as 1.2 keV are shown. The performance of JUNGFRAU 0.4 without the mask and also in the charge-sharing suppression configuration (with the mask, with a `software mask' or a `cluster finding' algorithm) is tested, compared and evaluated, in particular with respect to the removal of the charge-sharing contribution in the spectra, the detection efficiency and the photon rate capability. Energy-dispersive and imaging experiments with fluorescence X-ray irradiation from an X-ray tube and a synchrotron light source are successfully demonstrated with an r.m.s. energy resolution of 20% (no mask) and 14% (with the mask) at 1.2 keV and of 5% at 13.3 keV. The performance evaluation of the JUNGFRAU 0.4 prototype suggests that this detection system could be the starting point for a future detector development effort for either applications in the soft X-ray energy regime or for an energy-dispersive detection system.

Keywords: energy-dispersive detectors; hybrid detectors; instrumentation for FELs; instrumentation for synchrotrons; soft X-rays.

Figure 1

Photograph of the JUNGFRAU 0.4 chip and sensor on top of which a 150 µm-thick laser-drilled tungsten mask (Laser Zentrum Hannover eV, Hannover, Germany) with ∼28 µm-diameter holes is placed. The chip and sensor are about 3.6 mm × 3.6 mm in size, while the mask dimensions are about 3.3 mm × 3.3 mm. The total active area in the mask region is about 1.1 mm². The mask is placed on top of the sensor and the mask holes are aligned with the center of the pixels with an automatic sub-micrometer die bonder (Fineplacer Femto, Finetech GmbH & Co. KG, Berlin, Germany). The mask is attached with a drop of glue at each mask corner. The bottom right corner of the mask does not contain any holes for reference purposes.

Figure 2

Distribution of r.m.s. noise for JUNGFRAU 0.4 (a) with no additional noise filtering and (b) with noise filtering. (c) Gain distribution of JUNGFRAU 0.4.

Figure 3

Mean of the r.m.s. noise of JUNGFRAU 0.4 (no extra filtering) as a function of the acquisition time for a hybridized assembly (chip bump-bonded to sensor) at chip operating temperatures of 30°C and −10°C and for a bare chip at a chip operating temperature of 30°C.

Figure 4

Fluorescence spectra of Mo from single JUNGFRAU 0.4 pixels with the mask and without the mask. The noise of the JUNGFRAU 0.4 pixel can be determined from the spectrum of the masked pixel due to the absence of charge-sharing counts in the spectrum and is determined to be 48 e⁻.

Figure 5

JUNGFRAU 0.4 single-pixel spectra at a photon energy of 1.2 keV. The normalized spectrum of a pixel from a detector without a mask (black) and the spectrum of a pixel from a detector with a hole mask (red) are superimposed.

Figure 6

(a) JUNGFRAU 0.4 multi-photon spectrum of a single pixel at a single photon energy of 1.55 keV. (b) Linear pixel response.

Figure 7

Single-pixel fluorescence X-ray spectra acquired by JUNGFRAU 0.4 with and without the charge-sharing suppression mask. (a) Energy-dispersive measurements of fluorescence photons from a composite Cr, Fe, Cu and Ge target. (b) Fluorescence spectrum from a Mo target. The spectra of a masked (red) and an unmasked pixel (black) are superimposed for direct comparison; the spectra obtained from the software mask (blue) and the cluster-finding algorithm (gray) applied to the data of the unmasked pixel are included for the Mo fluorescence (b). (c) Zoom on the Mo peaks.

Figure 8

Flyscan images and spectrum of a fluorescence imaging target. The letters of ‘SOLEIL’ are made of Ni, while the material of the sun (with a Siemens resolution star at the center) is Au. (a) Full photon image of the Flyscan. (b) Gain-corrected integral photon spectrum of the entire Flyscan. (c)–(j) Two-color photon images where the Ni letters (left column) and the Au sun (right column) are selected by photon energy for JUNGFRAU 0.4 in combination with the mask (c, d), without the mask (e, f), the software mask (g, h) and the cluster-finding algorithm (i, j).